Dehiscence of the Tympanic Segment of the Facial Nerve

Abstract

A 45-year-old woman was admitted with a lifelong history of left hearing loss, aural fullness, and tinnitus without a history of previous otitis media, trauma, otorrhea, otalgia, or facial nerve dysfunction. Examination revealed a deep pars flaccida retraction without squamous debris with an otherwise normal-appearing middle ear cleft. An audiogram revealed a moderate low and mild midfrequency conductive hearing loss (CHL) in the left ear; the right ear was normal. Tympanometry was type C in the left ear and type A in the right ear, with absent reflexes on the left. No preoperative CT scan was obtained. Because of the deep pars flaccida retraction pocket, the patient underwent a tympanoplasty. Examination of the ossicular chain revealed normal ossicles and mobility without ossicular dislocation. The middle ear mucosa was normal. Intraoperative findings were significant for the aforementioned retraction pocket, which was easily averted, and a completely dehiscent tympanic segment of the facial nerve, which crossed underneath the lenticular process of the incus Figure 1. This finding was the probable cause of the patient's lifelong aural symptoms and audiometric profile. She recovered from her tympanoplasty without incident but continued to have a persistent CHL.

DISCUSSION

Facial nerve dehiscence is not an uncommon occurrence. Baxter 1 studied 535 temporal bones and found the incidence of facial nerve dehiscence to be 55%, with most located in the tympanic segment (91%). Fewer than 9% were located in the mastoid segment, and dehiscences rarely involve the labyrinthine segment or the area of the geniculate ganglion. Of dehiscent tympanic segment facial nerves, 83% were adjacent to the oval window, with 26% of facial nerves protruding from the canal. Embryologically, the facial canal is derived from 2 structures: the primordial otic capsule and Reichert's cartilage (second branchial arch). 2 Although ossification of these structures begins during gestation and is completed by the end of the first year of life, incomplete ossification results in exposure of part, or all, of the facial nerve.

CHL, as a result of marked protrusion of the facial nerve and contact with the incudostapedial joint and stapes superstructure, has been documented by many surgeons. 3 Dehiscence of the tympanic segment of the facial nerve is neither frequently considered nor frequently found as a cause of CHL. However, aside from this patient's pars flaccida retraction pocket, the remainder of her history, physical examination, audiometry, and intraoperative finding were typical for a patient with a dehiscence of the tympanic segment of the facial nerve. Heightened preoperative and intraoperative vigilance should prevent any iatrogenic facial nerve injuries to an exposed facial nerve.

Footnotes

Appendix

Fig 1.

Transcanal view with a 4-mm 0° degree rigid oto-scope showing a completely dehiscent tympanic seg-ment of the facial nerve (black arrows) crossing under-neath the lenticular process of the incus and abutting the stapes superstructure (white arrow).

References

Baxter

. Dehiscence of the fallopian canal J Laryngol Otol 1971;85:587–94

Anson

Harper

Warphea

. Surgical anatomy of the facial nerve Ann Otol Rhinol Laryngol 1963;72:713–34

Proctor

Nager

. The facial canal: Normal anatomy, variations and anomalies Ann Otol Rhinol Laryngol Suppl 1982;97:33–61