Impaired EMG inhibition elicited by tendon stimulation in dystonia

Objective: To test the effects of tendon stimulation on isometric voluntary contraction. Methods: Twenty patients with dystonia (12 patients with generalized and eight with task-specific dystonia) and 10 normal healthy subjects participated in the study. The tendon of the extensor carpi radialis muscle was stimulated with electrical stimuli at the wrist, and the electromyogram (EMG) signal was recorded during an isometric voluntary contraction. Results: In normal subjects, tendon stimulation elicited an excitatory phase (TE1), followed by a pronounced inhibitory phase (TI1) and a second excitatory phase (TE2). The three phases had similar perceptive thresholds, latencies, and durations in patients and control subjects. In patients with generalized dystonia, the TI1 area exceeded the control values (controls [mean +/- SE], 40.3 +/- 5.4; patients, 66.9 +/- 5.5; p = 0.0048, Mann-Whitney U test). In the patients with task-specific dystonia, the TI1 area was similar to control values (controls [mean +/- SE], 40.3 +/- 5.4; patients, 54.2 +/- 4.8; p = 0.7396, Mann-Whitney U test). Conclusions: The EMG suppression (TI1) after tendon stimulation is reduced in generalized dystonia, indicating a decreased group III-elicited presynaptic inhibition of Ia fibers. The impaired group III presynaptic inhibitory action from tendon afferents could contribute to the motor abnormalities present in dystonia. Dystonia causes widespread dysfunction of presynaptic inhibitory mechanisms in the spinal cord, involving Group I and III afferents.