The effect of changes in head posture on the patterns of muscle activity in cervical dystonia (CD)

Twelve patients with cervical dystonia (CD) and predominant rotation were studied to determine the effects of changes in head posture on the specific patterns of cervical muscle activity. Turns analysis was used to quantify muscle activity underlying head rotation, recorded simultaneously from the agonist and antagonist muscle pairs bilaterally (sternocleidomastoid [SCM] and splenius [SPL]). Muscle activity was compared between the uncompensated dystonic posture and during the maintenance of midposition. In addition, patients were separated into two groups (geste = 6; no geste = 6) based on whether they had a clinically efficacious geste to determine the effect of geste on patterns of cervical muscle activity. Muscle activity was measured during the maintenance of midposition with and without a clinical or simulated geste. Differences in muscle activity between the groups and postures were compared using repeated measure analysis of variance (ANOVA) analyses. The four muscles tested showed a significant difference in muscle activity in the uncompensated dystonic posture as a result of the increased activity in the agonist muscle pair (SCM and SPL responsible for the dystonic posture) (EMG amplitude: F[1,11] = 18.81, p = 0.0012; EMG frequency: F[1,11] = 32.07, p = 0.0001). Maintaining the head in the midposition was associated with a significant reduction in muscle activity compared with the uncompensated dystonic posture (EMG amplitude: F[1,9] = 6.36, p < 0.033; EMG frequency: F[1,9] = 10.96, p < 0.0091). This reduction in midposition muscle activity was significantly greater in the agonist muscle pair (EMG amplitude: F[1,10] = 19.70, p = 0.0013; EMG frequency: F[1,10] = 44.67, p < 0.0001). In the patients with clinically effective geste, there was no additional reduction in muscle activity observed in the midposition when they performed their geste (EMG amplitude: F[1,9] = 4.63, p = 0.060; EMG frequency: F[1,9] = 1.22, p = 0.298). These findings suggest that CD with rotation is characterized by predominantly increased agonist muscle activation. Patients with CD retain the ability to modulate this involuntary agonist muscle activity to maintain the head in the midposition. The maintenance of the midposition does not seem to be facilitated by geste.