MD Peter Wostyn
email: wostyn.peter@skynet.be

MD, PhD Kurt Audenaert

MD, PhD Peter Paul De Deyn

Alzheimer's disease and glaucoma: Is there a causal relationship?

Keywords: Alzheimer's disease, Glaucoma, Cerebrospinal fluid pressure, Intraocular pressure, Lamina cribrosa, Trans-lamina cribrosa pressure difference, Beta-amyloid, Tau protein

Evidence of a link between Alzheimer's disease (AD) and glaucoma has emerged from studies showing that patients with AD may have a significantly increased occurrence rate of glaucoma. In addition, it has been reported that patients with AD exhibit optic nerve degeneration and loss of retinal ganglion cells. In spite of intensive research, the clinical and genetic relationships between AD and glaucoma remain obscure. It is unclear whether the clinical correlation between the two diseases might be due to shared risk factors or the influence of one disorder on the other. Interestingly, certain observations may provide a clue towards a better understanding of the high rate of comorbidity reported between AD and glaucoma. In this article, we hypothesize that there may be a causal relationship between AD and glaucoma that may be explained by decreased cerebrospinal fluid pressure (CSFP) in patients with AD. A very recent study reported the intriguing new observation that mean CSFP was 33% lower in subjects with primary open-angle glaucoma compared with that of nonglaucomatous controls. It was noted that this observation supports the concept that an abnormal high trans-lamina cribrosa pressure difference, whether the result of elevated intraocular pressure, reduced CSFP, or both, plays an important role in glaucomatous optic nerve damage. Interestingly, it was also reported that a substantial proportion of AD patients have very low CSFP. Therefore, we hypothesize that an abnormal high trans-lamina cribrosa pressure difference may explain why patients with AD have a greater risk for developing glaucoma.

INTRODUCTION

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by cognitive and memory deterioration, as well as changes in personality, behavioral disturbances and an impaired ability to perform activities of daily living. [START_REF] Bullock | Realistic expectations: the management of severe Alzheimer disease[END_REF] AD is known to be the most common form of dementia and is a major public health problem throughout the world. [START_REF] Rocchi | Causative and susceptibility genes for Alzheimer's disease: a review[END_REF][START_REF] Sloane | The public health impact of Alzheimer's disease, 2000-2050: potential implication of treatment advances[END_REF] In addition to synaptic degradation and extensive neuronal cell loss, neuropathological characteristics of AD include extracellular senile plaques containing βamyloid (Aβ) derived from β-amyloid precursor protein (APP) after sequential cleavage by βsecretase and γ-secretase, and intracellular neurofibrillary tangles caused by abnormally phosphorylated tau protein. [START_REF] Selkoe | Alzheimer's disease: genes, proteins, and therapy[END_REF][START_REF] Sun | Hypoxia facilitates Alzheimer's disease pathogenesis by upregulating BACE1 gene expression[END_REF][START_REF] De Quervain | Glucocorticoid-related genetic susceptibility for Alzheimer's disease[END_REF] Early-onset familial AD caused by mutations in the APP, presenilin-1, and presenilin-2 genes accounts for less than 5% of the total number of AD cases. [START_REF] Rocchi | Causative and susceptibility genes for Alzheimer's disease: a review[END_REF] The majority of AD cases are sporadic with late onset and seem to result from a complex interaction of multiple genetic and environmental factors. [START_REF] Rocchi | Causative and susceptibility genes for Alzheimer's disease: a review[END_REF] An important genetic risk factor for late-onset AD is the epsilon 4 allele of the apolipoprotein E (APOE) gene located on chromosome 19. [START_REF] Horsburgh | The role of apolipoprotein E in Alzheimer's disease, acute brain injury and cerebrovascular disease: evidence of common mechanisms and utility of animal models[END_REF][START_REF] Tamura | High frequency of open-angle glaucoma in Japanese patients with Alzheimer's disease[END_REF] The pathogenesis of AD is complex, and involves many molecular, cellular and physiological pathologies. [START_REF] Laferla | Calcium dyshomeostasis and intracellular signalling in Alzheimer's disease[END_REF] Aβ is widely believed to be central to the pathogenesis in AD. [START_REF] Laferla | Calcium dyshomeostasis and intracellular signalling in Alzheimer's disease[END_REF] Glaucoma is one of the leading causes of vision loss worldwide. [START_REF] Tamura | High frequency of open-angle glaucoma in Japanese patients with Alzheimer's disease[END_REF] Open-angle glaucoma (OAG), the most common form of glaucoma, is characterized by a progressive loss of retinal ganglion cells (RGCs) and atrophy of the optic nerve, resulting in loss of visual field. [START_REF] Tamura | High frequency of open-angle glaucoma in Japanese patients with Alzheimer's disease[END_REF][START_REF] Wostyn | Alzheimer's disease-related changes in diseases characterized by elevation of intracranial or intraocular pressure[END_REF] Mechanical and vascular theories for the pathogenesis of glaucomatous optic neuropathy have been proposed. [START_REF] Wostyn | Alzheimer's disease-related changes in diseases characterized by elevation of intracranial or intraocular pressure[END_REF] Elevated intraocular pressure (IOP) is a strong risk factor, but a subset of glaucoma patients has normal IOP and is designated normal tension glaucoma. [START_REF] Tamura | High frequency of open-angle glaucoma in Japanese patients with Alzheimer's disease[END_REF][START_REF] Wostyn | Alzheimer's disease-related changes in diseases characterized by elevation of intracranial or intraocular pressure[END_REF] Clearly, factors other than IOP, including genetic factors, are likely to be involved in RGC death in glaucoma. [START_REF] Mckinnon | Glaucoma: ocular Alzheimer's disease?[END_REF] Some studies have implicated the ε4 allele of the APOE gene in the pathophysiology of OAG, but conflicting data exists. [START_REF] Tamura | High frequency of open-angle glaucoma in Japanese patients with Alzheimer's disease[END_REF][START_REF] Mabuchi | The apolipoprotein E gene polymorphism is associated with open angle glaucoma in the Japanese population[END_REF][START_REF] Lake | Normal tension glaucoma is not associated with the common apolipoprotein E gene polymorphisms[END_REF] There is a growing body of evidence demonstrating a link between AD and glaucoma.

However, the nature of this link remains obscure. Interestingly, lately published research may provide a clue towards a better understanding of the high rate of comorbidity reported between AD and glaucoma. In this article, we hypothesize that there may be a causal relationship between AD and glaucoma that may be explained by decreased cerebrospinal fluid pressure (CSFP) in patients with AD. Supportive evidence for this hypothesis is reviewed based on a PubMed literature search for the period 1986 to July 2008. The search keywords, used in different combinations, were "Alzheimer's disease", "glaucoma" and "cerebrospinal fluid pressure". Fifteen articles were selected. In addition, other relevant articles were identified from supplemental searches.

