Idiopathic spasmodic torticollis is not associated with abnormal kinesthetic perception from neck proprioceptive and vestibular afferences

Proceeding from recent evidence for a sensory involvement in the pathophysiology of idiopathic spasmodic torticollis (ST), we asked whether the abnormal head posture of these patients is associated with distortions of their internal spatial reference frames due to abnormal processing of neck proprioceptive and/or vestibular input. Twelve ST patients were instructed to estimate, by adjusting a light pointer in the dark, their head and trunk mid-sagittal directions (as representatives of ego-centric references) and to reproduce a remembered target location in space (space centric reference). They did so before and after horizontal head and trunk rotations, which evoked isolated or combined vestibular and/or neck stimulation. In ST patients, unlike in normal controls, pre-stimulus estimates of the head and trunk mid-sagittal directions (baselines) showed a pronounced across-subjects variability, with essentially normal mean values. Their post-stimulus estimates in all tasks, after correction for the individual baseline errors, were normal with respect to both amplitude and variability, independent of stimulus direction, modality and rotation dynamics. Our findings suggest that ST patients have a rather inaccurate knowledge of their head posture, but can effectively use neck proprioceptive input and vestibular cues when estimating head and trunk displacements in ego-centric and space centric spatial orientation tasks. We propose that an offset of a non-sensory set point signal in the neck proprioceptive loop for head-on-trunk control may be responsible for the pathological head deviation in ST.