Abnormal perception of the tonic vibration reflex in idiopathic focal dystonia

Although the pathophysiological basis of idiopathic focal dystonia (IFD) remains unclear, we recently reported abnormal perception of the tonic vibration reflex (TVR) in the biceps brachii in IFD. In this study we examined whether the abnormality affects muscles other than the biceps brachii. A 100-Hz vibration stimulating predominantly the muscle spindle afferent was transcutaneously applied to one muscle tendon of the triceps brachii, the wrist extensor and flexor muscles in 29 subjects with IFD (18 with torticollis, 9 with writer's cramp, 2 with blepharospasm) and 15 control subjects. The blindfolded subjects were instructed to copy any perceived movement with the opposite tracking arm. The elbow or wrist angle changes were quantified electronically. The TVR and subjects' perception of the TVR were evaluated by angle movements of the vibrated joint and of the tracking joint, respectively. Perception of the TVR was significantly reduced in dystonic subjects at both elbow and wrist joints, while magnitude of the TVR did not differ between the two groups. Abnormal central perception of the TVR is a feature of IFD, suggesting a widespread involvement of abnormal muscle spindle afferent processing in IFD.