Fabien Saint
email: fabiensaint@hotmail.com

Alexandra Masson-Lecomte

Saint Pr Fabien

Achieving disease free distal ureteral margin at the time of radical cystectomy: Why and for whom? (an overview of literature)

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Introduction

Radical cystectomy (RC) with pelvic lymph node dissection is the standard treatment option for non-metastatic, resectable muscle invasive bladder cancer (BCa) with a life expectancy rate close to 70 % at 5 years [START_REF] Shariat | Outcomes of radical cystectomy for transitional cell carcinoma of the bladder: a contemporary series from the Bladder Cancer Research Consortium[END_REF]. However, in order to maximize oncologic outcome, the surgical procedure has to be standardized [START_REF] Witjes | Updated 2016 EAU Guidelines on Muscle-invasive and Metastatic Bladder Cancer[END_REF]. Patients with BCa are at risk of experiencing upper urinary tract recurrences (UUTR) during the follow-up period [START_REF] Volkmer | Upper urinary tract recurrence after radical cystectomy for bladder cancer--who is at risk?[END_REF]. Upper urinary tract recurrence is frequently symptomatic, advanced disease, and associated with a metastatic evolution that severely affect survival expectancy [START_REF] Volkmer | Upper urinary tract recurrence after radical cystectomy for bladder cancer--who is at risk?[END_REF][START_REF] Furukawa | Upper urinary tract recurrence following radical cystectomy for bladder cancer[END_REF][START_REF] Sanderson | Upper tract urothelial recurrence following radical cystectomy for transitional cell carcinoma of the bladder: an analysis of 1,069 patients with 10-year follow up[END_REF].

Although patients with a positive ureteral intraoperative frozen section have been found to be at increased risk of developing UUTR during follow-up [START_REF] Raj | Significance of intraoperative ureteral evaluation at radical cystectomy for urothelial cancer[END_REF][START_REF] Osman | The value of frozen-section analysis of ureteric margins on surgical decision-making in patients undergoing radical cystectomy for bladder cancer[END_REF][START_REF] Tollefson | Significance of distal ureteral margin at radical cystectomy for urothelial carcinoma[END_REF][START_REF] Gakis | Sequential resection of malignant ureteral margins at radical cystectomy: a critical assessment of the value of frozen section analysis[END_REF], the optimal intraoperative management of patients with intraoperative positive ureteral margin is still under debate. Surgeons make the decision to evaluate margins intraoperatively on a case-bycase basis. Analysis of available literature questions both: 1/ the ability of intra operative frozen sections to properly predict final involvement of the ureters; 2/ the impact of invaded intra operative ureteral sections (with or without achieving negative ureteral margin) on upper urinary tract recurrences and oncological outcome; 3/ patients most likely to benefit of intra operative ureteral frozen section. In that regard, the latest EAU guidelines do not provide any recommendations [START_REF] Witjes | Updated 2016 EAU Guidelines on Muscle-invasive and Metastatic Bladder Cancer[END_REF]. The aim of this review was to evaluate frequency, reliability and impact of positive distal ureteral margin after radical cystectomy for bladder cancer on upper tract recurrence, cancer specific and overall survival, and to identify best candidates for intra operative frozen section analyses.

Material and Methods

A systemic review was performed following the PRISMA guideline using PubMed/Medline and Cochrane Library databases to identify eligible studies published between the creation of the databases and April 2020. The primary search string included the following items: bladder cancer OR cystectomy AND frozen section OR ureteral margin.

Filters were applied to exclude any studies not performed on humans. Additionally, a manual search of the reference lists of relevant meta-analysis was conducted to identify all available studies.

The reference lists from selected papers were also searched for relevant studies that may have been missed during the initial searches. Papers for inclusion analyzed at patient treated by radical cystectomy for urothelial carcinoma of the bladder (Bladder cancer, BCa).

The papers studied ureteral margin, prognostic factors associated with positive ureteral margin and their conversion to negative, and evaluated impact on upper urinary tract recurrence-free rate (UUTR), cancer-specific survival, and overall survival of these positive ureteral margins. Randomized controlled trials and prospective or retrospective analyses exploring patients with BCa treated by RC and studied on the basis of ureteral margin have been included. All previous reviews were automatically excluded, as were single case reports, editorial pieces, comment, and titles that were not available in English/French. The subsequently available papers were then systematically reviewed. The ultimate goal of this review was to assess any available papers that examined ureteral margin status at time of RC (frozen (FS) and permanent sections (PS)), that identified prognostic factors associated with positive ureteral margins, and that evaluated impact on upper urinary tract recurrence-free rate (UUTR), Cancer-Specific Survival (CSS), and Overall Survival (OS). The search technique is summarized in Figure 1.

Results

In total, thirty two relevant studies were identified. All studies published between 1989 and 2020 were retrospective, except the one published by Osman et al. [START_REF] Osman | The value of frozen-section analysis of ureteric margins on surgical decision-making in patients undergoing radical cystectomy for bladder cancer[END_REF]. Of these, seventeen studies reported specifically ureteral margin status (Table 1), and seventeen studies have recorded reliability of FS using PS as a reference (Table 2). Furthermore, twelve studies reported ureteral margin status (FS and/or PS) and upper urinary tract recurrence risk and three studies evaluated distal ureteral margin status as prognostic factor for cancer specific or overall survival after cystectomy (Table 3). Twelve studies explored prognostic factors associated with pathological positive ureteral margin at time of RC (Table 4). Potential studies bias have been reported Table 5.

Positive frozen section (FS) and permanent section (PS) rate

In contemporary series, the incidence of initial positive ureteral margin ranged between 1.1% to 25.4% for FS, and between 0.8% and 15% for PS (Table 1) [START_REF] Raj | Significance of intraoperative ureteral evaluation at radical cystectomy for urothelial cancer[END_REF][START_REF] Osman | The value of frozen-section analysis of ureteric margins on surgical decision-making in patients undergoing radical cystectomy for bladder cancer[END_REF][START_REF] Tollefson | Significance of distal ureteral margin at radical cystectomy for urothelial carcinoma[END_REF][START_REF] Gakis | Sequential resection of malignant ureteral margins at radical cystectomy: a critical assessment of the value of frozen section analysis[END_REF][START_REF] Johnson | Are frozen-section examinations of ureteral margins required for all patients undergoing radical cystectomy for bladder cancer?[END_REF][START_REF] Batista | Significance of ureteral carcinoma in situ in specimens of cystectomy[END_REF][START_REF] Schoenberg | Ureteral frozen section analysis during cystectomy: a reassessment[END_REF][START_REF] Silver | Ureteral carcinoma in situ at radical cystectomy: does the margin matter?[END_REF][START_REF] Lee | Significance of cancer involvement at the ureteral margin detected on routine frozen section analysis during radical cystectomy[END_REF][START_REF] Schumacher | Is there an indication for frozen section examination of the ureteral margins during cystectomy for transitional cell carcinoma of the bladder?[END_REF][START_REF] Touma | Ureteral frozen sections at the time of radical cystectomy: reliability and clinical implications[END_REF][START_REF] Gondo | Preoperative prediction of malignant involvement of resected ureters in patients undergoing radical cystectomy for bladder cancer[END_REF][START_REF] Zhou | Intraoperative frozen section evaluation of ureteral and urethral margins: studies of 203 consecutive radical cystoprostatectomy for men with bladder urothelial carcinoma[END_REF][START_REF] Kim | The clinical significance of intra-operative ureteral frozen section analysis at radical cystectomy for urothelial carcinoma of the bladder[END_REF][START_REF] Reder | Diagnostic accuracy of intraoperative frozen sections during radical cystectomy does not affect disease-free or overall survival: a study of 364 patients with urothelial carcinoma of the urinary bladder[END_REF][START_REF] Moschini | Effect on postoperative survival of the status of distal ureteral margin: The necessity to achieve negative margins at the time of radical cystectomy[END_REF][22].

Sensibility ranged from 45 to 100% and specificity from 83 to 100%. Positive Predictive Value (PPV) ranged from 25 to 100% and Negative Predictive Value (NPV) from 91 to 100%. Positive margin was successfully converted to negative margin during surgery for 39% to 100% patients (Table 2) [START_REF] Raj | Significance of intraoperative ureteral evaluation at radical cystectomy for urothelial cancer[END_REF][START_REF] Osman | The value of frozen-section analysis of ureteric margins on surgical decision-making in patients undergoing radical cystectomy for bladder cancer[END_REF][START_REF] Tollefson | Significance of distal ureteral margin at radical cystectomy for urothelial carcinoma[END_REF][START_REF] Gakis | Sequential resection of malignant ureteral margins at radical cystectomy: a critical assessment of the value of frozen section analysis[END_REF][START_REF] Schoenberg | Ureteral frozen section analysis during cystectomy: a reassessment[END_REF][START_REF] Silver | Ureteral carcinoma in situ at radical cystectomy: does the margin matter?[END_REF][START_REF] Schumacher | Is there an indication for frozen section examination of the ureteral margins during cystectomy for transitional cell carcinoma of the bladder?[END_REF][START_REF] Touma | Ureteral frozen sections at the time of radical cystectomy: reliability and clinical implications[END_REF][START_REF] Zhou | Intraoperative frozen section evaluation of ureteral and urethral margins: studies of 203 consecutive radical cystoprostatectomy for men with bladder urothelial carcinoma[END_REF][START_REF] Kim | The clinical significance of intra-operative ureteral frozen section analysis at radical cystectomy for urothelial carcinoma of the bladder[END_REF][START_REF] Reder | Diagnostic accuracy of intraoperative frozen sections during radical cystectomy does not affect disease-free or overall survival: a study of 364 patients with urothelial carcinoma of the urinary bladder[END_REF][START_REF] Moschini | Effect on postoperative survival of the status of distal ureteral margin: The necessity to achieve negative margins at the time of radical cystectomy[END_REF][22][START_REF] Gordetsky | Ureteral and urethral frozen sections during radical cystectomy or cystoprostatectomy: an analysis of denudation and atypia[END_REF][START_REF] Loeser | Frozen section analysis of ureteral margins in patients undergoing radical cystectomy for bladder cancer: differential impact of carcinoma in situ in the bladder on reliability and impact on tumour recurrence in the upper urinary tract[END_REF][START_REF] Hoang | Clinical significance of ureteric 'skip lesions' at the time of radical cystectomy: the M.D. Anderson experience and literature review[END_REF][START_REF] Cerf | Usefulness of frozen section exams during radical cystectomy for urothelial carcinoma[END_REF].

Positive ureteral margins and upper urinary tract recurrence (UUTR), cancer specific and overall survival.

