Visual deficits in Alzheimer's disease and dementia: This article considers visual deficits associated with Alzheimer's disease and dementia, and how these may affect patients in everyday life.
Dementia describes a set of symptoms including various changes in cognitive functions such as memory loss, problems with thinking, mental ability, language, understanding and judgment. Dementia primarily affects people over the age of 65 years. (1) However, some people may develop dementia at a younger age. There are different types of dementia, which include: Alzheimer's disease (AD); vascular dementia; and dementia with Lewy bodies. These types of dementia can have differing aetiology and symptoms; however, all types cause problems with memory and thinking. (2)
Types of dementia
AD is the most common of the subtypes of dementia, accounting for up to 60% of dementia cases, (3) and is caused by damage to neuronal structures in the brain. Evidence suggests a buildup of amyloid protein in plaques, and tubulin-associated unit (TAU) protein deposition in neurofibrillary tangles (NFT) within the brain (see Figure I). (4) The disease process is thought to start in the brain at least two to three decades before a person will experience any symptoms. (5) These changes cause memory loss and impairment in problem solving, planning, orientation and language.
Vascular dementia is the next most common type of dementia in the UK after AD; (6) this is caused by problems with blood supply, which is usually the result of changes in capillary vessels within the brain. Often, these changes in blood supply can occur after a series of small strokes. The symptoms most associated with this type of dementia are problems with a person's ability to think and plan ahead. Additionally, people may also experience changes in personality such as depression and being more emotional. Vascular dementia often presents with cognitive symptoms such as problems planning and organising, slower speed of thought and having difficulty concentrating.
Dementia with Lewy bodies (DLB) is less common, accounting for around 10-15% of all cases of dementia. (7) Lewy bodies are small deposits of protein inside the nerve cells in the brain, which is linked to problems with communication between these cells. Symptoms may present as problems with motor functions (similar to Parkinson's disease) and cognitive impairment (similar to AD). Patients suffering from DLB may also suffer hallucinations and delusions. As DLB is a much less common form of dementia, there is not extensive literature relating to changes in visual function and eye movements. It is also important to note that there are cases where it is difficult to distinguish between AD and vascular dementia. Mixed dementia is present in some patients with evidence of aetiological changes consistent with both AD and vascular dementia.
According to the Alzheimer's Society UK, there are an estimated 850,000 people diagnosed with AD in the UK at present. (8) Prevalence by age group in England in 2016 is shown in Figure 2. However, it is estimated that by 2051, the number of people with AD will increase to over 2 million. Early detection is necessary for a patient's long-term quality of life after diagnosis and in addition may result in savings in public and private expenditure. There is an inadequate detection rate of AD and other forms of dementia in primary care with failure rates of detecting mild cases of dementia reported to be as high as 91%. (9)
At present, screening is generally carried out using a cognitive assessment tool. The Mini-Mental State Exam (MMSE), a 30-point cognitive test, is the most widely used screening tool worldwide for both assessing and monitoring dementia. (10) The MMSE has been shown to lack reliability in providing valid diagnoses of dementia in low socioeconomic populations and in those with poor literacy and education levels; (11) additionally, ceiling effects exist in patients with higher levels of education. (12) Another sensitivity issue and arguably one of the most significant, is the difficulty this screening tool and others like it have in detecting the disease in the early stages. Assessments such as these cannot easily distinguish between cognitive impairment that is a result of the normal ageing process from signs that could be suggestive of the progression to dementia.
General visual deficits associated with dementia
Visual symptoms caused by dementia are often variable with some people experiencing many symptoms while others may not experience any at all. However, it may be that patients will come for an eye examination complaining of issues not directly related to their eyesight but actually due to deficits indicative of the early signs of dementia. It is key to note that patients with dementia will most likely have difficulty describing their symptoms, and many patients may put up with symptoms of visual decline due to their developing dementia. Patients with dementia are not only susceptible to the visual impairment noted with ageing, but they can also experience other types of visual deficiency associated with the disease. Although the research regarding visual changes in dementia is often inconsistent, patients with a diagnosis of dementia, or indeed those who are in the early stages of the disease without a diagnosis, may have problems with higher level visual processing including reading, object identification and visuospatial function.
The Prevalence of Visual Impairment in People with Dementia (PrOVIDe) study in 2016 sought to examine the prevalence of eye conditions causing visual impairment in people with dementia. (13) The main findings of this study showed that the prevalence of visual impairment (VI) is disproportionately higher in patients with dementia who are living in care homes compared to those with dementia not living in care homes. In this study, VI was defined as a binocular VA of 6/12 or poorer. The prevalence of VI in age-matched subjects was around 32%, whereas this figure was at 51% for patients with dementia.
