Cerebral blood flow patterns underlying the differential impairment in category vs letter fluency in Alzheimer's disease

Verbal fluency tasks are used to assess language functioning in Alzheimer's disease (AD), and performance typically declines as the disease progresses . However, several studies have shown that Category Fluency performance (pr oduce words from a category) declines faster than Letter Fluency performanc e (produce words beginning with a certain letter), which is not the case fo r other dementias. The purpose of this study was to determine if each of th ese two types of fluency tasks was associated with different patterns of ce rebral blood flow abnormality in AD. A resting, Xenon-inhalation regional c erebral blood flow measurement (Xe-133 rCBF) and neuropsychological evaluat ion was administered to 25 patients with probable AD and 24 healthy elderly controls. Stepwise regression using rCBF measures as predictor variables w as used to predict Category and Letter Fluency performance, in both a combi ned group of patients and controls, and in the patient group alone. Correla tions were also computed between rCBF variables and the difference between normatively corrected scores on each task for each subject, which character ized the extent of the discrepancy between them. In full sample regressions , both Category and Letter Fluency were predicted by education and the decl ine in left inferior parietal flow, a focal AD-related deficit. Additional variance in Category fluency, however, was predicted by global mean flow, w hile additional variance in Letter Fluency was predicted by frontal flow. W ithin the patient sample, in turn, the primary predictor of Category Fluenc y was mean flow; the primary predictor of Letter Fluency was left-sided fro ntal flow. Analysis of the fluency difference score revealed that relativel y greater impairment of Category Fluency was associated with more typical, AD-related deficits in posterior temporal and parietal perfusion. When the two were equivalently impaired, typical AD-related deficits were accompanie d by marked deficits in frontal perfusion. These findings are consistent wi th the underlying neuropsychology of these tasks, and suggest that Category Fluency's stronger association to the most typical CBF deficits of AD acco unt for its greater sensitivity to this disease. Letter Fluency deficits, o n the other hand, carry significant information about the degree to which p erfusion deficits have spread to frontal cortex. (C) 1999 Elsevier Science Ltd. All rights reserved.