Cranial Nerves

Facial Nerve

(Fig. 2-1)

Figure 2-1. Anatomy of the facial nerve.

Recording Electrode Placements (Fig. 2-2, Fig. 2-3)

Figure 2-2. Placements of recording electrodes and stimulator for facial nerve motor conduction.

Figure 2-3. Recording electrode placements in the nasalis muscle: E1 is active electrode and E2 is reference electrode.

E1 The surface recording electrode (E1) [10 millimeters (mm) in diameter] can be placed over the orbicularis oculi, nasalis, orbicularis oris, and frontalis muscles. When recording the motor response from any of the facial muscles, an initial positive deflection of compound muscle action potential (CMAP) is very common, but recording from the nasalis may yield the best result.
E2 The same type of E1 electrode is placed on the same muscle on the contralateral side of the face (E2).

Ground Place ground over the forehead or shoulder area.

Electromyography (EMG) Machine Settings

Filters: 2 Hz to 10 kHz
Sweep speed: 2 milliseconds (ms) per division (div)
Sensitivity: 500 microvolts (μV) to 1 millivolt (mV) per div
Current pulse duration: 0.2 ms

Stimulation of Facial Nerve (Fig. 2-5)

Preauricular stimulation: The stimulating cathode is placed over the anterior tragus in front of the ear. A volume-conducted response may occur with direct stimulation of the masseter muscle, but it can be minimized with the anode placed proximally to the cathode. We prefer stimulation at this site.
Postauricular stimulation (Fig. 2-5): The cathode is placed at the stylomastoid foramen, just behind and posterior to the lower ear and anterior to the mastoid process.

(Fig. 2-4) To supramaximally activate the normal facial nerve on pre- and postauricular stimulation, a moderate degree of current intensity is required. The stimulation is painful. We start the stimulation with 40 to 50 milliamperes (mA) with current pulse width 0.2 ms and monitor the CMAP amplitude

Only gold members can continue reading. Log In or Register to continue