Motor overflow and mirror dystonia

Motor overflow is an unintentional muscle contraction which accompanies, but is anatomically distinct from the primary dystonic movement. This phenomenological nosology has not been systematically studied in focal hand dystonia (FHD). We conducted a prospective, case-control study to characterize motor overflow and mirror dystonia in patients with FHD. We compared the performance of 30 patients with FHD and 40 healthy controls on a variety of motor tasks, such as writing, drawing a spiral, straight line and a sine wave, repetitive wrist flexion–extension, finger tapping, hand grasping, hand pronation–supination, and a finger-to-nose task with each hand. The assessments were videotaped, the edited video segments were randomized, and an independent investigator who was “blind” to the subject's diagnosis rated the ipsilateral and contralateral overflow and mirror dystonia twice, 6 months apart. Using the mean of the two ratings, ipsilateral overflow was identified in 8.5 ± 2.1 (28%) patients and in 1.5 ± 0.7 (4%) controls (p < 0.001), contralateral overflow in 2.5 ± 0.7 (8%) patients and in 1.5 ± 0.7 (4%) of controls (p = 0.138), and mirror movement in 20.0 ± 0.0 (67%) of patients and in 15.5 ± 4.9 (39%) of controls (p = 0.001). There was a statistically significant correlation of dystonia and overflow score (Pearson's r 0.713, p < 0.001). The relatively high frequency of ipsilateral overflow and mirror dystonia in patients with FHD has both pathophysiological and therapeutic implications. In this study, the severity of dystonia was significantly correlated with motor overflow in multiple tasks.