Facial Nerve

Diagrammatic representation of electrode placement for facial nerve (FN) monitoring. Paired needle electrodes are placed in the ipsilateral orbicularis oculi (A) and in the orbicularis oris muscles (B); needles should be advanced at a relatively sharp angle because the muscle layer is thin and superficial. Another paired needle electrode should be hooked to the contralateral orbicularis oris (C) or the trapezius (D) even if the FN is the only nerve at risk to provide a valuable control for nonspecific increases in EMG activity due to light anesthesia or other nonsurgical factors. A monopolar needle electrode is placed on the shoulder as a signal ground (E)

Notes:

Electrical stimulators in clinical use deliver rectangular pulses, the amplitude and duration of which vary within a wide range. Our preference is to use monopolar constant voltage stimulation, delivering pulses of 0.2 ms duration at a rate of 5–10/s. With these parameters, the threshold for an evoked EMG response from normal nerves is usually between 0.05 and 0.2 V, averaging about 0.1 V (Yingling and Ashram, 2005a).

Electrical stimulation can be used to distinguish between the facial nerve and other cranial nerves such as the trigeminal, vestibulocochlear and to a lesser extent the abducens and lower cranial nerves.

Stimulationof the motor fibers of the trigeminal nerve produces EMG responses in the masseter and temporalis muscles.

Stimulation of the trigeminal nerve produces EMG responses that are of a considerably shorter latency (3–4 ms to onset) than those to FN stimulation (6–8 ms), allowing these nerves to be distinguished (Moller, 1995).

Electrical stimulation can be used to evaluate the integrity of the facial nerve after the occurrence of an alarming episode of spontaneous activity during dissection. This evaluation requires a baseline measurement of threshold and amplitude proximal to the site of dissection. A change of threshold or amplitude from baseline after an episode of spontaneous activity is an indication that manipulation has caused a change in the functional status of the nerve. The use of electrical stimulation during dissection of large CPA tumors is an important adjunct to spontaneous EMG to give information about the condition of the nerve. In large tumors, axons of the FN can become significantly stretched and partially damaged, and thus be less likely to respond than healthy ones, producing little EMG when manipulated (Moller, 1995). In such cases, the absence of spontaneous EMG activity should not be relied upon as an indication of lack of mechanical irritation to the FN during dissection. Rather, electrical stimulation of the nerve should be used to assess its functional status by comparing responses with a previously recorded baseline (Yingling and Ashram, 2005a).

Illustrative examples of volume conducted activity on facial nerve (FN) monitoring channels that can be mistaken for FN responses.

Upper trace shows response from temporalis muscle after stimulation of the trigeminal nerve; middle trace is the same response (trigeminal nerve stimulation) recorded simultaneously on the orbicularis oculi channel. Note the short latency of the two responses (3 ms). Lower trace shows a response to FN stimulation on orbicularis oculi channel from the same patient (latency 6.2 ms). The trigeminal and FN responses can be differentiated by their latency and the use of multiple channels.

Responses on the orbicularis oris to stimulation of the nervus intermedius (upper) and FN (lower). Nervus intermedius response can be recognized by its longer latency, smaller amplitude, and higher threshold for stimulation.

A volume conducted response on the orbicularis oculi to stimulation of the abducens nerve (upper) compared to a FN response recorded from the same muscle (lower). Note the lower amplitude and shorter latency of the response to abducens nerve stimulation; the onset of the response is not clearly demarcated from the stimulus artifact. The vertical lines indicate the point of stimulation.

The nervus intermedius, or intermediate nerve, is the part of the facial nerve (cranial nerve VII) located between the motor component of the facial nerve and the vestibulocochlear nerve (cranial nerve VIII). It contains the sensory and parasympathetic fibers of the facial nerve. Upon reaching the facial canal, it joins with the motor root of the facial nerve at the geniculate ganglion.

Cranial Nerve VII - Facial Nerve- The facial nerve has four components with distinct functions.Branchial motor fibers constitute the largest portion of the facial nerve. The remaining three components are bound in a distinct fascial sheath from the branchial motor fibers. Collectively these three components are referred to as the nervus intermedius.

Brancial motor (special visceral efferent)

Supplies the muscles of facial expression; posterior belly of digastric muscle; stylohyoid, and stapedius.

Visceral motor
(general visceral efferent)

Parasympathetic innervation of the lcrimal, submandibular, and sublingual glands, as well as mucous membranes of nasopharynx, hard and soft palate.

Special sensory
(special afferent)

Taste sensation from the anterior 2/3 of tongue; hard and soft palates.

General sensory
(general somatic afferent

General sensation from the skin of the concha of the auricle and from a small area behind the ear.