Does motor subtype influence neurocognitive performance in Parkinson's disease without dementia?

The postural instability and gait difficulty (PIGD) motor subtype has been shown to represent a risk factor for development of dementia in Parkinson's disease. Whether this relationship extends to a more subtle cognitive dysfunction in patients is less clear. Therefore, we administered a battery of selected neuropsychological tests to two groups of non-demented patients with mild to moderate disease classified either as PIGD or as non-PIGD subtype and to a group of healthy controls. Groups were matched on potential confounders of neuropsychological performance. No significant differences were revealed between the two groups of patients in the performance of any of the administered neuropsychological tests. However, relative to controls there was a tendency towards a differential pattern of cognitive dysfunction. The PIGD group had slower performance in a test of psychomotor speed and cognitive flexibility, whilst the non-PIGD group performed worse in measures of verbal learning and visuo-spatial perception. In conclusion, the PIGD subtype was not associated with more severe cognitive deficits and may to a certain extent share common mechanisms of cognitive dysfunction with non-PIGD subtypes. Diverse pathological processes however may develop to account for unequal rates of dementia amongst different motor subtypes.