Aberrant facial nerve.

A 50-year-old man presented with a 5-year history of a right tympanic membrane perforation and associated conductive hearing loss. His otologic history was significant for otitis media, otorrhea, and otalgia. He had undergone no otologic surgery. His medical history was significant for poliomyelitis.

Examination revealed that the right ear was dry and the perforation was subtotal. The appearance of the left tympanic membrane was normal, and findings on the remainder of the head and neck examination were unremarkable.

The patient was taken to the operating room for a postauricular tympanoplasty. Canal incisions were made at 12 o'clock and 6 o'clock, and a tympanomeatal flap was elevated in the standard fashion. Neither a drill nor a curette was used. Approximately 3 to 5 mm from the annulus in the middle and interior portions of the canal, a trough was palpated. Inspection of that area detected fibers crossing parallel to and lateral to the annulus. A presumptive diagnosis of an aberrant facial nerve was made. An NIM-Response facial nerve monitor was used to stimulate these fibers, and a strong response was elicited at a very low stimulus setting, confirming that these fibers were facial nerve fibers.

The nerve was intact throughout its course. The remainder of the flap was elevated, and the middle ear was entered posterosuperiorly to avoid the aberrant facial nerve. The chorda tympani was identified in its usual course, passing over the incus as expected (figure). No ossicular abnormalities were identified. An underlay tympanoplasty was performed. The patient was awakened, and normal facial function was documented. Postoperatively, the tympanic membrane graft healed, and the conductive component of the hearing loss resolved.

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This aberrant dehiscent facial nerve was discovered during a routine tympanoplasty. The facial nerve passes through the bone in the posterior external auditory canal. Several studies have examined the anatomic relationship of the facial nerve to the tympanic annulus in an attempt to decrease the incidence of iatrogenic facial nerve injury. Litton et al used radiologic studies to document the variable relationship of the facial nerve to the tympanic annulus. (1) Nager and Proctor described aberrant coursing of the vertical portion of the facial nerve, further underscoring the potential for injury during a transcanal approach. (2) Adad et al drilled 37 temporal bones to directly examine the relationship of the facial nerve to the tympanic annulus, proving that the nerve runs lateral to the annulus in the posteroinferior quadrant in more than 70% of specimens. (3) Therefore, the tympanic annulus is not a reliable landmark to predict the course of the facial nerve.

Given the aberrant anatomy of the facial nerve in the external auditory canal, the transcanal approach used in many otologic procedures carries a rare but inherent risk of facial nerve injury. Awareness of the possibility of an anatomic variation of the facial nerve course and intraoperarive use of facial nerve monitoring can be useful adjuncts to meticulous dissection in order to avoid iatrogenic facial nerve injury.

References

(1.) Litton WB, Krause CJ, Anson BA. Cohen WN. The relationship of the facial canal to the annular sulcus. Laryngoscope 1969:79:1584-1604.

(2.) Nager GT, Proctor B. Anatomic variations and anomalies involving the facial canal. Otolaryngol Clin North Am 1991;24:531-53.

(3.) Adad B, Ragson BM, Ackerson L. Relationship of the facial nerve to the tympanic annulus: A direct anatomic examination. Laryngoscope 1999:109:1189-92.

Natalie P. Steele, MD; Andrea Vambutas, MD

From the Department of Otolaryngology, Long Island Jewish Medical Center. New Hyde Park, N.Y.