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Dementia’s Effect on the Visual System

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Published: Tue, 27 Feb 2018

Abstract

Recent evidence indicates that memory impairment and visual dysfunction are clearly linked in dementia, and that special testing for visual dysfunction can improve the early diagnosis and treatment of dementia. Visual function is divided in terms of anatomic, functional and cognitive areas respectively. Under normal circumstances these functions perform seamlessly together to produce a visual reality of what we call the external world. Alzheimer’s disease is the most common form of dementia and past research into this area has shown that sufferers show visual deficits in several key areas. Namely contrast sensitivity, motion, colour, depth perception as well as visual hallucinations. Thus by approaching the patient in a appropriate manor with regards to dementia, clinical professionals can detect visual dysfunction and memory impairment whilst also providing a vital role in secondary and tertiary preventative measures. Furthermore clinical professionals can provide aid in the treatment of dementia linked visual disorders. With current demographic trends, dementia is becoming increasingly prevalent due in the ageing population. Consequently there is an increased need for practitioners to have a sound knowledge of such dementia conditions. Improving the sufferers quality of life should be the practitioners main concern. By providing thorough treatments and suggestions on patient tailored environmental modifications this can be achieved.

(1) Introduction

Dementia is a loss of mental function in two or more areas such as language, memory, visual and spatial abilities, or judgment severe enough to interfere with daily life1. Dementia is not a disease itself, sufferers show a broader set of symptoms that accompany certain diseases or physical conditions1. Well known diseases that cause dementia include Alzheimer’s disease, Creutzfeldt-Jakob disease and multi-infarct dementia1.

Dementia is an acquired and progressive problem that affects cognitive functions, behavior, thinking processes and the ability to carry out normal activities. Vision is one of the most important primary senses, therefore serious or complete sight loss has a major impact on a individuals ability to communicate effectively and function independently. Individuals who suffer from both dementia and serious vision loss will inevitably be subject to profound emotional, practical, psychological and financial problems. These factors will also influence others around the sufferer and will extend to the family and the greater society. As we get older both dementia and visual problems inevitably become much more prevalent. Current demographic trends show the increase of the number of very old in our population. Therefore it is inevitable that dementia and serious sight loss either alone or together, will have important consequences for all of us1.

The vast majority of people are aware that dementia affects the memory. However it is the impact it has on the ability to carry out daily tasks and problems with behavior that cause particular problems, and in severe cases can lead to institutionalization. In the primary stages of dementia, the patient can be helped by friends and family through ‘reminders’. As progression occurs the individual will loose the skills needed for everyday tasks and may eventually fail to recognize family members, a condition known as prospagnosia. The result of such progression is that the individual becomes totally dependent on others. Dementia not only affects the lives of the individual, but also the family2.

Dementia can present itself in varying forms. The most common form of dementia in the old is Alzheimers disease, affecting millions of people. It is a degenerative condition that attacks the brain. Progression is gradual and at a variable rate. Symptoms of Alzheimer’s disease are impaired memory, thinking and changes in behaviour. Dementia with Lewy bodies and dementias linked to Parkinsons disease are responsible for around 10-20% of all dementias. Dementia with Lewy bodies is of particular interest as individuals3 with this condition not only present confusion and varying cognition, but also present symptoms of visual hallucinations2. Another common condition that causes dementia is multi-infarct dementia, also known as vascular dementia. It is the second most common form of dementia after Alzheimers disease in the elderly. Multi infarct dementia is caused by multiple strokes in the brain. These series of strokes can affect some intellectual abilities, impair motor skills and also cause individuals to experience visual hallucinations. Individuals with multi infarct dementia are prone to risk factors for stroke, such as high BP, heart disease and diabetes. Multi infarct dementia cannot be treated, once nerve cells die they cannot be replaced1.

In most cases the symptoms of dementia and serious sight loss develop independently. However some conditions can cause both visual and cognitive impairments, for example Down syndrome, Multiple sclerosis and diabetes. Dementia is most prevalent in the elderly, as is sight loss. Therefore it is inevitable that a number of people will present dementia together with serious sight loss.