SIMILARITIES BETWEEN ALZHEIMER'S DISEASE AND GLAUCOMA

It is intriguing to note that AD and glaucoma have many common features. [START_REF] Tamura | High frequency of open-angle glaucoma in Japanese patients with Alzheimer's disease[END_REF] Both are slow and chronic neurodegenerative disorders with a strong age-related incidence. [START_REF] Guo | Targeting amyloid-beta in glaucoma treatment[END_REF][START_REF] Wostyn | Can chronic increased intracranial pressure or exposure to repetitive intermittent intracranial pressure elevations raise your risk for Alzheimer's disease?[END_REF] Studies consistently report decreased levels of β-amyloid (1-42) and increased levels of tau in cerebrospinal fluid (CSF) from AD patients in comparison with healthy subjects. [START_REF] Engelborghs | Diagnostic performance of a CSFbiomarker panel in autopsy-confirmed dementia[END_REF][START_REF] Yoneda | Vitreous fluid levels of beta-amyloid((1-42)) and tau in patients with retinal diseases[END_REF] Recently, Yoneda et al. [START_REF] Yoneda | Vitreous fluid levels of beta-amyloid((1-42)) and tau in patients with retinal diseases[END_REF] suggested the possibility of a role for β-amyloid (1-42) and tau in the pathogenesis of glaucoma and diabetic retinopathy having found significantly decreased levels of β-amyloid (1-42) and significantly increased levels of tau in the vitreous fluid from patients with these disorders in comparison with the levels in a control group. Their findings suggested that the neurodegeneration processes in these ocular diseases might share, at least in part, a common mechanism with AD. [START_REF] Yoneda | Vitreous fluid levels of beta-amyloid((1-42)) and tau in patients with retinal diseases[END_REF] Lately, it was also demonstrated that abnormal tau AT8 is present in human glaucomas with uncontrolled elevated IOP. [START_REF] Gupta | Retinal tau pathology in human glaucomas[END_REF] Furthermore, evidence exists of build-up of Aβ in RGCs in experimental rat glaucoma. [START_REF] Mckinnon | Caspase activation and amyloid precursor protein cleavage in rat ocular hypertension[END_REF] Activation of caspases and abnormal APP processing, which includes production of Aβ, are important events in AD. [START_REF] Mckinnon | Caspase activation and amyloid precursor protein cleavage in rat ocular hypertension[END_REF] McKinnon and colleagues [START_REF] Mckinnon | Caspase activation and amyloid precursor protein cleavage in rat ocular hypertension[END_REF] detected a similar situation in experimental glaucoma. Indeed, in their study using a chronic ocular hypertensive rat glaucoma model, the authors found that caspase-3 is activated in RGCs, where it cleaves APP to produce neurotoxic fragments that include Aβ. [START_REF] Mckinnon | Caspase activation and amyloid precursor protein cleavage in rat ocular hypertension[END_REF] This suggested a new hypothesis for RGC death in glaucoma involving chronic Aβ neurotoxicity, mimicking AD at the molecular level. [START_REF] Mckinnon | Glaucoma: ocular Alzheimer's disease?[END_REF] A study published by Guo et al. in 2007 [14] provided evidence that targeting Aβ and blocking its effects with combination therapy may represent an effective treatment strategy in glaucoma. Further evidence of a link between AD and glaucoma has emerged from studies showing that patients with AD exhibit optic nerve degeneration and loss of RGCs. [START_REF] Hinton | Optic-nerve degeneration in Alzheimer's disease[END_REF][START_REF] Blanks | Retinal ganglion cell degeneration in Alzheimer's disease[END_REF] In addition, other studies found a significantly increased occurrence rate of glaucoma among patients with AD. [START_REF] Tamura | High frequency of open-angle glaucoma in Japanese patients with Alzheimer's disease[END_REF][START_REF] Bayer | High occurrence rate of glaucoma among patients with Alzheimer's disease[END_REF] In a nursing home based study in Germany, the prevalences of glaucoma (as defined by characteristic optic nerve or visual field changes) were reported to be 25.9% in patients with AD and 5.2% in controls. [START_REF] Bayer | High occurrence rate of glaucoma among patients with Alzheimer's disease[END_REF] In addition, the occurrence rate of ocular hypertension with normal visual fields and normal optic nerve heads in patients with AD was 0% compared to a prevalence of about 7% in the control subjects. [START_REF] Bayer | High occurrence rate of glaucoma among patients with Alzheimer's disease[END_REF] The authors assumed the optic nerve to be less resistant to elevated IOP levels in AD patients. [START_REF] Bayer | High occurrence rate of glaucoma among patients with Alzheimer's disease[END_REF] In another study, Tamura et al. [START_REF] Tamura | High frequency of open-angle glaucoma in Japanese patients with Alzheimer's disease[END_REF] found that the prevalence of OAG in Japanese patients with AD was 23.8%, which was significantly higher than that of the control subjects (9.9%). Furthermore, there was no significant difference between IOPs in AD patients with OAG and without OAG, and almost all AD patients with OAG showed normal tension. [START_REF] Tamura | High frequency of open-angle glaucoma in Japanese patients with Alzheimer's disease[END_REF] The authors concluded by suggesting that careful attention should be given to the potential for OAG in AD patients stressing the importance of not giving the impression to non-ophthalmic trained physicians or clinicians that OAG would be manifest through visual disturbance. [START_REF] Tamura | High frequency of open-angle glaucoma in Japanese patients with Alzheimer's disease[END_REF] In spite of intensive research, the clinical and genetic relationships between AD and glaucoma remain obscure. [START_REF] Tamura | High frequency of open-angle glaucoma in Japanese patients with Alzheimer's disease[END_REF] It is unclear whether the clinical correlation between the two diseases might be due to shared risk factors or the influence of one disorder on the other. There are several candidate common risk factors. For example, Tamura et al. [START_REF] Tamura | High frequency of open-angle glaucoma in Japanese patients with Alzheimer's disease[END_REF] evaluated the APOE ε4 allele as a common risk factor for AD and OAG. The percentage of AD patients with OAG who carried an APOE ε4 allele was not significantly different than that of AD patients without OAG. [START_REF] Tamura | High frequency of open-angle glaucoma in Japanese patients with Alzheimer's disease[END_REF] Their results suggested that APOE ε4 polymorphism may not be a common risk factor predisposing to both disorders. [START_REF] Tamura | High frequency of open-angle glaucoma in Japanese patients with Alzheimer's disease[END_REF] Recently, Helicobacter pylori infection has also been suggested to be involved in the pathogenesis of both AD and glaucoma. [START_REF] Kountouras | Normal-tension glaucoma and Alzheimer's disease: Helicobacter pylori as a possible common underlying risk factor[END_REF] To investigate whether primary open-angle glaucoma (POAG) is associated with increased risk of developing AD, Kessing et al. [START_REF] Kessing | No increased risk of developing Alzheimer disease in patients with glaucoma[END_REF] carried out a nationwide case register study comparing the rate of subsequent AD for patients with POAG (including normal tension glaucoma) with the rate for patients with primary angle-closure glaucoma (PACG), cataract, and osteoarthritis (OA) and with the rate for the general population. All patients included in the study were identified at hospital admission or outpatient contact during the period from 1977 to 2001 in Denmark. [START_REF] Kessing | No increased risk of developing Alzheimer disease in patients with glaucoma[END_REF] The authors did not confirm the hypothesis that patients with POAG have increased risk of developing AD. [START_REF] Kessing | No increased risk of developing Alzheimer disease in patients with glaucoma[END_REF] Indeed, patients with POAG did not have an increased rate of subsequent AD compared to patients with PACG, cataract, or OA or when compared with the general population. [START_REF] Kessing | No increased risk of developing Alzheimer disease in patients with glaucoma[END_REF] PRESENTATION OF THE HYPOTHESIS As noted above, we hypothesize that there may be a causal relationship between AD and glaucoma that may be explained by decreased cerebrospinal fluid pressure in patients with AD. In a retrospective case-control study, Berdahl et al. [START_REF] Berdahl | Cerebrospinal fluid pressure is decreased in primary open-angle glaucoma[END_REF] reported the intriguing new observation that mean CSFP was 33% lower in subjects with POAG (9.2 mmHg) compared with that of nonglaucomatous controls (13.0 mmHg; P<0.00005). Subjects were considered to have POAG if they were diagnosed with POAG by a glaucoma specialist, had characteristic optic nerve changes, and had visual field loss consistent with glaucoma. [START_REF] Berdahl | Cerebrospinal fluid pressure is decreased in primary open-angle glaucoma[END_REF] The criteria did not include IOP, because POAG occurs across the entire spectrum of IOP. [START_REF] Berdahl | Cerebrospinal fluid pressure is decreased in primary open-angle glaucoma[END_REF][START_REF] Pasquale | Low intracranial pressure: a tipping point in our understanding of primary open-angle glaucoma?[END_REF] The authors noted that their observation supports the concept that an abnormal high trans-lamina cribrosa pressure difference (a pressure difference across the lamina cribrosa), whether the result of elevated IOP, reduced CSFP, or both, plays an important role in glaucomatous optic nerve damage. [START_REF] Berdahl | Cerebrospinal fluid pressure is decreased in primary open-angle glaucoma[END_REF] The lamina cribrosa forms the bottom of the optic cup in the optic nerve head (optic disc) and acts as a pressure barrier between the intraocular pressure space and the retrobulbar CSF pressure space. [START_REF] Wostyn | Alzheimer's disease-related changes in diseases characterized by elevation of intracranial or intraocular pressure[END_REF][START_REF] Jonas | Anatomic relationship between lamina cribrosa, intraocular space, and cerebrospinal fluid space[END_REF] Normally, the IOP ranges from 10 to 21 mmHg, whereas the CSFP ranges from 5 to 15 mmHg (or 68 to 204 mmH 2 O). [START_REF] Berdahl | Cerebrospinal fluid pressure is decreased in primary open-angle glaucoma[END_REF] From a mechanical perspective, a similar posteriorly directed force is caused by either a lower pressure on the CSF side of the lamina or a higher pressure on the intraocular side. [START_REF] Berdahl | Cerebrospinal fluid pressure is decreased in primary open-angle glaucoma[END_REF] Berdahl et al. [START_REF] Berdahl | Cerebrospinal fluid pressure is decreased in primary open-angle glaucoma[END_REF] found that IOP, CSFP, and the trans-lamina cribrosa pressure difference all correlated with cup-to-disc ratio. Multivariate analysis showed that larger cup-to-disc ratio (an important parameter in the assessment of disc damage in glaucoma) was associated with lower CSFP. [START_REF] Berdahl | Cerebrospinal fluid pressure is decreased in primary open-angle glaucoma[END_REF] Importantly, the observation of Berdahl et al. [START_REF] Berdahl | Cerebrospinal fluid pressure is decreased in primary open-angle glaucoma[END_REF] may also shed new light on the link between AD and glaucoma. Silverberg et al. [START_REF] Silverberg | Elevated cerebrospinal fluid pressure in patients with Alzheimer's disease[END_REF] reported in 2006 on CSFP in patients with AD. In a clinical trial of low-flow CSF shunting for AD, AD subjects by National Institutes of Neurological and Communicative Diseases and Stroke-Alzheimer's Disease and Related Disorders Association criteria were carefully screened to exclude those with normal pressure hydrocephalus (NPH). [START_REF] Silverberg | Elevated cerebrospinal fluid pressure in patients with Alzheimer's disease[END_REF] As a final exclusion prior to shunt implantation, CSFP was measured supine under general anesthesia via the implanted ventricular catheter. [START_REF] Silverberg | Elevated cerebrospinal fluid pressure in patients with Alzheimer's disease[END_REF] During the initial implantation procedure, seven of the 181 subjects (3.9%) with no clinical or radiographic signs of NPH had an opening CSFP >200 mmH 2 O. [START_REF] Silverberg | Elevated cerebrospinal fluid pressure in patients with Alzheimer's disease[END_REF] These subjects were withdrawn from the remainder of the study, because of probable associated early NPH. [START_REF] Silverberg | Elevated cerebrospinal fluid pressure in patients with Alzheimer's disease[END_REF] For this AD-elevated CSFP group, the mean CSFP was 249 ± 20 mmH 2 O. [START_REF] Silverberg | Elevated cerebrospinal fluid pressure in patients with Alzheimer's disease[END_REF] AD patients with elevated CSFP were significantly younger and significantly less demented on the Mattis Dementia Rating Scale (MDRS) than those without elevated CSFP. [START_REF] Silverberg | Elevated cerebrospinal fluid pressure in patients with Alzheimer's disease[END_REF] The AD group without elevated CSFP consisted of 174 subjects (the remaining 96.1%). [START_REF] Silverberg | Elevated cerebrospinal fluid pressure in patients with Alzheimer's disease[END_REF] Mean opening CSFP in this group was 103 ± 47 mmH 2 O, which was statistically significantly lower when compared to the AD-elevated CSFP group and a somewhat younger non-demented control group of subjects with Parkinson's disease (140 ± 60 mmH 2 O). [START_REF] Silverberg | Elevated cerebrospinal fluid pressure in patients with Alzheimer's disease[END_REF][START_REF] Silverberg | Downregulation of cerebrospinal fluid production in patients with chronic hydrocephalus[END_REF] The distribution of CSFP in all AD subjects is shown in Figure 1. Remarkably, the frequency histogram of the CSFP distribution shows a substantial proportion of AD patients with very low CSFP. Indeed, it could be argued that there are really two subgroups of AD patients within the group without elevated CSFP: those with nearly normal CSFP and those with much lower CSFP (G. Silverberg, personal communication, 2008). An unexpected finding of this study was the relatively high (>30%) proportion of subjects with moderate to severe dementia as measured by MDRS total scores below 100, despite inclusion-exclusion criteria designed to capture subjects with mild to moderate dementia (Mini-Mental State Examination score between 15 and 24, inclusive). [START_REF] Silverberg | Elevated cerebrospinal fluid pressure in patients with Alzheimer's disease[END_REF][START_REF] Silverberg | Continuous CSF drainage in AD. Results of a double-blind, randomized, placebo-controlled study[END_REF] Cerebral atrophy associated with moderate to severe AD could be assumed to be the cause of the lower CSFP (G. Silverberg, personal communication, 2008). However, this has not been specifically investigated in this study (G. Silverberg, personal communication, 2008). Given the observation that a substantial proportion of AD patients has very low CSFP [START_REF] Silverberg | Elevated cerebrospinal fluid pressure in patients with Alzheimer's disease[END_REF], and given that a reduction in CSFP may produce an abnormal high translamina cribrosa pressure difference leading to glaucomatous damage [START_REF] Berdahl | Cerebrospinal fluid pressure is decreased in primary open-angle glaucoma[END_REF][START_REF] Pasquale | Low intracranial pressure: a tipping point in our understanding of primary open-angle glaucoma?[END_REF], we hypothesize that such a high pressure difference may explain why patients with AD have a greater risk for developing glaucoma. This obviously needs to be confirmed by future research. In addition, it should be stressed that it may be possible to hypothesize other factors that also might explain the link between AD and glaucoma.

