Dr Joanna M Jefferis
email: j.m.jefferis@ncl.ac.uk

Urs P Mosimann

Michael P Clarke

Cataract and Cognitive Impairment: A Review of the Literature

Keywords: Cataract, Dementia, Cognitive impairment, Visuo-perceptual impairment

Acquired cataract and cognitive impairment are both common age related problems, and ophthalmologists are increasingly likely to encounter patients who have both. Dementia types which display early visuo-perceptual impairment may present first to ophthalmology services. When these patients have coexisting cataract it may be difficult to distinguish visual complaints due to cataract from those due to dementia.

The interaction between visual impairment due to cataract, and neurodegenerative disorders affecting the central visual pathways, is not fully understood. Visual impairment due to cataract may stress impaired attentional mechanisms, and cataract extraction may improve cognitive performance in some patients with early cognitive impairment; however the benefits of cataract surgery in established dementia are less clear. Here we review the literature on this subject and consider the implications for practice.

INTRODUCTION

There are a number of areas of overlap between ophthalmology and dementia research. It has been postulated that assessing the eyes and vision could help with the diagnosis of dementia and the monitoring of dementia treatment. [START_REF] Fotiou | Cholinergic deficiency in Alzheimer's and Parkinson's disease: evaluation with pupillometry[END_REF][START_REF] Scinto | A potential noninvasive neurobiological test for Alzheimer's disease[END_REF] For example, glaucoma and dementia have been proposed to have a causal relationship; [START_REF] Wostyn | Alzheimer's disease and glaucoma: Is there a causal relationship?[END_REF] early age related macular degeneration and cognitive function seem to be related; [START_REF] Baker | Early age-related macular degeneration, cognitive function, and dementia: the Cardiovascular Health Study[END_REF] visual field defects are recognised as a presenting feature in some dementias [START_REF] Formaglio | Hémianopsie latérale homonyme et atrophie corticale postérieure [Homonymous hemianopia and posterior cortical atrophy[END_REF] and eye movements can distinguish between different causes of dementia. [START_REF] Mosimann | Saccadic eye movement changes in Parkinson's disease dementia and dementia with Lewy bodies[END_REF][START_REF] Garbutt | Oculomotor function in frontotemporal lobar degeneration, related disorders and Alzheimer's disease[END_REF][START_REF] Mosimann | Visual exploration behaviour during clock reading in Alzheimer's disease[END_REF] However, the interaction between age related cataract and neurodegenerative disorders affecting the central visual pathway is a neglected research area, despite its implications for clinical practice. Many of the issues raised here in regard to the interaction or coexistence of cataract and neurodegeneration will also be relevant to other age related eye diseases.

METHODS

Published literature was obtained by cross searching MEDLINE (1950MEDLINE ( to 2008) ) and EMBASE (1947 to 2008). A broad search strategy with appropriate descriptors and key words was used. Key words included: cataract, extraction, outcomes, dementia, cognitive impairment, visuo-perceptual, visuo-spatial impairment, ageing vision or a combination of those. No language or date restrictions were entered. Reference lists of identified primary reports and review articles were searched. Abstracts were carefully reviewed and discussed among the authors, and relevant papers were then considered in detail.

DEFINITIONS Age related Cataract

Age related cataract refers to acquired lens opacity with ageing. The pathophysiology and classification of acquired age related cataract have been reviewed elsewhere (e.g. [START_REF] Harding | Cataract: biochemistry, epidemiology and pharmacology[END_REF][START_REF] Sparrow | How should cataracts be measured?[END_REF])

Mild cognitive impairment

Mild cognitive impairment is characterised by a decline in cognitive ability (memory, language, attention/executive function or visual perception), without significant impairment in occupational or social functioning. Patients with mild cognitive impairment have an increased risk of developing dementia. [START_REF] Petersen | Mild cognitive impairment: clinical characterization and outcome[END_REF][START_REF] Petersen | Mild cognitive impairment as a diagnostic entity[END_REF] Dementia Dementia is an age related clinical syndrome which involves progressive cognitive decline for at least 6 months, sufficient to interfere with social or occupational functioning.

International classifications (DSM IVR and ICD-10) currently state that dementia must involve memory impairment and at least one other impaired cognitive domain e.g. language, judgement or visual perception. [START_REF]American Psychiatric Association, Diagnostic and Statistical Manual for Mental Disorders[END_REF][START_REF]World Health Organisation, The ICD-10 Classification of Mental and Behavioural Disorders[END_REF] This definition relates most closely to Alzheimer's disease (AD), but will be revised in the near future as memory impairment is often not a prominent initial feature in dementia. Dementias which show early visuo-spatial impairment with less prominent memory impairment include Dementia with Lewy Bodies, Parkinson's Disease Dementia and posterior cortical atrophy. [START_REF] Caine | Posterior Cortical Atrophy: A Review of the Literature[END_REF][START_REF] Hirono | Distinctive Neurobehavioral Features Among Neurodegenerative Dementias[END_REF][START_REF] Mckeith | Diagnosis and management of dementia with Lewy bodies. Third report of the DLB consortium[END_REF] EPIDEMIOLOGY: DO CATARACT AND DEMENTIA CO-EXIST IN ELDERLY POPULATIONS?

Epidemiology of cataract:

Age related cataract is the most common cause of reversible blindness worldwide. In the UK, a study of men aged 65 to 83 years old showed that 25% had vision 6/24 or worse attributable to cataract. [START_REF] Stocks | Prevalence of cataract in the Speedwell Cardiovascular study: a cross sectional survery of men aged 65-83[END_REF] Over 17% of Americans over 40 years have cataract. [START_REF] Congdon | Prevalence of cataract and psuedophakia/Aphakia among adults in the united states[END_REF] The prevalence of cataract increases with advancing age, with increased numbers of severe cataracts in elderly populations. [START_REF] Tan | Comparison of age-specific cataract prevalence in two population based surveys 6 years apart[END_REF][START_REF] Leibowitz | The Framingham Eye Study monograph: An ophthalmological and epidemiological study of cataract, glaucoma, diabetic retinopathy, macular degeneration, and visual acuity in a general population of 2631 adults, 1973-1975[END_REF][START_REF] Mitchell | Prevalence of cataract in Australia: the Blue Mountains eye study[END_REF] Cataract is a frequent cause of visual impairment and blindness in elderly, institutionalised patients. [START_REF] Taiel-Sartral | Acuité visuelle et pathologie oculaire chez le sujet âgé résidant en maison de retraite : étude orléanaise sur 219 personnes[END_REF][START_REF] Van Newkirk | Visual impairment and eye diseases in elderly institutionalised Australians[END_REF][START_REF] Tielsch | The Prevalence of blindness and visual impairment among nursing home residents in Baltimore[END_REF] Cataract surgery is a common procedure [START_REF] Williams | Longitudinal rates of cataract surgery[END_REF] and in the UK NHS it is the 3 rd most frequent main hospital intervention or procedure after childbirth and upper GI endoscopy (www.hesonline.nhs.uk).

Epidemiology of dementia:

There are an estimated 24.3 million patients with dementia worldwide, with 4.6 million new cases every year (one every 7 seconds). [START_REF] Ferri | Global prevalence of dementia: a Delphi consensus study[END_REF] 6-10% of those aged over 65 in the US suffer from dementia and if mild cognitive impairment is included, the prevalence doubles. [START_REF] Hendrie | Epidemiology of dementia and Alzheimer's disease[END_REF] Dementia prevalence increases exponentially with advancing age, with 1% of 65 year olds affected, 15% of 85 year olds, 30% of 90 year olds and upwards of 80% of 100 year olds [START_REF] Hofman | The prevalence of dementia in Europe: a collaborative study of 1980-1990 findings. Eurodem Prevalence Research Group[END_REF][START_REF] Blansjaar | Prevalence of dementia in centenarians[END_REF][START_REF] Heeren | Prevalence of dementia in the 'oldest old' of a Dutch community[END_REF][START_REF] Ritchie | Is senile dementia "age-related" or "ageing-related"? -evidence from meta-analysis of dementia prevalence in the oldest old[END_REF]. Alzheimer's Disease is the most common cause of dementia, followed by Dementia with Lewy Bodies and vascular dementia. [START_REF] Tomlinson | Observations on the brains of demented old people[END_REF][START_REF] Mckeith | Consensus guidelines for the clinical and pathologic diagnosis of dementia with Lewy bodies (DLB): report of the consortium on DLB international workshop[END_REF][START_REF] Burns | and Observations on the brains of non-demented old people[END_REF] Coexistence of cataract and dementia:

Cataract and dementia are both age related (Figure 1), and it follows that they will frequently coexist. Ophthalmology departments in the UK have more attendances for patients aged over 70 years than any other specialty (www.hesonline.co.uk). There are over 3 million ophthalmology outpatient attendances for patients over 60 years in the UK each year (www.hesonline.co.uk) of whom approximately 5% (i.e. 150,000) would be expected to have dementia. [START_REF] Ferri | Global prevalence of dementia: a Delphi consensus study[END_REF][START_REF] Hofman | The prevalence of dementia in Europe: a collaborative study of 1980-1990 findings. Eurodem Prevalence Research Group[END_REF][START_REF] Ritchie | Is senile dementia "age-related" or "ageing-related"? -evidence from meta-analysis of dementia prevalence in the oldest old[END_REF] This assumes no causative interaction between eye disease and dementia, and equal access to ophthalmology services for patients with dementia. The latter assumption is plausible for patients living in the community; however it may not apply for nursing home residents, where prevalence of dementia and visual impairment are high and access to medical care may be restricted. [START_REF] Taiel-Sartral | Acuité visuelle et pathologie oculaire chez le sujet âgé résidant en maison de retraite : étude orléanaise sur 219 personnes[END_REF][START_REF] Tielsch | The Prevalence of blindness and visual impairment among nursing home residents in Baltimore[END_REF][START_REF] Owsley | The visual status of older persons residing in nursing homes[END_REF] Insert figure 1 about here

