Decompression of Hemifacial Spasm with Intraoperative Monitoring Guidance using Electromyography and Brainstem Auditory Evoked Response

It is now well established that idiopathic hemifacial spasm is caused by vascular compression of the facial nerve at its root exit zone. Over the last 3 decades, microvascular decompression had proved to be a valuable procedure to definitively cure the disease.

The facial nerve has 5 branches in the face: temporal, zygomatic, buccal, mandibular and cervical (Fig 1). In a normal person electrical stimulation of a branch, for instance the zygomatic, causes contraction only of the orbicularis oculi muscle due to orthodromic conduction. In hemifacial spasm, stimulation of one branch also elicits contraction of other muscles which are not supplied by that branch. This constitutes the anomalous muscle response or lateral spread response. This phenomenon is due to abnormal axonal activity at the facial root exit zone secondary to compressive damage, inducing hyperexcitability of the facial nucleus. As a result electrical stimulation of a branch elicits antidromic conduction to the brainstem and facial nucleus with orthodromic conduction to the other branches of the facial nerve.

Fig 1: Branches of facial nerve in the face

There is a 10 to 15% risk of hearing loss during microvascular decompression. Monitoring with brainstem auditory evoked response (BAER) during surgery can decrease this risk. Prior to the use of BAER, the incidence of postoperative hearing loss was 11.9%, with 6.8% of patients having no residual hearing function. With the use of BAER 6.7% of patients had mild to moderate hearing impairment and no patient developed complete hearing loss.

Preoperatively we obtain BAER (Fig 2) and facial nerve electromyogram (EMG) recordings in the Electrodiagnostics Laboratory. We stimulate the zygomatic branch of the facial nerve and obtain orthodromic and antidromic responses from the frontalis and oris muscles respectively (Fig 3). The same setup was used for intraoperative monitoring. Baseline EMGs were recorded prior to decompression (Fig 4). During surgery the offending artery was identified and EMG recordings were obtained continuously during lifting and mobilisation of the artery. Several pieces of Teflon were inserted between the artery and facial root exit zone. Disappearance of the anomalous muscle response (Fig 5) provides electrophysiological evidence of adequate decompression of the facial nerve. BAERs were normal throughout the procedure with no significant latency or amplitude changes (Fig 6).

To summarise, intraoperative EMGs are useful to guide the surgeon in adequate decompression in hemifacial spasm. Concomittant BAER decreases the incidence of postoperative hearing loss.

Fig 2: Preop BAER

Fig 3: Preop facial nerve EMG (trace 4 = frontalis; trace 6 = oris)

 

Fig 4: Baseline facial nerve EMG (upper trace = frontalis; middle trace = oculi; lower trace = oris)

 

Fig 5: Absence of anomalous muscle response (lower trace) from oris with adequate decompression (upper trace = frontalis; middle trace = oculi)

Fig 6: Normal intraoperative BAER