TESTING THE HYPOTHESIS

To assess whether decreased CSFP is associated with OAG in patients with AD, CSFP could be compared in AD patients with OAG with that in AD patients without OAG. A significantly lower CSFP in AD patients with OAG compared with that in AD patients without OAG would support our hypothesis.

ACKNOWLEDGMENTS

We would like to thank Dr. Frank Kesteloot for his careful reading of this paper and his thoughtful comments.

CSFP (mm H2O)

Population (Subjects)

Histogram CSFP Across Population


Feltgen N, Bach M Lagrèze Wa

Prof. Dr. med W Lagrèze

Feasibility of intravitreal erythropoietin injections in humans

Keywords: Erythropoietin, neuroprotection, intravitreal injection, central retinal artery occlusion, anterior ischaemic optic neuropathy

Background/aims: Preclinical data suggest that intravitreally administered erythropoietin (EPO) is both neuroprotective and safe. In a small pilot series, we intended to assess the feasibility of intravitreal EPO injections in humans.

Methods: Three patients with acute vascular occlusion of the posterior pole received a single intravitreal EPO injection of 2000 U. Immediately before the injection and over the ensuing three months, these patient were closely monitored by measuring visual acuity, visual fields, intraocular pressure, the electroretinogram, the hematocrit and serum EPO levels.

Results: Over the observational period, most parameters remained unchanged except for a short-term rise of serum EPO levels, which, however, did not exceeded normal serum levels.

We did not observe injection-related toxicity.

Conclusion: Based on this limited set of data, a single EPO injection of 2000 U, a dose adapted from previous in vivo studies, is feasible and seems to induce no obvious damage.

Hence, further investigations of this therapeutic approach appear justified.

Introduction

Optic atrophy is the second most common condition leading to legal blindness. It is the final common pathway of inflammatory, ischemic, and other optic neuropathies, however most often of primary open angle glaucoma. These diseases cause irreversible loss of neurons and axons leading to a permanent functional impairment, which may impose a serious burden on the affected individuals and on health care systems in general. So far, no evidence based, causative therapy exists to decelerate axonal loss in the optic nerve except for lowering intraocular pressure in glaucoma, which is, however, sometimes insufficient.

Neuroprotection is a paradigm aiming at the reduction or even prevention of neuronal damage by pharmaceutical interventions or molecular genetic techniques. [START_REF] Lagrèze | Neuroprotection. Bases and possibilities of a future clinical use[END_REF] While being well established in vitro and in vivo, many clinical trials have failed for various reasons. It took until recently that randomized clinical trials yielded positive outcomes, i.e. with erythropoietin (EPO) in stroke, schizophrenia and multiple sclerosis [START_REF] Ehrenreich | Exploring recombinant human erythropoietin in chronic progressive multiple sclerosis[END_REF], with riluzole in amyotrophic lateral sclerosis [START_REF] Bensimon | A controlled trial of riluzole in amyotrophic lateral sclerosis. ALS/Riluzole Study Group[END_REF], and with memantine in Alzheimers disease [START_REF] Reisberg | Memantine in moderate-to-severe Alzheimer's disease[END_REF] and vascular dementia. [START_REF] Orgogozo | Efficacy and safety of memantine in patients with mild to moderate vascular dementia: a randomized, placebo-controlled trial (MMM 300)[END_REF] The hematopoietic growth factor EPO has been shown to have potent neuroprotective and neuroregenerative properties in the nervous system. We consider EPO an interesting neuroprotective candidate drug, because it is clinically available and its neuroprotective action has been well characterized. Over the last years, several groups have shown that EPO suppresses neuronal apoptosis and reduces inflammatory responses in different models of brain ischaemia [START_REF] Sakanaka | In vivo evidence that erythropoietin protects neurons from ischemic damage[END_REF] and inflammation. [START_REF] Brines | Erythropoietin crosses the blood-brain barrier to protect against experimental brain injury[END_REF] The action of EPO is conveyed by interference with intracellular Jak2/STAT3-and PI3K/Akt-pathways [START_REF] Kretz | Erythropoietin promotes regeneration of adult CNS neurons via Jak2/Stat3 and PI3K/AKT pathway activation[END_REF] mediated by EPOreceptor activation. [START_REF] Maiese | Erythropoietin in the brain: can the promise to protect be fulfilled?[END_REF] EPO crosses the blood brain barrier [START_REF] Brines | Erythropoietin crosses the blood-brain barrier to protect against experimental brain injury[END_REF] and non-hematopoietic mutants of EPO proved to be neuroprotective as well. [START_REF] Leist | Derivatives of erythropoietin that are tissue protective but not erythropoietic[END_REF] With regard to protection of retinal ganglion cells (RGC) it has been shown that EPO alleviates apoptosis in vitro [START_REF] Weishaupt | Effect of erythropoietin axotomy-induced apoptosis in rat retinal ganglion cells[END_REF]. Subsequent in vivo experiments in rats supported this finding. RGC survival was promoted by EPO after orbital transection [START_REF] Weishaupt | Effect of erythropoietin axotomy-induced apoptosis in rat retinal ganglion cells[END_REF], experimental autoimmune encephalomyelitis [START_REF] Diem | Combined therapy with methylprednisolone and erythropoietin in a model of multiple sclerosis[END_REF], chronically elevated intraocular pressure [START_REF] Zhong | Erythropoietin promotes survival of retinal ganglion cells in DBA/2J glaucoma mice[END_REF], and diabetic retinopathy. [START_REF] Zhang | Intravitreal injection of erythropoietin protects both retinal vascular and neuronal cells in early diabetes[END_REF] In these studies EPO was administered either systemically [START_REF] Zhong | Erythropoietin promotes survival of retinal ganglion cells in DBA/2J glaucoma mice[END_REF] or by injection into the vitreous. [START_REF] Weishaupt | Effect of erythropoietin axotomy-induced apoptosis in rat retinal ganglion cells[END_REF][START_REF] Zhang | Intravitreal injection of erythropoietin protects both retinal vascular and neuronal cells in early diabetes[END_REF][START_REF] King | Erythropoietin is both neuroprotective and neuroregenerative following optic nerve transection[END_REF] Over the last years, intravitreal injections became a new treatment regimen in retinal diseases such as age related macular degeneration, mostly, due to the success of VEGFinhibitors. Although not being without risks such as hemorrhages or endophthalmitis, they offer the advantage to deliver drugs locally in high concentrations. Hence, the question arises whether intravitreal injections of EPO may be possible in humans and have a neuroprotective potential on RGC and their axons in retinal and optic nerve diseases. Therefore, we conceived a small, prospective pilot trial encompassing three patients with vascular occlusions of the posterior pole in order to assess the feasibility of intravitreal EPO injections in humans.