After total cystectomy, Upper Urinary Tract Recurrence (UUTR) is an uncommon event and was recorded for 0.2% to 6.2% of patients (Table 3) [START_REF] Raj | Significance of intraoperative ureteral evaluation at radical cystectomy for urothelial cancer[END_REF][START_REF] Tollefson | Significance of distal ureteral margin at radical cystectomy for urothelial carcinoma[END_REF][START_REF] Gakis | Sequential resection of malignant ureteral margins at radical cystectomy: a critical assessment of the value of frozen section analysis[END_REF][START_REF] Johnson | Are frozen-section examinations of ureteral margins required for all patients undergoing radical cystectomy for bladder cancer?[END_REF][START_REF] Schoenberg | Ureteral frozen section analysis during cystectomy: a reassessment[END_REF][START_REF] Silver | Ureteral carcinoma in situ at radical cystectomy: does the margin matter?[END_REF][START_REF] Schumacher | Is there an indication for frozen section examination of the ureteral margins during cystectomy for transitional cell carcinoma of the bladder?[END_REF][START_REF] Kim | The clinical significance of intra-operative ureteral frozen section analysis at radical cystectomy for urothelial carcinoma of the bladder[END_REF][START_REF] Reder | Diagnostic accuracy of intraoperative frozen sections during radical cystectomy does not affect disease-free or overall survival: a study of 364 patients with urothelial carcinoma of the urinary bladder[END_REF][START_REF] Moschini | Effect on postoperative survival of the status of distal ureteral margin: The necessity to achieve negative margins at the time of radical cystectomy[END_REF][22].

Positive section was associated with higher rate of UUTR [START_REF] Raj | Significance of intraoperative ureteral evaluation at radical cystectomy for urothelial cancer[END_REF][START_REF] Tollefson | Significance of distal ureteral margin at radical cystectomy for urothelial carcinoma[END_REF][START_REF] Gakis | Sequential resection of malignant ureteral margins at radical cystectomy: a critical assessment of the value of frozen section analysis[END_REF][START_REF] Moschini | Effect on postoperative survival of the status of distal ureteral margin: The necessity to achieve negative margins at the time of radical cystectomy[END_REF]22]. However, intraoperative positive frozen section converted to negative was associated with lower risk of UUTR compared to non-converted [START_REF] Tollefson | Significance of distal ureteral margin at radical cystectomy for urothelial carcinoma[END_REF][START_REF] Gakis | Sequential resection of malignant ureteral margins at radical cystectomy: a critical assessment of the value of frozen section analysis[END_REF][START_REF] Moschini | Effect on postoperative survival of the status of distal ureteral margin: The necessity to achieve negative margins at the time of radical cystectomy[END_REF]22].

In some papers, specific attention was paid to discriminate specific incidence of invasive tumor in uretero-ileal anastomosis from UUTR. Anastomotic recurrence was uncommon and its occurrence ranged between 0% and 1% (Table 3) [START_REF] Raj | Significance of intraoperative ureteral evaluation at radical cystectomy for urothelial cancer[END_REF][START_REF] Tollefson | Significance of distal ureteral margin at radical cystectomy for urothelial carcinoma[END_REF][START_REF] Gakis | Sequential resection of malignant ureteral margins at radical cystectomy: a critical assessment of the value of frozen section analysis[END_REF][START_REF] Johnson | Are frozen-section examinations of ureteral margins required for all patients undergoing radical cystectomy for bladder cancer?[END_REF][START_REF] Schoenberg | Ureteral frozen section analysis during cystectomy: a reassessment[END_REF][START_REF] Silver | Ureteral carcinoma in situ at radical cystectomy: does the margin matter?[END_REF][START_REF] Schumacher | Is there an indication for frozen section examination of the ureteral margins during cystectomy for transitional cell carcinoma of the bladder?[END_REF][START_REF] Touma | Ureteral frozen sections at the time of radical cystectomy: reliability and clinical implications[END_REF][START_REF] Kim | The clinical significance of intra-operative ureteral frozen section analysis at radical cystectomy for urothelial carcinoma of the bladder[END_REF]22].

Regarding association with cancer specific mortality, results are equivocal. Cancer specific mortality attributable to concomitant carcinoma in situ on ureteral section appeared to be weak, probably hidden by bladder cancer specific mortality. Two contemporary retrospective studies (with more than 300 patients) failed to associate positive frozen section to survival [START_REF] Touma | Ureteral frozen sections at the time of radical cystectomy: reliability and clinical implications[END_REF][START_REF] Reder | Diagnostic accuracy of intraoperative frozen sections during radical cystectomy does not affect disease-free or overall survival: a study of 364 patients with urothelial carcinoma of the urinary bladder[END_REF], while Moschini et al associated positive frozen section with CSS and OS in one of the largest retrospective study published (n=1147) [START_REF] Moschini | Effect on postoperative survival of the status of distal ureteral margin: The necessity to achieve negative margins at the time of radical cystectomy[END_REF].

Prognostic factors associated with positive ureteral margins

Eleven studies reported prognostic factors associated with positive ureteral sections.

All but one included a multivariate analysis. No prospective study was identified.

Involvement of the prostatic urethra, history of bladder Cis, trigone involvement, hydronephrosis, clinical stage over pT2, tumor multifocality were advocated [START_REF] Raj | Significance of intraoperative ureteral evaluation at radical cystectomy for urothelial cancer[END_REF][START_REF] Osman | The value of frozen-section analysis of ureteric margins on surgical decision-making in patients undergoing radical cystectomy for bladder cancer[END_REF][START_REF] Gakis | Sequential resection of malignant ureteral margins at radical cystectomy: a critical assessment of the value of frozen section analysis[END_REF][START_REF] Touma | Ureteral frozen sections at the time of radical cystectomy: reliability and clinical implications[END_REF][START_REF] Gondo | Preoperative prediction of malignant involvement of resected ureters in patients undergoing radical cystectomy for bladder cancer[END_REF][START_REF] Zhou | Intraoperative frozen section evaluation of ureteral and urethral margins: studies of 203 consecutive radical cystoprostatectomy for men with bladder urothelial carcinoma[END_REF][START_REF] Kim | The clinical significance of intra-operative ureteral frozen section analysis at radical cystectomy for urothelial carcinoma of the bladder[END_REF][START_REF] Moschini | Effect on postoperative survival of the status of distal ureteral margin: The necessity to achieve negative margins at the time of radical cystectomy[END_REF]22,[START_REF] Loeser | Frozen section analysis of ureteral margins in patients undergoing radical cystectomy for bladder cancer: differential impact of carcinoma in situ in the bladder on reliability and impact on tumour recurrence in the upper urinary tract[END_REF][START_REF] Nakanishi | Management of concomitant ureteral carcinoma in situ at radical cystectomy[END_REF][START_REF] Masson-Lecomte | Predictive factors for final pathologic ureteral sections on 700 radical cystectomy specimens: Implications for intraoperative frozen section decision-making[END_REF] (Table 4). Bladder CIS was the most consistent factor associated with positive ureteral section (FS and/or PS) at time of radical cystectomy (odds ratio between 6.4 and 177.9 for FS and between 3.3 and 11.1 for PS (Table 4)).

Discussion

To our knowledge, this study is the first systemic extended review related to ureteral margin frequency, reliability and impact of positive distal ureteral margin on upper tract recurrence, cancer specific and overall survival. This systemic review have shown a significant degree of clinical and methodological heterogeneity across all studies (including patient groups, length of follow up and outcome measures), preventing pooled analysis of the results and making direct comparison between the papers sometime very challenging. Furthermore quality of these reports did not meet current standards of evidence-based medecine and were most of the time based on retrospective studies (Level 4).

Positive frozen section rate

Performing frozen section analysis of the distal ureteral margins at the time of cystectomy is daily routine in many urologic institutions. Positive results would lead to additional tissue resection in order to obtain a negative margin status. In old series, assiduous approaches using step-sectioning techniques have demonstrated that more than half of patients showed intra epithelial carcinoma or more advanced ureteric lesions for solid papillary bladder tumors, and more than one third showed carcinoma in situ or more advanced lesions in the ureteric stumps [START_REF] Schade | Morphological changes in the ureter in cases of bladder carcinoma[END_REF][START_REF] Farrow | Morphological and clinical observations of patients with early bladder cancer treated with total cystectomy[END_REF].

In contemporary series, we observed a high variability in the positive ureteral section rate (1.1% to 25.4%) [START_REF] Raj | Significance of intraoperative ureteral evaluation at radical cystectomy for urothelial cancer[END_REF][START_REF] Osman | The value of frozen-section analysis of ureteric margins on surgical decision-making in patients undergoing radical cystectomy for bladder cancer[END_REF][START_REF] Tollefson | Significance of distal ureteral margin at radical cystectomy for urothelial carcinoma[END_REF][START_REF] Gakis | Sequential resection of malignant ureteral margins at radical cystectomy: a critical assessment of the value of frozen section analysis[END_REF][START_REF] Johnson | Are frozen-section examinations of ureteral margins required for all patients undergoing radical cystectomy for bladder cancer?[END_REF][START_REF] Batista | Significance of ureteral carcinoma in situ in specimens of cystectomy[END_REF][START_REF] Schoenberg | Ureteral frozen section analysis during cystectomy: a reassessment[END_REF][START_REF] Silver | Ureteral carcinoma in situ at radical cystectomy: does the margin matter?[END_REF][START_REF] Lee | Significance of cancer involvement at the ureteral margin detected on routine frozen section analysis during radical cystectomy[END_REF][START_REF] Schumacher | Is there an indication for frozen section examination of the ureteral margins during cystectomy for transitional cell carcinoma of the bladder?[END_REF][START_REF] Touma | Ureteral frozen sections at the time of radical cystectomy: reliability and clinical implications[END_REF][START_REF] Gondo | Preoperative prediction of malignant involvement of resected ureters in patients undergoing radical cystectomy for bladder cancer[END_REF][START_REF] Zhou | Intraoperative frozen section evaluation of ureteral and urethral margins: studies of 203 consecutive radical cystoprostatectomy for men with bladder urothelial carcinoma[END_REF][START_REF] Kim | The clinical significance of intra-operative ureteral frozen section analysis at radical cystectomy for urothelial carcinoma of the bladder[END_REF][START_REF] Reder | Diagnostic accuracy of intraoperative frozen sections during radical cystectomy does not affect disease-free or overall survival: a study of 364 patients with urothelial carcinoma of the urinary bladder[END_REF][START_REF] Moschini | Effect on postoperative survival of the status of distal ureteral margin: The necessity to achieve negative margins at the time of radical cystectomy[END_REF][22]. It can be explained as much by some differences in pathologic evaluation and definition of ureteral sample (presence of carcinoma and CIS for most of the authors, associated with high grade dysplasia and atypia for others), as well as by differences in intraoperative management of ureteral length (shortening of the ureter probably minimizes the risk of positive surgical margins). Hence when the redundant ureter segment resected above the common iliac vessels was performed, carcinoma in situ was detected in only 0.1% on permanent section, suggesting that ureteral frozen section analysis is probably unnecessary in the majority of patients undergoing RC [START_REF] Schumacher | Is there an indication for frozen section examination of the ureteral margins during cystectomy for transitional cell carcinoma of the bladder?[END_REF].

