Cognitive impairment is among the most frequent non-motor symptoms in Parkinson's disease (PD). To clear the characteristics of cognitive impairment in patient with PD may be critical for the planning of effective rehabilitation interventions. However, a few studies have sought to investigate relationship between cognitive impairment and motor functions. The purpose of this study was to compare cognitive impairment of PD patients in relation to the motor impairment severity according to Hoehn and Yahr (HY) stages.

The subjects of this study were 78 patients with PD (33 males and 45 females). The Cognitive function was assessed using the Wechsler Adult Intelligent Scale-III (WAIS-III) and Wechsler Memory Scale-Revised (WMS-R). The HY scale was used to evaluate the motor impairment severity as I (no disability, n=11), II (mild, n=34), III (moderate, n=26), or IV–V (severe, n=7). The Kruskal–Wallis test was used to compare the cognitive impairment the between HY groups.

The difference in performance intelligence quotient (PIQ) of WAIS-III (I: 105.6±8.6, II: 94.9±17.7, III: 91.5±14.3, IV–V: 82.9±8.2) and general memory quotient (GMQ) of WMS-R (I: 105.5±14.4, II: 91.8±15.6, III: 90.0±18.2, IV–V: 86.1±13.0) between groups were significant, scores of IV-V stage were lowest. There were statistically significant differences in processing speed index and verbal memory quotient among the groups. The difference in other measurements was not significant the between groups.

Our present findings indicated that the cognitive impairment was observed in deterioration of the motor impairment in PD patients. Specifically, in the factor scale level, the low processing speed index and the low verbal memory quotient reduced each of the PIQ and GMQ in the lowest HY group patients. Therefore, we suggest that a careful assessment of cognitive impairments, especially processing speed and verbal memory, is important in identifying appropriate interventions for PD with severe stage.