Patients and methods

The investigation adhered to the declaration of Helsinki. Ethical and safety aspects were reviewed with the Institutional Review Board of the University Hospital Freiburg, which approved the limited patient number of three in this pilot case series. The inclusion criterion was an acute arterial occlusion of the retina or optic nerve head less than three day ago in patients between 50 and 80 years of age. Exclusion criteria comprised any kind of neoplasia, temporal arteritis, glaucoma, diabetic retinopathy, previous vascular occlusions of the posterior pole, and the eligibility for other interventions such intraarterial thrombolysis.

Accordingly, three patients with three different types of vascular occlusions of the posterior pole were prospectively included and gave fully informed, written consent. The first patient was male, 76 years old and had central retinal artery occlusion (CRAO) with intact cilioretinal perfusion 20 hours before presentation. The second patient was female, 79 years old and had complete CRAO 22 hours before presentation. The third patient was male, 65 years old and presented 30 hours after non-arteritic anterior ischemic optic neuropathy (NAION). Each patient was subjected to the following diagnostic procedures: Best corrected visual acuity

Results

Figures 1 to 3 summarize the clinical data of the three patients over the observational period.

The patient with CRAO and intact cilioretinal perfusion showed a small increase of visual acuity the day after injection. It remained stable over the ensuing three months. So did his visual field. The intraocular pressure did not change after the injection when measured one day thereafter. The ERG peak times remained constant. The ERG amplitudes rose during the week after the injection and declined thereafter. The serum EPO levels doubled, but did not exceed the upper normal limit.

-figure 1 -The patient with complete CRAO had a visual acuity in a very low range. It did not improve after the injection. Her visual fields were markedly depressed and showed central scotomata except for day seven. The intraocular pressure did not change. Her ERG parameters remained rather stable over the three months. After the injection, serum levels tripled within normal limits, and decreased again after one month.

-6 --figure 2 -The patients with NAION had variable visual acuities with a slight tendency towards improvement. A similar trend was seen in his visual fields. His intraocular pressure rose towards the end of the observational period, but remained with in normal limits. The ERG peak time was not changed, the amplitude decreased over three months. As seen in the other patients, his hematocrit did not change. Again, systemic serum EPO levels rose on the day after injection, but did not exceed normal limits.

-figure 3 -

Discussion

This is the first report about intravitreal injections of EPO into the human vitreous body, a potentially neuroprotective treatment regimen. In three patients with vascular occlusions of the posterior pole, we did not observe functional deterioration over three months of observation. Two of them showed a slight visual improvement. Hence, we conclude from this small pilot study that intravitreal EPO injections are feasible and probably safe and that this approach disserves further attention and clinical research.

Since we could not exclude a damaging effect, we only included patients with neuroretinal diseases having a rather unfavorable visual acuity and prognosis, i.e. arterial occlusions in the optic nerve head leading to NAION or CRAO. NAION has an incidence of 1/10.000 with resulting a mean visual acuity of 1.31 logMAR. After 2 years, 31% show an improvement in visual acuity of three lines or more. [START_REF]Ischemic Optic Neuropathy Decompression Trial: twenty-four-month update[END_REF] CRAO has an incidence of 0.1/10.000 and a poor prognosis with a resulting average acuity of hand motion. Only 3% show a visual recovery of three lines or more after 6 weeks. [START_REF] Atebara | Efficacy of anterior chamber paracentesis and Carbogen in treating acute nonarteritic central retinal artery occlusion[END_REF] In both entities, no evidence-based therapy has so far been established as efficacious in randomized clinical trials. [START_REF] Fraser | Interventions for acute non-arteritic central retinal artery occlusion. Cochrane database of systematic reviews (Online)[END_REF][START_REF] Buono | Nonarteritic anterior ischaemic optic neuropathy[END_REF] Because of this constellation and in the light of the hampered prognosis of these diseases, we consider our intervention as ethically justified.

-7 -

In the human vitreous body, Müller cells are the mainly source of endogenous EPO [START_REF] Fu | Up-regulated endogenous erythropoietin/erythropoietin receptor system and exogenous erythropoietin rescue retinal ganglion cells after chronic ocular hypertension[END_REF]. Its vitreal levels have been determined in three clinical investigations: Inomata et al. [START_REF] Inomata | Vitreous levels of erythropoietin in patients with macular oedema secondary to retinal vein occlusions: a comparative study with diabetic macular oedema[END_REF] found a mean EPO concentration of 34 (range 20 -64) mU/ml in subjects with macular holes and 881 (157 -1850) mU/ml in cases of proliferative diabetic retinopathy. They interpreted the ischaemia-induced, elevated EPO level as a possible endogenous neuroprotective reaction.

Garci-Arumi et al. [22] found a mean concentration of 25 (range 10 -75) mU/ml in patients with epiretinal membranes and macular holes and 430 (range 41 -3000) mU /ml in diabetic macular edema. Finally, Asensio-Sanchez et al. [START_REF] Asensio-Sanchez | Erythropoietin concentrations in the vitreous body from patients with proliferative diabetic retinopathy][END_REF] measured 25 ( range 5-201) mU/ml in non-ischaemic patients compared to 512 (120 -880) mU/ml in cases of proliferative diabetic retinopathy. Taken together, these data are rather consistent and indicate a 20fold increase in retinal ischaemia compared to non-ischaemic conditions. These increased EPO levels in retinal ischaemia are about 800 times lower than the concentration applied in our case series, in which we chose a single, intravitreal dose of 2000 IU. This dose was adapted from previous in vivo experiments. With regard to neuroprotection, intravitreal doses so far most effectively applied in rats were 2 IU [START_REF] Weishaupt | Effect of erythropoietin axotomy-induced apoptosis in rat retinal ganglion cells[END_REF], 7.65 IU [START_REF] Zhang | Intravitreal injection of erythropoietin protects both retinal vascular and neuronal cells in early diabetes[END_REF] or 10 IU. [START_REF] King | Erythropoietin is both neuroprotective and neuroregenerative following optic nerve transection[END_REF] Rats have an estimated vitreous volume of 0.013 ml [START_REF] Dureau | Quantitative analysis of intravitreal injections in the rat[END_REF], while humans have a vitreous volume of about 4 ml, thus being about 300 times higher. In a recent in vivo toxicity study in rabbits, intravitreal injections of up to 1000 U were considered safe by electroretinography, fluorescein angiography and histology. Vitreal half-life times were found to be 2 -3 days. [START_REF] Zhang | Pharmacokinetic and toxicity study of intravitreal erythropoietin in rabbits[END_REF] With regard to potential systemic side effects, we measured EPO levels in the blood serum of our patients and determined their hematocrit. The systemic EPO levels remained within normal clinical limits at all measurements. However, the dose applied in our patients was obviously sufficient to increase systemic EPO levels shortly after the injection. The hematocrit did not change. We therefore are not concerned that the EPO dose applied in this case series lead to unwanted systemic levels.

The potential use of intravitreal EPO is manifold, as several diseases affect the inner retina and the subsequent anterior visual pathway, but also the outer retina, causing neuronal loss which cannot be decelerated in many instances such as AION, CRAO, optic neuritis, photoreceptor dystrophies and sometimes glaucoma. However, one has to keep in mind that the potential benefit should outweigh the potential harm of such a treatment. Further, it is yet unknown, which EPO dose is most useful and whether repeated injections are feasible or necessary.

