Neck proprioception and spatial orientation in cervical dystonia

Neck muscle vibration is known to influence body orientation and posture during locomotion and stance in normal subjects. To verify the hypothesis that neck proprioceptive input can be misinterpreted in patients with cervical dystonia (CD), lateral continuous vibration was applied to the sternocleidomastoid muscle during both stepping-in-place and quiet stance, with eyes closed. The orienting responses of CD patients were compared with those of normal subjects. Vibration effects on body orientation during stepping and stance were apparently different from normal, since no effects were seen when all patients' data collapsed were analysed. However, while some patients did not respond to vibratory stimuli regardless of the vibrated side, others had a 'good' side, the stimulation of which produced effects on body orientation similar to those observed in normal subjects. Homogeneous groups within the patient population were identified, based on the vibration-induced responses under stepping conditions. The different orienting or postural responses observed in CD patients were correlated with disease-related features such as spontaneous head position, maximum range of voluntary head yaw, presence or absence of a botulinum toxin treatment and disease duration. Our data suggest that, in CD patients, the reference system used in the control of body orientation in space is either refractory to the lateralized proprioceptive neck input or modified such that the input from both sides produces an orientation shift in the same sense. This would depend on the pathogenesis of the disease or on an adaptive process connected to the head abnormal posture. It seems that this refractoriness spreads to both sides of the neck with the advancement of the disease, thereby possibly entraining a progressive shift from a reference system based on the head to a more reliable egocentric reference.