Sensibility and specificity of ureteral frozen section (FS)

Despite the risk of distal ureteric malignancy on radical cystectomy specimen [START_REF] Schumacher | Is there an indication for frozen section examination of the ureteral margins during cystectomy for transitional cell carcinoma of the bladder?[END_REF], the clinical value of routine FS remains questionable. Studies reported in this review showed that FS confirmed a very good specificity and a very good NPV [START_REF] Raj | Significance of intraoperative ureteral evaluation at radical cystectomy for urothelial cancer[END_REF][START_REF] Gakis | Sequential resection of malignant ureteral margins at radical cystectomy: a critical assessment of the value of frozen section analysis[END_REF][START_REF] Silver | Ureteral carcinoma in situ at radical cystectomy: does the margin matter?[END_REF][START_REF] Touma | Ureteral frozen sections at the time of radical cystectomy: reliability and clinical implications[END_REF][START_REF] Zhou | Intraoperative frozen section evaluation of ureteral and urethral margins: studies of 203 consecutive radical cystoprostatectomy for men with bladder urothelial carcinoma[END_REF][START_REF] Reder | Diagnostic accuracy of intraoperative frozen sections during radical cystectomy does not affect disease-free or overall survival: a study of 364 patients with urothelial carcinoma of the urinary bladder[END_REF]22,[START_REF] Loeser | Frozen section analysis of ureteral margins in patients undergoing radical cystectomy for bladder cancer: differential impact of carcinoma in situ in the bladder on reliability and impact on tumour recurrence in the upper urinary tract[END_REF][START_REF] Hoang | Clinical significance of ureteric 'skip lesions' at the time of radical cystectomy: the M.D. Anderson experience and literature review[END_REF]. Using "presence of carcinoma" as pathologic definition of ureteral sample increased the specificity without large compromises in sensitivity [START_REF] Reder | Diagnostic accuracy of intraoperative frozen sections during radical cystectomy does not affect disease-free or overall survival: a study of 364 patients with urothelial carcinoma of the urinary bladder[END_REF]. However, with its low positive predictive value, FS tends to underestimate disease at the ureteral margin [START_REF] Osman | The value of frozen-section analysis of ureteric margins on surgical decision-making in patients undergoing radical cystectomy for bladder cancer[END_REF][START_REF] Silver | Ureteral carcinoma in situ at radical cystectomy: does the margin matter?[END_REF][START_REF] Touma | Ureteral frozen sections at the time of radical cystectomy: reliability and clinical implications[END_REF].

From a surgical point of view: indeed, there is no detrimental effect of performing FS (it is not time consuming). However in case of resection of the redundant ureter segment above the common iliac vessels, some authors reported that carcinoma in situ was not detected for 99.9% patients on permanent section, suggesting that ureteral frozen section analysis is probably unnecessary in the majority of patients undergoing RC. One may consider that resecting 5 cm or 10 cm (to be above the common iliac vessels) doesn't take much more time to surgeons than performing FS. Moreover, this technical point should be useful to avoid damage inflicted on the ureter during distal surgical mobilization, and subsequent complications such as necrosis and stenosis. From a cost-effectiveness point of view, the direct and indirect costs associated with FS are substantial and should be considered given the questionable clinical value of this test. Therefore, for some authors FS should be advocated and performed for selected patients undergoing radical cystectomy [START_REF] Touma | Ureteral frozen sections at the time of radical cystectomy: reliability and clinical implications[END_REF]. From a Human Resources point of view, access conditions to FS are somewhat heterogeneous in terms of hospitalization structure and execution (e.g. Cryostat, Décimu), making the availability of this examination sometimes difficult.

Positive ureteral margins (PUM) and upper urinary tract recurrence (UUTR), cancer specific and overall survival.

Several retrospective studies have shown that a positive ureteral margin does predict upper urinary tract recurrence [START_REF] Raj | Significance of intraoperative ureteral evaluation at radical cystectomy for urothelial cancer[END_REF][START_REF] Tollefson | Significance of distal ureteral margin at radical cystectomy for urothelial carcinoma[END_REF][START_REF] Schumacher | Is there an indication for frozen section examination of the ureteral margins during cystectomy for transitional cell carcinoma of the bladder?[END_REF]22,[START_REF] Loeser | Frozen section analysis of ureteral margins in patients undergoing radical cystectomy for bladder cancer: differential impact of carcinoma in situ in the bladder on reliability and impact on tumour recurrence in the upper urinary tract[END_REF][START_REF] Umbreit | Multifactorial, site-specific recurrence model after radical cystectomy for urothelial carcinoma[END_REF] and may be associated with malignant ureteroenteric stricture occurring in approximately 0.2% to 1% [START_REF] Lee | Significance of cancer involvement at the ureteral margin detected on routine frozen section analysis during radical cystectomy[END_REF][START_REF] Westerman | Malignant ureteroenteric anastomotic stricture following radical cystectomy with urinary diversion: Patterns, risk factors, and outcomes[END_REF] of the cases. For Schumacher et al. when frozen sections were negative, 3% of patients developed UUTR, vs.17% when frozen sections were positive [START_REF] Schumacher | Is there an indication for frozen section examination of the ureteral margins during cystectomy for transitional cell carcinoma of the bladder?[END_REF]. However, frequency of UUTR was probably linked to differences in intraoperative management performed by surgeons, and when the redundant ureteral segment was resected above the common iliac vessels, UUTR was detected in only 0.3% of patients [START_REF] Schumacher | Is there an indication for frozen section examination of the ureteral margins during cystectomy for transitional cell carcinoma of the bladder?[END_REF]. Moreover, recurrence side did not always match with the positive ureteral section side [START_REF] Raj | Significance of intraoperative ureteral evaluation at radical cystectomy for urothelial cancer[END_REF][START_REF] Tollefson | Significance of distal ureteral margin at radical cystectomy for urothelial carcinoma[END_REF][START_REF] Lee | Significance of cancer involvement at the ureteral margin detected on routine frozen section analysis during radical cystectomy[END_REF][START_REF] Reder | Diagnostic accuracy of intraoperative frozen sections during radical cystectomy does not affect disease-free or overall survival: a study of 364 patients with urothelial carcinoma of the urinary bladder[END_REF].

Consequently, intraoperative analysis of ureteral margin should theoretically facilitate tumor removal, decrease upper urinary tract recurrence (UUTR) rate and improve cancer specific survival. On a practical level, in many cases, an initial positive margin on frozen section can be converted to a final negative margin by sequential sectioning of the ureter (more than 50% of cases) [START_REF] Osman | The value of frozen-section analysis of ureteric margins on surgical decision-making in patients undergoing radical cystectomy for bladder cancer[END_REF][START_REF] Schoenberg | Ureteral frozen section analysis during cystectomy: a reassessment[END_REF][START_REF] Silver | Ureteral carcinoma in situ at radical cystectomy: does the margin matter?[END_REF][START_REF] Schumacher | Is there an indication for frozen section examination of the ureteral margins during cystectomy for transitional cell carcinoma of the bladder?[END_REF][START_REF] Touma | Ureteral frozen sections at the time of radical cystectomy: reliability and clinical implications[END_REF][START_REF] Zhou | Intraoperative frozen section evaluation of ureteral and urethral margins: studies of 203 consecutive radical cystoprostatectomy for men with bladder urothelial carcinoma[END_REF][START_REF] Kim | The clinical significance of intra-operative ureteral frozen section analysis at radical cystectomy for urothelial carcinoma of the bladder[END_REF][START_REF] Reder | Diagnostic accuracy of intraoperative frozen sections during radical cystectomy does not affect disease-free or overall survival: a study of 364 patients with urothelial carcinoma of the urinary bladder[END_REF]22,[START_REF] Gordetsky | Ureteral and urethral frozen sections during radical cystectomy or cystoprostatectomy: an analysis of denudation and atypia[END_REF][START_REF] Hoang | Clinical significance of ureteric 'skip lesions' at the time of radical cystectomy: the M.D. Anderson experience and literature review[END_REF][START_REF] Cerf | Usefulness of frozen section exams during radical cystectomy for urothelial carcinoma[END_REF], even if for some authors sequential ureteral resection was associated with low intraoperative conversion rates [START_REF] Raj | Significance of intraoperative ureteral evaluation at radical cystectomy for urothelial cancer[END_REF][START_REF] Gakis | Sequential resection of malignant ureteral margins at radical cystectomy: a critical assessment of the value of frozen section analysis[END_REF]. So far, there is no definitive evidence that complete extirpation of malignant ureteric involvement (proven by negative frozen sections margin during RC) eliminates the risk for recurrence of UUT disease [START_REF] Furukawa | Upper urinary tract recurrence following radical cystectomy for bladder cancer[END_REF][START_REF] Sanderson | Upper tract urothelial recurrence following radical cystectomy for transitional cell carcinoma of the bladder: an analysis of 1,069 patients with 10-year follow up[END_REF][START_REF] Raj | Significance of intraoperative ureteral evaluation at radical cystectomy for urothelial cancer[END_REF][START_REF] Schumacher | Is there an indication for frozen section examination of the ureteral margins during cystectomy for transitional cell carcinoma of the bladder?[END_REF][START_REF] Kenworthy | The risk of upper tract recurrence following cystectomy in patients with transitional cell carcinoma involving the distal ureter[END_REF][START_REF] Sved | Upper tract tumour after radical cystectomy for transitional cell carcinoma of the bladder: incidence and risk factors[END_REF] even if conversion from positive FS to negative FS, and/or negative PS, may improve UUTR free status in several retrospective studies [START_REF] Tollefson | Significance of distal ureteral margin at radical cystectomy for urothelial carcinoma[END_REF][START_REF] Gakis | Sequential resection of malignant ureteral margins at radical cystectomy: a critical assessment of the value of frozen section analysis[END_REF][START_REF] Moschini | Effect on postoperative survival of the status of distal ureteral margin: The necessity to achieve negative margins at the time of radical cystectomy[END_REF]22]. Indeed, pagetoid spread of CIS makes it is impossible to guarantee that negative frozen sections ensure that CIS is not more proximal. Some data support the theory that, in addition to local field progression of the disease, a multicentric process is involved in the etiology of UUTR (due to intraluminal seeding and intraepithelial migration). Such hypothesis is also consistent with the fact that, in patients with bladder cancer CIS, upper tract involvement is associated with high rate of prostate involvement (67.6%) and bilateral ureteral disease (32%) [START_REF] Solsona | Upper urinary tract involvement in patients with bladder carcinoma in situ (Tis): its impact on management[END_REF]. Because urothelial carcinoma is associated with pan epithelial field defect and skip lesions are more common in patients with bladder CIS, tumor may occur anywhere in the urinary tract after treating an initial carcinoma [START_REF] Hoang | Clinical significance of ureteric 'skip lesions' at the time of radical cystectomy: the M.D. Anderson experience and literature review[END_REF]. Hoang et al. analyzed 660 patients who underwent RC and found 4.8% of ureteric skip lesion, half of them related to CIS [START_REF] Hoang | Clinical significance of ureteric 'skip lesions' at the time of radical cystectomy: the M.D. Anderson experience and literature review[END_REF]. Then, positive ureteral surgical margin did not necessarily translate to a large impact on patient outcome, and the risk of UUTR remained difficult to be evaluated (small cohorts, short follow-ups and competing risk with dying from bladder cancer) [START_REF] Sanderson | Upper tract urothelial recurrence following radical cystectomy for transitional cell carcinoma of the bladder: an analysis of 1,069 patients with 10-year follow up[END_REF][START_REF] Raj | Significance of intraoperative ureteral evaluation at radical cystectomy for urothelial cancer[END_REF][START_REF] Umbreit | Multifactorial, site-specific recurrence model after radical cystectomy for urothelial carcinoma[END_REF][START_REF] Tran | Longitudinal risk of upper tract recurrence following radical cystectomy for urothelial cancer and the potential implications for long-term surveillance[END_REF].