It is well known that intravitreal injections cause a transient rise of intraocular pressure, which may compromise retinal circulation, especially in the diseases chosen herein. Hollands et al. measured intraocular pressure before and 2, 5, and 30 minutes after injection of an identical volume of bevacizumab. [START_REF] Hollands | Short-term intraocular pressure changes after intravitreal injection of bevacizumab[END_REF] The mean intraocular pressures were 14, 36, 26, and 16 mm Hg, respectively. Accordingly, one has to assume that the injection of 0.05 ml significantly raises the pressure for about half an hour, which may affect the perfusion pressure. In order to avoid this, a paracentesis may be performed. The patient with the NAION indeed noted a short term, reversible deterioration of vision for one hour after the injection. Intravitreal injections have been performed in NAION before. The clinicaltrials.gov data base lists a current trial in the recruiting phase, in which patients with NAION receive intravitreal injections of ranibizumab (NCT00561834).

In conclusion, this small pilot study with a single injection of 2000 U EPO and its limited set of data may stimulate further clinical research regarding intravitreal injections aiming at neuroprotection. The dosage, which was adapted from preclinical in vivo studies, exceeded endogenous intravitreal EPO levels by far, but induced no obvious damage. The intervention seemed feasible, and did not reveal, as expected, any positive effect for the three patients.

  chart), Goldmann applanation tonometry, Goldmann kinetic perimetry (stimulus III4e and I4e), indirect funduscopy, full field photopic electroretinography (ERG) according to ISCEV standards (Toennies Physiologischer Verstärker, Hoechberg, Germany) and measurement of serum EPO levels (IMMULITE 2000 EPO assay, Siemens, Germany; range of normal values 3.7 -31.5 mU / ml) and hematocrit. The two patients with retinal vessel occlusion additionally underwent fluorescence angiography. On the day of presentation, each of the three patients received one single injection of 0.05 ml Epoetin alfa containing 2000 I.U. (Erypo FS 40 000 I.E./ml, Janssen-Cilag, Germany) via a 30 gauge needle 4 mm posterior to the limbus into the temporal superior quadrant after local desinfection and topical anaesthesia. All diagnostic procedures listed above, except fluorescence angiography, were performed directly before, and one day, one week, and one and three months after the injection.

Figure 1 :

 1 Figure 1: Top left: Fundus image of a patient with central retinal artery occlusion and intact cilioretinal perfusion showing retinal oedema sparing the papillomacular bundle. Top middle: Fluorescence angiogram showing attenuated and delayed venous filling 38 sec after dye injection. Top right: Fundus image 90 days after treatment. Middle: Table indicating clinical and laboratory parameters (abbreviations: IOP = intraocular pressure, ERG = electroretinogram, Hkt = hematocrit, EPO = erythropoietin). Bottom: Goldmann kinetic visual field on days 0, 1, 7, 30, and 90 (Outside border isopter Goldmann III4e, inside isopter I4e, dashed area scotoma for stimulus III4e).

Figure 2 :

 2 Figure 2: Top left: Fundus image of a patient with central retinal artery occlusion retinal oedema and a foveal red spot. Top middle: Fluorescence angiogram showing massively reduced circulation 40 sec after dye injection Top right: Fundus image 90 days after treatment showing cessation of oedema and optic atrophy. Middle: Table indicating clinical and laboratory parameters (abbreviations: IOP = intraocular pressure, ERG = electroretinogram, Hkt = hematocrit, EPO = erythropoietin). Bottom: Goldmann kinetic visual field on days 0, 1, 7, 30, and 90 (Outside border isopter Goldmann III4e, inside isopter I4e).

11 Figure 3 :

 113 Figure 3: Top left: Fundus image of a patient with anterior ischaemic optic neuropathy showing optic disc swelling. Top right: Fundus image 90 days after treatment showing cessation of papilloedema and optic atrophy. Middle: Table indicating

  clinical and laboratory parameters (abbreviations: IOP = intraocular pressure, ERG = electroretinogram, Hkt = hematocrit, EPO = erythropoietin). Bottom: Goldmann kinetic visual field on days 0, 1, 7, 30, and 90 (Outside border isopter Goldmann III4e, inside isopter I4e, dashed area scotoma for stimulus III4e).


MD Pilar Casas-Llera
email: casasdellerapilar@gmail.com

PhD Gema Rebolleda

PhD Francisco J Muñoz-Negrete

PhD Francisco Arnalich-Montiel

MD Marta Pérez-López

MD Roberto Fernández-Buenaga

Ramón

Pilar Casas De Llera

Tomás López

Visual Field Index Rate and Event-Based Glaucoma Progression Analysis: Comparison in a Glaucoma Population Key words: Glaucoma, Glaucoma progression analysis

Keywords: visual field index, trend analysis, visual fields

Aims: To compare event-based glaucoma progression analysis (GPA) I with new GPA II software and pattern deviation-based trend analyses (visual field index [VFI]) to detect progression in a glaucoma population.

Methods: A retrospective study that included 90 eyes of 90 patients with a minimum of five reliable visual field tests and a follow-up period of at least 2 years.

Results:

Event-based GPA II detected progression in 16.7% of eyes in which trend analysis VFI failed. GPA detected progression 6.8 months earlier than VFI. GPA I and II showed excellent agreement (k=0.94). Agreement between VFI and mean deviation (MD) linear analysis and with GPA criteria was k=0.52 and k=0.48, respectively. Mean rates of progression of MD and VFI were -0.41 dB and -1.30% annually, respectively (rho=0.824; P < 0.0001). Using VFI, mean follow-up time was 6.12 and 4.89 years (P= 0.004) and mean number of visual field tests was 7.33 and 6.01 (P= 0.023) in eyes with and without progression, respectively.

Conclusions:

Event-based software GPA I and II had an excellent agreement. Event analysis showed earlier and greater sensitivity for detecting progression than VFI analysis and both had only moderate agreement. Trend analysis VFI is likely to detect progression in patients with a greater number of visual field tests and a longer follow-up time. The VFI analysis seems to be more accurate than MD analysis for determining rate of progression.

The ability to detect progression of visual field defects remains one of the most challenging aspects of glaucoma management. Attention has been focused lately on developing methods to detect early glaucoma damage by evaluation of a longitudinal series of visual field tests. [START_REF] Giangiacomo | Diagnosing glaucoma progression: current practice and promising technologies[END_REF] Techniques such as the judgment of expert clinicians, defect classification systems, trend analyses, and event-based analyses have been evaluated for determining visual field progression . [START_REF] Birch | Determining progressive visual field loss in serial Humphrey visual fields[END_REF][START_REF] Heijl | Early Manifest Glaucoma Trial Group. Reduction of intraocular pressure and glaucoma progression: results from the Early Manifest Glaucoma Trial[END_REF][START_REF] Spry | Identification of progressive glaucomatous visual field loss[END_REF][START_REF] Diaz Aleman | Detection of visual field deterioration by glaucoma progression analysis and threshold noiseless trend programs[END_REF] Event-based analysis has been proposed as an advantageous approach for detecting progression of glaucoma. [START_REF] Birch | Determining progressive visual field loss in serial Humphrey visual fields[END_REF] This method relies on stable glaucoma population-derived variability limits. Glaucoma Progression Analysis (GPA), included in the software of the Humphrey Visual Field Analyzer (VFA) (Carl-Zeiss Meditec, Dublin, CA), is an example of event-based analysis using pattern standard deviation (PSD) values.

Determining the rate of disease progression in every patient is fundamental. [START_REF] Caprioli | The importance of rates in glaucoma[END_REF] Nouri-Mahdavi et al. [START_REF] Nouri-Mahdavi | Prediction of visual field progression in glaucoma[END_REF] reported that the faster progression was the strong predictive factor for further progression. Up-to-date mean deviation (MD) trend analysis was the standard index for estimating the glaucoma progression rate. The MD is affected not only by progression of glaucoma but also by cataract. Arnalich-Montiel et al. [START_REF] Arnalich-Montiel | Performance of glaucoma progression analysis software in a glaucoma population[END_REF] reported that trend analysis of the MD is poorly correlated with clinical judgment and event-based analysis, is less sensitive, and ignores the detailed spatial information within the computerized field test. Thus, Bengtsson and Heijl [START_REF] Bengtsson | A visual field index for calculation of glaucoma rate of progression[END_REF] described a technique, the visual field index (VFI), to measure the rate of visual field deterioration in glaucoma that is based largely on the Humphrey perimetry PSD analysis. The GPA II software displays the linear regression of the VFI.

The purpose of the current study was to compare the commercially available eventbased analysis GPA I with the new GPA II software, including PSD-based trend analyses (VFI) to detect disease progression in a glaucoma population.

Patients and Methods

Analyses were performed using the Humphrey VFA database in the glaucoma unit of our hospital. This retrospective study, approved by the institutional review board of the hospital, included 90 eyes of 90 patients with a definitive diagnosis of primary open-angle glaucoma who had a minimum of five visual field tests that could undergo GPA analysis and a follow-up period of at least 2 years.

The visual field tests were considered reliable if the fixation losses, the false negative results, or the false positive results were 25% or less. The inclusion criteria also included a visual acuity (VA) that exceeded 20/100 and a baseline MD of -15 decibels (dB) or better. When both eyes were eligible, one eye was randomly selected.

We excluded patients with visual field loss from causes other than glaucoma, those who developed visual field loss for reasons other than glaucoma during follow-up, and patients with disorders that affect the visual fields. Patients were included in the study who had no substantial lens opacity as determined by an ophthalmologist at the baseline clinical examination and throughout the study.

