Clinical neurophysiology of dementia

Abstract

The role of EEG in the study of the dementias is to help in the differential diagnosis of the multiple causes of this syndrome. EEG is useful in differentiating early on between treatable and as of now untreatable forms of dementia. Space-occupying lesions that give rise to dementia are reliably detected by EEG. Infectious, toxic, and metabolic processes are associated with early and severe electroencephalographic abnormalities. The "slow virus" infections show characteristic electrical patterns that reliably distinguish them from the cortical or subcortical dementias. Finally, the EEG may contribute to distinguishing between Alzheimer's disease and MID, two commonly occurring forms of dementia. The paucity of substantial early EEG abnormalities in Alzheimer's disease, although helping to differentiate it from other dementias, leaves us without a currently available physiologic test that provides positive evidence for this condition. Recent studies of EPs, however, suggest that some intermediate latency VEP components may be delayed in patients with Alzheimer's disease when compared with normal subjects. This is encouraging, as latencies in VEPs are more reliable and less variable than amplitude that has previously been reported as "abnormal" in some early Alzheimer patients. Long latency ERPs and CNV also show early abnormalities in Alzheimer's disease. Tests of eye movements such as ERPs are psychophysiologic tests requiring some degree of patient cooperation. Performance on tests of ocular smooth pursuit correlate highly with severity of the dementia syndrome in Alzheimer's disease. In contrast, smooth pursuit testing is usually normal in elderly patients with pseudodementia of depression, suggesting this test may be of some value in differentiating these two clinical disorders. Some evidence exists that smooth pursuit eye movements are also normal, at least in the early and middle stages, in Pick's disease, again suggesting that eye movement testing may prove to have some utility in differentiating this form of dementia from Alzheimer's disease. Ocular scanpaths are abnormal in dementia. They typically are poorly organized and at times perseveratory. In addition, the average durations of eye fixations during directed visual search are altered in dementia as compared with normals. The average eye fixation durations are longer with Alzheimer's disease and briefer in patients with frontal lobe tumors as compared with elderly normal controls. These group differences suggest differing scanning strategies for these two forms of dementia.(ABSTRACT TRUNCATED AT 400 WORDS)