As for UUTR, it remains controversial if clearing the margins improves cancer survival. Given the low number of upper tract recurrences and the weak association between such recurrences and death, it is unlikely to have a significant impact on reducing recurrences on cancer specific survival and overall survival outcome. Various groups suggested that ureteral carcinoma in situ in ureteral surgical margin barely contributes to cancer specific survival and overall mortality in patients who undergo radical cystectomy [START_REF] Tollefson | Significance of distal ureteral margin at radical cystectomy for urothelial carcinoma[END_REF][START_REF] Schoenberg | Ureteral frozen section analysis during cystectomy: a reassessment[END_REF][START_REF] Lee | Significance of cancer involvement at the ureteral margin detected on routine frozen section analysis during radical cystectomy[END_REF][START_REF] Touma | Ureteral frozen sections at the time of radical cystectomy: reliability and clinical implications[END_REF][START_REF] Reder | Diagnostic accuracy of intraoperative frozen sections during radical cystectomy does not affect disease-free or overall survival: a study of 364 patients with urothelial carcinoma of the urinary bladder[END_REF]22].

Only one large retrospective study, published by Moschini et al. reported the beneficial effect of sequential sectioning FS on cancer specific survival and overall survival, only for patients who were found N0, R0, and <pT3 [START_REF] Moschini | Effect on postoperative survival of the status of distal ureteral margin: The necessity to achieve negative margins at the time of radical cystectomy[END_REF].

Prognostic factors for positive ureteral margins

Targeting the high-risk subgroups for positive margins could confirm the utility, the clinical significance and the impact on UUTR, cancer specific survival and overall survival of

FS at time of radical cystectomy (RC).

Based on step-sectioning, radical cystectomy specimen was associated with grade 3 carcinomas mucosal involvement (CIS and high grade dysplasia) adjacent to and remote from the visible tumor for 30 to 50% of patients [START_REF] Farrow | Morphological and clinical observations of patients with early bladder cancer treated with total cystectomy[END_REF][START_REF] Kakizoe | Significance of carcinoma in situ and dysplasia in association with bladder cancer[END_REF]. Trigone region and contiguous posterior wall were particularly affected, and 57.1% patients were concerned by ureteral involvement (demonstrating direct intramucosal spread of malignant epithelial cells process) [START_REF] Farrow | Morphological and clinical observations of patients with early bladder cancer treated with total cystectomy[END_REF]. Finding published by Johnson et al. and Schumacher et al. suggested that patients with CIS of the bladder can have concomitant CIS of the distal ureter, in as many as 24% of patients [START_REF] Johnson | Are frozen-section examinations of ureteral margins required for all patients undergoing radical cystectomy for bladder cancer?[END_REF][START_REF] Schumacher | Is there an indication for frozen section examination of the ureteral margins during cystectomy for transitional cell carcinoma of the bladder?[END_REF].

Long ago, Johnson et al emphasized the importance of careful examination of the ureteral margin prior to constructing ureteroenteric anastomoses in patients with multifocal carcinoma in situ involving either the bladder or prostatic ducts [START_REF] Johnson | Are frozen-section examinations of ureteral margins required for all patients undergoing radical cystectomy for bladder cancer?[END_REF]. Two contemporary studies with low numbers of patients [START_REF] Osman | The value of frozen-section analysis of ureteric margins on surgical decision-making in patients undergoing radical cystectomy for bladder cancer[END_REF][START_REF] Gondo | Preoperative prediction of malignant involvement of resected ureters in patients undergoing radical cystectomy for bladder cancer[END_REF] and with low frequency CIS within the bladder (20% and 27.7% respectively) [START_REF] Gondo | Preoperative prediction of malignant involvement of resected ureters in patients undergoing radical cystectomy for bladder cancer[END_REF]22], showed no association with positive ureteral margin [START_REF] Osman | The value of frozen-section analysis of ureteric margins on surgical decision-making in patients undergoing radical cystectomy for bladder cancer[END_REF][START_REF] Gondo | Preoperative prediction of malignant involvement of resected ureters in patients undergoing radical cystectomy for bladder cancer[END_REF]22]. However, even if some problem for histopathological diagnosis may exist, in several retrospective recent studies, ureteral CIS involvement was significantly associated with the presence of the CIS within the bladder [START_REF] Raj | Significance of intraoperative ureteral evaluation at radical cystectomy for urothelial cancer[END_REF][START_REF] Touma | Ureteral frozen sections at the time of radical cystectomy: reliability and clinical implications[END_REF][START_REF] Zhou | Intraoperative frozen section evaluation of ureteral and urethral margins: studies of 203 consecutive radical cystoprostatectomy for men with bladder urothelial carcinoma[END_REF][START_REF] Kim | The clinical significance of intra-operative ureteral frozen section analysis at radical cystectomy for urothelial carcinoma of the bladder[END_REF][START_REF] Moschini | Effect on postoperative survival of the status of distal ureteral margin: The necessity to achieve negative margins at the time of radical cystectomy[END_REF][START_REF] Loeser | Frozen section analysis of ureteral margins in patients undergoing radical cystectomy for bladder cancer: differential impact of carcinoma in situ in the bladder on reliability and impact on tumour recurrence in the upper urinary tract[END_REF][START_REF] Nakanishi | Management of concomitant ureteral carcinoma in situ at radical cystectomy[END_REF][START_REF] Masson-Lecomte | Predictive factors for final pathologic ureteral sections on 700 radical cystectomy specimens: Implications for intraoperative frozen section decision-making[END_REF]. Schumacher et al. reported that close to 80% of patients with CIS in the ureter have CIS in the bladder [START_REF] Schumacher | Is there an indication for frozen section examination of the ureteral margins during cystectomy for transitional cell carcinoma of the bladder?[END_REF], and Raj et al. reported 30% of patients with bladder CIS have ureteric involvement in the RC versus only 9% of patients without [START_REF] Raj | Significance of intraoperative ureteral evaluation at radical cystectomy for urothelial cancer[END_REF]. Unfortunately, the presence of CIS in TURBT and RC specimens was also predictive and correlated with ureteric skip lesion (odds ratio [OR] 4.15, 95% CI 2.00-8.55, p<0.001) and [OR] 7.87, 95%CI 3. 19-19.23, p<0.001], respectively [START_REF] Hoang | Clinical significance of ureteric 'skip lesions' at the time of radical cystectomy: the M.D. Anderson experience and literature review[END_REF].

Due to the retrospective nature of studies, we were unable to determine whether CIS was focal or diffuse and if the extent of CIS was associated with an altered risk of ureteral involvement, upper recurrence or survival outcomes. However, factors associated with an increasing risk ratio for UUTR, included history of CIS, history of recurrent bladder cancer, cystectomy for noninvasive bladder cancer [START_REF] Volkmer | Upper urinary tract recurrence after radical cystectomy for bladder cancer--who is at risk?[END_REF][START_REF] Solsona | Upper urinary tract involvement in patients with bladder carcinoma in situ (Tis): its impact on management[END_REF]. Typically, Cis is present in close to 10% of TUR specimens versus 30 to 50% after cystectomy and final examination. This can be explained by the fact that less material was analyzed during TUR and by the difficulty and dedication required to determine the presence of concomitant Cis at the morphologic level (inter observer variability). However some qualitative concepts, associated with adherence to specific National and International guideline, must be reminded to really rely on TUR specimen [START_REF] Witjes | Updated 2016 EAU Guidelines on Muscle-invasive and Metastatic Bladder Cancer[END_REF]. Initial treatment for bladder cancer involves transurethral resection (TUR) of tumors to remove all possible tumors and obtain material for histological examination. As a result, resection patients are started on treatment pathways that reflect the nature and the potential of their disease. If TUR should include a deeper part, which has to be sent in separately, incomplete TUR should be exceptional (technical considerations) to avoid re-TUR necessity (morbidity and cost implication) and to reflect as close as possible the nature of the disease. Additional bladder biopsy should be associated for bladder abnormal-looking urothelium and when cytology is positive and when a non-papillary appearance was found. In the 21st century (equipment shall be available), photodynamic diagnosis could be considered to identify associated carcinoma in situ (Cis), and since the presence of Cis may lead to a modified treatment plan (eg. chemo radiation) [START_REF] Witjes | Updated 2016 EAU Guidelines on Muscle-invasive and Metastatic Bladder Cancer[END_REF].

In orphan retrospective studies, other prognostic factors were discussed like tumor location (trigone), tumor multifocality, clinical stage, number of tumor, intramural invasion, mucosal or ductal prostatic urothelial cancer, and hydronephrosis [START_REF] Gakis | Sequential resection of malignant ureteral margins at radical cystectomy: a critical assessment of the value of frozen section analysis[END_REF][START_REF] Gondo | Preoperative prediction of malignant involvement of resected ureters in patients undergoing radical cystectomy for bladder cancer[END_REF][START_REF] Kim | The clinical significance of intra-operative ureteral frozen section analysis at radical cystectomy for urothelial carcinoma of the bladder[END_REF]22,[START_REF] Masson-Lecomte | Predictive factors for final pathologic ureteral sections on 700 radical cystectomy specimens: Implications for intraoperative frozen section decision-making[END_REF].

Studies limitations

Most of the results published were derived from retrospective, observational data. Due to several limitations like patient-selection factors, surgical technique, multiplicity of surgeons, frequency follow-up, neoadjuvant chemotherapy before RC, imaging modalities used -which improved during last decade, and limited number of UUTR, the statistical relevance should be questioned. In retrospective studies, potential errors introduced by incomplete data acquisition and patient selection bias should be discussed [START_REF] Gondo | Preoperative prediction of malignant involvement of resected ureters in patients undergoing radical cystectomy for bladder cancer[END_REF]. In context, differences in survival recorded in the subgroup of patients with negative soft tissue margins and negative node metastases could be an indirect effect of uncontrolled factors [START_REF] Moschini | Effect on postoperative survival of the status of distal ureteral margin: The necessity to achieve negative margins at the time of radical cystectomy[END_REF].

Conclusion

Literature data suggest that ureteral frozen section analysis is probably unnecessary in the majority of patients undergoing RC, and even more so when the redundant ureteral segment was resected above the common iliac vessels. For patients with CIS within the bladder, frozen section (FS) at the time of RC can accurately detect ureteral involvement with reliable specificity.

Several retrospective studies have shown that conversion from positive FS to negative may improve UUTR free status. Only one retrospective study showed the beneficial effect of sequential sectioning FS on cancer specific survival and overall survival only for patients (<pT3, R0, N0).

Adequate powered randomized controlled trial, based on large structured collaborative studies, are necessary to confirm the clinical significance and utility (UUTR, Cancer Specific Survival and Overall Survival) of FS at time of radical cystectomy.

Legends

Table 1 .

 1 Figure 1. PRISMA flow diagram Studies reporting ureteral margin status.

Table 2 .

 2 Reliability of Frozen section using permanent section as a reference (PPV: positive predictive value; NPV: negative predictive value; FS: frozen section).

Table 3 .

 3 Studies reporting ureteral margin status and upper urinary tract recurrence (UUTR), Cancer Specific Survival (CSS) and Overall Survival (OS).

Table 4 .

 4 Predicting pathological positive ureteral margin at time of RC (FS: frozen section; PS: permanent section).

Table 5 .

 5 Potentially associated biases (IVP: intravenous pyelogram; CTS: computerized tomographic scan).

	Table 2.

Table 4 .

