Cognitive decline affects subject attrition in longitudinal research.

We evaluated prospectively 210 patients with idiopathic Parkinson's disease (PD) to determine whether cognitive deterioration and disease disability affect subject drop out. Subjects who refused to return for follow-up testing had a greater degree of bradykinesia and overall disability, more advanced disease, fewer years of education and greater depressive symptomatology. However, discriminant analysis indicated that performance on the neuropsychological measures, rather than PD severity, significantly predicted whether patients return for follow-up testing. Our findings indicate that cognitive impairment uniquely contributes to subject attrition, which may distort dementia estimates in PD.