Reappraising the role of motor surround inhibition in dystonia.

BACKGROUND: Surround inhibition (SI) in the motor system has been described to be decreased in patients with focal hand dystonia (FHD) but no evidence currently exists for patients with cervical dystonia (CD). OBJECTIVE: To characterise the SI profiles in three groups of participants: healthy volunteers, patients with FHD and patients with CD. To provide sample size calculations for future studies. METHODS: SI was assessed using Transcranial Magnetic Stimulation (TMS) in 31 right-handed healthy participants, 11 patients with CD and 12 patients with FHD. In addition data of SI in patients with FHD were extracted from previously published and analysed for sample size calculations and assessment of SI variability. RESULTS: No statistically significant difference in SI was found amongst the groups (healthy, FHD, CD). Analysis of combined current and previous data suggests that our study and all prior studies were underpowered. At least 26 participants in each group are required for a simple comparison of two groups. Analysis of published data indicated that SI is more variable in FHD patients compared to healthy controls. CONCLUSIONS: The highly variable SI in patients with dystonia can confound statistical comparisons of mean differences. Larger studies are needed to assess SI in dystonia and to explore the origins of its variability.