38.1 Somatosensory Evoked Potentials and Electromyography

The major risks of MVD are neurological injury to CN V, VII and, especially, VIII. An added concern is nerve injury from the lateral decubitus patient positioning most often used for the surgery. Such positioning allows for easier access to the retromastoid area. The cerebellum becomes displaced downward under gravity, requiring reduced retraction. However, this position may result in the compression of the contralateral arm, axilla and brachial plexus. ¹ SSEP monitoring is widely used for detection of potential iatrogenic injury to neuronal structures. We monitor SSEPs before and after patient positioning to assess any potential changes to baseline waveforms and inform the surgeon of any necessary adjustments in positioning. SSEPs also detect global systemic changes such as body temperature, anesthetic equilibrium, and blood flow to the extremities and to vital neural structures (▶ Fig. 38.1).

During the procedure, retraction around the nerves, ischemia, and heat from electrocautery puts CN V and VII at risk of injury. Functional integrity of the motor portions of these CNs is evaluated by free-running electromyography (EMG) that detects real-time nerve activity from surgical manipulation. Triggered EMG with a hand-held probe also provides additional nerve health information to the surgeon.

To monitor free running EMG, two subdermal needle electrodes are placed in each muscle innervated by the respective CNs: the masseter/temporalis for the TN and the following muscles for the facial nerve: frontalis, orbicularis oculi, orbicularis oris, and mentalis (▶ Fig. 38.2a). Iatrogenic EMG activity can be classified, based on the response amplitude and duration, as spikes, bursts, and trains. Three types of trains, in decreasing severity, have been identified: A-trains are the most clinically significant and are usually associated with post-operative facial paresis. ² They have a distinct sinusoidal waveform of high frequency sound, short or long duration, and amplitudes ranging from 100–200 µV (▶ Fig. 38.2c).