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 w ere studied to determine the effects of changes in head posture on the specific patterns of cervical muscle activity. Turns analysis was use d to quantify muscle activity underlying head rotation, recorded simul taneously from the agonist and antagonist muscle pairs bilaterally (st ernocleidomastoid [SCM] and splenius [SPL]). Muscle activity was compa red between the uncompensated dystonic posture and during the maintena nce of midposition. In addition, patients were separated into two grou ps (geste = 6; no geste = 6) based on whether they had a clinically ef ficacious geste to determine the effect of geste on patterns of cervic al muscle activity. Muscle activity was measured during the maintenanc e of midposition with and without a clinical or simulated geste. Diffe rences in muscle activity between the groups and postures were compare d using repeated measure analysis of variance (ANOVA) analyses. The Fo ur muscles tested showed a significant difference in muscle activity i n the uncompensated dystonic posture as a result of the increased acti vity in the agonist muscle pair (SCM and SPL responsible for the dysto nic posture) (EMG amplitude: F[1,11] = 18.81, p = 0.0012: EMG frequenc y: F[1,11] = 32.07, p = 0.0001). Maintaining the head in the midpositi on was associated with a significant reduction in muscle activity comp ared 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 redu ction in midposition muscle activity was significantly greater in the agonist muscle pair (EMG amplitude: F[1,10] = 19.70, p = 0.0013; EMG f requency: F[1,10] = 44.67, p < 0.0001). In the patients with clinicall y effective geste, there was no additional reduction in muscle activit y observed in the midposition when they performed their geste (EMG amp litude: 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 r etain the ability to modulate this involuntary agonist muscle activity to maintain the head in the midposition.The maintenance of the midpos ition does not seem to be facilitated by geste.