There have been numerous reports of the impairment of specific cognitive functions in patients with Parkinson's disease. Some of the impairments described are often only identifiable by specially designed methods of assessment, but some are revealed by tests of cognitive function that are in widespread use. Mortimer and his colleagues have reported a very high prevalence of cognitive impairment—93 per cent in a substantial group of patients with Parkinson's disease. (l) Examination of their data showed neither a clear distinction between impaired groups nor the presence of subtypes of Parkinson's disease in which cognitive impairment was a more frequent occurrence. Their findings led them to propose that cognitive impairment in Parkinson's disease lies on a continuum of severity, rather than arising as a feature of particular subgroups. The impairments identified include deficits in memory, language, visuospatial functioning, abstract reasoning, slowness in intellectual tasks, and difficulty in shifting from task to task. Not only are these deficits widespread among patients with Parkinson's disease but they have been shown to occur at a very early stage of the disorder.(23)
A proportion of patients with Parkinson's disease show impairment of a range of cognitive functions, which is more akin to the global impairment seen in dementing disorders such as Alzheimer's disease.(4) However, the pattern of this impairment is frequently less severe than that seen in Alzheimer's disease where the pathological changes in the brain are known to be widespread. This observation, together with the occurrence of cognitive impairment in a range of movement disorders where the main neuropathological changes reside in the subcortical region of the brain, has led to the concept of 'subcortical dementia'—a form of intellectual impairment of lesser degree than in Alzheimer's disease, but affecting several cognitive functions and associated with a disorder of movement. In 1974 Albert and his colleagues gave a description of the syndrome in which the main features were listed as emotional or personality changes, impaired memory, defective ability to manipulate acquired knowledge, and a striking slowness in the rate of information processing. (5)
This concept has carried some conviction, as the impairment seen in many subjects with disease in the subcortical region of the brain shows a pattern of cognitive impairment distinctly different from that of Alzheimer's disease. However, many issues have arisen as to the nature of 'subcortical dementia'. Is subcortical dementia a clinical or a pathological concept? Is the difference between this and other forms of dementia simply one of degree? Do the pathological changes occur in the subcortical region of the brain alone? Is the syndrome of cognitive impairment distinctly different from other dementias or does the presence of motor features of the disorder simply give the intellectual impairment a distinct character? Is subcortical dementia a stable condition or a transitional state leading eventually to global dementia? Opinion has ranged from full acceptance of 'subcortical dementia' as a distinct form of cognitive impairment to scepticism. (67 and 8)
McHugh(9) has gone further than Albert et al.,(5) suggesting that the subcortical region subserves functions not only in motor control and cognitive function but also in the control and display of mood. He suggests that some syndromes arising from subcortical disease represent a 'subcortical triad' of symptoms. This combination of symptoms is most convincingly seen in Huntington's disease. A notable difference between this concept and that of Albert and his colleagues is that the pathological disturbance of mood is only intermittently present, whereas the motor and cognitive changes are persistent.
Cummings(l.0) has suggested a useful development of the concept of 'subcortical dementia'. He believes that the concept is applicable to disorders of movement to varying degrees. He suggests that cognitive impairment in Parkinson's disease takes three forms: a form which is relatively mild and meets the criteria for subcortical dementia, a more severe form showing wider impairment of cognitive function but neuropathologically distinct from senile dementia Alzheimer type ( SDAT), and a severe form which shows neuropathological changes in both the subcortical region of the brain and in the cortex, the latter of Alzheimer type. This proposal does not resolve some of the questions that have arisen over the nature of subcortical dementia, but it does provide a basis for viewing cognitive changes in Parkinson's disease, albeit provisional.
Many reports have suggested that global dementia occurs in Parkinson's disease. Whether such a severe change in cognitive function can be regarded as an intrinsic feature of this disease, whether it implies an extension of a neuropathological process more widely in the brain, or whether it suggests a different neuropathology from the outset which initially presented with disordered movement is, as yet, uncertain.
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