Each subject underwent periodic examinations that included measurement of the best-corrected visual acuity, slit-lamp biomicroscopy, gonioscopy, applanation tonometry, dilated funduscopy examination, and visual field testing, the last performed on a Humphrey VFA using the 24-2 program with the Swedish interactive threshold algorithm standard strategy.

As baseline, we used the first two abnormal but reliable visual field tests either displaying a glaucoma hemi-field test outside the normal limits or a corrected PSD of 5% probability or worse.

GPA I Definition of Progression

GPA I printouts were obtained by one author (PC-L) who changed the baseline tests as needed by discarding unreliable visual field test or those affected by the learning curve.

Event based GPA I used statistical criteria designed for the Early Manifest Glaucoma Trial [START_REF] Leske | Early Manifest Glaucoma Trial: design and baseline data[END_REF] that were used to identify incident progression of visual field defects. When significant deterioration (P<0.05) was evident on the PSD probability maps of the GPA I printouts at the same three or more points on two consecutive follow-up tests, the GPA I software interpreted this as possible progression, whereas if deterioration occurred in three consecutive follow-up tests, the software interpreted that as likely progression. The points did not necessarily need to be clustered together to satisfy either criterion. In the GPA I printout, the follow-up probability maps were sometimes marked with Xs at specific locations, indicating that these locations had baseline threshold values that were too low for an effective comparison. Therefore, these locations were not used to assess progression. GPA I printouts also showed a summary plot of the MD for each test in the analysis plus a MD linear regression analysis. An eye was classified with progressive damage if a negative linear regression slope was significant with a P < 0.05.

GPA II Definition of Progression

GPA II printouts were obtained by one author (MP-L) who eliminated the last test if it was not included in the GPA I in order to evaluate the same visual fields in both analyses. There are no differences between event based GPA I and II definitions of progression, but in contrast with GPA I, GPA II did not include in its analysis the first visual field if a learning effect was detected automatically. GPA II also did not include patients with a MD worse than -20 dB, because PSD analysis may suggest artifactual visual field improvements despite further visual field deterioration. [START_REF] Bengtsson | A visual field index for calculation of glaucoma rate of progression[END_REF] GPA II provides the VFI, which is based on the PSD probability maps. For this index, visual field data are age adjusted and expressed as a percentage of a normal visual field and intended to calculate the rate of functional loss. The VFI of a perimetrically normal visual field was set to 100%, and the VFI of a perimetrically blind visual field was set to 0%. Estimates of the cortical representation of the spatial relationships of vision were used to adjust the VFI to be more heavily weighted to central areas of the visual field. [START_REF] Bengtsson | A visual field index for calculation of glaucoma rate of progression[END_REF][START_REF] Heijl | Visual field interpretation with empiric probability maps[END_REF] Besides event-based analysis, GPA II printouts also showed a trend analysis with summary plot of the VFI index for each test in the analysis plus linear regression analysis of the VFI, i.e., the VFI progression rate. Both event and trend analysis are complementary of each other. An eye was classified with progressive damage if a negative linear regression slope was significant with a P < 0.05.

Statistical Analysis

Kappa statistics were used to estimate the agreement between the different approaches to assess glaucoma progression. The continuous variable correlation was calculated using Spearman's coefficient. We compared how follow-up time and mean number of visual field tests affected the likelihood of detecting progression by the different methods using the U-Mann-Whitney test. The total level of significance was set to P < 0.05 (two-sided). SPSS 13.0 software for Windows (SPSS Inc, Chicago, IL) was used to provide all of these data and to calculate progression rates and demographic data.

Rate of Progression

The rates of progression of visual field loss based on the VFI linear regression, the MD linear regression, and the event based GPA I and II progression are shown in Table 2.

Based on the trend analysis VFI and event based GPA II analyses, 63 eyes (70%) were stable and the percentage of eyes classified as the same (either with progressive damage or stable) was 82.2%. Event based GPA I and II showed similar progression rates (31.1% and 28.9%, respectively). The possible progression rate was slightly higher with GPA I than GPA II (12.2% vs. 10%, respectively).

Parameter

Table 1 .

 1 Demographic Data of the Study Population MD = mean deviation; PSD = pattern standard deviation; dB = decibel; VFI = visual field index.

Table 2 .

 2 Prevalence of Progression

		Mean	Standard	Range
			Deviation
	Age (years)	72.54	7.8	40 to 86
	Follow-up (years)	5.06	1.55	2 to 11
	VF number	6.19	1.44	5 to 11
	Basal MD (dB)	-6.26	3.02	-13.14 to -1.49
	Basal PSD (dB)	5.24	3.04	0.87 to 15.40
	Basal VFI (%)	87.13	8.9	55 to 99

GPA = glaucoma progression analysis; VFI = visual field index; MD: mean deviation.

Table 3 .

 3 Differences between GPA I and II Progression Criteria

	Progression	GPA I (%)	GPA II (%)
	Possible	11 (12.2)	9 (10)
	Likely	17 (18.9)	17 (18.9)
	None	62 (68.9)	62 (68.9)
	Wrong basal visual field	Not applicable	2 (2.2)
	included
	MD < -20 dB	Not applicable	2 (2.2)

GPA = glaucoma progression analysis; dB = decibel; MD = mean deviation.

Study approved by the institutional review board of the Ramón y Cajal Hospital. grant on behalf of all authors,

As previously reported [START_REF] Arnalich-Montiel | Performance of glaucoma progression analysis software in a glaucoma population[END_REF], we also evaluated the MD linear regression analysis using previous-generation GPA I software, which showed that 76.7% of eyes remained stable, while 23.3% had a significantly worsening slope. There was poor agreement compared with event based GPA II (k = 0.22; p=0.031)

No patients had progressive visual field damage detected by the trend analysis VFI when the event based GPA II analysis indicated stability. The only discrepancy occurred in an eye with a MD worse than -20 dB and as mentioned previously, GPA II does not apply in this case. However, event based GPA II detected progression of damage in 15 of 90 eyes (16.7%) that did not have based on the VFI trend analysis. In 11 patients (12.2%) who had progression using these both criteria, event analysis detected progression 6.8 months earlier than the trend analysis .

Rate of Progression

The mean rate of MD progression was -0.41 dB annually. The mean rate of VFI progression was -1.30% annually. There was a significant correlation between both rates of progression (rho= 0.824; P < 0.0001) Figure 1 shows the length of the follow-up related to agreement in the detection of progression by event-based analysis (classified as likely or possible progression) and the VFI.

Using trend analysis VFI, the mean numbers of visual field test in eyes with and without progression were 7.33 and 6.01, respectively (P=0.023); the mean follow-up times in eyes with and without progression were 6.12 and 4.89 years, respectively (P =0.004).

The mean confidence limits in eyes with progression and no progression were ±2.70% and ±3.24% (P =0.614), respectively. The mean confidence limits were weakly inversely correlated with the follow-up time (rho=-0.32; P=0.002). There was no correlation between the number of visual field tests and the mean confidence limits (P=0.23). The mean VFI confidence limits in eyes with and without progression by event-based GPA were ±4,15% and ±2,73%, respectively (P=0.044). Moreover, the mean confidence limits of the VFI rate were ±2.72% and ±5.23% when methods yield identical results and inconsistent results respectively (P=0.004).

Fourteen eyes (15.5%) had VFI rate of progression confidence intervals that exceeded 5%; GPA showed progression in seven eyes (50%) of these eyes.

Agreement between Progression Criteria

Discussion

Several methods have been developed to detect visual field progression. [START_REF] Birch | Determining progressive visual field loss in serial Humphrey visual fields[END_REF][START_REF] Heijl | Early Manifest Glaucoma Trial Group. Reduction of intraocular pressure and glaucoma progression: results from the Early Manifest Glaucoma Trial[END_REF][START_REF] Spry | Identification of progressive glaucomatous visual field loss[END_REF][START_REF] Diaz Aleman | Detection of visual field deterioration by glaucoma progression analysis and threshold noiseless trend programs[END_REF] Eventbased GPA was designed primarily to detect whether progression has occurred. However, most patients with glaucoma would eventually show some progression if followed for a sufficiently long time. [START_REF] Caprioli | The importance of rates in glaucoma[END_REF] The objectives of the current study were to compare previous and actual software event-based PSD analysis (GPA I and II) with PSD-based trend analysis (VFI) of the visual fields to detect glaucoma progression in routine clinical ophthalmic practice.

