Central Adaptation after Peripheral Vestibular Injury

Abstract

This thesis examines how the human brain adapts after peripheral vestibular injury. Vestibular perceptual function is used as a probe of cortical vestibular function. A paradigm determining vestibular perceptual thresholds to yaw axis rotation by a method of limits is described. Asymmetry in the thresholds is induced in normal subjects with galvanic vestibular stimulation. In patients with acute vestibular neuritis, perceptual thresholds were bilaterally elevated, with less asymmetry when compared to the brainstem reflexive function. Thresholds were measured in a prospective longitudinal study in vestibular neuritis patients, assessed acutely and at follow-­‐up (n=16). Assessments comprised vestibular caloric testing, visual dependency measures, questionnaire measures of symptom load, anxiety, depression and fear of body sensations. Clinical recruitment found a low rate of correct diagnoses by referring clinicians. Symptomatic outcome at follow-up was associated with increased visual dependence, asymmetric caloric function, increased anxiety and depression. It was also associated with increased fear and anxiety of body sensations present acutely, suggesting this may be predisposing. The anatomical substrate of central compensation was investigated in patients with bilateral vestibular failure (n=12) and normal controls (n=15) using functional MRI. A novel air turbine-powered vibrating device was developed to provide high and low levels of proprioceptive stimulus to neck rotator muscles. This was combined with a horizontal visual motion paradigm in a factorial design. A lateralised interaction was found in the lateral occipital visual processing areas in the avestibular patients. In addition to the known visual-vestibular interaction, this demonstrates a visuo-proprioceptive interaction, which may reflect compensation after vestibular injury. Conclusions: Vestibular perceptual function can be measured in disease, and is elevated in patients with acute peripheral vestibulopathy. Specific psychological and physiological factors associated with clinical recovery after vestibular neuritis are proposed. Functional MRI shows that proprioceptive signals interact with visual motion signals in patients with vestibular failure.