THE RELATIONSHIP BETWEEN COGNITIVE IMPAIRMENT AND VISUAL IMPAIRMENT

There is a strong association between cognitive test scores and visual acuity [START_REF] Lindenberger | Sensory functioning and intelligence in old age: a strong connection[END_REF] with longitudinal data showing the rate of change of vision and the rate of change of cognition to be significantly correlated. [START_REF] Anstey | A latent growth curve analysis of late-life sensory and cognitive function over 8 years: Evidence for specific and common factors underlying change[END_REF] There are essentially three hypotheses for this association:

1. Decreased vision impairs a subject's ability to see and interpret the test material.

Cognitive tests are therefore biased against visually impaired subjects and give artificially poor scores for patients with low vision. This is known as the "cognitive resource" theory. This first hypothesis was supported by Dickinson and Rabbitt [START_REF] Dickinson | Simulated visual impairment: Effects on text comprehension and reading speed[END_REF] who showed that simulating visual impairment with lenses impaired healthy volunteers' recall of text passages, despite being able to accurately read text at the time of presentation. The authors concluded that the additional effort required for reading under impoverished stimulus conditions consumed processing resources needed for effective memory. Kempen and Krichevesky [START_REF] Kempen | Effect of visual impairment on neuropsychological test performance[END_REF] showed that subjects with uncorrected refractive error and decreased near vision did worse than controls with corrected vision on facial recognition and visual form discrimination tasks. Both studies show that visual impairment due to uncorrected refractive errors impairs cognitive performance in cognitively intact volunteers. This emphasises the importance of testing visual acuity and correcting refractive errors before cognitive testing.

Cognitive test scores are low in patients with poor vision because they have a true cognitive impairment. A common factor underlies both cognitive decline and visual

decline. Impaired vision is therefore a marker of cognitive impairment. This is known as the "common factor" theory.

The close relationship between cognitive and visual decline has been used in itself to argue that there must be common factors underlying both. [START_REF] Lindenberger | Sensory functioning and intelligence in old age: a strong connection[END_REF][START_REF] Anstey | A latent growth curve analysis of late-life sensory and cognitive function over 8 years: Evidence for specific and common factors underlying change[END_REF] In further support of this second hypothesis, Lindenberger and colleagues [START_REF] Lindenberger | The strong connection between sensory and cognitive performance in old age: not due to sensory acuity reductions operating during cognitive assessment[END_REF] showed that simulating "old age" vision in middle aged patients did not affect their performance on a variety of cognitive tasks. "Old age vision" did not give subjects "old age cognition" suggesting that poor vision alone did not account for decreased test scores in elderly patients. Drobny et al [START_REF] Drobny | Visual Memory Testing in Older Adults with Age-Related Visual Decline: A Measure of Memory Performance or Visual Functioning[END_REF] showed that patients with visual impairment did worse on both visually presented tasks and aurally presented tasks. Patients with decreased vision performed poorly on cognitive tasks which did not require vision, suggesting that their low vision was accompanied by a real cognitive impairment. These observations have led to the suggestion that a common factor (which may be normal ageing) underlies both visual and cognitive decline.

A combined mechanism.

There is still debate as to whether impaired cognitive performance in association with visual impairment is simply due to degraded stimulus representations or whether there is a common factor underlying cognitive and visual decline. The co-existence of the two problems is not proof of a common causation (although it may be suggestive). It seems probable that both mechanisms play a role.

IS THERE A COMMON AETIOLOGY UNDERLYING CATARACT AND DEMENTIA?

Cataract and dementia are degenerative processes and have been proposed to share common aetiological mechanisms. [START_REF] Harding | Cataract, Alzheimer's disease, and other conformational diseases[END_REF][START_REF] Goldstein | Cytosolic beta-amyloid deposition and supranuclear cataracts in lenses from people with Alzheimer's disease[END_REF] Risk factors for cataract include advancing age, female gender, smoking and lower socioeconomic class (and associated lower educational attainment). [START_REF]Risk factors associated with age-related nuclear and cortical cataract: A case-control study in the Age-Related Eye Disease Study[END_REF][START_REF] West | Epidemiology of risk factors for age-related cataract[END_REF][START_REF] Leske | The Lens Opacities Case-Control Study. Risk factors for cataract[END_REF] These are also risk factors for dementia. [START_REF] Fillit | Cardiovascular risk factors and dementia[END_REF][START_REF] Qiu | Epidemiology of Alzheimer's disease: occurrence, determinants, and strategies toward intervention[END_REF][START_REF] Schmidt | Sex differences in Alzheimer's disease[END_REF][START_REF] Anstey | Smoking as a risk factor for dementia and cognitive decline: a metaanalysis of prospective studies[END_REF] Vascular risk factors contribute to both vascular dementia and Alzheimer's Disease. They also seem to be important for the generation of cataract. Diabetes, smoking and obesity are risk factors for age related cataract [START_REF] Tan | Influence of diabetes and cardiovascular disease on the long-term incidence of cataract: the Blue Mountains eye study[END_REF][START_REF] Schaumberg | Relations of body fat distribution and height with cataract in men[END_REF] as well as dementia. [START_REF] Fillit | Cardiovascular risk factors and dementia[END_REF] Statins may be protective for dementia [START_REF] Fillit | Cardiovascular risk factors and dementia[END_REF] and cataract. [START_REF] Tan | Statin use and the long-term risk of incident cataract: the Blue Mountains Eye Study[END_REF][START_REF] Leske | Nuclear Cataract: Do Statins Reduce Risk?[END_REF] Anti-hypertensives are protective in dementia [START_REF] Fillit | Cardiovascular risk factors and dementia[END_REF] and there is some association between hypertension and cataract. [START_REF] Tan | Influence of diabetes and cardiovascular disease on the long-term incidence of cataract: the Blue Mountains eye study[END_REF][START_REF] Kahn | The Framingham Eye Study. II. Association of ophthalmic pathology with single variables previously measured in the Framingham Heart Study[END_REF][START_REF] Schaumberg | A prospective study of blood pressure and risk of cataract in men[END_REF] The ApoE4 allele, which is known to be a genetic risk factor for Alzheimer's Disease, is not associated with increased cataract risk. [START_REF] Utheim | Apolipoprotein E genotype and risk for development of cataract and agerelated macular degeneration[END_REF][START_REF] Zetterberg | Apolipoprotein E polymorphism in patients with cataract[END_REF] Given many common risk factors, dementia patients may be expected to have a higher incidence of cataract. However, a single study of self reported previous cataract surgery found no association between cataract and cognitive impairment. [START_REF] Grodstein | Cataract extraction and cognitive function in older women[END_REF] NEURODEGENERATIVE DISORDERS WITH PROMINENT VISUAL SYMPTOMS Patients with dementia often experience visual phenomena, including visual hallucinations, visual complaints and visuo-perceptual impairments. If these symptoms appear early in the course of the disease, they may help distinguish between different causes of dementia. Many of the visual symptoms which are seen in dementia have complex terminology attributed to them. The definitions of these terms are given in table 1.

Symptom Explanation

Visual agnosia An inability to recognise familiar objects by sight Proposagnosia A specific form of visual agnosia, an inability to recognise faces Optic ataxia An inability to reach for objects in the visual environment

Simultanagnosia

The patient only perceives one element of a scene at a time and is unable to absorb the whole Palinopsia Images persist to some extent even after their corresponding stimulus has left

Oculomotor apraxia An inability to explore a scene and to shift gaze from one area to another Balint's syndrome A combination of simultanagnosia, optic ataxia and oculomotor apraxia Table 1: Some visual symptoms described in neurodegenerative diseases and their definitionsDementia with Lewy Bodies is characterised by prominent visuo-spatial impairment and persistent, well-formed visual hallucinations. [START_REF] Mckeith | Diagnosis and management of dementia with Lewy bodies. Third report of the DLB consortium[END_REF][START_REF] Mckeith | Consensus guidelines for the clinical and pathologic diagnosis of dementia with Lewy bodies (DLB): report of the consortium on DLB international workshop[END_REF] Compared to Alzheimer's Disease, patients with Dementia with Lewy Bodies have less memory impairment, but more visuo-spatial impairment, [START_REF] Mosimann | Visual perception in pParkinson disease dementia and dementia with Lewy bodies[END_REF][START_REF] Mori | Visuoperceptual Impairment in Dementia with Lewy Bodies[END_REF][START_REF] Gnanalingham | Clock-face drawing to differentiate Lewy body and Alzheimer type dementia syndromes[END_REF][START_REF] Walker | Neuropsychological performance in Lewy body dementia and Alzheimer's disease[END_REF] which helps to separate Dementia with Lewy Bodies from other causes of dementia. [START_REF] Hirono | Distinctive Neurobehavioral Features Among Neurodegenerative Dementias[END_REF] There is an association between visual hallucinations and visuo-perceptual impairment, in that patients with more severe perceptual impairment are more likely to experience visual hallucinations. [START_REF] Mosimann | Visual perception in pParkinson disease dementia and dementia with Lewy bodies[END_REF][START_REF] Mori | Visuoperceptual Impairment in Dementia with Lewy Bodies[END_REF] Patients with idiopathic Parkinson's disease (PD) are at increased risk of developing dementia, with the 4-year prevalence being nearly 3 times higher than in non-Parkinson's subjects. [START_REF] Aarsland | Prevalence and Characteristics of Dementia in Parkinson Disease: An 8-Year Prospective Study[END_REF] Parkinson's Disease Dementia shows considerable overlap with Dementia with Lewy Bodies, both having prominent visuo-spatial impairment at the onset of the disease, [START_REF] Mosimann | Visual perception in pParkinson disease dementia and dementia with Lewy bodies[END_REF] fluctuations in visual attention and extra-pyramidal motor features . [START_REF] Ballard | Fluctuations in attention: PD dementia vs DLB with parkinsonism[END_REF][START_REF] Gnanalingham | Motor and cognitive function in Lewy body dementia: comparison with Alzheimer's and Parkinson's diseases[END_REF] Visual symptoms, such as visual hallucinations, can be an indication of the progression from Parkinson's Disease to Parkinson's Disease Dementia. [START_REF] Williams | Visual hallucinations in the diagnosis of idiopathic Parkinson's disease: a retrospective autopsy study[END_REF] Posterior Cortical Atrophy is also known as visual variant Alzheimer's Disease. It affects the occipital and parietal cortex and visual symptoms are present from the onset of the disease.

