Case of the Week
Section Editors: Matylda Machnowska1 and Anvita Pauranik2
1University of Toronto, Toronto, Ontario, Canada
2BC Children's Hospital, University of British Columbia, Vancouver, British Columbia, Canada
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April 29, 2021
Mixed Hearing Loss due to Aberrant Position of the Right Facial Nerve
- Background:
- In the temporal bone, the facial nerve (FN) might show various anomalies in its natural course. The most common is congenital bony dehiscence of the FN canal that occurs in up to 55% of otherwise normal temporal bones, predominantly involving the tympanic portion (91%).
- Anomalies involving the facial canal frequently are encountered in malformations of the temporal bone. These anomalies include aberrations of the course of 1 or all of the segments of the canal; abnormal relation to the oval and round windows; bifurcations and trifurcations of the nerve; and associations with dysplasia of the stapes, oval window, external ear canal, and auricle (some of them are responsible for transmission deafness).
- Rarely, the facial nerve may be hypoplastic or totally absent.
- Two abnormal vessels occasionally may accompany the FN in the fallopian canal: a persistent stapedial artery and a persistent lateral capital vein.
- Due to the close anatomic relationship among the FN, stapes, and oval window (OW), intratemporal FN displacement may likely influence the choice of surgery for hearing improvement.
- An anomalous tympanic FN can be a manifestation of several congenital syndromes with temporal bone malformations. It has been reported in 29% to 88% of patients with CHARGE syndrome, along with other temporal bone anomalies, and similar circumstances can be encountered in patients with Pierre Robin sequence, oculo-auriculo-vertebral sequence, and Down syndrome.
- The exquisite detail afforded by a dedicated CT scan of the temporal bone allows evaluation of osseous abnormalities that may have previously gone unnoticed.
- The aberrant intratemporal FN is the most troublesome condition during middle ear surgery and is important to recognize prior to otologic surgery. Failure to recognize the malpositioned FN can be devastating.
- Clinical Presentation:
- Most commonly asymptomatic
- Unilateral or bilateral mixed hearing loss
- Middle and inner ear malformations
- Key Diagnostic Features:
- Dedicated high-spatial-resolution CT scan of the temporal bone allows:
- Identification of relationships between the fallopian canal and the surrounding bony structures.
- Evaluation of the whole intratemporal facial nerve from the labyrinthine segment to the mastoid segment.
- Oval window atresia and malformed stapes.
- Dedicated high-spatial-resolution CT scan of the temporal bone allows:
- Differential Diagnoses:
- Oval window atresia: Especially on axial and coronal planes the oval window niche is obliterated, and imaging may reveal either concentric narrowing along the medial tympanic wall or osseous obliteration by a thick bony plate. With regard to stapes malformation and also association with an incus malformation. Stapes malformations include failure to identify both crura on axial CT images or the stapes being displaced from the oval window niche. A more inferior course of the horizontal segment of the facial nerve canal can be found with also lack of bony coverage.
- Congenital stapes fixation: The stapes and oval window area can appear abnormal on the CT images without development of the annular ligament. This results in footplate ankyloses.
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Congenital malformation of middle ear: On high-resolution temporal bone CT, the FN can be displaced, including hanging over the OW, bifurcated, and crossing over the promontory.
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Persistent stapedial artery: Features on CT include the absence of the ipsilateral foramen spinosum and a soft-tissue prominence in the region of the proximal tympanic segment of the facial nerve. An aberrant ICA is often associated with a persistent stapedial artery.
- Treatment:
- Careful avoidance at time of middle ear surgery
- Surgeries for the purpose of hearing improvement include stapodotomy + piston implantation, labyrinthotomy, labyrinthotomy + total ossicular replacement prosthesis implantation, and Vibrant Soundbridge implantation.
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