 4

	Authors	Year of	Pronostic factors for	Analysis	Type of study	Odds ratio
	[Ref]	publication	positive PS			OR (CI 95%)
		Patients
		(n)
	Nakanishi et al.	2006	CIS within bladder	Multivariate	Retrospective	3.33, CI : 1.05-10.53 (PS) p=0.04
	[27]
		n=158
	Raj et al.	2006	CIS within bladder	Multivariate	Retrospective	3.99, CI : 1.87-8.51 p<0.0005
	[6]					PS+:
		n=1330				30% (with bladder Cis) and 9% (without)
	Osman et al.	2006	FS(+)	Univariate	Prospective	p<0.01
	[7]		Male gender			p=0.01
		n=100	CIS within bladder			12.9% (no bladder Cis) versus 26.7%
						(bladder Cis) NS
	Touma et al.	2010	CIS within bladder	Multivariate	Retrospective	p<0.02
	[16]		CIS of prostatic urethra			p<0.02
		n=301
	Gakis et al.	2011	FS(+)	Multivariate	Retrospective	p< 0.0001
	[9]		Tumor multifocality			p< 0.04
		n=218
	Gondo et al.	2013		Multivariate	Retrospective	(*) for FS(+) and/or PS(+)
	[17]		Tumor trigone location			CI : 1.116-5.200 p=0.0252
		n=197
			Clinical stage > or = cT3			*3.190, CI : 1.432-7.105 p=0.0045
			Number of tumor (> or			*3.245, CI : 1.431-7.363 p=0.0048
			=3)
	Loeser et al.	2014	CIS within bladder	Multivariate	Retrospective	0.85% (no bladder Cis) versus 35.6%
	[24]					(bladder Cis) p<0.0001
		n=243
	Zhou et al.	2014	CIS within bladder	Multivariate	Retrospective	0% (no bladder Cis) versus 15.5% (bladder
	[18]					Cis) p<0.001
		n=203
	Kim et al.	2015	CIS within bladder	Multivariate	Retrospective	11.1, CI : 2.3-54.03 p=0.003
	[19]
		n=402	Intramural invasion			7.82, CI : 1.97-31.04 p=0.003
			Lymp node status			5.01, CI : 1.19-21.03 p=0.02
			FS(+)			177.91, CI : 36.27-872.67 p<0.001
	Moschini et al.	2016	CIS within bladder	Multivariate	Retrospective	3.83, CI : 1.83-8 (PS) p<0.001
	[21]					6.40, CI : 2.86-14.3 (FS) p<0.001
		n=1447
	Satkunasivam et	2016	Mucosal or ductal	Multivariate	Retrospective	*1.78, CI : 1.11-2.86
	al. [22]		prostatic urothelial			(*) for FS(+)
		n=2047	cancer
	Masson et al.	2017	CIS within adder	Multivariate	Retrospective	Cohort A
	[28]		Hydronephrosis			Hydronephrosis: Odds ratio [OR]=4.9,
		Cohorte A n=441				p=0.004
						CIS: OR=3.9, p=0.01
						Cohort B
		Cohorte B				Hydronephrosis: [OR]=5.9, p=0.01
		n=307				CIS: OR=3.4, p=0.10

Table 5 .

 5


Robin Baudouin
email: robin.baudouin@aphp.com

Abeer Alali

Marc Blumen

OSAS and upper pharynx surgery: does basilingual collapsus always rhyme with failure?

Keywords: Obstructive sleep apnea, Tonsillectomy, Pharyngoplasty, Upper-airway surgery

Objective: Drug-induced-sedation endoscopy (DISE) has proved superior to awake clinical examination for diagnosis of upper-airway obstruction sites and surgical planning. Our question is: does multilevel obstruction on DISE systematically entail failure for surgery limited to the upper pharynx? Material & Methods: We conducted a retrospective single-center study in patients with obstructive sleep apnea syndrome (OSAS) treated by single-level surgery of the upper pharynx (tonsillectomy with or without pharyngoplasty). Preoperative assessment included polysomnography (PSG) and DISE. Surgical efficacy was assessed on postoperative PSG.

Treatment response was defined by postoperative apnea-hypopnea index (AHI) <20 events/h with 50% reduction, and cure by AHI <10 (patients with preoperative AHI ≤10 being excluded). Efficacy was compared between groups without (group A) and with basilingual or laryngeal collapsus on DISE (group B).

Results:

We analyzed 63 patients, with mean preoperative AHI 33.8±17.9 events/h. The two groups (A, n=36; B, n=27) were clinically comparable. The success rate was 66.7% in group A (mean reduction in AHI, 57.3±36.2%) and 59.3% in group B (mean reduction, 53.9±39.2%). Cure rates were respectively 48.5% and 48.1%. There was no statistically significant difference between the two groups (p>0.1).

Conclusions:

Oropharyngeal surgery can alleviate associated obstructive sites found on DISE in the lower pharynx, and step-by-step treatment shows efficacy equal to that of single-step multilevel surgery.

INTRODUCTION

Drug-induced-sedation endoscopy (DISE) is effective and reliable in diagnosing upper-airway obstruction sites causing obstructive sleep apnea syndrome (OSAS). Superiority to awake clinical examination has been demonstrated [START_REF] Zerpa | Druginduced sedation endoscopy versus clinical exploration for the diagnosis of severe upper airway obstruction in OSAHS patients[END_REF].

Dynamic study of upper-airway obstruction sites has become more precise, enabling personalized surgery. However, interpretation of the endoscopic imaging remains a subject of discussion.

Two attitudes prevail: a multi-site strategy seems logical, simultaneously treating all observed obstructions, while a single-site strategy presumes that some obstruction sites are secondary to others and that single-site treatment can beneficially impact other sites. Indications follow the joint guidelines of the French Sleep Research and Medicine Society (SFRMS), French Society of ENT (SFORL) and French-Language Society of Pneumology (SPLF) [START_REF]Recommandations pour la Pratique Clinique[END_REF] and the present state of knowledge.

Hsu et al. reported equivalent AHI decrease and response rate between 11 patients with complete basilingual obstruction on DISE and 19 without, all undergoing palatopharyngoplasty [START_REF] Hsu | Does Sleep Endoscopy Staging Pattern Correlate With Outcome of Advanced Palatopharyngoplasty for Moderate to Severe Obstructive Sleep Apnea?[END_REF]. They considered certain obstructions on DISE to be secondary, improved by sequential surgery restricted to the velopharyngeal site. The same hypothesis emerges from the study by Victores et al., where a nasopharyngeal tube significantly reduced lateral obstruction seen on DISE in the pharynx (by 86%), epiglottis (by 55%) and tonguebase (by 50%) [START_REF] Victores | Interventional Drug-Induced Sleep Endoscopy: A Novel Technique to Guide Surgical Planning for Obstructive Sleep Apnea[END_REF].

The present study focused on surgery for sleep respiratory disorder in everyday practice, addressing the following question: does tongue-base obstruction on the VOTE criteria [START_REF] Kezirian | Drug-induced sleep endoscopy: the VOTE classification[END_REF] on DISE (by Propofol [START_REF] Blumen | Drug-induced sleep endoscopy: A new gold standard for evaluating OSAS? Part I: Technique[END_REF]) systematically entail failure of surgery limited to the velopharyngeal level?

PATIENTS AND METHODS

A single-center retrospective study included OSAS patients undergoing oropharyngeal surgery (tonsillectomy with or without palatopharyngoplasty).

Inclusion criteria comprised: age ≥ 18 years, OSAS diagnosed on French Health Authority criteria, implementation of DISE, and oropharyngeal surgical treatment.

Exclusion criteria comprised: acute or chronic respiratory, cardiovascular or neuromuscular pathology independent of OSAS, multilevel surgery in the upper airway (basiglossectomy, tongue-base tonsillectomy, hyoid bone suspension, 12 th cranial nerve stimulator implantation) or bone foundations (bimaxillary advancement, apophysis geni advancement), or untreated nasal respiratory disorder. Patients failing to undergo postoperative polysomnography (PSG) were considered lost to follow-up.

Preoperative work-up included awake clinical examination by an ENT sleep specialist, with flexible endoscopy, recording symptoms and body-mass index (BMI), and the Epworth selfadministered questionnaire. Type 2 or 3 PSG was required for OSAS diagnosis, and was repeated in case of excessive interval since last PSG, significant weight-change or other event found on interview liable to affect sleep respiration. Apnea and hypopnea episodes were recorded on the 2012 American Academy of Sleep Medicine criteria. Systematic DISE was performed in theater following the department's protocol, with incremental continuous Propofol perfusion. Interpretation used the VOTE classification (Velum, Oropharynx (i.e., palatine tonsils and lateral pharyngeal walls), Tongue base, Epiglottis). Retrobasilingual obstruction was diagnosed if the glottic plane was masked by more than 50%, and considered moderate for 50-75% and complete for >75% masking. Velar obstruction was considered complete for ≥75% and partial for <75% stenosis. Endoscopy results were recorded at end of examination: i.e., ahead of and blind to surgery.

Surgery was indicated on DISE by the ENT sleep specialist, after refusal or failure of continuous positive airway pressure (CPAP) ventilation and/or mandibular advancement orthosis for severe OSAS (AHI ≥30), or as first-line alternative for mild-to-moderate OSAS (AHI 5-30). Tonsillectomy was proposed whatever the severity of OSAS in case of severe tonsillar hypertrophy (Friedman grade III or IV). Indications for palatopharyngoplasty were at the surgeon's discretion.

All surgeries were oropharyngeal. Tonsillectomy was by extra-capsular cold-instrument dissection conserving the posterior and anterior pillars; no closure of the fossa was performed.

No simple velectomy was associated. Any associated palatopharyngoplasty was according to our department's habits. After tonsillectomy, full-thickness sectioning of the palatopharyngeal muscle was performed in the superior third, sparing the mucosa; the inferior part was isolated from the mucosa for ≥1 cm and sutured laterally and superiorly to the palatoglossal muscle to allow lateral and anteroposterior enlargement of the pharynx. Any nasal respiration surgery (septoplasty, turbinoplasty, adenoidectomy) was performed in the same step.

Surgical efficacy was assessed on PSG at >3 months. One patient was classified as responder, with AHI <20 for ≥50% reduction. One was classified as cured, with AHI <10, patients with preoperative AHI ≤10 being excluded. Efficacy against OSAS was compared between groups without (group A) and with basilingual or laryngeal collapsus (group B) on DISE.

The Mann-Whitney was used to compare physical and PSG characteristics, and the Fisher exact test for response and cure rates. The significance threshold was set at p ≤0.05.

RESULTS

Between November 2007 and July 2017, 63 patients were included, with mean AHI 33.8 ±179 events/h. Thirty-two (50.8%) had severe, 24 (38.1%) moderate and 7 (11.1%) mild OSAS.

Palatine tonsillectomy was performed in 61 patients (96.8%), including 40 (63.5%) with associated palatopharyngoplasty, and 21 (33.3%) with isolated tonsillectomy; 2 patients (32%) had isolated palatopharyngoplasty. Nasal surgery was associated in 8 cases, with 2 septoplasties, 7 inferior turbinoplasties, and 2 adenoidectomies.

Thirty-six patients (57.1%) showed no associated retrobasilingual obstruction (group A).

Twenty-seven (42.9%) showed low-airway obstruction (group B): 4 with total (14.8%) and 15 with partial (55.6%) basilingual obstruction and 8 (29.6%) with epiglottic tilt. Low-airway obstacles were isolated in 21 patients (87.8%), and comprised associated basilingual collapsus and epiglottic tilt in 6 (22.2%).