Patients with Posterior Cortical Atrophy often present to eye care providers with visual complaints, before the diagnosis is established. [START_REF] Kiyosawa | Alzheimer's disease with prominent visual symptoms. Clinical and metabolic evaluation[END_REF] Other features include simultanagnosia (the patient only perceives one element of a picture or object at a time and is unable to absorb the whole), hemispatial neglect and hemianopia. Posterior Cortical Atrophy is one of the causes of Balint's syndrome (see table 1). Unlike typical Alzheimer's Disease, day-to-day memory, language skills and affect remain relatively intact (reviewed by [START_REF] Caine | Posterior Cortical Atrophy: A Review of the Literature[END_REF][START_REF] Fletcher | Ophthalmological aspects of Alzheimer's disease[END_REF])

In typical Alzheimer's Disease, perceptual and visuo-spatial abilities tend to be relatively spared at the onset of disease, compared to DLB and PDD, and memory impairment is the most prominent feature. [START_REF] Mosimann | Visual perception in pParkinson disease dementia and dementia with Lewy bodies[END_REF][START_REF] Mori | Visuoperceptual Impairment in Dementia with Lewy Bodies[END_REF][START_REF] Walker | Neuropsychological performance in Lewy body dementia and Alzheimer's disease[END_REF] Visual hallucinations and visuo-spatial impairment are less common in typical early-stage Alzheimer's Dementia, and if present suggest comorbidity (e.g. delirium) or another diagnosis. [START_REF] Tiraboschi | What best differentiates Lewy body from Alzheimer's disease in early stage dementia?[END_REF] There are reports of complex visual disturbances in Alzheimer's Disease, [START_REF] Mendez | Complex visual disturbances in Alzheimer's disease[END_REF][START_REF] Sadun | Assessment of visual impairment in patients with Alzheimer's disease[END_REF][START_REF] Holroyd | A Study of Visual Hallucinations in Alzheimer's Disease[END_REF] however most of these studies were done before the Dementia with Lewy Body consensus criteria were established in 1996, [START_REF] Mckeith | Consensus guidelines for the clinical and pathologic diagnosis of dementia with Lewy bodies (DLB): report of the consortium on DLB international workshop[END_REF] and the Parkinson's Disease Dementia consensus criteria in 2005. [START_REF] Mckeith | Diagnosis and management of dementia with Lewy bodies. Third report of the DLB consortium[END_REF] It is therefore likely that some of these early studies included other causes of dementia in their Alzheimer's Disease samples or assessed advanced disease. Visual hallucinations can occur in established Alzheimer's Disease [START_REF] Hirono | Distinctive Neurobehavioral Features Among Neurodegenerative Dementias[END_REF][START_REF] Holroyd | A Study of Visual Hallucinations in Alzheimer's Disease[END_REF] and may be exacerbated by ocular pathology such as cataract or other causes of age related eye disease. [START_REF] Chapman | Association Among Visual Hallucinations, Visual Acuity, and Specific Eye Pathologies in Alzheimer's Disease: Treatment Implications[END_REF] Visual field defects have been reported in Alzheimer's Disease, [START_REF] Trick | Visual field loss in senile dementia of the Alzheimer's type[END_REF] most frequently inferior arcuate field defects. This creates potential diagnostic confusion with normal tension glaucoma, which has been observed to be more frequent in Alzheimer's Disease patients. [START_REF] Bayer | High occurrence rate of glaucoma among patients with Alzheimer's disease[END_REF][START_REF] Tamura | High frequency of open-angle glaucoma in Japanese patients with Alzheimer's disease[END_REF] The rare Heidenhain variant of sporadic Creutzfeldt-Jakob disease is characterised by visual symptoms predominating at presentation. These include decreased vision, blurred vision and visual distortions. [START_REF] Cooper | Isolated visual symptoms at onset in sporadic Creutzfeldt-Jakob disease: the clinical phenotype of the "Heidenhain variant[END_REF] THE EFFECT OF CATARACT SURGERY ON COGNITIVE PERFORMANCE AND

NEUROPSYCHIATRIC FEATURES

Cognitive decline and visual decline are associated and may be related, and it therefore follows that reversal of visual impairment (as in cataract surgery) may improve cognitive function or visual hallucinations.

A number of studies have tested the hypothesis that cataract surgery can improve cognitive function with varying results: these are summarised in table 2. The evidence from Gray and colleagues [START_REF] Gray | Recovery of visual and functional disability following cataract surgery in older people: Sunderland Cataract Study[END_REF] and similar results from Ishii et al [START_REF] Ishii | The impact of cataract surgery on cognitive impairment and depressive mental status in elderly patients[END_REF] support the hypothesis. However, the modest improvement (of just 2 points or less) on the MMSE score demonstrated in both studies may not be clinically significant, and may reflect a learning effect, particularly as in the Gray study participants performed better at 6 months post-operatively than 2 months post-operatively. In a small prospective study of patients with known cognitive impairment, Tamura et al [START_REF] Tamura | Improvement in cognitive impairment after cataract surgery in elderly patients[END_REF] showed cataract surgery improved cognitive performance, however, the design of the study has been criticised. [START_REF] Rosen | Cognitive impairment and cataract surgery[END_REF] Hall et al [START_REF] Hall | Effect of Cataract Surgery on Cognitive Function in Older Adults[END_REF] showed that patients without cataract had no change in cognitive scores at one year but that patients with cataract, whether in the surgery group or control group, had improved cognitive scores at one year follow up. These results are difficult to interpret, but may represent a preferential learning effect on cognitive tests by individuals with poor vision, and do not support the hypothesis that cataract surgery in itself improves cognition. Results from Grodstein [START_REF] Grodstein | Cataract extraction and cognitive function in older women[END_REF] and Anstey [START_REF] Anstey | The effect of cataract surgery on neuropsychological test performance: A randomized controlled trial[END_REF] were more common in patients with cataract, and the investigators hypothesised that correcting poor vision with cataract surgery might also improve visual hallucinations.

In summary, whilst we know that visual function, mood and visual quality of life improve following cataract surgery; [START_REF] Gray | Recovery of visual and functional disability following cataract surgery in older people: Sunderland Cataract Study[END_REF][START_REF] Ishii | The impact of cataract surgery on cognitive impairment and depressive mental status in elderly patients[END_REF] there is, at present, no conclusive evidence for improvement in cognitive function or neuropsychiatric features.

CONSENT FOR CATARACT SURGERY IN PATIENTS WITH DEMENTIA

Patients with established dementia may lack the capacity to consent according to the requirements for consent laid out in the Mental Capacity Act 2005. In these patients it is up to the clinician to decide whether surgery is in the patients best interests taking into account the wishes and beliefs of the patient when competent, their current wishes and general well-being. Deciding whether cataract surgery is in an individual patients best wishes is also likely to involve discussion with people close to them. Some of the legal, ethical and moral issues surrounding informed consent for cataract surgery in dementia patients have been reviewed previously. [START_REF] Piazza | Cataract surgery in the patient with dementia: justified paternalism?[END_REF] HOW DOES COGNITIVE DECLINE INFLUENCE OUTCOMES FROM CATARACT SURGERY?

