Parkinson's disease in twins: A follow-up study

Objective: To reevaluate concordance rates in 9 monozygotic (MZ) and 12 dizygotic (DZ) twin pairs with PD 8 years after the initial study. Background: Longitudinal investigations increase accuracy in clinical diagnosis of PD. Methods: Follow-up by personal interview and clinical examination. Results: Concordance rates were not different between MZ (3/9) and DZ (5/12) twin pairs at follow-up, even when PD-associated dementia and isolated postural tremor were considered diagnostic of familial Lewy body parkinsonism. Evaluation of medical history, personality traits, and blink rate did not reveal putative early or premorbid parkinsonism in 9 co-twins who were motor-asymptomatic during the follow-up interval. However, these co- twins had reduced semantic fluency in comparison with a healthy control group of similar age. None of 7 co-twins without motor signs who underwent PET investigation 6 years previously showed signs of extrapyramidal disease at follow-up, but verbal memory continued to be reduced in 5 of these co-twins. Conclusion: Based on concordance rates only, the findings in our twin sample do not support a major genetic impact for the motor expression of PD.