The lesions of severe cerebral trauma, if they result in dementia, are found in the cerebral convolutions (mainly frontal and temporal poles), corpus callosum, and thalamus. In some cases, there is widespread degeneration of the deep cerebral hemispheres, because of a mechanical disruption of the deep white matter termed axonal shearing. Most traumatic lesions that produce dementia are quite extensive, making localization difficult. Our experience suggests that the thalamic lesions are important, but many authorities view the diffuse axonal shearing lesions in cerebral white matter as the primary cause of traumatic dementia. The special problem of chronic traumatic encephalopathy is addressed in Chap. 34.
Mechanisms other than the overt destruction of brain tissue may operate in some cases of dementia. Chronic hydrocephalus, regardless of cause, is often associated with a general impairment of mental function. Compression of the cerebral white matter is probably a factor, but this has not been settled. The extrinsic compression of one or both of the cerebral hemispheres by chronic subdural hematomas may have the same effect. A diffuse inflammatory process is at least in part the basis of dementia in syphilis, cryptococcosis, other chronic meningitides, and viral infections such as HIV encephalitis, herpes simplex encephalitis, and subacute sclerosing panencephalitis; presumably, there is a loss of neurons and an inflammatory derangement of function in the neurons that remain. The prion diseases (e.g., Creutzfeldt-Jakob disease) cause a widespread loss of cortical neurons, replacement gliosis, and spongiform change and produce special patterns of cognitive dysfunction.
The adult forms of leukodystrophy (see Chap. 36) also give rise to a dementing state, generally a “subcortical” dementia syndrome with prominent frontal lobe features. Or extensive lesions in the white matter may be the result of advanced multiple sclerosis, progressive multifocal leukoencephalitis, or some of the vascular dementias already mentioned (Binswanger disease and CADASIL [cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy; Chap. 33]). Last, several of the metabolic and toxic disorders discussed in later chapters, for example, vitamin B12 deficiency, may interfere with nervous function over a period of time and create a clinical picture similar, if not identical, to that of the dementias. One must suppose in those cases that the altered biochemical environment has affected neuronal function in critical areas.
Classification of the Dementing Diseases
Conventionally, the dementing diseases have been classified according to cause, to the specific pathologic changes, or by the most prominent clinical feature, for example, memory loss in Alzheimer disease. Another practical approach, which follows from the method by which much of the subject matter is presented in this book, is to divide the diseases into categories on the basis of the neurologic signs and associated clinical and laboratory features of medical disease: (1) dementia with medical disease, (2) dementia that is accompanied by other principal neurologic signs, and (3) dementia as the sole or predominant feature of the illness (Table 20-3). Once it has been determined that the patient suffers from a dementing illness, the illness may be characterized from these medical, neurologic, and ancillary data discussed in the following text. This classification may seem somewhat dated and less based on genetic and molecular models of degenerative disease, but it is likely to be more useful to the physician who must confront the many processes that cause dementia.