Post-operative visual acuities following cataract surgery decline with advancing age, [START_REF] Jay | Effect of age on visual acuity after cataract extraction[END_REF][START_REF] Jaycock | The Cataract National Dataset electronic multicentre audit of 55 567 operations: updating benchmark standards of care in the United Kingdom and internationally[END_REF] even when recognisable ocular pathologies are taken into account. [START_REF] Westcott | Effect of age on visual outcome following cataract extraction[END_REF] Elderly patients may also have difficulty converting improved vision to improved functioning following cataract surgery. [START_REF] Bernth-Petersen | Cataract extraction in the 'nineties[END_REF][START_REF] Elliott | Vision-enhancing interventions in nursing home residents and their short-term effect on physical and cognitive function[END_REF] Other age related pathology, such as macular degeneration and glaucoma may contribute to visual impairment in elderly patients, however, there is increasing evidence that decline in cerebral function is partly responsible for age related visual deterioration. [START_REF] Del-Viva | Visual Spatial Integration in the Elderly[END_REF] Given that cerebral changes may be partly responsible for decreasing post-operative visual acuities following cataract surgery in elderly patients, it follows that cognitively impaired elderly patients may have worse post-operative visual acuities than non-cognitively impaired patients. Patients may also have visual symptoms which are due to cognitive their symptoms attributed to cataract and underwent cataract extraction before a diagnosis of sCJD was made. [START_REF] Cooper | Isolated visual symptoms at onset in sporadic Creutzfeldt-Jakob disease: the clinical phenotype of the "Heidenhain variant[END_REF] sCJD is a devastatingly rapid, progressive form of dementia with decline to death typically within 6 months. Removing a cataract for symptoms due to sCJD would become rapidly apparent as an error; however, removing a cataract, or a lens with a normal degree of age related opacity, for symptoms due to a more gradual, progressive dementia would be more difficult to detect. It is likely, on statistical grounds and given the absence of routine cognitive testing prior to cataract surgery that some patients have cataract surgery for symptoms due to early forms of dementia, but the extent to which this occurs is not yet clear.

IMPLICATIONS FOR PRACTICE

The symptoms of cataract and dementia overlap, and there is therefore a risk that without thorough pre-operative clinical assessment, inappropriate cataract surgery could be performed on patients with cognitive impairment. In patients with visual symptoms due to both cataract and cognitive impairment, a lack of awareness of the cognitive component of the symptomatology will lead to unrealistic expectations of the improvement in symptoms which would follow cataract surgery. Furthermore, persistent visual symptoms following cataract surgery may be incorrectly attributed to other pathology, such as cystoid macula oedema or age related macular degeneration, leading to inappropriate management. Given the large numbers of elderly patients presenting to ophthalmology services, many patients are likely to be encountered with visual symptoms and dementia. Eye care services may be more accessible to elderly patients with perceptual disturbances than old-age psychiatry services, and ophthalmic assessment may be more acceptable than cognitive assessments.

This highlights the need for awareness by ophthalmologists of the potential for cognitive impairment to cause visual symptoms, and a multi-disciplinary approach in managing such patients with good lines of communication between ophthalmology, neurology and old-age psychiatry services. Figure 2 gives a model of how such a multi-disciplinary team may be accessed and what it may involve.

Insert Figure 2 about here SUMMARY/KEY POINTS

• Large numbers of elderly patients are seen in ophthalmology departments and ophthalmologists will encounter patients with cognitive impairment and dementia, some of whom have yet to be diagnosed.

• Visual symptoms in cataract and dementia may show considerable overlap and this may lead to diagnostic difficulty.

• Major causes of dementia with prominent visual symptoms at the outset include Dementia with Lewy Bodies, Parkinson's Disease Dementia, Posterior Cortical Atrophy and sporadic Creutzfeldt-Jakob Disease.

• Cataract surgery has been hypothesised to improve cognitive function in patients with established dementia but further evidence is needed to support this view.

• Patients with dementia and cognitive impairment may achieve poorer postoperative outcomes and need to be counselled about this prior to surgery.

• There is a need for a multidisciplinary approach for patients with visual symptoms, ophthalmic pathology and dementia developed areas of the world with reliable prevalence data. [START_REF] Williams | Longitudinal rates of cataract surgery[END_REF] Below, the prevalence rates of cataract in the Blue Mountain's eye study, 1 st and 2 nd cohorts which included 3654 and 3509 patients respectively in Australia. [START_REF] Congdon | Prevalence of cataract and psuedophakia/Aphakia among adults in the united states[END_REF] For each cohort prevalence rates of any type of cataract (nuclear, cortical and PSC) including patients who had had previous cataract surgery are shown. Cataract was assessed from lens photographs and without regard to patient symptoms.

Cataract prevalence

Cross section 1

Cross section 2

Directed history taking to include features of cognitive impairment:

•Do your eyes ever play tricks on you?

Figure 1 :

 1 Figure 1: Above, the prevalence rates of dementia from a Delphi consensus study for 3

Figure 2 :

 2 Figure 2: A proposed model for elderly patients presenting for cataract surgery. This gives advice on which patients would require further assessment by a multi-disciplinary team (MDT) including ophthalmologists, neurologists and old age psychiatrists.

Table 2 :

 2 demonstrate no benefit of cataract surgery on cognitive performance. Does cataract surgery improve cognitive performance? Review of the literature showing evidence both for and against improved cognition following cataract surgery Cataract surgery may also improve visual hallucinations. A study of patients with established Alzheimer's Disease experiencing visual hallucinations showed that improving vision with refraction relieved 4/6 patients of visual hallucinations.[START_REF] Chapman | Association Among Visual Hallucinations, Visual Acuity, and Specific Eye Pathologies in Alzheimer's Disease: Treatment Implications[END_REF] Visual hallucinations

	Author	Study design	Sample and control group (if any)	Cognitive Assessments	Outcome
	Gray et al[80]	Prospective, observational (without control group)	92 cognitively normal patients (mean age 78.1 +/-6.5SD) undergoing either 1 st eye, 2 nd eye or sequential cataract surgery	MMSE score 2 weeks pre-operatively, and 2 and 6 months after surgery	Improvement in MMSE scores at 6 months post-op (although not 2 months post-op)
	Tamura et al[82] Prospective, observational (with control group)	20 patients (mean age 82.1 +/-6.0SD) with cognitive impairment having cataract surgery. 20 controls (mean age 84.3, range 70-93) with surgery cognitive impairment not having cataract	Revised Hasegawa Dementia Scale (HDS-R) pre-and post-operatively	Significant improvement in surgery group post-operatively but not in control group
	Ishii et al[81]	Prospective, observational (without control group)	102 cognitively normal patients (mean age 75.3+/-8.2SD) undergoing cataract surgery	MMSE score before and 2 months post cataract surgery	Significant improvement in MMSE score following surgery
	Hall et al[84]	Nonrandomized control study	92 cognitively normal patients with no cataract (mean age 66.8 +/-5.9SD), 122 patients with cataract who elected to have surgery (mean age 70.9+/-6.8SD), 87 patients (mean age 71.1+/-5.4) with cataract who declined cataract surgery	Mattis Organic Mental Syndrome Screening Examination (MOMSSE) scores at baseline and 1 year post-op	No cataract group had no change in cognitive scores at follow up. Cataract/surgery and cataract/no surgery groups had significantly better cognitive scores at follow-up
	Grodstein et al[60]	Cross-sectional epidemiological study	16, 197 women in the community aged 70-79 years. 3,597 women (mean age 75.1) self reported previous cataract surgery, 12,600 (mean age 73.9) reported no previous surgery	6 cognitive tests asked over telephone (no tests of visual memory) and questioned about previous cataract surgery	No difference in the cognitive scores between 2 groups.
	Anstey et al[85]	Randomised control study	56 cognitively normal patients with bilateral cataract randomised to either cataract surgery (mean age 73.4 +/-5.9SD) or waiting list control group (mean age 76.5 +/-8.45SD)	6 cognitive tests including both visually and aurally presented tasks at baseline and follow-up	No improvement in cognitive scores for either intervention or control group

  •Do you have difficulty finding your way around your home? •Do you have difficulty reaching for objects? •Do you have difficulty recognising faces or objects?

		Patient referred for cataract surgery
	Ophthalmic Examination
	No features of cataract on examination; history positive for symptoms of cognitive impairment	Clear features of cataract on examination; history negative for symptoms of cognitive impairment	Features of cataract on examination AND history positive for symptoms of cognitive impairment
	Cognitive assessment with MMSE. Refer to MDT team	Proceed with cataract work-up as per usual	Cognitive assessment with MMSE. Refer to MDT team
		•Dementia clinic referral if indicated •Formal Cognitive examination and extra-ocular movements •Further ophthalmic examination including visual fields •Neurological assessment and examination •Detailed history from patient and informant MDT assessment to include:	Explain guarded prognosis of cataract surgery allowing patient to make informed choice.

ACKNOWLEDGEMENTS

The Authors would like to thank Ava Tan and the Blue Mountains eye study group for kindly sharing their cataract prevalence data with us. This work was supported by the UK NIHR Biomedical Research Centre for Ageing and Agerelated disease award to the Newcastle upon Tyne Hospitals NHS Foundation Trust

COPYWRIGHT LICENCE STATEMENT

The Corresponding Author has the right to grant on behalf of all authors and does grant on behalf of all authors, an exclusive licence (or non-exclusive for government employees) on a worldwide basis to the BMJ Publishing Group Ltd, and its Licensees to permit this article (if accepted) to be published in THE BRITISH JOURNAL OF OPHTHALMOLOGY and any other BMJPGL products and to exploit all subsidiary rights, as set out in our licence.

impairment but which they perceive as being due to eye pathology and so expect to improve following cataract surgery.

SYMPTOM COMPLEXES IN CATARACT AND DEMENTIA: DO THEY OVERLAP?

