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measured by the latency of the first onset of EMG potential . Similar monitoring techniques can be applied to extra-ocular nerves by placing the needle electrodes in the appropriate muscle. For the trigeminal nerve, monitoring can be performed by placing the electrodes in the temporal muscle. Similarly, electrodes can be applied to the accessory nerve over the sternocleidomastoid muscle. The fifth through twelfth cranial nerves can be recorded in this manner. The surgeon must exert caution, however, when stimulating nerves supplying large muscles because strong contractions may lead to gross movement when perfect steadiness is required for intracranial surgery.
All recordings are perforrned using subdermal needle electrodes. Electrodes that are not in the operative field, but that are on the scalp and not accessible during surgery, are either sutured or stapled in place. Electrodes on the face, which are placed for recording EMG activity, are taped in place. Electrodes in the operative field are placed by the surgeons using sterile technique, usually early in the procedure. The electrodes are checked for impedance values and are accepted if the impedance is less than 10,000 ohms. If electrodes fail the impedance test and all connections are intact, the faulty electrodes are identified and replaced. In all cases, every effort is made to reduce noise and artifact to obtain a robust and consistent potential.
Baseline responses are obtained before draping the patient and compared to the preoperative evaluation. Significant differences must be accounted for, since signal deterioration may be due to patient positioning. The Cranial nerve function is monitored continuously during skull base surgery for two reasons: first, to establish the location and orientation of the cranial nerves in the operative field; and second, to preserve functioning in the cranial nerves and their related brain stem nuclei.
The major observed variables are the electromyograms (EMGs) from the appropriate muscle group innervated by the cranial nerves of interest. The cranial nerves, along with the associated muscle groups, that are usually monitored using EMG techniques are: the facial nerve (VII), and the orbicularis oculi, oricularis oris, and the mentalis muscles innervated by the zygomatic branch, the buccal branch, and the mandibular branch, respectively; the abducens nerve (VI) and the lateral rectus muscle; the trigeminal nerve (V) and the masseter muscle; the trochlear nerve (IV) and the superior oblique muscle; and the oculomotor nerve (III) and the medial and inferior rectus, and inferior oblique muscles of the eye. When appropriate, the functioning of the glossopharyngeal (IX), vagus (X), the spinal accessory (XI), and the hypoglossal (XII) cranial nerves are monitored by placing electrodes in the stylopharyngeus, the cricothyroid, the trapezius, and the intrinsic muscles of the tongue, respectively. In general, the cranial nerves ipsilateral to the operative side are monitored; however, when appropriate, bilateral activity is monitored.
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