Side onset influences motor impairments in Parkinson disease.

In right-handers, the left hemisphere has greater ipsilateral control of the upper extremities than the right hemisphere. This asymmetry is not as profound among left-handers. We sought to determine whether Parkinson disease (PD) maintains this pattern of ipsilateral control and affects motor impairment. Using right- and left-hand sub-scores from the Unified Parkinson Disease Rating Scale (UPDRS), we calculated a lateralized impairment score to compare ipsilateral influence among four groups: right-handed-right-sided onset (RH-RSO), right-handed-left-sided onset (RH-LSO), left-handed-right-sided onset (LH-RSO), and left-handed-left-sided onset (LH-LSO). We hypothesized that right-handed patients with RSO (left hemisphere dysfunction) would have lower scores, hence more deficits in both hands, than the other groups. Among right-handers, the RSO patients (left dominant hemisphere dysfunction) had a significantly lower mean lateralized impairment score than LSO patients (micro = 2.09 versus micro = 4.06; p < 0.001). Further, RHRSO also had lower scores than both the LHLSO (micro = 4.52, p < 0.05) and LHRSO (micro = 4.27, p < 0.05) groups. The symmetry of impairments indicates deficits in both hands and supports more ipsilateral involvement of the affected left hemisphere in RH-RSO patients. Therefore, we suggest that PD does retain an asymmetric ipsilateral influence. The low numbers of left-handed subjects and the significant percentage of left-handers with a dominant left hemisphere prevented a clear interpretation of our findings in left-handed subjects.