Visual symptoms in cataract Patients with cataract may experience deterioration in visual acuity, loss of contrast sensitivity, difficulty reading, recognising faces, watching television, glare and altered colour recognition. Crabtree et al [START_REF] Crabtree | Measuring visual symptoms in British cataract patients: the cataract symptom scale[END_REF] explored the subjective symptoms of patients aged over 65 with cataract. They excluded patients with MMSE scores less than 25. They found that the symptoms which patients complain of can be grouped into sight related mobility, recognition and perception and hand eye co-ordination. Table 2 shows some of the subjective symptoms elicited by semi-structured interviews which led to the development of the Cataract Symptom Scale. [START_REF] Crabtree | Measuring visual symptoms in British cataract patients: the cataract symptom scale[END_REF] Visual symptoms in Dementia Visual symptoms may be the presenting symptom in idiopathic, dementing disease. [START_REF] Cogan | Visual disturbances with focal progressive dementing disease[END_REF][START_REF] Hof | Object recognition deficit in Alzheimer's disease: Possible disconnection of the occipito-temporal component of the visual system[END_REF][START_REF] Armstrong | Visual field defects in Alzheimer's disease patients may reflect differential pathology in the primary visual cortex[END_REF][START_REF] Lee | Neuro-ophthalmic Findings in the Visual Variatn of Alzheimer's Disease[END_REF][START_REF] Atchison | The Woman who needed a pet[END_REF] Visual symptoms complained of by patients with dementia include difficulty reading, visuospatial difficulties, blurred vision and visual hallucinations. [START_REF] Kiyosawa | Alzheimer's disease with prominent visual symptoms. Clinical and metabolic evaluation[END_REF][START_REF] Mendez | Complex visual disturbances in Alzheimer's disease[END_REF][START_REF] Sadun | Assessment of visual impairment in patients with Alzheimer's disease[END_REF][START_REF] Lee | Neuro-ophthalmic Findings in the Visual Variatn of Alzheimer's Disease[END_REF] Glare has also been reported as an early symptom in a patient presenting with Alzheimer's Disease. [START_REF] Kiyosawa | Alzheimer's disease with prominent visual symptoms. Clinical and metabolic evaluation[END_REF] Visual symptoms which may be present in dementia are shown in table 3, divided into the same groups as the cataract groups for ease of comparison. 3: Visual symptoms in cataract (from [START_REF] Crabtree | Measuring visual symptoms in British cataract patients: the cataract symptom scale[END_REF]) and visual symptoms reported in dementia disease (from [START_REF] Kiyosawa | Alzheimer's disease with prominent visual symptoms. Clinical and metabolic evaluation[END_REF][START_REF] Mendez | Complex visual disturbances in Alzheimer's disease[END_REF][START_REF] Sadun | Assessment of visual impairment in patients with Alzheimer's disease[END_REF][START_REF] Lee | Neuro-ophthalmic Findings in the Visual Variatn of Alzheimer's Disease[END_REF]) Symptoms reported in dementia in the elderly show a large degree of overlap with those described in children with cerebral visual impairment. [START_REF] Dutton | Cerebral visual impairment in children[END_REF] As in children, a structured history taken from a close friend or relative, for evidence of cognitive and visual perceptual visual dysfunction, may reveal visual difficulties which the patient is unable to describe.

[100] This is particularly important in dementia patients because anosognosia (unawareness of symptoms) is not uncommon. [101] Visual symptoms seen in cataract and dementia have a large degree of overlap (see table 3). This creates diagnostic difficulties in distinguishing which symptoms are due to cataract and which due to cognitive impairment, particularly in the elderly where most patients will have some degree of lens opacity on examination (see figure 1).

CAN VISUAL SYMPTOMS ASSOCIATED WITH COGNITIVE IMPAIRMENT BE CONFUSED WITH SYMPTOMS DUE TO CATARACT?

A retrospective review of 22 patients with the Heidenhain variant of sporadic Creutzfeldt-Jakob Disease (sCJD), in which visual symptoms predominate at presentation, showed that


Nicklas Forsell
email: nicklas.forsell@mines-paristech.fr

Gilles Guerassimoff

Dimitris Athanassiadis

² Edi Assoumou¹

Sub-national TIMES model for analyzing regional future use of Biomass and Biofuels in France and Sweden

In the context of mitigating climate change and increase energy security, the utilization of biomass in the energy sector is expected to play a major role. However, to estimate the possibility of fulfilling national goals concerning the future use of biomass sources and to estimate the future use of biomass sources in the energy sector, the current energy system models needs to be further developed to consider the high sub-national variances in the supply and cost of the biomass sources. In this paper we present a sub-national MARKAL/TIMES model for estimating regional utilization of biomass sources and the future development of the energy system. The proposed model is evaluated for two case studies, France and Sweden, for which the future utilization of biomass is evaluated utilizing numerous scenarios of the potential supply of biomass, cost of biomass, and end-use demand. Our results show that the limit of national biomass potentials for energy purposes in France is approximately 35 Mtoe, while all demand scenarios for Sweden could be fulfilled by national biomass potentials. Furthermore, the results show that there are large differences in the regional utilization level of biomass sources, even when the total utilization level of biomass sources is high.

Introduction

Renewable energy sources such as biomass and biofuels are increasingly being seen as important energy sources in Europe. To promote the usage of renewable energy sources and thereby mitigate climate change and increase energy security, the European Union (EU) has set the following targets: by 2020 the share of renewable energy sources on the final energy consumption should be 20% and the share of biofuels on the final consumption of energy in transport should be 10%. The long-term target is to reduce the green house gas emission (GHG) by 50% by 2050. To reach these targets, biomass is expected to play a major role and an increased use of biomass is considered essential [START_REF] Faaij | Bio-energy in europe: changing technology choices[END_REF].

Biomass is currently the main source of renewable energy accounting for 66% of the gross domestic consumption of renewables in the current 27 countries in EU (EU27) [START_REF]Eurostat, statistical office of the european communities[END_REF]. Most of the biomass currently being used for energy purposes is wood, and in absolute terms France (9.3 Mtoe) and Sweden (8.2 Mtoe) are the second and third largest users of woody biomass for energy purposes.

Even though the usage of biomass has increased significantly during the last years, a number of studies suggest that there is a potential for further increasing the usage of biomass [START_REF] Faaij | A roadmap for biofuels[END_REF][START_REF] Londo | The refuel eu road map for biofuels in transport: Application of the project's tools to some short-term policy issues[END_REF][START_REF] Berndes | The contribution of biomass in the future global energy supply: a review of 17 studies[END_REF]. Sources of biomass that traditionally have not been utilized in the energy sector, for example unutilized agricultural land and forest residues, are starting to play an important role in the energy sector. However, there is still considerable uncertainty as to how commercially available the different sources of biomass are. Furthermore, existing energy system models that may be used to estimate the future use of biomass typically do not consider important divergences in the structural characteristics of the cost and supply of biomass. Also, the models are typically aggregated on national levels and thereby unable to consider sub-national or regional variances and unable to analyze regional differentiations in the development of the energy system.

The main objective of this paper is twofold. Firstly, to present a sub-national MARKAL/TIMES model specifically developed for estimating the future use of biomass in the energy system. Secondly, to study the future use of biomass in France and Sweden. The proposed model is a MARKAL/TIMES model which are based on a partial-equilibrium, linear-programming approach for which the short-, medium-, or long-term development of an energy system can be represented and analyzed. The model is driven by the demand of some commodities and is based on a technology rich, bottom-up approach in which typically a large number of technologies can supply the different demands. The proposed model considers the heat, electricity, and biofuel sectors, and can be utilized to analyze and estimate the national utilization levels of biomass, the mix of different sources of biomass to be utilized, the utilization of biomass in the different sectors, mix of technologies used to produce the demand as well as the introduction rate of new technologies. One of the main features of the proposed model is that it is sub-national and thereby able to consider aspects on both national and regional/county levels. The supply, cost, and demand of biomass and biofuels can thereby be expressed with enhanced precision, and the development of the energy system and the future use of biomass can be studies on several levels. Furthermore, the proposed model is able to consider important features of the cost and supply of biomass such as: regional and variable transport distances, numerous agricultural and forest biomass supply assortments, regional distributions of the size of forest clear-cutting areas, supply and exploitation cost of surplus agricultural land and forest biomass in mountainous, hilly, and flat landscape areas.

The proposed model was applied to two case studies, one for France and one for Sweden. These two countries were selected as they both are large users of forest biomass in the energy sector, and as statistics at a very detailed level are available for the agricultural and forest sectors. The case studies demonstrate two methods for enhancing the expression of the cost and supply of biomass to include aspects having a major influence on the cost of supplying biomass to the energy system. For the case study of France, the available areas for production of biomass is divided into four land classes according to the difficulty of performing management activities due to aspects such as soil, land slope, and accessibility. The supply of each land classes is defined per region, thereby facilitating the differentiation of mountainous regions from regions with a flat landscape. For the case study of Sweden, the cost of supplying forest residues is modeled on a detailed level utilizing regional costsupply curves. The cost considers numerous aspects having a major impact on the cost of supplying biomass such as cost of harvesting, cost of transporting to road, and cost of transporting the biomass to the factory. The regional cost-supply curves thereby represent the supply of biomass in the region and where they are located in terms of distances to the factories utilizing the biomass. Sources of biomass nearby and far from the factories are thereby differentiated.

Two sources of biomass are considered in the two case studies: surplus agricultural land and forestry biomass. The utilized sources of biomass are sources that are not utilized in other sectors and which utilization does not hinder the development of sectors such a food and livestock. They thereby do not hinder other sectors. The surplus agricultural land potential corresponds to the utilization of agricultural land that is no longer required in the food and livestock sectors, and can thereby be utilized for the production of biofuel and biomass production. The forest biomass corresponds to the forestry biomass that may be utilized for energy purposes and that is not utilized in other sectors.