Colour vision defects in dementia are variable, (14) and it may be that patients with dementia have problems with using colour information generally. Studies have shown that patients with dementia can have red-green and / or blue-yellow colour vision defects. The literature has identified that patients with AD perform worse than age-matched controls when colour is used as a visual cue. (15)
Stereoscopic vision can also be affected in dementia. Patients with AD and vascular dementia may struggle with judging distances and viewing objects in three dimensions. Research shows that AD patients when compared to age-matched control subjects have impaired performance on tests of local and global stereopsis, motion parallax and monocular depth perception. (16) Stereo acuity has been found to be poor in patients with vascular dementia. (17) Symptoms relating to problems with stereoscopic vision may present in everyday tasks such as climbing stairs or placing objects down. Additionally, driving and ability to park a car may become problematic. Falls are also more prevalent in patients with dementia, and while the causes of falls are multifactorial, reduced depth perception in these patients has been implicated as a contributing factor. (18)
Problems with identifying familiar objects and faces are also noted in those with dementia. Patients with AD and vascular dementia have demonstrated problems in identifying common objects, familiar faces and spatial locations. Notably, patients with AD seem to be significantly more impaired on these areas than vascular dementia patients. (19) Patients with AD also show problems with locating a known object in an unknown location. (20)
There is a growing amount of research showing people diagnosed with dementia demonstrate deficits in eye movements, in particular saccades and fixational eye movements. (21) Humans are required to make eye movements in order to react appropriately to the environment around them. Eye movements are an important evolutionary mechanism to allow for a specific area of the world to be brought into focus and stabilised onto the fovea. In turn, this gives the opportunity to redirect and fixate our eyes as we look around us and react visually to potentially important situations in everyday life as they happen. Saccades are a conjugate eye movement involving fast, darting movements of the eyes. Generally, the main purpose of a saccadic eye movement is to bring a particular part of the visual scene onto the fovea, allowing visual attention to be directed to this area. Saccades can be initiated reflexively by an unexpected new object of interest appearing in our field of view, or voluntarily during a task asking us to look in a specific direction. Saccades can also be memory-guided to a remembered target position, or predictive in anticipation of a target appearing. Saccades range in amplitude; small saccades are executed while reading whereas larger saccades are produced when someone freely gazes around an open space.
Fixation of the eyes on a specific object or aspect of the environment happens in between saccadic eye movements. During the attempted fixation on an object, small involuntary movements called fixational eye movements repeatedly change the gaze position of our eyes. We generally do not notice these tiny eye movements nor are we even aware that we are making them, as our eyes seem stationary when fixating on something. However, these small movements of the eye are important to keep the fovea stimulated.
While saccades and fixational eye movements enable vision to be redirected to project and focus an object onto the fovea, another type of eye movement called a smooth pursuit allows for the object to be tracked and maintained on the fovea if and when it moves around within the environment. These eye movements are much slower than saccades and their function is to track a moving stimulus and keep it projected on the fovea. Smooth pursuits are classed f as voluntary eye movements as we can choose whether to look at and track a moving object or not.
Efficacy of eye tracking in dementia diagnosis
The majority of studies examining eye movements in dementia utilise eye trackers (see Figure 3). Put simply, eye tracking involves the measurement of the movement and position of the eyes in real time. It does not require subjective responses to make suggestions about physiological changes. Eye tracking can provide information on which areas of a visual stimulus are attracting the most attention or being ignored, the amount of time we spend looking at these areas, and how quickly reactions are made when something changes within a visual scene. A huge advantage of eye tracking is that it is non-invasive and is well suited for patient studies as a clinical diagnostic tool.
Saccadic eye movements in patients with dementia
Tasks that monitor saccadic eye movements are used to assess clinical populations. These tests will generally ask a subject to fixate on a central fixation cross and direct their attention to a target stimulus presented in the periphery. Tasks such as these are used to examine the ability to generate voluntary prosaccades in response to stimuli, mirroring what happens in everyday life as new important features enter the visual scene. Dr Trevor Crawford's research involves examining eye movements as a possible diagnostic marker for AD. (22) His research along with other studies examining saccades show patients with
AD and vascular dementia take longer to make a saccade towards a target. Furthermore, patients are often less accurate when making saccades and the saccades do not always reach the target location. (23) Patients with both AD and vascular dementia also produce more errors such as leaving inaccurate saccades uncorrected, continually fixating on a target, and repeatedly making saccades to target locations from previous trials. (24) These impairments are found compared to age-matched control subjects and may be caused by problems disengaging and reorienting attention. They also may explain everyday symptoms noted such as disorientation and confusion.
