**1. Introduction**

Dementiaposesasignificantrisktothoseovertheageof65,affectingupto46.8millionindividuals globally, a number that is expected to increase to approximately 131 million by 2050 [https:// www.alz.co.uk/research/WorldAlzheimerReport2015.pdf]. Currently, no reliable treatments exist, although it has been estimated that slowing disease progression by just 5 years would reduce the number of dementia-related deaths by almost half [https://www.alzheim‐ ers.org.uk].Alzheimer'sdisease (AD)is anage-relatedneurodegenerativedisorderofthebrain, and the most common cause of dementia amongst the elderly [1]. AD is typified by progres‐ sive memory loss and significant cognitive decline amongst other complications that ultimate‐ ly leads to death [2].

A major pathological feature of AD is misfolding and aggregation of the naturally occurring amyloid beta (Aβ) family of proteins. A variety of different Aβ peptides are generated by successive proteolytic cleavage of the amyloid precursor protein (APP). These accumulate as large insoluble aggregates in AD brains and are referred to as 'senile plaques'. The amyloid cascade hypothesis proposes that Aβ plays a central role in AD [2]. However, AD and dementia are complex diseases, and the role of Aβ and other disease factors are still incompletely understood. Studies are also hampered by the brain's relative inaccessibility, and clinical diagnosis typically occurs many years after disease onset [3]. Consequently, there is consid‐ erable impetus towards developing non-invasive, reliable and cost-effective diagnostic methods so those at risk may be identified at relatively early stages to maximise the chances of clinical intervention. Most studies into AD and dementia are primarily focused on memory loss and cognitive decline [4, 5]. However, many AD patients are also reported to suffer from a variety of visual complications, which by contrast has received comparatively little attention. The recent discovery that Aβ deposits in the ageing human retina correlate with complex retinopathies such as age-related macular degeneration (AMD) has given support to the hypothesis that shared pathologies may exist between the brain and senescent retina.

In this chapter, we provide a comprehensive review of the latest findings reporting visual abnormalities in patients with AD and dementia. The methodology for this review is based on searches conducted in the NCBI PubMed (http://www.ncbi.nlm.nih.gov/pubmed) database using keywords 'dementia and retina' (396 articles) and 'AD and retina' (457 articles) in June 2016. These numbers contrast starkly with larger numbers of studies in areas related to memory loss and cognitive decline. For example, use of keywords 'dementia and memory loss' (12,383 articles) or 'AD and cognitive impairment' (19,883 articles) yield many more citations; highlighting the as yet limited interest in systematically reporting visual abnormalities in AD and dementia patients. Additionally, we used keywords such as 'retina and Aβ' (111 articles) and 'retinal pigment epithelium (RPE) and amyloid' (59 citations) to include specific articles related to ophthalmology and dementia, and to describe studies in ex vivo and animal models that provide insights into Aβ-mediated pathogenesis. Centring on these articles, specific information in hyperlinks, as well as insights from our own work, we discuss the role of Aβ in driving retinal degeneration and neurodegeneration, and propose that the eye may provide a powerful model to study Aβ pathology. We suggest that the retina may act as an anatomical window into the Alzheimer's brain through which early stages of neurodegeneration could non-invasively be identified.
