Neurophysiological Characteristics of Cranial Nerve XII EMG in Endoscopic Endonasal Approach Skull Base Surgery: Technical Report

Objective: Neurological deficits due to damage to cranial nerves (CNs) or surrounding neurovasculature are a risk of endoscopic endonasal skull base surgery (EEA). Damage to the hypoglossal nerve (CN XII) can cause difficulty speaking, eating, or breathing, impacting quality of life. Intraoperative neurophysiological monitoring with electromyography (IOM) can assist in identification and protection of CNs during surgery. Changes in induction threshold on triggered EMG (t-EMG) may warn of risk of CN damage. However, normative data on latency and amplitude of t-EMG responses during EEA has not been described. The goal of this study is to describe normative t-EMG response data to reduce neurological deficits in CN XII after endoscopic endonasal approach neurosurgery at the skull base.

Methods: We retrospectively reviewed EEA procedures performed at the University of Pittsburgh Medical Center between 2009 and 2015. T-EMG was recorded using subdermal needle electrodes placed on the tongue and stimulated using a monopolar stimulator with voltage intensity ranging from 0.5 to 2 V. The sensitivity, time base, and bandwidth were established at 50 µV/division, 5 ms/division, and 3 Hz to 1 KHz for recording the t-EMG responses. T-EMG waveforms were extracted and analyzed for time to response onset and degree of muscle contraction measured via peak amplitude difference of waveforms. Medical records for all patients were reviewed to determine if any new neurologic deficits were present postoperatively.

Results: Eighteen patients aged 5 to 74 (average age: 47 ± 19 years) underwent EEA with IOM of CN XII. 26 total nerves were monitored. 11 (61%) had neurological deficits before surgery. 2 (11%) had new postoperative neurological deficits. There were 5 cases (28%) of preoperative CN XII deficit and 3 cases (17%) of post-operative CN XII deficit, none of which were new. T-EMG data and findings are provided in Table 1. Notably, peak to peak amplitude was observed to increase with increasing stimulation voltage, an expected result consistent with CN neurophysiology.

Conclusion: Waveforms recorded from t-EMG of CN XII during EEA have recognizable and consistent characteristics. We recommend neurophysiologists consult these normative values when monitoring CN XII in EEA to ensure accuracy of recordings. This may help neurosurgeons identify and protect CN XII during endoscopic endonasal skull base surgery.

Table 1