There have been many studies into the prevalence of dementia in the UK. An estimate for the prevalence of dementia in people over 75 years of age is 15% of the population2. The Alzheimer’s society suggest that 775,200 people in the UK suffer from dementia (figures taken 2001). The Alzheimer’s society also calculates that the prevalence of dementia in the 65-75 years age group is 1 in 50, for 70-80 years 1 in 20 and for over 80 years of age 1 in 5. Estimates suggest that by 2010 approximately 840,000 people will become dementia sufferers in the UK. Estimates suggest that around 40% of dementia sufferers are in residential institutions. One study from 1996 showed that dementia sufferers are 30 times more likely to live in an institution than people without dementia. At 65 years of age men are 3 times more likely than women to live in an institution and at 86 men and women are equally likely to be institutionalized4.

Visual impairments are not associated general diagnostic features of dementia. However recent research has shown the change in visual function and visual processing may be relevant. Alzheimer’s disease patients often present problems with visual acuity, contrast sensitivity, stereo-acuity and color vision. These problems are believed to be more true of cognitive dysfunction rather than any specific problems in the eye or optic nerve9. Early diagnosis is essential to both dementia and sight loss patients, as drug treatments are becoming more and more available. Therefore maximizing the treatment and care for the individual. On the other hand early diagnosis of visual conditions is also essential, so that progression is slowed and treatment is commenced, therefore further progression is prevented if plausible2.

The Mini-Mental State examination MMSE, is the most commonly used cognitive test for the diagnosis of dementia. It involves the patient to undertake tests of memory and cognition. It takes the form of a series of questions/answers and uses written, verbal and visual material. Poor vision or blindness is the most common cause of poor performance on this test other than dementia itself2.

Visual deterioration can occur simultaneously with memory loss in most dementia sufferers. Therefore early recognition of dementia through vision tests has become of importance. Table 1 shows few possible tests that might be useful for such purpose

Table 1 : Vision tests for possible early detection and monitoring of Alzheimer’s disease

 

Use

Benton visual retention test

  • Might be able to predict risk for AD 10-15 years before the onset of the disease
  • Tests visual memory

Contrast sensitivity

  • AD patients have selectively reduced CS for distinguishing large objects and faces

Useful field of view

  • Tests processing speed, divided attention and selective attention
  • Facilitates detection of “attentional dysfunction”; patients suffering from this problem complain of poor vision and inability to identify someone in a group or an object on a patterned background
  • Could be useful to assess fitness to drive

Facial recognition

  • AD patients do not recognize faces with large features and low contrast
  • AD patients do not recognize familiar faces (due to impaired memory)
  • Tests that use facial expressions with progressively diminished degree of contrast

The aim of this paper is to provide information about current knowledge on the topic of visual function & dementia. With regards to Alzheimer’s disease, there will be an inclination to several main foci of research. Namely anatomical/structural changes, functional visual changes, cognitive brain changes and other changes such as the effects of diagnostic drugs on Alzheimer’s disease patients.

(2) Alzheimer’s disease

Alzheimer’s disease is the most common cause of dementia amongst older adults. The Alzheimer’s research trust estimates that 700,000 individuals in the UK currently are afflicted. This number will inevitably increase exponentially in the near future with the trend of an increasingly aging UK population. Therefore it must be of the utmost of importance worldwide to have an understanding all behavioral, anatomical and physiological aspects of this disease.

Alzheimer’s disease is a degenerative disease that attacks the brain, it begins gradually and progresses at a variable rate. Common signs are impaired thinking, memory and behavior. Health professionals and care givers agree that the memory deficit is usually the initial sign of the disease. However researchers have long known that Alzheimer’s disease is characterized by impairments of several additional domains, including visual function5.

