The effect of processing speed on cognitive functioning in patients with familial bipolar I disorder and their unaffected relatives.

Background: Despite increasing evidence of cognitive dysfunctions in bipolar I disorder, there is no specific neuropsychological profile of the disorder.

Sampling And Method: The aim of the present study was to investigate the effect of processing speed on other cognitive functions in a population-based sample of 32 familial bipolar I disorder patients, their 40 unaffected first-degree relatives and 55 controls. A neuropsychological test battery was administered to all participants, and the effect of processing speed on other cognitive functions was analyzed with the digit symbol subtest of the Wechsler Adult Intelligence Scale-Revised both in within- and between-group comparisons.

Results: After adjusting for the effect of processing speed, only small differences were detected in short-delay cued recall and in long-delay memory between patients and controls, as well as between patients and relatives. Relatives scored better than controls only in verbal ability. Processing speed had a significant effect on nearly all scores, differing by group when patients, relatives and controls were examined separately, the effect being most extensive in patients.

Conclusions: These results support the view that impaired processing speed in particular contributes to a range of cognitive dysfunctions in bipolar disorder. However, it may not be specific to bipolar I disorder and can possibly be considered a shared endophenotype with other mental disorders.