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Facial nerve mimicking the fibrocartilaginous annulus of the tympanic membrane.

Facial nerve damage is the most feared complication of otologic operations. This risk increases with the dehiscence of the facial nerve canal. The main causes of facial nerve damage are variation of the normal developmental process, long-standing inflammation, previous otologic surgeries, and traumas and pressure of tumoral lesions such as cholesteatoma. (1-3) In this article, we introduce the mobilization of the mastoid segment of the facial nerve during the elevation of the tympanic annulus by a straight microneedle in a myringoplasty operation.

A 44-year-old female patient with a central, dry, posteroinferior perforation of her left ear drum was prepared for a myringoplasty operation in our clinic. The pure-tone air thresholds on the right and left were 10 dB and 25 dB, respectively, and the pure-tone bone thresholds on the right and left were both 5 dB. There was no evidence of inflammation or cholesteatoma on the preoperative computed tomography scan of the temporal bone.

With the patient under general anesthesia, a postauricular incision into the middle ear was made under a microscope. We noticed a structure that was thicker than the fibrocartilaginous annulus of the tympanic membrane and which was mobilized with the mobilization of the chorda tympani. We recognized that this structure was the mastoid segment of the facial nerve, and that the full length of the mastoid segment was exposed (figure). The myringoplasty operation was completed, with care taken to preserve this structure.

Politzer was the first person to describe facial canal dehiscence (FCD) in 1894. (4) Since then, many normal and diseased ears have been analyzed in terms of facial canal dehiscence. Moreano et al examined a total of 1,000 temporal bones without evidence of inflammation or cholesteatoma to study the prevalence of FCD. (5) They found that 56% of the temporal bones had gross dehiscence. The most common site of dehiscence was around the oval window (73%). Nearly all gaps (98%) were located in the horizontal course of the nerve, whereas only 1.6% of the dehiscence was located in the mastoid segment.

In another study, Di Martino et al compared the appearance of the facial nerve canal in 357 routine ear operations with 300 temporal bone specimens from 150 autopsies. (1) They detected that the rate of FCD was 6.4 and 29.3% for intraoperative and autopsy examinations, respectively. The oval window niche was the most affected site. There was no dehiscence at the mastoid segment.

The relative risk of FCD is much higher in ears with cholesteatoma. (2,3) Sheehy et al analyzed 1,024 primary cases of mastoid surgery for cholesteatoma. (2) In 15% of cases, the facial nerve was congenitally exposed, and a further 17% of cases had FCD due to cholesteatoma.

In another study, Moody and Lambert examined the incidence of FCD in 416 cases of cholesteatoma. (3) They found tympanic segment dehiscence in 78 cases and mastoid segment dehiscence in 5 cases. Only one of the cases of mastoid segment dehiscence was thought to be related to a congenital variation; the rest were associated with cholesteatoma. Bayazit et al similarly found that the dehiscence rate was higher in patients with cholesteatoma, followed by adhesive otitis media, chronic otitis media, and tympanosclerosis. (6) The canal dehiscence was mainly seen in the tympanic segment and the second genu and was rarely seen in the mastoid segment. The mastoid segment of the facial nerve, as it moves forward distally, takes a more lateral and anterior course and crosses the plane of the annulus in its inferior third. (7)

During transcanal surgery, the facial nerve is mainly sensitive at the posteroinferior quadrant because the annulus is not a steady landmark for the facial nerve. (8) In our case, the mastoid segment of the facial nerve was mobilized with a microelevetor at the posterior quadrant, but by taking utmost care, we were able to preserve the facial nerve.

Mastoid segment involvement in FCD is rare. To the best of our knowledge, no other case in which the facial nerve was mobilized during the elevation of the annulus has been reported. Surgeons, especially those who are inexperienced, must be aware of this possibility in ear surgery.

References

(1.) Di Martino E, Sellhaus B, Haensel J, et al. Fallopian canal dehiscences: A survey of clinical and anatomical findings. Eur Arch Otorhinolaryngol 2005;262(2): 120-6.

(2.) Sheehy JL, Brackmann DE, Graham MD. Cholesteatoma surgery: Residual and recurrent disease. A review of 1,024 cases. Ann Otol Rhinol Laryngol 1977;86(4 Pt l):451-62.

(3.) Moody MW, Lambert PR. Incidence of dehiscence of the facial nerve in 416 cases of cholesteatoma. Otol Neurotol 2007;28(3):400-4.

(4.) Politzer A. Diseases of the Ear and Adjacent Organs. For Students and Practitioners. London: Balliere Tindell and Cox; 1894.

(5.) Moreano EH, Paparella MM, Zelterman D, Goycoolea MV. Prevalence of facial canal dehiscence and of persistent stapedial artery in the human middle ear: A report of 1000 temporal bones. Laryngoscope 1994;104(3 Pt 1):309-20.

(6.) Bayazit YA, Ozer E, Kanlikama M. Gross dehiscence of the bone covering the facial nerve in the light of otological surgery. J Laryngol Otol 2002; 116(10):800-3.

(7.) Zaghal ZA, Raad RA, Nassar J, et al. Anatomic relationship between the facial nerve and the tympanic annulus. Otol Neurotol 2014;35(4):667-71.

(8.) Adad B, Rasgon BM, Ackerson L. Relationship of the facial nerve to the tympanic annulus: A direct anatomic examination. Laryngoscope 1999; 109(8):1189-92.

Hande Senem Deveci, MD; Tulay Erden Habesoglu, MD; Cem Karatas, MD; Selami Uzun, MD

From the Department of Otorhinolaryngology, Fatih Sultan Mehmet Education and Research Hospital, Istanbul, Turkey.

Caption: Figure. In this perioperative microscopic image, the microelevator is under the mastoid segment of the facial nerve (green arrow). The aspirator is pulling forward the chorda tympani (black arrows).
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Title Annotation:OTOSCOPIC CLINIC
Author:Deveci, Hande Senem; Habesoglu, Tulay Erden; Karatas, Cem; Uzun, Selami
Publication:Ear, Nose and Throat Journal
Article Type:Report
Date:Apr 1, 2017
Words:966
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