Eye movements during visual search in patients with dementia
Attending to relevant visual information within a scene requires accurate eye movements, a full visual field and the cognitive ability to perform the action. Finding relevant information within a scene is called visual search and is extremely important for normal visual function. Visual search forms part of the way we react effectively within our environment; it allows us to find an important object or look out for a specific person within an array of complex distractors.
During the ageing process, there are mechanisms that interfere with efficient visual search strategies to some extent; however, studies examining visual search strategies comparing patients with AD to age-matched controls have found that those with AD are poorer at these tasks (see Figure 4). Findings in visual search studies show generally that patients with AD correctly detect fewer targets than healthy people of a similar age. (25) Patients with AD are also slower to detect targets in general. Furthermore, the lengthening of fixation duration during visual search is increased for AD patients but not for healthy age-matched normal participants, which may suggest a change in the style of visual search specific to this condition. (26) Visual search deficits appear to be due to a reduced information processing capacity within the region of fixation and an inability to disengage attention from peripheral target locations.
Reading in patients with dementia
Reading requires the combination of many cognitive subsystems: attention, oculomotor control, word identification, and language comprehension. Information about the length, spelling/grammar and how an upcoming word should sound is available during fixations of the previous words.
Reading ability appears to be maintained in the early stages of AD and vascular dementia, while semantic processing of the words seems to be affected; this means that patients have the ability to read written material but are shown to have slower eye movements while doing so, which may be due to patients not understanding what words mean; hence it may take AD and vascular dementia patients longer to read than other people of their age. (27)
Research also shows patients with dementia have increased fixation duration with longer words, and decreased fixation duration with short predictable words; this may mean AD and vascular dementia patients have impairment in their ability to make predictions about words, (28) which further affects the speed at which they read.
Several key changes in visual function may occur as a result of one or more of the different forms of dementia. These include: O Higher prevalence of visual impairment--the prevalence of visual impairment is higher in patients with dementia with the added difficulty that patients with dementia may not be able to describe their symptoms very easily O Impaired visual search strategies--patients have difficulty in locating objects or people, for example having trouble finding their keys lying on a table surrounded by other items, or trying to find a friend / relative within a crowd of people O Difficulty identifying objects and people--problems in identifying common objects and in some cases patients may not recognise familiar faces O Impaired stereoscopic vision--patients may experience difficulties in navigating stairs, placing their cup of tea down on a table, parking the car O Impaired reading--patients may notice a slower pace of reading and may be unable to predict upcoming words.
Practitioners are likely to encounter an increasing number of patients presenting with AD and vascular dementia as the population ages. It is important to recognise the visual difficulties that these patients may encounter.
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About the authors
* Julia Blair graduated from the University of Strathdyde in 2014 with a degree in psychology. She then went on to study for an MSc in Research Methods of Psychological Sciences at the University of Glasgow and graduated with distinction in 2015. Ms Blair is now a PhD candidate at Glasgow Caledonian University developing a screening test to help diagnose dementia in the early stages using eye movements.
Julia Blair MSc, BA (Hons) and Dr Laura E Sweeney PhD, BSc, MCOptom, Dip Tp(IP), FHEA
* Dr Laura Sweeney is a lecturer in Vision Sciences at Glasgow Caledonian University. Her primary research interests are oculomotor function and binocular vision. Her main teaching responsibilities are centred on ocular disease and independent prescribing. Dr Sweeney is a College of Optometrists' council member for Scotland.
Course code: C-57817 Deadline: 9 February 2018
* Be able to elicit relevant detail from patients presenting with Alzheimer's disease and vascular dementia (Group 1.1.2)
* Be aware of the impact of Alzheimer's disease and vascular dementia upon visual function (Group 6.1.13)
* Understand the assessment of visual function in patients presenting with Alzheimer's disease and vascular dementia (Group 7.1.5)
* Be able to elicit relevant detail from patients presenting with Alzheimer's disease and vascular dementia (Group 1.1.2)
& Be aware of the impact of Alzheimer's disease and vascular dementia upon visual function (Group 8.1.4)
Caption: Figure 1 Neuronal changes caused by Alzheimer's disease
Caption: Figure 2 Dementia prevalence (%) by age group in England (2016)
Caption: Figure 3 Eyelink 1000 used in many eye tracking research studies (top), remote video eye tracker, which can be used with a laptop (bottom, left) and a head mounted eye tracker with head tracking (bottom, right)
Caption: Figure 4 Example of a visual search task--a target appears followed by an array of six objects and the patient must correctly identify the target within the array
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|Author:||Blair, Julia; Sweeney, Laura E.|
|Date:||Jan 1, 2018|
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