However these findings have not yet appeared in the diagnostic guides consulted by healthcare professionals, for example the most recent addition of the Diagnostic & Statistical manual of mental disorders states that few sensory signs occur in early Alzheimer’s disease2. Therefore we still have a limited understanding of the true extent to which visual impairments affect Alzheimer’s disease sufferers. The current web site of the Alzheimer’s association1 and National Institute of Aging6 make no mention of the topic of sensory changes in Alzheimer’s disease. It has even been said that patients with Alzheimer’s disease report visual problems to their healthcare professionals less frequently than do healthy elderly individuals7. Nevertheless visual function is impaired in Alzheimer’s disease8. In terms of cognitive changes, the neuropathology of this disorder affects several other brain areas which are dedicated to processing low level visual functions, as well as higher level visual cognition and attention5.These neuropathological cognitive changes are more dominant however in the visual variant of Alzheimer’s disease known as posterior cortical atrophy. However visual problems are also present in the more common Alzheimer’s disease.

Alzheimer’s disease begins when there are deposits of abnormal proteins outside nerve cells located in the brain in the form of amyloid. These are known as diffuse plaques, and the amyloid also forms the central part of further structured plaques known as senile or neurotic plaques1. Buildup of anomalous filaments of protein inside nerve cells in the brain can also take place. This protein accumulates as masses of filaments known as neurofibril tangles. Atrophy of the affected areas of the brain can also occur as well as the enlargement of the ventricles1. There is also a loss of the neuro transmitter Serotonin, Acetylcholine, Norepinephrine and Somatostatin. Attempts have been made to try to slow the development of the disease by replacing the neurotransmitters with cholinesterase inhibitors, such as donepezil (Aricept), rivastigmine (excelon), galantamine (Reminyl) and memantine (Namenda)1. These drugs work by increasing the levels of transmitters between cells, which otherwise become lacking in Alzheimer’s disease. The National Institute for Clinical Excellence NICE conducted a review of these drugs in March 2005 and concluded that none of these drugs provided sufficient enough advantages to the patient in order to justify their cost. They recommended against the use of such drugs in the Nhs, though the Department of Health later overturned this ruling.

(3) Visual Changes in Alzheimer’s Patients

Loss of vision is a key healthcare dilemma amongst the elderly. By the age of 65 approximately one in three people have a vision reducing eye disease. Dementia, Alzheimer’s disease patients and elderly patients, consequently have many visual conditions in common.

Alzheimer’s disease impairs visual function early in the course of the disease and functional losses correlate with cognitive losses. There are several common visual functional deficits that are frequently identified in Alzheimer’s disease. There is evidence for deficits in Motion perception9,10 contrast sensitivity11 colour discrimination of blue short wavelength hues34 and performance on backward masking tests31.In Alzheimer’s disease the secondary point of damage is usually the visual association cortex and other higher cortical areas, as well as the primary visual cortex 13,14.

(3.1) Some of the main changes that occur in the eye with aging include:

  • The crystalline lens increases in thickness, therefore decreasing its transparency and elasticity; therefore there is a tendency for cataracts to appear.
  • The conjunctiva can become thicker and wrinkled, therefore is subject to deposits such as pinguecela.
  • The iris can atrophy, therefore pupils become constricted and their response to light becomes sluggish. The eyes ability to dark/light adapt is affected.
  • Refractive index of the cornea decreases and it becomes less transparent. Arcus senilis can appear.
  • The ocular globe and eyelids can shrink leading to conditions such as entropian, ectropian and trichiasis. Also while the lachrymal production is reduced the puncta lachrymalis can become stenosed and provide less drainage which gives rise to chronic watering of the eyes
  • Anterior chamber usually becomes more shallow and the sclera more rigid, increasing the prospects of glaucoma.

(3.2) Visual changes due to Alzheimer’s disease reported in literature are outlined below:

(3.2) Anatomic

  • Abnormal nerve fiber layer and retinal ganglion cells (Blanks et al, 1989); (Tsai et al, 1991); (Hedges et al, 1996

Imaging of the nerve fibre layer can be conducted via three techniques. These include Optical coherence topography (OCT), Scanning laser polarimetry and Confocal laser topography. Parisi et al16 conducted research upon the optic nerve fibre layer thickness using OCT. 17 Alzheimer’s disease individuals and 14 age matched healthy individuals were used. The findings of this study showed a definite relationship between the thickness of the nerve fiber layer and the prevalence of Alzheimer’s disease. There was a significant decrease in the nerve fiber layer thickness in Alzheimer’s individuals when compared to healthy age matched particpants.

