Dementia is the term used to describe the decline in neurological functioning which causes difficulties in daily life. It is an increasingly common phenomenon in our ageing population, and is often extremely distressing for both the patient and their family and carers. Whilst the most common dementia diagnosis is Alzheimer’s disease, similar deterioration in mental functioning can be caused by a wide-range of other problems. Differentiating between specific causes of dementia can be challenging, and this lack of knowledge can make it very difficult to plan effective support and treatment.
One of the more common specific causes of dementia is Parkinson’s disease – commonly occurring several years after the diagnosis has been made. In Parkinson’s disease dementia (PDD) the decline is caused by abnormal build-up of protein within the brain, which affects how messages are transmitted, and thus how it functions. This problem is initially localised to the parts of the brain involved in organising and directing movements, before later spreading to other areas and thus causing dementia . Similar protein build-ups are also seen in Lewy body dementia, where they occur across all areas of the brain from the start of the condition . Thus in the later stages Parkinson’s disease dementia and Lewy body dementia (LBD) may appear very similar – but in Parkinson’s disease dementia the movement problems came first.
The global deterioration in mental processing can show subtle differences between different types of dementia, with some diseases affecting some parts of the brain earlier or more severely. Visual problems are some of the more common, and perhaps less well recognised. Visual hallucinations are common – particularly in Parkinson’s disease dementia and Lewy body dementia – as are a range of other visual processing problems.
Visual problems in dementia, and how they are tested
There are many and varied visual problems found in patients with dementia. The production and interpretation of a visual image is an extremely complicated process, involving the eye, retina, optic nerve and a variety of areas of the brain depending on the type of image or information being assessed. Assessment and interpretation of movement is handled separately from the identification of well-known forms, which is again differently handled from distinguishing between (for example) sizes or lengths of objects. As any part of the brain can be affected by dementia, any or all of these processes can be damaged in dementia patients. Such problems with visual perception can cause or enhance problems in interpreting and coping with the outside world – particularly as in some cases the patient may not even really understand what is going wrong. Testing for visual perception problems can help clinicians and carers to understand some of the disorientation experienced by the patient, and therefore support them more effectively.
As visual processing is so varied, testing for problems is clearly quite challenging. The testing will usually involve a range of tasks designed to distinguish between different functions. It is obviously important to check for, or correct obvious visual acuity problems before attempting to test for more complex visual disturbances. The patient may then be asked to look at two images and determine which is the larger, or in which the components are moving more quickly. Other tests may involve the visual recognition of familiar items (animals, household items) or identifying which can be found in a more complex image. All of this needs to be handled carefully to try and ensure the patient does not become distressed, or too tired by the length of testing required. In addition, time pressures should be removed, and the way in which responses are given needs to be thought about carefully, as problems with movement or coordination may make pressing keyboards or buttons difficult for the patient.
Differences in visual perception between Alzheimer’s and other types of dementia
Sufferers of all types of dementia can experience problems with visual perception. As the disease progresses and more and more areas of the brain are affected these problems may become both more common and more obvious. Interestingly though it seems that not all forms of dementia are the same when it comes to visual perception. A group in the UK worked with a group of dementia patients who were similar in their overall level of function and tested their visual perception . They looked at some patients with Alzheimer’s, some with Lewy body dementia (LBD) and some with Parkinson’s disease dementia (PDD) – they then compared their results to patients who were healthy, and those with Parkinson’s disease (and no dementia). Looking at around 25 patients in each group they found that problems with visual perception were significantly more likely (and worse) in patients with either Parkinson’s disease dementia or Lewy body dementia. Patients with Alzheimer’s disease had significantly worse memory than the others, but their visual processing (although slightly worse than both groups of ‘control’ patients) was relatively intact. The difficulties were found in all categories - Parkinson’s disease dementia and Lewy body dementia patients had more difficulty in determining size, speed and identifying familiar objects than Alzheimer’s patients. Other studies have found similar findings, also identifying particular difficulty with visual memory in patients with Lewy body dementia .
Although the numbers were small, the UK group also noted that overall visual perception was significantly worse in these patients who regularly experienced visual hallucinations – perhaps indicating a link between abnormal and absent processing in the parts of the brain concerned with vision.
It is difficult to be sure of the reasons for this, although the findings do match up with some studies looking at blood flow in different areas of the brain of dementia patient which found relatively lower blood flow in the visual cortex (occipital lobe) of patients with both Lewy body dementia and Parkinson’s disease dementia . It seems as though the parts of the brain responsible for visual processing are particularly badly affected – and targeted early in both Parkinson’s disease dementia and Lewy body dementia. That the two conditions show similar problems should probably not come as a surprise, since both exhibit similar areas of abnormal protein build-up called Lewy bodies. These affect brain functioning by causing a fall in some of the brain’s signalling molecules, such as Acetylcholine. Acetylcholine is particularly used by the brain during processing of visual images – perhaps explaining the reason for these findings.
Implications and future directions
Visual perception disorders seem to be more severe in patients with Lewy body dementia and Parkinson’s disease dementia than in those with Alzheimer’s disease and a similar degree of overall function. Increased awareness of and testing for visual perception disorders in these patients may help the patient, carers and carers better understand the problems encountered by the patient, and therefore help them cope. Improved understanding may hopefully lead to improved treatment options in the future.
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