Hemifacial Spasm Due to Bony Stenosis of the Internal Auditory Meatus: Look Beyond the Loop

Hemifacial spasm (HFS) is a neuromuscular disorder resulting from cranial nerve VII compression at the root entry zone, characterized by brief, involuntary, progressive spasms of muscles on one side of face. The cisternal part of cranial nerve VII myelinated by Schwann cells is considered relatively resistant to compression. Rarely, direct compression over this segment without coexistent root entry zone compression may also result in HFS. An aberrant vessel posterior inferior cerebellar artery/anterior inferior cerebellar artery loop remains the leading cause of compression at this location. Cerebellopontine angle tumors or cysts may affect cranial nerve VII distally. However, bony meatal stenosis with pure distal facial nerve compression leading to HFS in the absence of other clinical symptoms has not been reported.

A 53-year-old woman presented with worsening left HFS for 9 years despite multiple trials of medical therapy, which severely impeded her social life and occupation. Temporal bone computed tomography revealed severe stenosis of the left internal auditory meatus (2.36 mm) compared with the right side (4.67 mm). Under three-dimensional exoscope guidance, a left retrosigmoid suboccipital craniotomy was performed, the posterior bony wall of the internal auditory canal was drilled to decompress the canal, and durotomy was performed to release the contents. Her symptoms resolved without developing facial weakness or hearing deficits.

An aberrant anterior inferior cerebellar artery vascular loop is usually the most frequent lesion causing compression of the distal cisternal part of the facial nerve. However, other purely distal or coexistent lesions must be actively sought for both in preoperative radiologic images and during surgery.