Intraoperative monitoring during surgery for hypoglossal schwannoma.

A 54-year-old man presented with an intracranial schwannoma of the hypoglossal nerve between the medulla and the left hypoglossal canal. The condylar fossa approach was used with intra-operative electromyography (EMG) monitoring of the lower cranial nerves. The tumor was then removed carefully without decreasing the tongue EMG responses. EMG monitoring enabled us to remove the tumor while maintaining the function of the hypoglossal nerve. Tongue EMG was easily recorded by stimulating the hypoglossal nerve fibers, which was useful in identifying the hypoglossal nerve and evaluating its function. This suggests that tongue EMG is a useful monitoring tool to enhance neurological outcome following removal of tumors in this region.

Clipping of a vertebral artery aneurysm behind the hypoglossal nerve under the monitoring of lower cranial nerves.

Under an operative view, an aneurysm of the vertebral artery is located behind the lower cranial nerves. To prevent neurological deficits we employed electrophysiological monitoring while clipping an aneurysm of the vertebral artery. A 64-year-old woman had suffered a sudden severe headache in the morning. Computed tomography (CT) revealed a subarachnoid hemorrhage (SAH) and CT angiography revealed an aneurysm at a branching point of the left vertebral artery. The condylar fossa approach was taken while recording electromyography (EMG) of the lower cranial nerves. The aneurysm was located just behind the hypoglossal nerve and could not be clipped without strong traction of the hypoglossal nerve. Therefore, the hypoglossal nerve was divided to separate the lower two bundles of the hypoglossal nerve from the other bundles, and the clip was applied to the aneurysm between the nerve bundles without any change of the tongue EMG. The patient went home 10 days after operation with no neurological deficit. In conclusion, we report a case of a ruptured aneurysm of a vertebral artery, which was clipped while monitoring the lower cranial nerves. Tongue EMG monitoring enabled us to clip the aneurysm without nerve injury and revealed that the hypoglossal nerve near the hypoglossal canal can be divided into several bands without neurological deficit.

[Evoked potential monitoring in an operation of neurosurgery].

In the neurosurgical field, the evoked potential is employed for the monitoring of intraoperative nerve function. During evoked potential monitoring, surgical manipulation-related nerve dysfunction is detected, and functional localization/nerves in the cerebral cortex are identified to prevent postoperative neurological complications. It is important to reduce the contact resistance to 2 kOmega or less on the application of plate electrodes used for recording and prevent noise by bundling electrode leads, to ensure a stable evoked potential during surgery. In our laboratory, intraoperative monitoring, such as ABR to prevent auditory disturbance, SEP to detect cerebral ischemia, cortical SEP and MEP to prevent motor paralysis, and evoked electromyography to identify/maintain the cranial nerves including the facial, trigeminal, oculomotor, and abducens nerves, is performed based on requests from the Department of Neurosurgery.