Abnormal vibration-induced illusion of movement in idiopathic focal dystonia: an endophenotypic marker?

The frequency of symptomatic dystonia in relatives of patients with idiopathic focal dystonia (IFD) is higher than expected from epidemiologic studies implying that genetic factors may be involved. Perception of the vibration-induced illusion of movement (VIIM) is subnormal in patients with IFD compared with healthy volunteers and the abnormality corrects with volitional fatigue of the vibrated arm. The aim of the study was to establish the heritability of the abnormality of VIIM. The perception of illusion of movement elicited by vibration of the biceps brachii tendon before and after fatigue of the muscles was investigated in 30 patients with torticollis, 57 of their first degree relatives, and 19 healthy volunteers. VIIM did not change after fatigue in healthy controls. Before fatiguing the muscles, patients' perception of VIIM was less than healthy controls, (P < 0.01, unpaired t-test). After fatigue, the illusion of movement perceived by patients increased, so that it did not differ any more from that of the healthy control subjects (P < 0.05, repeated measures ANOVA). First degree relatives' response to vibration varied; 45% of parents, 60.7% of siblings, and 63.6% of children had an "abnormal" response to vibration compared with 21% of healthy volunteers. In contrast to patients' response, the "abnormality" did not correct after volitional fatigue of the vibrated arm. The results suggest that abnormal VIIM may represent an endophenotypic marker for IFD, which interacts with other factors including central motor learning and compensation mechanisms in the expression of the dystonic phenotype.