The two case studies were used to study the mix of utilized sources of biomass and the amount of biomass utilized in the different energy sectors. A base case scenario as well as alternative scenarios was developed to analyze the future development of the energy system under aspects such as differences in the cost and supply of biomass as well as differences in the end-use demand of heating, electricity and biofuels. We are thereby able to study the stability of the mix of utilized sources of biomass, the demand level that can be fulfilled by national supplies, and the introduction rate of second generation conversion technologies. Furthermore, we look closely at regional differences and national trends in the utilization of the different sources of biomass and the development of the energy system. The paper is structured as follows. In section 2 we describe the proposed model. In section 3 we describe the data utilized for the two case studies and how the case studies were created. Section 4 presents the results of the case studies. A discussion of the results ends the paper in section 5.

Methodology

MARKAL/TIMES models

A MARKAL/TIMES (Market Allocation) model [START_REF] Fishbone | MARKAL, a linear-programming model for energy systems analysis: technical description of the BNL version[END_REF][START_REF] Kanudia | Advanced bottom-up modelling for national and regional energy planning in response to climate change[END_REF][START_REF] Kanudia | The World-MARKAL model and its application to cost-effectiveness, permit sharing, and cost-benefit analyses[END_REF][START_REF] Loulou | Documentation for the TIMES Model[END_REF] is a demand driven linear programming model that can be used to represent, optimize and analyze energy systems over a mid-to long-term planning horizon (20 to 100 years). The energy system may be on a local, regional, national or global scale and the planning horizon may be divided into multiple planning periods of variable length. It was developed and is being maintained by the international ETSAP (Energy Technology System Analysis Programme) [START_REF]ETSAP end-of-term report[END_REF][START_REF]ETSAP implementing agreement, iea[END_REF] organization, which is an implementation agreement with the International Energy Agency (IEA) [START_REF]International energy agency, implementing agreements[END_REF].

The MARKAL/TIMES model represents the energy system via a set of technologies that are linked together by the flows of commodities. A technology, or process, is a typically a physical device that consumes and/or produces commodities, while a commodity is anything produced/consumed by the technologies. Examples of technologies are gas extraction, oil refinery, and coal-based electricity plant. Examples of commodities are gas, crude oil, electricity, and heat. Technologies are characterized by their input and output of commodities, efficiency, costs and environmental impacts, while commodities are characterized by their potential availability, cost of extraction, demand, and which technologies they can be produced and/or consumed by.

The MARKAL/TIMES model is driven by the demand of some commodities and is based on a technology rich bottom-up approach in which typically a large number of technologies can supply the different demands. To fulfill the demands, available energy resources are extracted or imported, and in a series of steps, converted from one commodity into another commodity, finally resulting in the availability of the demanded commodities. The commodities can be divided into three general groups according to the chain of conversion of commodities: primary commodities that are extracted (or imported) and that previously have not been subjected to any conversion or transformation process, secondary commodities which are the processed or transformed commodities, end-use commodities that are the commodities that are demanded. An example of a primary commodity is rapeseed, which can be processed into the secondary commodity bio-oil, which in turn can be refined into the end-use commodity bio-diesel. Each conversion of a commodity into another commodity occurs through a technology and the link between the different technologies is represented by a special type of network diagram, known as a Reference Energy System (RES). The RES provides a representation of the energy system by presenting the link between the technologies and the commodities. For a general example of an RES, see Figure 1. For a more detailed description of the RES concept, we refer to [START_REF] Marcuse | A dynamic time dependent model for the analysis of alternate energy policies[END_REF]. Each transformation step is a linear input/output process and the RES provides an extended linear model of the whole energy system. The second dimension of the MARKAL/TIMES model is that it is a linear programming model that optimizes the fulfillment of the demands to the least total discounted cost of the system, via decisions concerning the activity of the technologies, trade between regions, technological investments to increase capacity and lifetime of the technologies, etc. The objective function being minimized represent aspects such as fixed and variable utilization costs of the technologies, investment and/or dismantling costs, taxes and subsidies, salvage values, costs and revenues due to import and exports of commodities etc. The constraints cover a large number of aspects, ranging from fulfillment of demands, to maximal available of resources for extraction, maximal production capacities of the technologies, lifetime of the technologies, bounds on the utilization rate of technologies, environmental and policy goals etc. The model is commonly based on a perfect foresight assumption for which the input energy prices, the end-use demands, and the development of the technologies are known. The long-term development of the energy sector is estimated according to a deterministic future development, and the development of the energy sector is commonly analyzed according to numerous scenarios of the future development of the system. The MARKAL model can thereby evaluate the effect of a range of aspects, for example the effect of different supply levels of commodities, climate policies, trading opportunities between countries etc. The MARKAL/TIMES model is formulated in the GAMS language and is operated with the help of an interface implemented in the IEA/ETSAP framework.

In MARKAL/TIMES, the user provide projections of the current and future demand of the end-use commodities, the supply of primary commodities, characteristics of the technologies, currently installed capacities of the technologies, political and environmental policies etc. Supply of the primary commodities is described in the form of cost-supply curves, expressing the cost of supplying a specific amount of the commodity. Renewable energy resources are commonly expressed in terms of annual production potentials, while fossil fuel resources are commonly expressed in terms of cumulative potentials over the planning horizon.

Sub-national MARKAL/TIMES model

As there are large sub-national/regional differences in both the potential supply and cost of utilizing sources of biomass, we propose the utilization of a regional MARKAL/TIMES model in order to evaluate the future utilization of biomass and the development of the energy system. The proposed model contains a detailed description of numerous sources of biomass, ranging from agricultural crops, agricultural waste, and forestry harvesting products. Each source of biomass is defined per region in terms of supply, cost, and geographical location. Regional cost and supply levels of the different sources of biomass are thereby considered. The general outline of the model is that the biomass resources are extracted on a regional level, thereafter through a series of steps, converted to fulfill demands on an aggregated national level. The proposed model considers the heat, electricity, and biofuel sectors. Extracted sources of biomass can thus be utilized in these three sectors, and a detailed description of the possible conversions and utilization of the different sources is considered in the model. For a general overview of the model, see Figure 2.

Figure 2: The RES of the proposed model

A current concern is that the increased demand of bioenergy will induce land use change, resulting in agricultural land currently utilized for food and livestock production instead being utilized for growing crop specifically dedicated for energy purposes. A number of issues have been raced concerning land use change. To avoid conflicting land use issues, the proposed model only considers land not utilized in the food and livestock sectors as potential land for energy purposes. The potential land for energy purposed thereby does not conflict with the food and livestock sectors. The land utilized in the food and livestock sectors has in the last years been decreasing in both France and Sweden, thereby freeing up land for energy purposes. Furthermore, agricultural land is also likely to become available for energy crops and biofuels production due to future changes in the production intensity, trade of agricultural products as well as development in food demand. Assuming that the self-reliance level of agricultural products will be kept in the future, the amount of freed up agricultural land that can be utilized for growing crops for energy purposes can be estimated. Note that the same underlying assumption is also utilized for the potential forest biomass for the energy sector. The forest potential considered in the model is only forest biomass that is not utilized in other sectors (pulp/paper, construction, industry, etc).

A number of technologies to convert the biomass sources into biofuels, heat, and electricity are considered in the model. A set of "mean" technologies were created to represent the rich and diverse set of current and future technologies. Each implemented technology thereby represented a specific type or a specific size of a transformation technology. A total of eleven technologies for biofuel conversion and five for heat and electricity cogeneration production were available in the model to choose between. The possible conversions of the biomass sources can be seen in Table 1. To get an accurate picture of the economy of the different conversion technologies and thereby which technologies that will be utilized, the byproducts of the various conversion technologies were considered in the model. The price of the different byproducts were set according to current prices and assumed to be fixed over the planning horizon. The byproducts considered in the model were: glycerin, brewer's spent grain, press cake, propane, naphtha, carbon dioxide, wine product, and electricity.

Potentials Generation of conversion technology

Biodiesel

Bioethanol FT-diesel

The proposed MARKAL/TIMES model allows for international import of commodities such as wood and biofuels, this in order for the model to always be able to fulfill national demands for bioenergy.

As the main aim of the proposed model is to identify possible national production levels, the cost of importing sources of biomass were arbitrarily set to be higher than domestic prices. Domestic sources of biomass were thereby always utilized before import of biomass was selected.

Case studies

The planning horizon of the proposed models was from the year 2000 until the year 2050, divided into time periods of five years. The years 2000 and 2005 were used as reference periods, according to which the model was calibrated. The model had no freedom in the selection of the variables for these two periods as the variables in the linear-programming are fixed. The calibration of the model to reference periods is important as it calibrated the capacity of the technologies, commodities in storage, trade between regions, emissions etc.

Case study of France

The model of France was created utilizing nine regions, dividing the national biomass potential and cost of harvesting the potential into regional potentials and costs (see Figure 3). The regional potential and cost of the different sources of biomass were that of the data collected and compiled for the VALERBIO project [START_REF] Valerbio | valorisation énergétique de la biomasse, etude technico-économiques et prospectives, analyse des concurrences potentielles entre différents usages actuels et futurs de la biomasse, projet realisé par l'IFP[END_REF]. In the VALERBIO project, data concerning both the agricultural and forestry sectors were collected, cost and potential of the different biomass sources, as well as scenarios of the end-use demand. For the cost of agricultural crops, the cost of transport varied between the different crops, while a fixed national average transport distance of 150 km was utilized for all crops. The costs (harvesting and transport costs) of the agricultural crops were set to be constant over the planning horizon.

