Intraoperative neurophysiological monitoring of the ocular vestibular evoked myogenic potential (oVEMP): a novel implementation to detect vestibular and oculomotor pathway dysfunction during brainstem surgery

Walsh, Peter (2022) Intraoperative neurophysiological monitoring of the ocular vestibular evoked myogenic potential (oVEMP): a novel implementation to detect vestibular and oculomotor pathway dysfunction during brainstem surgery. Doctoral thesis (DClinSci), Manchester Metropolitan University.

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Abstract

Different intraoperative neurophysiological monitoring techniques can be used to assess the functional integrity of the brainstem during neurosurgical procedures that put the delicate neurovascular structures of the brainstem at risk. Whilst each individual technique has its benefits and limitations, multimodal monitoring can offer a near real-time comprehensive assessment of the neural pathways under investigation. However, not all the pathways that are at risk during tumour resection are able to be assessed with the currently available techniques. The pathways that convey the vestibulo-ocular reflex, which is responsible for stabilisation of a steady visual image on the retina during movement of the head and body can be disrupted during surgery to remove tumours within the cerebellar-pontine fossa. This disturbance in transmission can lead to disabling gaze deficits and vestibular disturbances. The ocular vestibular evoked myogenic potential (oVEMP) that can be recorded from the contralateral inferior oblique muscle after air conducted stimulation describes the excitation of this reflex. The oVEMP is mediated via the ascending utricle fibres that connect to the ipsilateral vestibular nuclei and project, via a crossed reflex pathway in the medial longitudinal fascicule in the brainstem, to the contralateral oculomotor nucleus to elicit an excitatory muscle response. Objective: To determine if oVEMPs could be recorded intraoperatively. To investigate whether changes in amplitude and/or latency correlated with post-operative vestibular pathway dysfunction in those patients undergoing cerebellopontine angle surgery. Methods: This observational study incorporated 37 patients who were monitored neurophysiologically during brainstem surgery with additional oVEMPs recordings. The sensitivity and specificity of the amplitude decrement of the oVEMP was determined. Results: Intraoperative oVEMPs were able to be recorded in 31 patients. 21/22 patients who did not show any changes in the oVEMP did not experience any immediate or long term relevant clinical deficit. Eight patients showed immediate post-operative vestibulo-ocular dysfunction with seven of them showing accompanying oVEMP changes. The sensitivity, specificity, and positive and negative predictive values for the oVEMPs to detect vestibular ocular dysfunction in the immediate post-operative period were 75%, 91.3%, 75% and 91.3% respectively, with a test accuracy of 87.1%. Conclusion: Changes in the oVEMP amplitude <50% (which did not reverse), or loss of the potential intraoperatively, predict post-operative deficits of the vestibulo-ocular pathway. Significance: The oVEMP is able to be recorded in patients undergoing brainstem surgery to monitor the integrity of the vestibular portion of the eighth cranial nerve, the medial longitudinal fascicule within the brainstem and the oculomotor cranial nerve pathway. Monitoring of this potential intraoperatively may enhance the safety of cerebellar pontine angle surgery.