Not surprisingly, we did not find significant differences in the prevalence of glaucoma progression when comparing GPA I and II event analysis. This prevalence agrees with other published studies. [START_REF] Katz | A comparison of the pattern-and total deviation-based Glaucoma Change. Probability programs[END_REF][START_REF] Boden | Patterns of glaucomatous visual field progression identified by three progression criteria[END_REF] The differences are related to the fact that GPA II does not include patients with a MD worse than -20 dB, because PSD analysis may suggest artifactual visual acuity improvements despite further visual field deterioration. [START_REF] Bengtsson | A visual field index for calculation of glaucoma rate of progression[END_REF] However, in the current study, because eyes had to have a MD of -15 dB or better in the basal visual fields to be included, we could not emphasize that difference. In addition, GPA II analysis does not include the first visual field if a learning effect is detected. This In the current study, VFI trend analyses showed moderate agreement (k=0.48) with event-based analysis GPA II. Twenty-six eyes (28.9%) were classified by event GPA II as having progressive damage. In fifteen of these twenty-six eyes (57,6%),the worsening slope of the trend VFI was not significant. Therefore, event analysis GPA seems to be more sensitive for detecting progression than trend analysis VFI. One reason could be that there were few visual fields during follow-up, because if we analyse these eyes, only two of the fifteen (13.3%) had more than eight visual field test. Otherwise, the VFI rate confidence interval was wider in patients with progression detected by event-based GPA and in patients in whom results were inconsistent between methods. Thus, the VFI worsening slope would not be significant when there was great variability while the event GPA analysis is not.

In patients who had progression by both criteria, event-based GPA II detected the progression 6.8 months earlier than trend VFI. Therefore event GPA II showed earlier detection than trend analysis VFI, just as we expected because of event-based methods design.

Regression analysis of any VFI requires a sufficiently large number of tests to detect progression reliably and predictively.[6] Indeed, we found that VFI is likely to detect progression in patients with a greater number of visual field tests and a longer follow-up period. However the follow-up time showed only a weak inverse correlation with the mean confidence limits of the VFI progression rate. Thus, not only follow-up but also variability can affect the estimating of VFI progression rate.

A primary limitation of event-based analysis methods is the detection of very focal progression based on marked worsening of one point within the central 10 degrees, [START_REF] Diaz Aleman | Detection of visual field deterioration by glaucoma progression analysis and threshold noiseless trend programs[END_REF][START_REF] Arnalich-Montiel | Performance of glaucoma progression analysis software in a glaucoma population[END_REF] since GPA does not consider the location of the point of progression of damage but the presence of progression on at least three different points. In the current study, one patient had this kind of central progression. Neither event GPA nor trend VFI detected progressive damage in this patient.

MD linear regression showed poorer agreement with event-based analysis GPA II (k=0.22) than VFI linear regression (k=0.50). Therefore, VFI seems to be better than MD linear regression analysis for determining a patient's rate of progression. Nevertheless, when comparing MD and VFI trend analysis, we found that agreement was moderate (0.52), and there was a strong positive correlation (rho=0.824). This may be explained because only patients without significant lens opacity at the baseline and throughout the study were included.

VFI trend analysis has two main limitations: 1) the need for longer follow-up and more visual field tests than event-based methods to detect progression and to estimate the progression rate if high variability is present, and 2) despite the fact that VFI emphasizes the functional importance of central vision, [START_REF] Caprioli | The importance of rates in glaucoma[END_REF][START_REF] Bengtsson | A visual field index for calculation of glaucoma rate of progression[END_REF] it cannot resolve the main GPA limitation [START_REF] Diaz Aleman | Detection of visual field deterioration by glaucoma progression analysis and threshold noiseless trend programs[END_REF][START_REF] Arnalich-Montiel | Performance of glaucoma progression analysis software in a glaucoma population[END_REF], namely, detection of very focal central progression.

In summary, event-based GPA detected glaucoma progression earlier and was more sensitive for detecting progression than VFI trend analysis, and both had only moderate agreement. The mean VFI rate of progression confidence intervals were wider in patients whom results were inconsistent between GPA and VFI. Trend analysis VFI is likely to detect progression in patients with a greater number of visual field tests and a longer follow-up period. The VFI rate of progression seems to be better than MD linear regression analysis for determining a patient's rate of progression, although both methods had a strong positive correlation. We also found that event-based analysis software GPA I and II identification of progression has a high level of agreement.


S Saidkasimova

D Mitry

J Singh

D Yorston

D G Charteris

Retinal detachment in Scotland is associated with affluence

Keywords: retinal detachment, socioeconomic factors, population characteristics, incidence, Scotland

Purpose. Socio-economic deprivation plays a major role in health and disease, however its role in retinal detachment has not been studied. We used data from the Scottish Retinal Detachment Study to investigate any association between retinal detachment, macular status at presentation, and deprivation,. Methods. Prospective multicentre population based observational study. Data was collected on all patients with primary retinal detachment presenting in Scotland between 1 November 2007 and 31 October 2008. Every patient was allocated a validated deprivation ranking according to their postcodes based on the Scottish Index of Multiple Deprivation (SIMD), which takes into account income, employment, health, education, housing, geographic accessibility and crime in 6505 postcode based datazones. The patients were divided into four quartiles according to their SIMD ranking.

Results. 572 patients were included. The annual incidence of detachment declined from 15.4/100,000 in the most affluent quartile to 13.6/100,000 in the second, 9.3/100,000 in the third and 6.9/100,000 in the most deprived (χ 2 for trend = 50.2, p<0.0001). The trend was more marked for men than women, but was present in both sexes. There were no differences in the rate of macula-off detachments, nor the number of quadrants detached. Rates of pseudophakic and myopic detachment were evenly distributed across all quartiles.

Conclusions. Retinal detachment appears to be associated with affluence. This has not been previously reported and may be partly responsible for the variation in estimates of the incidence of retinal detachment. It may also have implications for service planning. The reason for this association is unknown.

INTRODUCTION

Rhegmatogenous retinal detachment (RRD) is a potentially blinding condition with an average reported annual incidence of 10 per100,000 population [START_REF] Mitry | Rhegmatogenous retinal detachment in Scotland: research design and methodology[END_REF][START_REF] Rowe | Retinal detachment in Olmsted County, Minnesota, 1976 through 1995[END_REF][START_REF] Wong | Racial difference in the incidence of retinal detachment in Singapore[END_REF][START_REF] Polkinghorne | Northern New Zealand Rhegmatogenous Retinal Detachment Study: epidemiology and risk factors[END_REF][START_REF] Li | Incidence and epidemiological characteristics of rhegmatogenous retinal detachment in Beijing, China[END_REF] . Myopia, age, pseudophakia and certain collagen disorders are known risk factors which may predispose to RRD. Visual recovery and successful surgical repair of RRD are affected by delayed presentation which leads to a more extensive detachment and involvement of the macula. Improvements in the outcome of retinal detachment repair are more likely to arise from earlier diagnosis than advances in re-attachment surgery [START_REF] Wong | Are we getting better at treating retinal detachment? Technology, referral pattern or primary care?[END_REF] Social deprivation is known to have a significant impact on many diseases [START_REF] Eachus | Deprivation and cause specific morbidity: evidence from the Somerset and Avon survey of health[END_REF] . Low socioeconomic status is associated with an increased incidence of blindness and visual impairment 8;9 . Social deprivation has been associated with delayed presentation in sight threatening diabetic eye disease [START_REF] Bachmann | Socioeconomic inequalities in diabetes complications, control, attitudes and health service use: a cross-sectional study[END_REF] , glaucoma 11;12 , and anisometropic amblyopia [START_REF] Smith | Social deprivation and age at presentation in amblyopia[END_REF] . We undertook this study to examine the influence of social deprivation on the incidence of retinal detachment, and to determine if deprivation is associated with delayed presentation.

METHODS

Participants

A prospective multicentre study was established to investigate the clinical and epidemiological features of retinal detachment. This paper is part of this large study, and subsequent publications will address presentation, aetiology, and genetic issues in more detail.

Data was collected on all incident cases of RRD presenting to one of six vitreoretinal surgical sites in Scotland (Glasgow, Edinburgh, Dundee, Aberdeen, Ayr and Inverness) over a one year period between 1 November 2007 and 31 October 2008. Every patient presenting with primary RRD to these units was invited to participate in the study. All cases were confirmed by clinical examination by a consultant vitreoretinal surgeon. Demographic data (age, sex, address, and ethnicity) as well as clinical data (duration of symptoms, configuration of RRD, past ocular history and intraocular surgery, lens status) were collected on all consenting participants. In patients with macula-on detachments, the initial symptoms were often minor, and it was difficult to obtain a precise estimate of the duration of the detachment. Because the duration of symptoms was unreliable,, we used the proportion of macula-off detachments, and the number of quadrants detached as indicators of delayed presentation. This was based on the natural history of rhegmatogenous detachment, which usually originates in one quadrant and extends to involve the remaining retina. Full details of the study methodology have been published elsewhere [START_REF] Mitry | Rhegmatogenous retinal detachment in Scotland: research design and methodology[END_REF] . We have not included outcomes data, as this study was designed to examine the aetiology, presentation and epidemiology of retinal detachment rather than the outcomes of treatment.

Population data for each datazone was taken from the 2001 census (Scottish Neighbourhood Statistics (http://www.sns.gov.uk)

Deprivation score: SIMD ranking

The Scottish Government has developed the Scottish Index of Multiple Deprivation (SIMD).

