Odor identification deficits identify Parkinson's disease patients with poor cognitive performance

Olfactory dysfunction is a prodromal and prevalent nonmotor symptom of Parkinson's disease. Unlike olfactory dysfunction in Alzheimer's disease, it is believed to be unrelated to cognitive impairment. However, recent research has implicated cholinergic denervation in Parkinson's disease hyposmia and linked it to verbal memory. This research hypothesized that severe odor identification deficits may identify patients with Parkinson's disease at risk for cognitive impairment. The current study tested this hypothesis by comparing 24 functionally anosmic, nondemented patients with Parkinson's disease and 39 nonanosmic, nondemented patients with Parkinson's disease with 29 healthy control participants on composite scores of memory, processing speed, executive function, and language. The functionally anosmic group had significantly poorer visual and verbal memory than the nonanosmic group, which was indistinguishable from the control group. Furthermore, the functionally anosmic group had reduced processing speed compared with the nonanosmic patients with Parkinson's disease, who, in turn, were outperformed by the control group. On the composite language score, the score of the functionally anosmic group was significantly reduced compared with that of the control group, whereas the nonanosmic group scored in the medium range. The 2 patient groups did not differ on executive functioning. These findings demonstrate co-occurrence between reduced cognitive function and olfactory deficits in functionally anosmic patients with Parkinson's disease and support the notion of more severe cognitive deficits in this group.