Figure 3: Regional division of biomass sources in France

The representation of the regional potentials of forestry biomass was further enhanced utilizing four types of accessibility groups (Easy, Medium, Difficult, Very Difficult) and three types of woody products (Big wood, Medium wood, Small wood). The potential and cost of forestry biomass was thereby defined per region, accessibility group, and woody product. The accessibility groups represent the accessibility and the difficulty of performing management activities in the stands, while the woody products represent the different part of a tree that may be utilized. Big and medium wood represents the stem of the tree, while the small wood represents the crown and branches. For energy purposes, small wood is more interesting as it is currently not utilized in other industrial sectors and is therefore highly available. While big and medium wood can be utilized in the energy sector, they are also demanded in other industrial sectors, resulting in a lower potential for energy purposes. As the management of the stands varies between the accessibility groups, the potential of forestry biomass also varied between the accessibility groups. The cost of harvesting biomass was estimated as a linear function of the cubic meters of biomass harvested for each accessibility group and woody product. For each region, an average transport distance was utilized to represent the according to a literature study of the current costs of agricultural crops [START_REF]Förutsättningar för nya biobränsleråvaror[END_REF][START_REF] Börjesson | Production conditions of bioenergy in swedish agriculture[END_REF]. The cost of the forestry and agricultural biomass sources were assumed to be fixed over the planning horizon. As the potentials of agricultural crops collected and computed in the RES2020 project were for each general type of crop (oil, sugar, starch, woody, and grassy) and not in terms of each species of the crop (rapeseed, sunflower soy bean, palm oil, jatropha), the cost and potentials in the case study of Sweden were expresses in terms of the general types of crop.

The potential and cost of forestry biomass for energy purposes was enhanced utilizing two wood assortments: pulpwood and woody chips produced from forest residues. Woody chips are in Sweden almost exclusively utilized for energy purposes and is most commonly produced from forest residues such as crown and branches of the tree. While short rotation coppice are also chipped and may also be sold as woody chips, the two assortments are in the proposed model separated with different potentials and costs. Pulpwood is a wood assortment commonly produced from either small trees or parts of the stem that cannot be sold as timber. For energy purposes, forest residues is more interesting as it is commonly cheaper than pulpwood, and while there is a high demand for pulpwood in the industry and construction sectors, the demand of forest residues for non-energy purposes is small.

A detailed description of the potential and cost of forest residues on a county level was utilized, while the potential and cost of pulpwood was defined on a national level. The potential and cost of forest residues was defined for each of the 21 counties in Sweden according to detailed linear costsupply curves [START_REF] Athanassiadis | Harvesting potential and procurement costs of logging residues in sweden[END_REF]. A cost-supply curve was thus defined for each county (see Figure 4). Each costsupply curve was estimated according to geographical estimation where in the county harvesting operations would occur and how much forest residues it would result in. Environmental, technical and economical restrictions were utilized when evaluating the harvesting potentials in the counties. The cost of harvesting biomass considered aspects such as: harvesting operations, compensation to forest owner, chipping, transport to forest collection site, and transport to factory. The cost of road transport of the biomass was computed utilizing the distance between the forest collection site and the nearest population centre of at least 10000 inhabitants. Chipping of forest residues was assumed to always be performed at roadside and was estimated as a linear function of the amount of harvested biomass. Note that forest residue was only considered from final felling and that after chipping, no differentiation was made between the biomass provided from different tree species.

Figure 4: Regional cost-supply curves for forest residues [18]

To evaluate the impact of the cost and supply of the biomass sources on the development of the energy sector, a number of scenarios were developed. The scenarios reflect different future developments of the industrial and energy sectors as well as high volatility in costs. Note that all these scenarios were developed under the previously defined assumptions. The developed scenarios are as follows: two scenarios for the potential of woody products, two scenarios for the potential of short rotation coppice (SRH), two scenarios for the potential of agricultural crops, and two scenarios for the price of agricultural crops.

Potential of forestry products:

 Business as usual (BAU).  All for industry (AFI): assumes that a low amount of wood can be used for energy purposes as few forest owners are willing to sell the forest residues for energy purposes. The forest residue potential in each harvesting site is 20% less than that of the business as usual scenario.

Potential and price of agricultural crops:  Business as usual (BAU).  All for energy (AFE): assumes that a high amount of agricultural crops can be used for energy purposes. The potential is 20% higher than that of the business as usual scenario.

 Low cost: assumes low costs of the agricultural crops.  High cost: assumes high costs of the agricultural crops.

Potential of short rotation coppice:  Average availability.  High availability. The potential is 20% higher than that of the average potential scenario.

Four combinations of the different scenarios were created as seen in Table 3.

Global scenario name

Forestry potential

Agricultural potential

Agricultural cost SRC potential

BAU AFI BAU AFE Average High Average High P1 X X X X P1b X X X X P2 X X X X P2b
X X X X Table 3: Scenarios for development of the agricultural and forestry products

End-use energy demand from biomass source in France and Sweden

The proposed model considers contribution of biomass sources to the heat, electricity, and biofuels sectors. An end-use demand was specified for each of these three sectors. While both France and Sweden are large users of biomass for energy purposes, there are large differences between the countries in the end-use energy demand from biomass sources and what the different sources of biomass are utilized for. While two thirds of the woody biomass harvested in France is utilized for direct heating of private houses, the major part of woody biomass harvested in Sweden is utilized in the industry sectors (pulp/paper, construction, industry, etc) and only a small portion of the woody biomass harvested in Sweden is directly utilized for heating of private houses. Of the roughly 40 millions of ton dry mass of wood (MTon DS) harvested in Sweden during 2007, only roughly 3 MTon DS were utilized for direct heating of private houses [START_REF]Swedish forestry agency[END_REF]. In Sweden, heating generated from CHP plants instead constitutes to a major part of the end-use heat demand from biomass energy sources. As there are large differences between the countries end-use demand, the demand of heat was separated into two demands, one corresponding to heat from CHP and one corresponding to heat from private biomass heating. The end-use demand of electricity was specified as single demand. For Sweden, the end-use demand of the four sources of biofuels was expressed in term of one demand for ethanol and one aggregated demand for diesel. For France, the demand for biofuels was aggregated over the four sources of biofuels and expressed as a single demand for biofuels.

To evaluate the effect of the end-use demands on the development of the energy sectors, numerous scenarios concerning the development of the end-use demand were created. For France, three scenarios of the end-use demands were defined. The scenarios were created based on the objectives announced in the 10 th operational comity for renewable energy development with environmental high quality (ComOp 10). The ComOp 10 was provided in the framework of the French "Grenelle de l'environnement" (environment round table), and announced that by 2020, the total end-use demand of biomass in the heating, electricity and biofuel sectors could be 20 Mtoe. The defined scenarios are as follows:  D3: Total end-use demand of biomass by 2050 is 40 Mtoe. The ComOp 10 targets are reached in 2020, after which the demand for bio-energy continues to grow up to 2050. The demand after 2020 incorporates demand for bio fuel jet. The demand for biofuels is assumed to cover 20% of the total demand of fuels in the transport sector (air and road).

Figure 5: Structure of the demand scenarios for France

For Sweden, three scenarios of the end-use demands were defined. The scenarios were created based on the Swedish Energy Agency's long-term energy projection 2008 [START_REF]Långsiktsprognos[END_REF], which provide long term projections of the end-use energy demand in Sweden up to 2030. The scenarios were defined as follows:

 D1: Business as usual (BAU).  D2: High economic growth. The scenario corresponds to a higher economic growth than that of the BAU scenario, resulting in a slightly higher demand for heating.

 D3: High prices of fossil fuels. The scenario corresponds to a future projection in which the prices of fossil fuels are higher than those in the BAU scenario. The higher fossil fuels were assumed to decrease the economic growth and give a higher demand for electricity, heating, and biofuels.

Results

We will now present and discuss some of the results that were attained from the MARKAL/TIMES models of France and Sweden. Our discussion will be focused on the differences between the results for the two countries and on regional differences within the countries.

Production of biofuels

Our results show that while national potentials are sufficient to fulfill the demand for biofuels in Sweden, import of ethanol is required to fulfill the demand for biofuels in France (see Figure 7 and Figure 8). The amount of imported ethanol varies between the demand scenarios and while the amount of import is relatively small for demand scenario D1, it is high for scenarios D2 and D3. As the import price for ethanol was set to be higher than the national production cost of biomass, import was only selected as a last resource to fulfill the demand. The demand levels for France thereby show the limits of the French production.

Biodiesel production from oil crops shows to an important source of biofuels in both France and Sweden. The production levels are fairly stable over the scenarios and after 2020, the production level of biodiesel increases over time. However, while the production of biodiesel increases by time, its share of the total production of biofuels decreases in Sweden. Biodiesel's share of the total production is fairly stable in France for demand scenarios D1 and D2, but decreases in the case where there is a demand for national production of bio jet fuels (D3).

