Modulation of reflex responses in hand muscles during rhythmical finger tasks in a subject with writer's cramp

The objective of this study was to examine phase- and task-dependent modulation of stretch reflexes during repetitive finger movements in writer's cramp, and compare them with normal controls from our previous study. A subject with writer's cramp conducted two rhythmic tasks, index finger abduction (RFA) and a pen-squeezing (RPS) task akin to handwriting. Stretch reflexes were evoked by mechanical perturbations at random phases of each task. Surface electromyograms (EMG) were recorded from two hand muscles, first dorsal interosseous (FDI) and flexor digitorum superficialis (FDS). The reflex response and background EMG activity of each muscle were modulated in a phase-dependent manner in both tasks. However, they varied largely in phase during the RFA task, but in approximately inverse phase-relationship during RPS. Reflex sensitivity, as represented by the slope of the linear regression between response and background, was much lower for both muscles in the 'writing' task (RPS) than in the RFA task with its positively correlated responses. These phase- and task-related modulation patterns differed dramatically from those observed in our control subjects, where reflex responses were modulated largely in phase with background activity and reflex sensitivity was much higher, particularly in FDI during RFA and FDS during RPS. The altered reflex modulation patterns in writer's cramp may reflect deficiencies of integration of proprioceptive afferent inputs and reduced inhibition at cortical and spinal levels during writing performance. Results from this case study support clinically identified task-specific feature of focal hand dystonia.