Chapter 74 Vestibulo-spinal neurophysiology in spasmodic torticollis

Abstract

Publisher Summary Spasmodic torticollis (ST) is a neurological movement disorder, causing involuntary repetitive or sustained neck muscle contractions, leading to abnormal head and neck postures and impaired head movement control. Control of head postures and movements is modulated by afferent somatosensory, visual, and vestibular input from the neck. Because the vestibular system seems to play a role in stabilizing the head, it has been hypothesized that the vestibular system and its projections are implicated in the pathophysiology of ST. Imbalance in the vestibular system and its connections can produce abnormal head and neck postures under experimental conditions. Several deficits in vestibular reflexes have been described in these patients. The vestibulo-ocular reflex shows a directional preponderance with the slow phase of the nystagmus induced by rotational and caloric stimuli being more active ipsilateral to the chin rotation. This abnormality in ST patients, indicating a tonic bias of neural activity, affecting cervical muscles and vestibuloocular balance in the same direction persists, if the head is fixed during vestibular stimulation or when the head position is normalized following botulinum toxin injections. Thus, although there are vestibulo-ocular reflex abnormalities, the vestibulo-collic reflex (VCR), that is important for head stabilization in space, seems to be unimpaired in ST. However, in ST patients there seems to be a problem of integrating vestibular reflexes with voluntary neck movement. This chapter discusses abnormal vestibulo-voluntary interaction and abnormal postural responses to neck muscle vibration. There is evidence that dysfunction of the basal ganglia plays a role in ST patients. ST can be observed either in isolation or as a part of a more wide-spread hemidystonia after lesions of the basal ganglia or its connections. ST bilateral pallidal stimulation has been reported to be effective in improving symptoms in patients with complex ST not responding to other treatments.