Ethanol from cereals is a very important conversion technology for biofuel production in Sweden, consisting of a major part of the total production of biofuels. Ethanol production from cereals increases over the years in both Sweden and France. Ethanol from cereals takes a growing share of the total production of biofuels in France in all the scenarios. However, ethanol's growing share of the total production of biofuels is in Sweden reduced by the second generation ethanol production from wood. In particular 2050, second generation ethanol production technologies may substitute a large share of first generation ethanol production technologies from cereals. For France, the BTL conversion technology shows to be an important technology for production of biofuels. The technology is based on a multistep process in which the biomass is gasified after which a Fischer-Tropsch process is used to produce the diesel fuel. Our results show that the technology is especially important in France when the demand for biofuels is high (D2 and D3). For Sweden, however, second generation ethanol technologies producing ethanol from lignocelluloses straw/woody material is important while the BTL conversion technology is not utilized. The BTL conversion technology is not utilized as biodiesel conversion of oil crops is in Sweden sufficient to produce the demanded diesel level. Introduction of the second generation ethanol production technologies in Sweden varied with the demand scenarios: for demand scenarios D3, the conversion technology is already utilized 2020, while for demand scenarios D1 and D2, the conversion technology is not utilized to a high degree until the 2050.

Utilization of SRC and forestry biomass

Our results show for both France and Sweden, that when short rotation coppice is available, it is utilized to a high degree and thereby reduces the utilization of standard forestry biomass sources for energy purposes (see Figure 9 and Figure 10). The utilization of SRC is stable over the three considered demand scenarios for both France and Sweden. Furthermore, for demand scenarios where the total use of woody biomass is small, the utilization of forestry biomass decreases rather than the utilization of SRC. The utilization of SRC increases over the years in both France and Sweden. When the demand for biomass sources is small in France (D1), the utilization of SRC becomes larger than the utilization of forestry biomass by 2040. The utilization of SRC becomes larger that the utilization of forestry biomass in Sweden by 2040 in all the demand scenarios. This shows the importance of SRC and its possible future contribution to the energy systems in both France and Sweden.

In all the demand scenarios for France, the utilization of forestry biomass peaks by 2030, and thereafter decreases to give way for SRC. In demand scenarios D2 and D3, the amount of forestry biomass utilized for energy purposes increases drastically until 2030. Even though the amount of forestry biomass utilized for energy purposes decreases after 2030, the utilization rate is still high in comparison to current utilization rates. For demand scenarios D1 and D2 for Sweden, the utilization of forestry biomass peaks by 2030, and thereafter decreases to give way for SRC. However, the increase in the utilization of forestry biomass is relative small while the decrease after 2030 is high, resulting in 50% decrease in the utilization level during the period 2030-2050. However, in demand scenario D3 for Sweden, the utilization of forestry biomass continues to increase up to 2050.

Regional differences in harvesting of forest residues in Sweden

As the potential of forest residue in Sweden was defined individually for each of the 21 counties, we were able to study the difference in the harvesting levels of forest residues between the counties. Figure 11 shows the percentage harvest of the total potential of forest residue in the counties. Our results show that there is a difference between the counties in the harvest level of forest residues and that generally the difference between the counties increases as the marginal price of forest residues decreases. Furthermore, there are three counties that stand out with a lower percentage outtake of forest residue (Norrbotten, Västerbotten, Jämtland).

MTon

DS

Forest biomass SRC

When the national harvesting level of forest residue is high and close to the total potential (for example year 2020 and 2030), we observe that the differences in the harvesting level of the resources in the counties is relatively small, only the counties Norrbotten, Västerbotten, and Jämtland do not have a 100% harvesting rate. The harvesting level in these three counties is consistently lower than the harvesting level of the other counties as the harvesting cost of the most expensive harvesting sites in these counties is high. The demand for forest residue thus has to be high for there to be an economical insinuative to harvest forest residue from these sites.

When the national harvesting level of forest residues is low in comparison to the total potential (for example year 2040 and 2050), we observe a high difference in the utilization rate between the counties. This is specially the case in year 2050 for demand D1 and scenario P2, for which we observe that the utilization rate of the counties ranges between 4% and 100%. The high difference in the harvesting levels between the counties can be explained by the high differences in the cost-supply curves. The MARLAL/TIMES model assumes that forest residue is only harvested from areas with a harvesting cost lower than the marginal price of forest residues. As seen in Figure 12, for the marginal price of forest residue year 2050 (scenario P1 and P2), there is a large variance between the counties in the portion of their cost-supply curves that is above the marginal price and thus will not be harvested.

Conclusions

We have in this paper presented a MARKAL/TIMES model that can be used to study and analyze the possible fulfillment of national objectives concerning the utilization of biomass sources, mix of biomass sources to be utilized, development of the energy system, and the introduction and mix of utilization of first and second generation conversion technologies. As the proposed model is subnational and considers the biomass potentials on a regional level, the potentials and harvesting costs can be expressed utilizing detailed information. Furthermore, the model enables the study of regional differences in the mix of utilized biomass sources and in the regional utilization levels.

The presented model was utilized to study the utilization of biomass resources in France and Sweden. Utilizing numerous scenarios concerning potential and demand of biomass sources, we were able to analyze the development of the energy system and the fulfillment of national biomass objectives. While all the demand scenarios for Sweden could be fulfilled by national production potentials, import was required to fulfill the 40 Mtoe demand of biomass in France. The considered scenarios of national biomass potentials and technology availabilities shows that while the 20 Mtoe demand of biomass in France can be fulfilled by national potential, import is required to fulfill the 40 Mtoe demand of biomass in France. Our results show that the limit of national biomass potentials for energy purposes in France is approximately 35 Mtoe.

Our results show that there will be a high demand for forest residue in Sweden. Also, there are large differences between the counties in the utilization levels of forest residues. Norrbotten, Västerbotten, and Jämtland consistently had a lower percentage outtake of forest residue in comparison to the other counties in Sweden. While the transport cost corresponds to a large part of the harvesting cost of forest residues, the transport cost differs heavily between the counties and between the harvesting sites and is therefore difficult to approximate with a single transport distance. As the harvesting costs varies heavily between the harvesting sites, it is important to consider sub-national cost-supply curves for biomass sources. This especially when comparing the utilization of biomass sources to sources such oil and gas which cost-supply are approximately linear. From our results, one can see that while the counties are can be seen as price takers on commodities such oil and gas as they cannot dictate the price of the commodity but have to buy the commodities at the market price, they can be seen as price setters for biomass sources and can dictate the price of the commodity. This as the price of the biomass is highly dependent on the harvest levels, which can be selected by the counties.

Figure 1 :

 1 Figure 1 : A general view of the reference energy system.

:

  D1Total end-use demand of biomass by 2050 is 20 Mtoe. The scenario corresponds to a pessimistic extension of the ComOp 10 targets, following that the targets should instead be met by 2050.  D2: Total end-use demand of biomass by 2050 is 40 Mtoe. The ComOp 10 targets are reached in 2020, after which the demand for bio-energy continues to grow up to 2050. The demand after 2020 incorporates demand for bio-electricity due to the penetration of electric vehicles.

Figure 6 :

 6 Figure 6 : Structure of the demand scenarios for Sweden

Figure 7 :Figure 8 :

 78 Figure 7: Conversion technologies for production of biofuels in France

Figure 9 :Figure 10 :

 910 Figure 9: Utilization of forest biomass and short rotation coppice (SRC) in France

Figure 11 :Figure 12 :

 1112 Figure 11: Percentage utilization of forest residues in the counties in Sweden

Table 1 : The modeled conversions for the biomass sources

 1

	Bio-HVO

Acknowledgement

The authors would like to thank numerous people at IFP (D. Lorne, F. Bouvart, A. Prieur), FCBA (A. Berthelot, A. Thivolle-Cazat, C. Ginet, N. Nguyenm-The), INRA (G. Millet, F. Jacquet) for help with data collection and scenario cunstructions. Also, the TUCK foundation for founding the VALERBIO project. This research was supported by the Chair Modeling for sustainable development, driven by MINES ParisTech, Ecole des Ponts ParisTech, AgroParisTech, and ParisTech; and supported by ADEME, EDF, RENAULT, SCHNEIDER ELECTRIC and TOTAL.

distance between the forest collection site and the plant utilizing the forest biomass. Chipping of forest residues was assumed to always be performed at roadside. The costs (harvesting and transport costs) of the different forestry products were set to be constant over the planning horizon.

To evaluate the impact of the cost and supply of the biomass sources on the development of the energy sector, a number of scenarios were developed. The scenarios reflect different future developments as well as high volatility in the costs. Note that all these scenarios were developed under the previously defined assumptions. The developed scenarios are as follows: three scenarios for the potential of woody products, three scenarios for the potential of short rotation coppice (SRH), two scenarios for the potential of agricultural crops, and two scenarios for the price of agricultural crops.

Potential of forestry products:

 Business as usual (BAU).  All for energy (AFE): assumes that a high degree of wood can be used for energy purposes as the wood industry is not growing fast.

 All for industry (AFI): assumes that a low degree of wood can be used for energy purposes as the wood industry is growing fast. The wood potential is 20% less than that of the business as usual scenario.

Potential and price of agricultural crops:  Business as usual (BAU).  All for energy (AFE): assumes that a high amount of agricultural crops can be used for energy purposes.

 Average cost: assumes average costs of the agricultural crops.  High cost: assumes high costs of the agricultural crops.

Potential of short rotation coppice:  Not available.  Average availability.  High availability.

Six combinations of the different scenarios were created as seen in Table 2.

Global scenario name

Forestry potential Agricultural potential

Agricultural cost SRC potential

Case study of Sweden

For the model of Sweden, the forestry potential was defined for each of the 21 counties in Sweden, while the agricultural potentials were defined on an aggregated national level. The national potential of agricultural crops was defined according to the RES2020 project [START_REF]RES2020[END_REF], while the costs were set