inevitably be lost. Hearing will be preserved if both waves remain unaffected.

Cranial Nerve Monitoring

The most extensive experience with brainstem and cranial nerve monitoring has come from procedures involving the CPA. Various sensory and motor functions of the cranial nerves can be monitored in an attempt to preserve function or to assist intraoperative decision making. As previously, sensory nerves are typically monitored with evoked potentials and motor nerves with EMG recordings. The theoretical rationale for monitoring spontaneous EMG activity relies on the property that thermal, mechanical or metabolic irritation of the intracranial portion of a cranial motor nerve will lead to a predictable and measurable activity in the innervated muscle.

Logistically, intraoperative EMG monitoring of muscles innervated by the various cranial nerves is relatively simple. Needle insertion into the appropriate muscle is preferable to surface electrode placement for increasing the sensitivity and specificity of the system. Intramuscular electrodes increase the sensitivity of detection for spontaneous EMG activity, while surface electrodes are more appropriate for the assessment of compound muscle action potentials (CMAPs). Most systems amplify and convert the muscle action potential to audible signals that are immediately available to the surgical team. A variety of probes are available for the purposes of stimulation. Both constant current and constant voltage stimulation paradigms exist, and both have been used effectively and safely. Monopolar and bipolar stimulating electrodes are available, with the latter providing more focal stimulation. Finally, appropriate communication with the anesthesiologist is crucial, as EMG potentials will be abrogated by significant neuromuscular blockade.

The efficacy of facial nerve monitoring in reducing post-operative facial palsy during CPA tumor surgery is well established. Monitoring during vestibular schwannoma resection is particularly crucial, as up to 78% of these cases involve an impairment of the facial nerve. Facial nerve activity is usually recorded from the ipsilateral frontalis, orbicularis oculi, orbicu-

laris oris, and/or mentalis muscles. EMG responses have been classified as either spontaneous or evoked. Spontaneous activity is common with the onset of monitoring, generally characterized by low-amplitude, low-density unit potentials presenting as steady trains or small repetitive bursts. Evoked responses, which are more common, are further subdivided into three patterns. "Pulse patterns" result from purposeful stimulation of the facial nerve with the stimulating probe and have a frequency identical to that generated by the stimulator. A second response, the "burst pattern", results from mechanical, chemical or thermal stimuli. Such alterations of nerve firing occur soon after the inciting event, with the observed pattern consisting of short (<1 s) bursts of synchronous motor activity. The final type of activity, known as the train pattern, presents as groups of asynchronous discharges with durations of several minutes. These potentials may have a latency of seconds to minutes after the aggravating event occurs. Traction on the nerve is usually the causative event, but chemical or thermal irritation may also be responsible. Activity may also be seen if cold irrigation fluid is used. The train pattern is most concerning as an indicator of potential or current damage to the nerve; the effect is typically delayed and outlasts the stimulus that incited it.

Post-operative function of the facial nerve can be predicted based on stimulation studies performed prior to wound closure. Inability to produce facial motion with high current stimulation is predictive of post-operative paralysis. When stimulation of more than 3.0 mA in a constant voltage setting is required to produce facial movement, post-operative palsy of the seventh cranial nerve is expected. However, potential for recovery exists if the nerve is intact. Good movement induced by 0.05-0.2 mA at the brainstem has been correlated with normal or minimal paresis of the facial nerve. Using a constant current setting, elevation of stimulus threshold to 0.2 or 0.3 V following the procedure is usually associated with good postoperative function of the nerve. However, threshold potentials greater than 0.3 V may be indicative of post-operative loss of function.

Antidromic stimulation of the facial nerve is also possible via the use of a hand-held electrode placed in the surgical field, with nerve stimulation at the stylomastoid foramen. This

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