[Heterogeneity of Alzheimer's disease]

Abstract

Despite the original distinction between Alzheimer's disease (AD) and senile dementia, both are currently considered to be one single disease based on their common neuropathological findings. We have reviewed the literature to investigate whether there is proof to assume the existence of subgroups of AD. Historically, there is a division based on the age at onset. Although typical cortical symptoms may be more prominent in early onset cases they may be encountered in senile cases too. Cortical neuronal loss may be more severe in early onset patients, but hippocampal neuronal loss is equally severe in both groups. In older patients cerebral reserve is reduced by age-related neuronal loss and a small amount of AD-type lesions might be sufficient to cause dementia. Aphasia has been proposed as indicator of a subgroup. However, it probably occurs in all AD patients when the cortical degenerative process progresses. AD cases presenting with aphasia are rare and more often prove to be Pick's disease or primary progressive aphasia. Extrapyramidal signs are present in 25% of patients without neuroleptics and in 90% of all patients. They are usually of mild intensity, appear during the course of the disease and are found to be of extranigral origin. Myoclonus may also be encountered at any stage, although more frequent in early onset AD. Its presence is often associated with a more progressive course. Most AD cases are sporadic and some are familial, suggesting an autosomal dominant transmission. Molecular genetics reveals that some patients with familial early-onset AD have a mutation on chromosome 21. Other genes, probably on chromosome 19, may be associated with late onset familial cases, suggesting heterogeneity in familial AD. White matter involvement on computerized tomography or magnetic resonance imaging has been reported to be more prominent in a subgroup of AD patients, with later onset and confusional symptoms. The typical bilateral temporo-parietal hypometabolism on positron emission tomography and hypoperfusion on single photon emission computed tomography, is not found in all AD cases but may be indicative of a subgroup. Based on this review of the literature one homogeneous subgroup emerges: "probable" AD patients displaying memory disturbances with predominant cortical signs (especially aphasia), with a low prevalence of confusion and white matter involvement, exhibiting symptoms at a relatively early age but not exclusively below 65 years, and with a higher prevalence of genetic predisposition and more widespread neuropathological lesions at postmortem examination.