Parkinson disease: Handedness predicts asymmetry

Objective: To determine the proportion of individuals in a clinic-based setting that present with asymmetric Parkinson disease (PD) and identify predictive factors associated with asymmetric symptoms. Methods: The authors examined right vs left difference scores on the Unified Parkinson Disease Rating Scale motor subscale in a consecutive clinical series of 1,277 individuals diagnosed with PD. Predictors of asymmetry included sex, symptomatic disease duration, age at onset, initial motor symptom laterality, handedness, and medical history variables (e.g., family history of PD). Results: Nearly half the sample (46%) met criteria for asymmetric disease based on a right vs left difference score of ≥5 points, and 12% of the sample had a difference score of ≥10 (difference score: mean = 4, SD = 3.4). All three cardinal features of PD showed characteristics of asymmetric disease presentation. Multiple regression analyses showed that an increased discrepancy between right- and left-sided symptoms was significantly associated with a shorter disease duration, younger age at symptomatic onset, asymmetric initial symptom onset, hand dominance, and a positive self-reported family history of "other" neurodegenerative disorder. Hand dominance was related to the side of asymmetric disease such that left-handed individuals tended to have more severe disease on the left side of the body. Conclusion: Asymmetric presentation of Parkinson disease features was a common occurrence in the clinical cohort. Asymmetry was reliably predicted by several clinical characteristics, although the moderate level of explained variance (i.e., between 16 and 23%) highlighted the need for additional research examining predictive models of asymmetric disease. Recommendations for the classification and measurement of asymmetric disease are discussed.