  • Macular cell loss (Blanks et al, 1990)

Research has shown a definite decrease of the number of retinal ganglion cells located in the maculae of Alzheimer’s disease sufferers in comparison to age matched control individuals. It was found that the loss of retinal ganglion cells varied with eccentricity from the central macula17. Results obtained by Blanks et al, 1990 showed a 28% loss of neurons from retinal ganglion cells at 0-0.5mm from the foveola, 24% loss at 0.5-1.0mm and 47% loss at 1.0mm to1.5mm from the foveola. These losses of retinal ganglion cells were constantly greater than those seen in age matched healthy individuals.

  • Supranuclear cataract (Goldstein et al, 2003)

Cataract removal could improve not only the visual acuity but may be an important tool in helping those patients suffering from visual hallucinations (Chapman et al, 1999); however, no prospective study has been carried out to prove the role of vision improvement through cataract surgery on the well-being of patients suffering from AD;

  • Exfoliation (Janciauskien and Krakau, 2001)
  • Abnormal pupillary innervation [109-113]
  • Glaucomatous optic nerve cupping (Bayer et al, 2002)

(3.3) Functional

  • Decreased visual acuity (Holroyd and Shepherd, 2001)
  • Rapid loss of visual field in patients with AD and glaucoma (Bayer and Ferrari, 2002)
  • Visual field loss (inferior) (Trick et al, 1995)
  • Reduced contrast sensitivity (Holroyd and Shepherd, 2001)
  • Abnormal colour discrimination (blue, short-wavelength hues) (Cronin-Golomb et al, 1991)
  • Abnormal flash visual evoked potentials (VEPs) (Holroyd and Shepherd, 2001)
  • Delayed saccadic eye movements (Holroyd and Shepherd, 2001)

(3.4) Cognitive

  • Abnormal visual sustained/divided/selective attention and visual processing speed (Rizzo et al, 2000)
  • Inability to recognize depth (Holroyd and Shepherd, 2001)
  • Impaired face recognition (van Rhijin et al, 2004)

(3.5) Other

  • Excessive pharmacological mydriasis/miosis [109-113]

These changes summed together not only diminish the quality of vision, but many of them also make the examination of the eye much more complicated. In conjunction with the general visual symptoms of aging, Alzheimer’s patients can also experience visual disturbances caused by the brain rather than the visual system alone. This means that they can have problems and difficulties perceiving what they see rather than how clearly they see it3. Difficulties are usually experienced in the areas mentioned earlier, namely depth, motion, color, and contrast sensitivity. Visual hallucinations are also a common problem linked to loss of vision in Alzheimer’s disease patients18. Another common disorder linked to patients with Alzheimer’s disease is a variant of motion blindness. The patient can appear to be confused and lost; the individual will see the world as a series of still frames19.

Visual changes in Alzheimer’s disease may also be dependent upon which brain hemisphere is more severely damaged; this factor can often be overlooked. An individual with Alzheimer’s disease could have damage to a greater extent on their left brain hemisphere from plaques and tangles. This would therefore cause subsequent retinal changes in only the left hemi-retinas of each eye i.e. the right visual fields. The right eye visual field would be affected in the temporal side (right) and the left eye visual field would be affected nasally (right)20. When only half the retina is impacted, smaller regions of the optic nerve and nerve fiber layer show losses. The left eye with affected temporal retina would show optic nerve damage in differing regions of the nerve than the right eye with nasal retinal damage20.