The country is divided into 6,505 geographic zones, based on postal codes. The average population of each datazone is 780. The datazones are ranked according to a global deprivation score, which is a weighted sum of the seven domain scores. These scores are derived from 37 different indicators. The domain weightings used in SIMD 2006, expressed as a percentage of the final score are: current income (28%), employment (28%), health (14%), education (14%), geographic access (9%), crime (5%) and housing (2%). The score is a ranking only, so a score of 4 does not mean that the area is twice as deprived as a zone with a score of 2. Further details of the SIMD can be found on the Scottish Government website: http://www.scotland.gov.uk/Topics/Statistics/SIMD/ From the patient's post code it was possible to identify their SIMD datazone. For this study the datazones were grouped into four quartiles, with the first quartile being the most deprived and the fourth the least deprived.

Statistical analysis

Categorical data was analysed with chi-squared test, and the chi-squared for trend.

RESULTS Demographics

Clinical and postcode data was obtained for 572 patients. The difference in incidence of RRD between the lowest and highest quartiles is shown in Table 1. The annual incidence increased from 6.9/100,000 in the most deprived quartile to 15.4 in the least deprived quartile. The trend appears to be stronger in males than females, but is significant in both genders. There were no differences in the proportion of macula-off detachments ( see Table 2), nor the number of quadrants detached (Table 3). In two patients the macular status was unknown, and in 14 patients the extent of the detachment was not documented. The lens status was not recorded in four eyes. 431 eyes were phakic, 125 were pseudophakic, and 12 were aphakic. The 12 aphakic patients were included in the pseudophakic group for analysis. There was no difference in the proportion of patients who had previous cataract surgery.

Pseudophakia

Myopia.

Among phakic patients, there was no difference in the prevalence of significant myopia (at least -3D) (see Table 5). In 60 patients, the refraction prior to RRD was unknown.

DISCUSSION

In this study, the overall incidence of RRD was comparable to other reports 2-5;14 . We found no association between SIMD quartile and either the number of quadrants involved, or the proportion of macula-off detachments. This implies that delayed presentation is not linked to socioeconomic deprivation.

Socioeconomic status is known to be an important determinant of the risk of blindness and visual impairment 9;15 . However, the inverse association between deprivation and retinal detachment was unexpected and has not been previously reported.

We used a postcode based methodology to assess deprivation, and this is necessarily an approximation as there may be deprived households in affluent areas, and some households in deprived areas may be affluent. However, these anomalies are unlikely to lead to systemic bias on the scale reported in this study.

Only patients resident in Scotland were included in the study. Scotland is known to have highly significant disparities in life expectancy and other health indicators according to socioeconomic factors [START_REF] Mccarron | Deprivation and mortality in Glasgow: changes from 1980 to 1992[END_REF] . Our findings may not be applicable to other countries, and it would be interesting to replicate the study elsewhere.

Nineteen patients refused to participate in the study, but even if they were all from the most deprived quartile, this would not alter the results.

This finding has implications for research into the epidemiology of RRD. The reported annual incidence varies from 4 17 to 17.9 2 per 100,000 population . One cause of this variation may be differing levels of social deprivation. Observed differences in incidence between ethnic groups may also be partly caused by socioeconomic factors [START_REF] Wong | Racial difference in the incidence of retinal detachment in Singapore[END_REF] . There are also implications for service planning, as there is likely to be a greater need for vitreoretinal services in areas with less deprivation.

We have considered a number of possible explanations. On a global scale, cataract surgery is associated with affluence -the cataract surgery rate is much higher in wealthy industrialised countries than in poor countries [START_REF] Yorston | High-volume surgery in developing countries[END_REF] . Within developing countries poverty and low socioeconomic status reduce access to cataract surgery 19;20 . Even in the UK, with universal provision of free health care, there is evidence to suggest that access to cataract surgery is influenced by social deprivation [START_REF] Goyal | Referrals for cataract surgery: variations between different geographic areas within a Welsh Health Authority[END_REF] . If more affluent zones have more cataract surgery, they might be expected to be at greater risk of RRD. If this was the main reason for the association of RRD with affluence, we would expect to see more pseudophakic detachments in the least deprived group. However aphakic and pseudophakic detachments were evenly distributed between the four quartiles.

Myopia has been associated with IQ, and thus higher income and socioeconomic status 22;23 .

As myopia is a risk factor for RRD, a higher prevalence of myopia in the most affluent quartile could explain the increased incidence of RRD. However there was little difference between the quartiles in the proportion of RRD associated with 3 dioptres or more of myopia.

Deprived areas in Scotland have higher standard mortality ratios [START_REF] Mccarron | Deprivation and mortality in Glasgow: changes from 1980 to 1992[END_REF] . RRD is associated with age [START_REF] Zou | Epidemiology survey of rhegmatogenous retinal detachment in Beixinjing District, Shanghai, China[END_REF] and is most common in the sixth decade. High mortality rates mean that fewer people will live long enough to be at greatest risk of retinal detachment. However, data from the Scottish Census show that the percentage of the population aged over 60 shows little variation -21.5% in the most deprived group, 22.8% in the second quartile, 21.4% in the third, and 18.7% in the most affluent. It appears that the higher risk of RRD in the least deprived cannot be explained by greater life expectancy.

The Scottish RRD survey only records patients on presentation to hospital, and it is possible that patients from areas of greater deprivation do not attend hospital, and are excluded from the study. However, sudden loss of vision is a dramatic symptom which is unlikely to be ignored. Secondly, if there were large numbers of untreated RRD in Scotland, we would expect to encounter them when patients attend their optometrist, or an eye clinic, for some other reason. In fact it is rare to detect untreated RRD in primary care. Finally, there is no evidence of delayed presentation in the more deprived patients, which suggests they are no more likely to ignore a detached retina than the most affluent.

We have not found any satisfactory explanation for the greater incidence of RRD in less deprived patients. It is possible that a combination of small differences in known risk factors, such as myopia, and previous cataract surgery, account for the difference. However we cannot exclude the possibility that affluence is associated with some other, hitherto unknown, risk factor for RRD. The Scottish RRD survey will continue until November 2009, and we hope that additional data may provide an explanation for this unexpected finding.

Contributors

The authors would like to acknowledge the work of all the contributors to the Scottish Retinal

Table 1 .

 1 Annual incidence of RRD.

	SIMD Quartile	1	2	3	4	Total
	Total population
	No of detachments	89	117	169	197	572
	Total population	1,284,075 1,260,842 1,239,562 1,277,532 5,062,011
	Annual incidence / 100,000	6.9	9.3	13.6	15.4	11.3
	Males
	No of detachments	48	67	106	119	340
	Total population	603,610	602,622	603,052	623,210 2,432,494
	Annual incidence / 100,000	8.0	11.1	17.6	19.1	14.0
	Females
	No of detachments	41	50	63	78	232
	Total population	680,465	658,220	636,510	654,322 2,629,517
	Annual incidence / 100,000	6.0	7.6	9.9	11.9	8.8

Total population: χ 2 for trend = 50.17, p <0.0001 Male population: χ 2 for trend = 34.71, p <0.0001 Female population: χ 2 for trend = 15.03, p =0.0001

Table 2

 2 Macula status and deprivation quartile

	Quartile	Macula on	Macula off	Total
	1	41	46.1%	48	53.9%	89
	2	49	42.2%	67	57.8%	116
	3	63	37.3%	106	62.7%	169
	4	90	45.9%	106	54.1%	196
	Total	243	42.6%	327	57.4%	570
	χ 2 = 3.28, DF = 3, p = 0.35
	Table 3 Number of quadrants detached and deprivation quartile
	Quartile	One quadrant	Two quadrants	Three quadrants	Four quadrants	Total
	1	21 24.4%	40 46.5%	20 23.3%	5	5.8%	86
	2	35 31.0%	54 47.8%	19 16.8%	5	4.4%	113
	3	29 17.3%	87 51.8%	40 23.8%	12	7.1%	168
	4	56 29.3%	90 47.1%	33 17.3%	12	6.3%	191
	Total	141 25.3%	271 48.6%	112 20.1%	34	6.1%	558
	χ 2 = 11.30, DF = 9, p = 0.26

Table 4 .

 4 Distribution of the patients with Pseudophakia in the quartiles.

	Quartile	Phakic		Pseudophakic / aphakic	Total
	1	73	82.0%	16	18.0%	89
	2	86	74.8%	29	25.2%	115
	3	120	71.0%	49	29.0%	169
	4	152	78.0%	43	22.0%	195
	Total	431	75.9%	137	24.1%	568
	(χ 2 = 4.56, DF = 3, p=0.21).

Table 5

 5 Significant myopia (at least -3D) in phakic eyes

	Quartile	Myopic		Not myopic Total
	1	27	41.5%	38 58.5%	65
	2	35	45.5%	42 54.6%	77
	3	39	36.8%	67 63.2%	106
	4	64	48.1%	69 51.9%	133
	Total	165	43.3%	216 56.7%	381
	χ 2 = 3.31, DF = 3, p=0.35

  Detachment Survey: Kurt Cornish, Anna-Louise Murphy, Noemi Lois, Hatem Atta, Ian Whyte, Simon Hewick, John Ellis, Graham Cormack, Harry Bennett, John Murdoch, Harry Hammer, Alan Cox, Arvind Singh, Karen Madill, Zachariah Koshy, Harry Campbell, Alan Wright, and Brian Fleck