Groups were comparable on preoperative data (Table 1). Six patients in each group showed concentric velar palate collapsus. Surgeries in Group A comprised palatine tonsillectomy (n=15, 41.7%), tonsillectomy+palatopharyngoplasty (n=20, 55.6%) and isolated velopharyngoplasty (n=1, 2.8%), and in Group B palatine tonsillectomy (n=5, 18.5%), tonsillectomy+palatopharyngoplasty (n=21, 77.8%) and isolated palatopharyngoplasty (n=1,

3.7%).

Pre-and post-operative data are compared between groups in Table 2.

The postoperative sleep examination took place at a mean 8.5 ± 11.5 months.

Response rate was 66.7% in Group A, with mean drop in AHI of 57.3 ± 36.2%, and 59.3% in Group B, with 53.9 ± 39.2%. Cure rates were respectively 48.5% and 48.1%. There were no significant intergroup differences (p>0.1) (Table 3). Results were similar according to severity (Table 3). Two of the 4 patients with complete basilingual obstruction were cured and 3 responded, with mean drop in AHI of 65.2 ± 27.6%.

Twelve patients (6 in each group) showed concentric velar palate obstruction, with poorer results for uvulopalatopharyngoplasty (UPPP). Five of these were responders in Group A, and 2 in Group B.

Twelve patients in group B showed complete velar obstruction, with 5 cures (41.7%) and 7 responses (58.3%) with AHI drop of 52.2 ± 36.3%. Five patients showed partial velar obstruction, with no cures and 1 response (20.0%) with AHI drop of 21.0 ± 48.0%. There were no significant intergroup differences.

DISCUSSION

DISE findings of multilevel obstruction are not infallibly predictive of failure of surgery restricted to the oropharynx or velar palate.

Even so, multiple and especially retrobasilingual obstruction is frequent, and appears logically to be a risk factor for failure of limited surgery [START_REF] Lin | The Efficacy of Multilevel Surgery of the Upper Airway in Adults With Obstructive Sleep Apnea/Hypopnea Syndrome[END_REF]. The literature is largely retrospective, but lower obstructions are reported in most cases after failure of velar or oropharyngeal surgery [START_REF] Kezirian | Nonresponders to pharyngeal surgery for obstructive sleep apnea: Insights from drug-induced sleep endoscopy[END_REF]. Preoperative findings of multilevel obstruction may lead to over-treatment by multisite surgery, incurring greater iatrogenic risk for only slight efficacy at tongue-base level [START_REF] Blumen | Surgical treatment of obstructive sleep apnea syndrome[END_REF], or, in contrast, to mistaken abstention from any surgery.

Surgery is one of the three classical attitudes in OSAS, alongside CPAP and mandibular advancement, but is the only one that is not dependent on patient compliance. Efficacy is poorer than for CPAP, but improvement and cure are possible in selected cases [START_REF] Verse | The surgical treatment of sleep-related upper airway obstruction[END_REF]. The issue is thus to refine phenotypic diagnosis of obstructive sleep disorder so as to be able to predict the outcome of the chosen surgical option.

DISE is the gold-standard for analysis of airway obstruction. It consists in reproducing sleep

disorder within a medical environment, enabling precise diagnosis of obstruction levels. It takes account of sedation impact on muscle tonus and the impact of decubitus on OSAS episode onset [START_REF] Kotecha | Drug induced sleep endoscopy: its role in evaluation of the upper airway obstruction and patient selection for surgical and non-surgical treatment[END_REF]. It has been shown to be superior to wake clinical examination, significantly affecting indications for surgery [START_REF] Eichler | Does drug-induced sleep endoscopy change the treatment concept of patients with snoring and obstructive sleep apnea?[END_REF]; notably, it is more effective in diagnosing velar and especially retrobasilingual obstruction [START_REF] Zerpa | Druginduced sedation endoscopy versus clinical exploration for the diagnosis of severe upper airway obstruction in OSAHS patients[END_REF] [START_REF] Blumen | Drug-induced sleep endoscopy: A new gold standard for evaluating OSAS? Part II: Results[END_REF]. However, do findings of low pharyngeal obstruction on DISE in a patient in whom velum or pharynx surgery is indicated preclude implementing velotonsillar surgery?

Although DISE is an effective and reliable diagnostic tool, certain limitations have to be borne in mind. There is no consensus on practical implementation. Sedation can use Midazolam or Propofol, and these differ in their action, notably on muscle tonus, and in their perfusion concentrations, which moreover are not consensually defined. Secondly, there are no guidelines for examination time, despite this being a crucial parameter for the detection of obstruction sites: too short an examination risks overlooking some site, while too long an examination risks false-positives due to cumulative sedative concentration [START_REF] Heo | Time-dependent changes in the 2732 obstruction pattern during drug-induced sleep endoscopy[END_REF]. Thus, no particular sedation level has been set as limit or as target, although it would seem that the deeper the sedation the lower the tonus and the more likely DISE is to reveal multiple severe obstacles at different retrobasilingual levels, which are in fact false-positives biasing treatment planning [START_REF] Hong | Change of obstruction level during drug-induced sleep endoscopy according to sedation depth in obstructive sleep apnea: Change of DISE According to Sedation Depth[END_REF]. Thirdly, numerous read-out and interpretation grids are available, for none of which is there consensus.

A mistaken indication for tongue-base surgery or mistaken abstention from limited surgery on an obvious obstacle are the scylla and charybdis to be avoided in managing multiple obstruction. The present failure rates of 33.3% in Group A and 40.7% in Group B (nonsignificant difference) may not be due simply to retrobasilingual obstruction seen on preoperative DISE. Concentric velar collapsus is also a risk factor for failure of velotonsillar surgery [START_REF] Iwanaga | Endoscopic examination of obstructive sleep apnea syndrome patients during drug-induced sleep[END_REF], which was, however, neutralized in the present series by identical incidence (n=6) in either group. Other factors for success/failure need to be screened for.

In point of fact, retrobasilingual pharyngeal obstruction during sleep, if associated with velar or oropharyngeal obstruction accessible to surgery, can be alleviated by surgery limited to the upper pharynx. Hsu et al. reported equal efficacy of palatopharyngoplasty in reducing AHI between one group of patients with associated retrobasilingual obstruction and another without [START_REF] Hsu | Does Sleep Endoscopy Staging Pattern Correlate With Outcome of Advanced Palatopharyngoplasty for Moderate to Severe Obstructive Sleep Apnea?[END_REF]. Kwon et al. reported no significant difference in efficacy (44.4% vs 40.7%) between patients with (n=27) and without (n=27) epiglottic obstruction undergoing multilevel upper-airway surgery without laryngeal procedure [START_REF] Kwon | Is epiglottis surgery necessary for obstructive sleep apnea patients with epiglottis obstruction? The Laryngoscope[END_REF]. Victores et al., in a prospective study, found alleviation of retrobasilingual obstruction in 50% of 41 patients using a nasopharyngeal tube that eliminated upper pharyngeal obstruction. Results were especially clear in case of complete velar collapsus, while the tube did not significantly improve lower sites in case of partial velar collapsus, where the retrobasilingual obstruction seemed independent of higherlevel obstruction and to be unaffected by velopharyngeal surgery [START_REF] Victores | Interventional Drug-Induced Sleep Endoscopy: A Novel Technique to Guide Surgical Planning for Obstructive Sleep Apnea[END_REF]. The present study confirmed this to the extent of a statistical trend, but was not able to demonstrate significant effects: in Group B (with retrobasilingual obstruction), rates of cure, response and AHI reduction were non-significantly lower in partial versus complete velar obstruction.

Physically, it is understandable that treating a higher level site might improve retrobasilingual obstruction. The physiology of OSAS can be seen in terms of a Sterling resistance model associating a fluid (air) flowing under negative pressure (inspiration) in a collapsible tube (upper-airway collapsibility varying with neuromuscular tonus, varying in turn with depth of sleep, the specific elastic properties of each part of the airway, decubitus, and fatty or aqueous tissue infiltration) with weak zones (tongue base, epiglottis, lateral pharyngeal walls) and anatomic stenoses (palatine or lingual tonsillar hypertrophy, macroglossia) (<tel-00012061> Institut National Polytechnique de Grenoble -INPG, 2005). Airway freedom is ensured by a balance between inspiration pressure and anti-collapsus factors [START_REF] Isono | Interaction of cross-sectional area, driving pressure, and airflow of passive velopharynx[END_REF]. The narrower the upstream airway (nose, oropharynx), the greater the negative pressure of inspiration (Venturi effect) and the risk of it inducing downstream (tongue-base) collapsus. Upstream widening by velar or oropharyngeal surgery lowers the negative pressure on the airway and can resolve a retrobasilingual obstruction revealed only at high negative pressure. It is thus a mistake to systematically rule out surgery limited to an obvious velar or oropharyngeal obstacle on the grounds that some lower obstruction would compromise efficacy.

CPAP and mandibular advancement are effective treatments for OSAS, but presuppose constant compliance by the patient [START_REF] Bizieux-Thaminy | Long term use of nCPAP therapy in sleep apnoea patients[END_REF], whence the advantage of surgery [START_REF] Rotenberg | Reconsidering first-line treatment for obstructive sleep apnea: a systematic review of the literature[END_REF]. The issue in phenotypic diagnosis is to determine optimal individual medical or surgical treatment according to morphology [START_REF] Eckert | Phenotypic approaches to obstructive sleep apnoea -New pathways for targeted therapy[END_REF]. DISE improves OSAS diagnosis and treatment by refining the indications for surgery, but its limitations have to be borne in mind, and prediction of surgical efficacy from endoscopic data must be made with caution [START_REF] Blumen | Are the obstruction sites visualized on drug-induced sleep endoscopy reliable?[END_REF].

CONCLUSION

  Surgery restricted to the velum or oropharynx can alleviate collapsus in retrobasilingual obstruction sites. Surgical improvement of upper pharyngeal ventilation reduces the inspirational negative pressure underlying retrobasilingual collapsus. Retrobasilingual obstruction on DISE is not an infallible predictor of failure of surgery limited to the upper pharynx. Interpretation should take into account all the obstruction sites observed and the patient's morphology. The aim is to achieve reliable prediction of which medical or surgical treatment will be most effective for the individual patient. Retrobasilingual obstruction on DISE should not rule out surgery on easily accessible upper pharyngeal sites nor, on the contrary, indicate excessive multisite surgery involving the tongue base and larynx.

TABLE 1 .

 1 Preoperative patient data by group

		Group A	Group B
		Without lower airway	With lower airway
				P
		obstruction	obstruction
		n=36	n=27
	Physical
	examination
	age (years)	40.4 ± 10.1	40.7 ± 10.9
	BMI (k/m 2 )	27.0 ± 3.6	27.3 ± 4.1	NS
	Epworth	12.0 ± 4.4	9.7 ± 4.8
	Friedman grade
	III	17 (47.2)	13 (48.1%)
				NS
	IV	12 (33.3%)	5 (18.5%)
	Polysomnography
	SaO2min(%)	80.7 ± 8.8	81.4 ± 6.8
				NS
	AHI(/h)	33.1 ± 20.1	33.1 ± 15.2
	Severity
	Mild	7 (19.4%)	3 (11.1%)
	Moderate	13 (36.1%)	9 (33.3%)
	Severe	16 (44.4%)	15 (55.6%)

TABLE 2 .