Alzheimers patients commonly show selective degeneration of large ganglion cell axons located in the optic nerves. This suggests that there would be impairment of broadband channel visual function. Conversely studies have shown that broadband visual capabilities are not selectively impaired in Alzheimer’s disease. The magnocellular and parvocellular neurons are greatly affected in Alzheimer’s patients, this has been proved by studies of the dorsal Lateral geniculate nucleus(LGN)1. The geniculostirate projection system is split both functionally and anatomically into two sections. They include the parvocellular layers of the Lateral geniculate body and also incorporates the magnocellular layers. These systems are mainly divided in the primary visual cortex and go through further segregation in the visual association cortex. They conclude in the temporal and paritetal lobes1.

The parvocellular layers contain smaller, centrally located receptive fields that account for high spatial frequencies (acuity), they also respond well to color. On the other hand these cells do not respond well to rapid motion or high flicker rates. The magnocellular cells have larger receptive fields and respond superiorly to motion and flicker. They are however comparatively insensitive to color differences. The magnocellular neurons generally show poor spatial resolution, although they seem to respond better at low luminance contrasts. To summarize the parvocellular system is superior at detecting small, slow moving, colored targets placed in the centre of the visual field. Meanwhile the magnocellular system has the ability to process rapidly moving and optically degraded stimuli across larger areas of the visual field1.

The parvocellular system projects ventrally to the inferior temporal areas, which are involved in visual research, pattern recognition and visual object memory. The magnocellular system projects dorsally to the posterior parietal and superior temporal areas. These are specialized for motion information processing. The cerebral cortical areas to which the parvocelluar system projects receives virtually no vestibular afferents. Alternatively the cerebral areas to which the magnocelullar system projects receives significant vestibular and other sensory inputs. These are believed to be involved in maintaining spatial orientation. Research shows shows that the magnocellular system is more involved in Alzheimer’s disease1

Oddly, many individuals experience difficulties at low spatial frequencies instead of high frequencies as in old age. This suggests that areas controlling the low spatial frequency processing in the primary visual cortex would be affected more than those for higher frequencies processing21 After neuropathilogical studies in 1997 by Hof et al were carried out on brains with visual impairments they concluded that cortical atrophy dominated on the posterior parietal cortex and occipital lobe22.

Glaucoma is also a neurodegenerative disease that has similar effects on the visual system. Lower spatial frequencies in the contrast sensitivity, deficits in the blue short wavelength color range as well as reductions in motion perception are all linked to glaucomatous patients23. When patients diagnosed with Alzheimer’s disease also have glaucoma, the deterioration of vision related to glaucoma is much more rapid and progression is more aggressive than in people with glaucoma solely and not Alzheimer’s disease as well24.Glaucoma is different from Alzheimer’s disease in that it affects the visual function at the early sites of neural activity, namely, the retinal ganglion cells. Glaucoma destroys the afferent axons at the nerve fiber layer in the retina. This loss of axons ultimately leads to added atrophy further up the visual pathway due to decreased neuronal input. Alternatively Alzheimer’s disease impacts the cells that are located terminally or intermediary in the visual pathway of the brain. The result is again reduced neuronal input due to loss of nerve fibre connections and atrophy along the visual pathway. When the two diseases exist in the same individual together it can be seen that there is likely to be a greater disruption to the visual system25. One key difference between the two diseases is that they affect the visual pathway at different points. Glaucoma is a degenerative disease starting at the beginning of the visual pathway, whereas Alzheimer’s disease is a degenerative process starting relatively late in the visual pathway. When the two diseases coexist then the neuronal and functional losses of vision are cumulative.

(4) Optometric examination of dementia patients

Dementia patients present special problems for optometrists. A standard eye test can be an audile to even the best of us. The patient is placed in an unfamiliar environment surrounded by unusual equipment, machinery and is subjected to probing questions about their medical history which will without doubt tax their already flawed memory. Dementia patients are most likely to be from the elderly. Therefore several difficulties are presented while conducting an ocular examination. The patient is required to sustain a position and has to maintain concentration throughout the testing procedures, which can be very difficult. Subjective examination requires responses from the patient, they are expected to remember and follow complex instructions given to them by the optometrist as well as make many precise discriminatory judgments in a short space of time. The multiple tasks required to be completed during the examination are often beyond dementia patients as they are limited by the disease. Therefore it is common that patients with even a minor degree of dementia fail to provide valid answers, provide unpredictable responses to the subjective examination and retreat into an apathetic state1,2.

