[Executive function in Parkinson's disease]

Executive function declines in Parkinson's disease (PD). However, it has not been clearly shown at what stage in PD, this decline starts to occur. We here report a study aiming to answer this question. We conducted Wisconsin Card Sorting Test (WCST) and Trail Making Test A and B (TMT A, B) in three normal control groups (young, adult, and aged) and three PD patients groups (Hoehn & Yahr stage I, II, and III). We intend to analyze at what age or stage the decline in executive function would take place. The score of all the normal subjects and PD patients were 25 points or above by Mini-Mental State Examination. WCST in stage I PD patients (mean age 60.8, n = 9) was essentially same as those of young normal (mean age 23.1, n = 9) and adult normal (mean age 61.5, n = 10) subjects. Elderly normal subjects (mean age 75.8, n = 4) showed a significantly lower mean category achievement score (3.0 as the mean score) and higher numbers of errors and perseverations compared with those of young and adult normal subjects. Stage II (mean age 62.6, n = 8) and III PD (mean age 62.9, n = 8) patients showed significantly lower mean category achievement scores (2.4 and 2.1, respectively as the mean scores) and higher numbers of errors and perseverations compared with those of adult control subjects (5.4 as a mean score). TMT (B-A) in elderly normal subjects revealed significantly longer score (209 seconds as the mean score) compared with those of young and adult normal subjects (20 and 45 seconds, respectively as the mean scores). TMT (B-A) in stage III PD patients was significantly longer (219 seconds) compared with that of adult normal subjects (45 seconds as the mean), however TMT (B-A) in stage II PD patients (102 seconds as the mean) did not show prolongation. TMT (B/A) showed essentially similar results as TMT (B-A), however, stage II PD patients showed significant prolongation compared with that of normal adult subjects. Therefore, TMT (B/A) appears to be a more sensitive indicator of decline in executive function in PD. Between WCST and TMT, the former appeared to be a more sensitive indicator. Our results indicate that the decline in executive function takes place in normal ageing. In PD, this decline starts much earlier than the normal subjects. The onset in this decline coincides with the stage of PD, in which bilateral symptoms start to present. Anatomo-chemical subsrate of cognitive decline in PD is still to be debated, however, we believe that involvement of nigro-caudatal projection is at least in part responsible, as nigroputaminal pathway is mainly involved in motor functions. We also point out the importance of age factor in the evaluation of cognitive-executive function in PD, as this function is age-dependent in normal subjects.