 2 Pre-to post-operative comparison, by group.

	Group A
	Without lower airway
		Preoperative	Postoperative	P
	obstruction
	n=36
	Physical examination
	BMI (k/m 2 )	27.0 ± 3.6	26.8 ± 3.5	NS
	Epworth	12.0 ± 4.4	8.8 ± 4.2	NS
	Polysomnography
	SaO2 min (%)	80.7 ± 8.8	86.5 ± 4.8	NS
	AHI (/h)	33.1 ± 20.1	12.5 ± 11.2	<0.05
	Group B
	With lower airway
		Preoperative	Postoperative	P
	obstruction
	n=27
	Physical examination
	BMI (k/m 2 )	27.3 ± 4.1	26.3 ± 4.1	NS
	Epworth	9.7 ± 4.8	8.2 ± 5.4	NS
	Polysomnography
	SaO2 min (%)	81.4 ± 6.8	85.9 ± 4.8	NS
	AHI (/h)	33.1 ± 15.2	12.6 ± 9.4	<0.05

BMI: body-mass index; AHI: apnea-hypopnea index; h: hour of sleep

TABLE 3 .

 3 * n=17 (3 patients with preoperative AHI < 10 /h in group A). AHI: apnea-hypopnea index; h: hour of sleep

	Endpoint results.
		Group A
			Group B
		Without lower airway
			With lower airway obstruction	P
		obstruction
			n=27
		n=36
	AHI decrease
	/h	20.6 ± 18.6	20.9 ± 18.9	NS
	%	57.3 ± 36.3	53.9 ± 39.2	NS
	Response rate
	(S20)
		24 (66.7%)	16 (59.3%)	NS
	Cure rate
		16* (48.5%)	13 (48.1%)	NS
	Non-response
		12 (33.3%)	11 (40.7%)	NS
	rate
	Mild to moderate	n=20	n=12
	AHI decrease (/h)	10.1 ± 10.5	10.1 ± 12.1
				NS
	AHI decrease (%)	50.9 ± 43.0	42.1 ± 41.6
				NS
	Response	13 (65%)	6/12 (50.0%)
	Cure	10** (58.8%)	5/12 (41.7%)
	Non-response	7 (35%)	6 (50.0%)
	Severe	n=16	n=15
	AHI decrease (/h)	33.8 ± 18.2	29.6 ± 19.1	NS
	AHI decrease (%)	65.2 ± 24.2	63.4 ± 35.1

*

Disclosure of interest: the authors have no conflicts of interest to disclose.


Dancheng Chen

Paul-Louis Fabre

Olivier Reynes
email: reynes@chimie.ups-tlse.fr

Nadia Chouini-Lalanne

Valérie Sartor

Electrocarboxylation of chloroacetonitrile mediated by a Ni(I) terpyridine complex: voltammetric and spectroscopic studies

Keywords: Electrocatalysis, Nickel bis(terpyridine) complex, carbon dioxide, chloroacetonitrile, cyanoacetic acid, spectroelectrochemistry

Cyclic voltammetry and spectroelectrochemistry (e.p.r.) were used to investigate the mechanisms implied in the electrocarboxylation of chloroacetonitrile (Cl-CH2-CN, noted RCl) mediated by [Ni(II)(terpy)2] 2+ . The oxidative addition of chloroacetonitrile on the electrogenerated [Ni(I)(terpy)] + yields an alkyl-nickel R-Ni(III) complex which is reduced into an alkyl-nickel(II). E.p.r study shows that the homolytic nickel-carbon bond cleavage seems to be the more probable pathway to form the organic radical R • , which is reduced to R -. Under carbon dioxide atmosphere, cyclic voltammetry studies show that CO2 was added on the alkyl-Ni(III) complex giving probably a CO2 adduct which is reduced to RCO2-Ni(II) complex allowing to release the carboxylate RCO2 -and the active [Ni(I)(terpy)] + complex.

Introduction

In order to valorise carbon dioxide which is an abundant and relatively inexpensive waste, the electrocarboxylation of alkyl halides (RX) into carboxylated products (R-COO -) has been extensively studied [1][2][3][4][5]. Among all the electrocarboxylation methods, the direct electroreduction of alkyl halides under carbon dioxide atmosphere at various metallic electrodes has been studied. High faradic yields were obtained but required highly negative working potentials associated to passivation phenomena [6,7]. However, due to the specific reactivity of silver towards halides, when a silver electrode was used at cathode, good faradic yields were obtained with a low working potential [8]. On the other hand, it is confirmed that the use of catalytic systems based on transition-metal complex is also an efficient pathway for the electrocarboxylation of alkyl halides. Several nickel [9][10][11], palladium [12] and cobalt [13][14][15] complexes have been reported to prepare carboxylated products. Carboxylation of chloroacetonitrile (Cl-CH2-CN) has received much attention due to the relevant interest of the resulting cyanoacetic acid which is used in many industrial processes [16,17]. Actually, the chemical preparation of cyanoacetic acid is particularly environmentally unfriendly and toxic [START_REF] Pollak | Ullmann's Encyclopedia of Industrial Chemistry[END_REF]. In order to develop a safer process, the electrocarboxylation of chloroacetonitrile using nickel [START_REF] Isse | [END_REF] and cobalt [20,21] complexes have been studied. The best result was been obtained with the Ni(salen) planar complex giving the cyanoacetic acid with a yield of 71 % under an applied potential of -1.7 V/SCE [START_REF] Isse | [END_REF].

The catalytic processes go through the oxidative addition of the alkyl halide on the metallic centre and require at least one free coordination site on the metal. Recently, full coordinated Co(II) complexes have also shown interesting properties toward electrocarboxylation of chloroacetonitrile due to the loss of a ligand allowing the oxidative addition of alkyl halide [20,21]. Considering these works, many full coordinated transitionmetal complexes, which have never been studied before, can show interesting catalytic properties to give cyanoacetic acid.

We report, in this paper, the electrocatalytic properties of hexacoordinated [Ni(II)(terpy)2] 2+ complex for the electrocarboxylation of chloroacetonitrile Cl-CH2-CN. The overall process is studied by cyclic voltammetry and spectroelectrochemical experiments in order to propose a catalytic cycle. Studies were conducted under argon and carbon dioxide atmospheres. The influence of the applied potential on the faradic and carboxylated yields were studied through bulk electrolyses.

Materials and methods

Reagents and synthesis

Acetonitrile (HPLC grade), used as solvent, was stored on molecular sieves (3 Å).

Tetrabutylammonium hexafluorophosphate (TBAPF6), used as support salt, was recrystallized from ethyl acetate and dried at 80 °C under vacuum. 2,2':6',2''-Terpyridine, nickel (II) chloride hexahydrated, chloroacetonitrile, cyanoacetic acid, succinonitrile and potassium hexafluorophosphate (KPF6) were used as received. The [Ni(terpy)2]Cl2, synthetized as described in literature [22], was converted to its corresponding hexafluorophosphate salt by metathesis with KPF6. The resulting complex was dissolved in dry acetonitrile and crystals of [Ni(terpy)2](PF6)2 were grown by low diffusion of diethylic ether into acetonitrile solution.

Instrumentations

Voltammetric experiments were conducted in a three-electrode cell in dry acetonitrile + TBAPF6 0.1 mol.L -1 under argon or carbon dioxide atmospheres at room temperature by using an Autolab 20 potentiostat (EcoChemie). The working electrode was a glassy carbon disc (diameter 3 mm). The reference electrode was a saturated calomel electrode (S.C.E) equipped with a double junction. The counter electrode was a platinum wire. Controlled potential electrolyses were carried out as previously described [20]. The cyanoacetic acid was extracted using the procedure previously reported [START_REF] Isse | [END_REF]. In order to identify and quantify cyanoacetic acid formed during electrolysis, standard solutions of commercial cyanoacetic acid were previously analysed by HPLC using a HP 1100 HPLC equipped with a UV-Vis detector operating at 200 nm and a Sugar SH1011 column eluted by 5.10 -3 mol.L -1 H2SO4. For e.p.r. spectroelectrochemical experiments, a Bruker EMX-8/2.7 (9.86 GHz) equipped with a high-sensitivity cavity (4119/HS 0205) and a gaussmeter (Bruker, Wissembourg, France) was used for the analysis of radicals trapped by PBN. PBN, N-tert-butyl-αphenylnitrone, is a spin trap already used for the detection of radicals during photolysis experiments of alkylcobaloximes [23]. Two types of measurements were carried out: (i) In the e.p.r. flat quartz cell the electrolysis

(Eapplied = -1.2 V) of [Ni(terpy)2] 2+ 2.10 -3 mol.L -1
in presence of chloroacetonitrile 10 -2 mol.L -1 and PBN 10 -2 mol.L -1 was achieved in order to study the R-Ni(II) cleavage. The e.p.r. spectra were recorded during the electrolysis.

[Ni(terpy)2] 2+ , 2.10 -3 mol.L -1 . After exhaustive electrolysis, a green solution is obtained and 0.01 mol.L -1 of chloroacetonitrile was added; the solution turned red. After addition of PBN 10 -2 mol.L -1 , the e.p.r. spectra were recorded.

Results and discussion

Activation of chloroacetonitrile under argon atmosphere

As already described, [Ni II (terpy)2] 2+ complex exhibits four electronic transfers in the cathodic domain [-2 V ; 0 V] under an argon atmosphere in acetonitrile (figure I, curve 1) [24 -27].

FIGURE I: Cyclic voltammograms, recorded at a GC electrode under argon, in CH3CN +

TBAPF6. v = 100 mV/s. (1, -) 1 mmol.L -1 [Ni(II)(terpy)2] 2+ , (2, --) 1 mmol.L -1 [Ni(II)(terpy)2] 2+ and 2 mmol.L -1 chloroacetonitrile. [Ni(II)(terpy)2] 2+ + e - [Ni(I)(terpy)2] + (1)
The second electrochemical system observed at E2 = -1.45 V, corresponds to the reduction of terpyridine ligand associated to the loss of a terpyridine ligand.

[Ni(I)(terpy

)2] + + e - [Ni(I)(terpy) • -] + terpy (2)
The third reduction at E3 = -1.72 V is due to the reduction of Ni(I) metallic center to form the neutral Ni(0) complex.

[Ni(I)(terpy

) • -] + e - [Ni(0)(terpy) • -] - (3)
The last reduction which is irreversible at E4 = -1.95 V corresponds to the reduction of free terpyridine ligand release during reaction (2).

terpy + e - terpy • - complex to give an organo-nickel(III) complex (reaction 1 and 5) [28][29][30]. For the electrogenerated [Ni(I)(terpy)2] + complex, the oxidative addition of chloroacetonitrile implies the release of a terpyridine ligand to form the undercoordinated [Ni(I)(terpy)] + complex as already described for hexacoordinated cobalt complexes [21,31].

[Ni(I)(terpy)2] + + RX [RNi(III)(terpy)] 2+ + X -+ terpy (5) Associated to the loss of reversibility, when the concentration of chloroacetonitrile increases, the cathodic peak rises to 5 molar equivalents of RX. Moreover, the potential cathodic peak shifts to less negative potentials (Ep,c = -1,2 V) when the scan rate was increased and no reverse oxidation peak was observed.

This behaviour shows that the oxidative addition, reaction (5) -EC scheme, is coupled with another reaction. The increase in cathodic currents in presence of chloroacetonitrile at low scan rate is too high for a simple EC scheme. These observations show a second electronic transfer corresponding to the reduction of the organo-nickel complex at a less negative potential compared to Ni(II)/Ni(I) complex.