During the visual examination of Alzheimer’s disease patients, several key visual problems can be detected. Moderate dementia patients will often experience problems such as topographic agnosia, alexia without agraphia, visual agnosia and prospagnosia1. Such patients often cannot describe individual components of photos and routinely fail to recognize family members. The degree to which such problems are experienced is consistent with the level of cytochrome oxidase deficits in the associated cortical area. In conjunction with these problems dementia patients often have problems with texture discrimination and blue violet discrimination1.

Throughout the examination of the elderly dementia patients there are two contradictory requirements, firstly is ‘assurance’. The patient’s responses will be delayed and the patient may feel anxious in such an unfamiliar situation. Thus constant reassurance is required and they cannot be rushed. Alternatively time constraints are important, a dementia/elderly patient is likely to have a short attention span. Consequently the two factors above much be considered and balanced. The examination must be thorough yet carried out as quickly as possible. Often when examining a dementia patient a family member of the carer must be present in order to aid the communication between optometrist and patient, for example difficulties are likely to occur when recording history and symptoms without a carer present. All factors need to be considered such as family history, medication, eye treatment and knowledge of any medical conditions and if so how long they have suffered from them.

In terms of an external examination firstly, gross observations should be recorded for example does the patient have an abnormal head position or is there any lid tosis. Many external observations can also be detected with the aid of pupil reflexes. Upon carrying out the external examination the optometrist must be carful to explain exactly what each procedure will involve so as not to intimidate the patient.

(4.1) Internal ocular health examination

Internal examination of an elderly patient often presents many problems. Older patients tend to have constricted pupils and often opacities in the media such as cataract. All of which make opthalmoscopy a much more complex task for the optometrist. Patients with dementia also show poor fixation as well as lack of concentration. Pupil dilation is often used to aid external examination however many older patients can have a poor response to the insertion of mydriatic eye drops. fddfdffdg

There have been many studies into the affects of diagnostic mydriatic and miotic drugs. Many studies have shown excessive mydriatic pupil response to trompicamide (a pupil dilating drug) in patients with Alzheimer’s disease when compared to control individuals26-30. On the other hand studies into the use of Miotic drops, particularly Pilocarpine have shown an increased response of pupil constriction in Alzheimer’s disease patients upon comparison to normal control patients. These findings suggest a defect in pupillary innervation with Alzheimer’s disease individuals. Studies of post mortem individuals with exaggerated mydriatic pupil responses to Tropicamide found a definte disruption to the Edinger-Westphal nucleus. The Edinger-Westphal nucleus is one of the key structures of the brain involved in the autonomic nervous system, it mediates the sympathetic and para-sympathetic pupil responses. Research by Scinto et al found amyloid plaques and neurofibrillary tangles in all individuals tested with excessive mydriatic pupil responses. The conclusion was that the Edinger-Westphal nucleus is targeted early in the progression of Alzheimer’s disease.

In terms of intraocular pressures use of the goldman an Perkins tonometers will be limited for the elderly dementia patients, due to health and safety reasons. Sudden movements whilst carrying out pressure tests on such equipment may be dangerous. Therefore this can be overcome to a degree by the use of handheld instruments such as the pulseair. However even with the pulseair problems can still be faced with uncooperative patients.

(4.2) Objective Refraction examination

With uncooperative and awkward patients objective refraction through retinosopy may be difficult. Factors such as opacified media, miotic pupils, and poor fixation will influence the accuracy of the refraction. The recent introduction of hand held optometers has contributed to somewhat overcoming such problems. Instruments such as thee Nikon Retinomax are excellent for obtaining an objective refraction of the elderly patient with miotic pupils and cloudy media.

When presenting the Snellen chart to a patient, the quality of their response will inevitably depend upon the degree of their dementia. Depending on which stage of dementia they are suff


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