Severity and specificity of cognitive impairment in Alzheimer's, Huntington's, and Parkinson's diseases and progressive supranuclear palsy.

To investigate differences in severity and specificity of cognitive impairment among various neurodegenerative diseases, we tested groups of patients presenting with senile dementia of the Alzheimer type (SDAT; 44), progressive supranuclear palsy (PSP; 45), Huntington's disease (HD; 35) and Parkinson's disease (PD; 164), with an extensive neuropsychological battery. We found dementia, as defined by a global intellectual performance 2 standard deviations lower than mean control values, in 93% of SDAT, 66% of HD, 58% of PSP, and 18% of PD patients. Specific features of cognitive impairment distinguished the four groups of patients once they were matched for level of intellectual deterioration: remote memory and linguistic disorders in SDAT, frontal lobe-like abnormalities in PSP, concentration and acquisition disorders in HD. There was no specific alteration in demented PD patients. This study demonstrates the frequency of dementia in predominantly subcortical degenerative diseases and indicates that "subcortical dementia," rather than being a homogeneous entity, should be divided into specific subtypes of cognitive impairment related to different underlying specific lesions of each disease.