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

Post-operative neurological deficits after surgical resection of brainstem lesions can have a devastating effect on the patient’s quality of life. During the removal of posterior fossa masses the normal anatomical structures and landmarks are distorted and the normal course of the cranial nerve are displaced and can become embedded within the infiltrating mass. Whilst the operative mortality has decreased, and the survival rates have increased, many survivors will still experience significant impairments in neurological function post-operatively. During the removal of brainstem tumours excessive manipulation can cause stretch and compression of neural structures by inadvertent retraction required to expose the mass and can cause permanent dysfunction of the adjacent neuronal tissue. Coagulation of the mass can cause ischaemic changes to major vessels and perforating vessels that are unable to be recognised even under microscopic observation. The implementation of intraoperative neurophysiological monitoring during these procedures can afford the surgeon, and their patients, the benefit of continual assessment of the functional integrity of cranial nerves, their brainstem nuclei and the white matter tracts, to minimise the risk of post-operative deficit. Although the use of intraoperative neurophysiological monitoring has been of an oncological (enabling extensive resection) and neurological (preserving function) benefit to the patient, not all the intrinsic and vital brainstem pathways and their reflex activities are currently able to be monitored intraoperatively. In particular the oculomotor system remains inaccessible to real time monitoring. The ocular vestibular evoked myogenic potential (oVEMP) is an excitatory muscle potential that can be recorded non-invasively from electrodes placed beneath the eye that record the response from the contralateral inferior oblique muscle and is used in the out-patient setting to record and interpret the function of the oculomotor reflex pathways, but they have not been attempted intraoperatively. oVEMPs can be recorded after brief stimulus that is delivered via the ears, the nerveim pulse travels along the nerve that controls balance (the vestibular portion of the eight cranial nerve), it then passes up through the brainstem (via the medial longitudinal fascicule) to connect onto the cranial nerve that controls movement of the eyes (the third cranial nerve). However, the development and implementation of ocular vestibular evoked myogenic potential recordings may give the patients the assurance that as much as possible is being done for them at the time of surgery so that vestibular and oculomotor function is preserved.