The aforementioned alterations of intellect and behavior are the direct consequence of neuronal loss in certain parts of the cerebrum. In other words, the symptoms are the primary manifestations of neurologic disease. However, some symptoms are secondary; that is, they may represent the patient’s reactions to his mental incapacity. For example, a demented person may seek solitude to hide his affliction and thus may appear to be asocial or apathetic. Again, excessive orderliness may be an attempt to compensate for failing memory; apprehension, gloom, and irritability may reflect a general dissatisfaction with a necessarily restricted life. According to Goldstein, who has written about these “catastrophic reactions,” as he called them, even patients in a state of fairly advanced deterioration are still capable of reacting to their illness and to persons who care for them.
In the early and intermediate stages of the illness, neuropsychologic tests aid in the quantitation of some of these abnormalities, as indicated in the later part of this chapter.
Subcortical Dementia Associated With Diseases of the Basal Ganglia and White Matter
McHugh, who introduced the concept of subcortical dementia, pointed out that the cognitive decline of certain predominantly basal ganglionic diseases—such as progressive supranuclear palsy, Huntington chorea, and Parkinson disease—is different in several respects from the cortical dementia of Alzheimer disease. In addition to the obvious disorders of motility and involuntary movements, there are degrees of mild forgetfulness, slowed thought processes, lack of initiative, and depression of mood. Relatively spared, however, are vocabulary, naming, and praxis. By contrast, the “cortical dementias” (exemplified by Alzheimer disease) are distinguished by more severe disturbances of memory, language, and calculation, prominent signs of apraxia and agnosia, and impaired capacity for abstract thought.
The pathologic changes underlying subcortical dementia predominates in the basal ganglia, thalamus, rostral brainstem nuclei, and in the ill-defined projections in the white matter from these regions to the cortex, particularly to the frontal lobes. However, it would be overly simplistic to attribute the dementia to changes in these areas. One of the problems with the concept of subcortical dementia is the name itself, implying as it does that symptoms of dementia are due to lesions confined to subcortical structures (non-cortical dementia may be more apt). None of the neurodegenerative dementias is strictly cortical or subcortical. In a similar way, the changes of Alzheimer disease may extend well beyond the cerebral cortex, involving the striatum, thalamus, and even cerebellum. Also, functionally, these lesions produce their effects by interrupting neural links to the frontal and other parts of the cerebral cortex. Further ambiguity arises when one considers the dementias caused by Lewy-body disease (probably second in frequency only to Alzheimer disease) and by normal-pressure hydrocephalus; here there are parkinsonism and dementing features that could be construed as both cortical and subcortical in nature.
Certain authors, notably Mayeux and Stern and Mayeux and colleagues as well as Tierney and coworkers, have been critical of the concept of subcortical dementia. They argue that the distinctions between cortical and subcortical dementias are not fundamental and that any differences between them are probably attributable to differences in the relative severity of the pathologic processes. Nevertheless, many clinical studies indicate that the constellations of cognitive impairments in the two groups of dementias differ along the lines indicated earlier (see Pillon et al) and the clinical distinction between cortical and subcortical dementia based on a relative sparing of cortical functions is very useful.
Pathogenesis of Dementia