[RNi(III)(terpy)] 2+ + e - [RNi(II)(terpy)] + E5

Recently, many works have shown that the Nickel-Alkyl complexes, synthesized chemically, were stable and have been isolated [28][29][30][31]. However, others works have shown that the electrogenerated alkyl-nickel(II) complexes, coming from the reduction of alkylnickel(III) complex, are generally unstable [32,34,36], and underwent a rapid homolytic (7) or heterolytic (8) Ni-R bond cleavage.

[RNi(II)(terpy

)] + R • + [Ni(I)(terpy)] + Homolytic (7) [RNi(II)(terpy)] + R -+ [Ni(II)(terpy)] 2+ Heterolytic ( 8
)
spectroscopies were carried out. The complex R-Ni(II) was synthesized by exhaustive electrolysis of a solution of CH3CN + 0.1 mol.L -1 TBAPF6 containing the complex Ni(terpy)2(PF6)2 (2.10 -3 mol.L -1 ) at -1,2 V to electrogenerate the [Ni(I)(terpy)] + complex in presence of 10 -2 mol.L -1 of chloroacetonitrile and the spin trap PBN (10 -2 mol.L -1 ). The e.p.r. The e.p.r. experiment shows that the homolytic Ni-R bond cleavage of [RNi(II)(terpy)] + complex seems to be the more probable pathway (7) to give the [Ni(I)(terpy)] 2+ complex and organic radical R • . Note that the heterolytic alkyl-Ni(II) bond cleavage cannot be totally excluded.

[RNi(II)(terpy)] 2+ [Ni(I)(terpy)] + + R •

Through reaction ( 5), the electrochemical process presents the characteristics of an E Ccatalytic behaviour which explains the increase in the cathodic peak current under low potential scan rate. The homolytic dissociation of the alkyl-nickel(II) (7) gives the [Ni(I)(terpy)] + complex which then turns in a new catalytic cycle. The radical R • is then [35]). The resulting carbanion R -can then react with chloroacetonitrile via a selfprotonation reaction or with electrophilic species [38].

reduced to R -(NC-CH2 • /NC-CH2 -E° = -0.69 V versus SCE
Whatever the cleavage, the global electron exchange is bielectronic and catalytic.

Indeed, after the reduction of the alkyl-nickel(III) ( 6 the intensity of the cathodic peak is strongly enhanced under carbon dioxide atmosphere in presence of chloroacetonitrile and the forward potential peak shifts to more cathodic potential (Ep,c = -1.3 V) compared to -1.2 V observed under argon atmosphere with chloroacetonitrile. These observations imply that carbon dioxide enters the catalytic cycle and interferes in the reduction cycle of chloroacetonitrile [20].

Moreover, under CO2-atmosphere, the mono-electronic reduction of [RNi(III)(terpy)] 2+ complex is not observed and the shift of forward peak potential indicates that another complex is implied in the catalytic cycle. Taking into account, the behaviour of Ni or Pd complexes used as catalysts for electrocarboxylation [10,12], it seems reasonable to assume that CO2 was added on R-Ni(III) complex.

[RNi(III)(terpy

)] 2+ + CO2 [RNi(III)(terpy)CO2] 2+ (9)
The carboxylated complex is then reduced to [RNi(II)(terpy)CO2] + at a more cathodic potential (reaction 10) compared to the potential reduction of [RNi(III)(terpy)] 2+ .

[RNi(III)(terpy)CO2] 2+ + e - [RNi(II)(terpy)CO2] + (10) After addition of CO2 (9) and reduction of alkyl carboxylated Ni(III) complex (10), it was assumed that the carboxylated complex [RNi(II)(terpy)CO2] + underwent a coupling (11) between alkyl R and CO2 to give, after dissociation, the carboxylate anion RCO2 -and the undercoordinated complex [Ni(II)(terpy)] 2+ .

[RNi(II)(terpy

)CO2] + [Ni(II)(terpy)RCO2] + (11)
[Ni(II)(terpy

)RCO2] + RCO2 -+ [Ni(II)(terpy)] 2+ (12)
The peak current (Epc = -1.3 V) is not related to square root of scan rate. When the scan rate increases, the current peak normalized to square root of scan rate decreases showing that a chemical reaction limits the process. Taking into account the high concentration of carbon dioxide ([CO2] = 0.28 mol.L -1 in CO2-saturated solution [35]), the reaction (9) must be favoured. It is assumed that the limiting reaction is the dissociation of [Ni(II)(terpy)RCO2] + (11).

The overall activation of CO2 was:

[RNi(III)(terpy)] 2+ + CO2 + e - RCO2 -+ [Ni(II)(terpy)] 2+

Note that the carbanion R - [40] yielded from the chloroacetonitrile activation can also react directly with CO2 (13):

R -+ CO2 RCO2 - (13)
The spectroscopic and voltammetric studies show that [Ni(II)(terpy)2] 2+ complex is a good candidate for the electrocarboxylation of chloroacetonitrile into cyanoacetic acid.

Bulk Electrolyses

In order to quantify the products formed, preparative electrolyses at different applied potentials were carried out in CO2-satured solution in the presence of chloroacetonitrile (ten-fold excess) and nickel complex. The faradic and carboxylated yields obtained for different electrolysis potential are summarized in table I. Note that acetonitrile formed during electrolysis cannot be determined. The limitation of faradic yield is also due to the reactivity of electrogenerated [Ni(I)(terpy)2] + complex towards CO2 [39]. In CO2-saturated solution without chloroacetonitrile, at a -1.2 V applied potential, the charge passed (22 C) is higher than the theorical charge (2,4 C) needs for the mono-electronic reduction of [Ni(II)(terpy)] 2+ complex.

Conclusion

Electrochemical experiments have shown the oxidative addition of chloroacetonitrile onto the electrogenerated [Ni(I)(terpy)] + active complex to give an alkyl-nickel(III) complex which is then reduced to [RNi(II)(terpy)] + . E.p.r experiments show that alkyl-nickel(II) complex undergoes a homolytic dissociation to liberate R • radical which is reduced to R - carbanion and the active complex [Ni(I)(terpy)] + which enters a new catalytic cycle. Under CO2 atmosphere, it is assumed that the addition of carbon dioxide on [RNi(III)(terpy)] 2+ results in a carboxylate complex which is reduced to more cathodic potential. The [Ni(II)(terpy)RCO2] + complex after dissociation gives the carboxylate anion RCO2 -.

Electrolysis experiments have shown that at an applied potential of -1.3 V and -1.4 V, the faradic yields in cyanoacetic acid were good (around 60%) with an overpotential much lower (0.4 V) than those applied for the electrocarboxylation mediated by Ni(salen) [START_REF] Isse | [END_REF].

[RNi(III)(terpy)] 2+ + terpy [RNi(II)(terpy)]

  In presence of increasing amounts of chloroacetonitrile (Cl-CH2-CN, noted RCl) under argon atmosphere, the voltammogram of [Ni(II)(terpy)2] 2+ , drawn until -2 V, was strongly modified (Figure I, curve 2). The anodic backward peaks observed for the three first reductions disappeared. Associated to the loss of reversibility, an increase in the cathodic forward peak was observed. These observations show the reactivity between the electrogenerated complexes, obtained from reactions (1) to (3), and chloroaceonitrile and the activation of RCl by Ni(II)/Ni(I) redox couple. For the sake of clarity, the report is restricted to the first mono-electronic reduction of [Ni II (terpy)2] 2+ (figure II).

FIGURE II :

 II FIGURE II: Cyclic voltammograms, recorded at a GC electrode under argon, in CH3CN + TBAPF6. v = 100 mV/s. (1, -) 1 mmol.L -1 [Ni(II)(terpy)2] 2+ , (2, • • • •) 1 mmol.L -1 [Ni(II)(terpy)2] 2+ and 2 mmol.L -1 chloroacetonitrile, (3, --) 1 mmol.L -1 [Ni(II)(terpy)2] 2+ and 5 mmol.L -1 chloroacetonitrile.

  spectrum of the solution shows a signal corresponding to the R-PBN • radical: a triplet characterized by the hyperfine constants aN = 13.4 G, corresponding to the coupling of the electron spin with the core spin of the PBN nitrogen atom associated to the nuclear spin of the PBN hydrogen (aH = 1.7 G) which splits the triplet further into doublets (figure III). The signal intensity of R-PBN • radical was weak showing that the dissociation was slow.

FIGURE

  FIGURE III: e.p.r spectrum of a solution containing the electrogenerated [Ni(terpy)2] + (2.10 -3

  Scheme: Mechanisms implied in electrocatalytic properties of [Ni(II)(terpy)2] 2+ under

Figure

  Figure IV shows the voltammograms recorded in a CO2-saturated solution.

Table I . Electrocarboxylation of chloroacetonitrile in acetonitrile + 0.1 mol.L -1 TBAPF6 solution a

 I either with CO2 to give cyanoacetic acid or is deactivated by protonation to give acetonitrile. Note that succinonitrile, coming from the radical R • dimerization, is not detected in bulk electrolysis showing that the radical reduction is the preferential pathway.

	carbanion which reacts
	Entry	E b (V) Charge passed (C)	RCO2H (%) c	Faradic yield (%) d
	1	-1.2	50	37 ± 1 %	37 ± 1 %
	2	-1.3	60	53 ± 0.5 %	50 ± 0.5 %
	3	-1.4	80	61 ± 1 %	61 ± 1 %
	4 e	-1.2	22	0 %	0 ± 1 %

Acknowledgements

The authors would like to thank Pierre Perio (Laboratoire PHARMA-DEV, UMR IRD 152, Université de Toulouse) for his technical assistance in the e.p.r. experiments.

a Electrolyses were performed in an undivided cell at room temperature in the presence of 10 -2 mol.L -1 and Cl-CH2-CN 10 -3 mol.L -1 of [Ni(II)(terpy)2](PF6)2. The electrolytic solution was saturated with CO2. The theoretical charge is 48 C for the total conversion of the substrate in a 2 e -process. Good faradic and carboxylated yields (around 50 -60 %) obtained for applied potential to -1.3 V and -1.4 V show the interesting electrocatalytic properties of complex. Compared to Ni(salen) complex [START_REF] Isse | [END_REF], the carboxylate yields were lower. However, the over potential is much lower, about 0.4 V. When the electrolysis potentials were smaller than -1.3 V, the faradic and carboxylated yields decrease dramatically to 37 %. These results are in accordance with voltammetric studies. Indeed in the potential range [-1.3 ; -1.4 V],

[RNi(III)(terpy)CO2] 2+ complex is reduced to [RNi(II)(terpy)CO2] + which is unstable and gives RCO2 -and [Ni(II)(terpy)] 2+ . In this range of potential, Ni(II) complex is then reduced to Ni(I) complex which can react with RCl and CO2 to form [RNi(III)(terpy)CO2] 2+ complex. The faradic yields never reach 100%. This limitation of faradic yield is due to competition between the reduction of [RNi(III)(terpy)] 2+ and the formation of [RNi(III)(terpy)CO2] 2+ . Cyclic voltammetry and e.p.r studies show that the [RNi(III)(terpy)] 2+ complex reduction to [RNi(II)(terpy)] + gives radical R • which either dimerizes to give the assumed dimer R-R or is reduced to R -