Facial Nerve Bifurcation in Congenital Oval Window Atresia
We present a case of an 18-year-old woman with unilateral left conductive hearing loss diagnosed at age 1. She also had an atrial septal defect which was surgically corrected at 6 months old. There were no other major anomalies or syndromic features. She presented to our service seeking to improve her single sided deafness which had been stable since diagnosis. Her external ear, external auditory canal, and tympanic membrane were normal. Audiogram showed left maximal conductive hearing loss with an air-bone gap of 50 dB HL. Left acoustic reflex was absent with ipsilateral presentation of stimulus but present with contralateral presentation of stimulus. High-resolution computer tomographic (HRCT) revealed an absent left oval window, a dysplastic left stapes with poorly seen crura (Fig. 1C) and the left facial nerve was displaced inferiorly slightly, partially lying over the oval fossa (Fig. 1B). Consensus after discussion with several otologists was to undergo exploratory tympanotomy and perform a laser stapedotomy if anatomy was favorable. During surgery, the chorda tympani was encountered in a normal position after entering the middle ear. However, there was an anomalous bifid facial nerve. The larger branch of this bifid nerve was attached to one crus of a dysplastic stapes while the smaller branch lay directly over the oval fossa (Fig. 2). Both branches were confirmed with a facial nerve stimulator. Further surgery was aborted in view of these findings. HRCT scans were re-examined and bifurcation of the tympanic facial nerve was noted retrospectively (Fig. 1A). Both branches rejoined to form the mastoid segment.
Oval window atresia is associated with anomalies of the stapes and facial nerve. The stapes is usually dysplastic and can have an anomalous bony connection to the facial canal. The facial nerve course is also aberrant, running more inferiorly than usual. This patient's surgical findings can be explained by embryology. Division of her facial nerve occurred after the end of the 5th gestational week, hence her chorda tympani has been differentiated from the main facial nerve. Around the 6th week, abnormal division of the facial nerve occurred amidst developing temporal bone structures causing one segment of the developing nerve to be displaced anteriorly and inferiorly to lie between the stapes blastema and oval window (1). This abnormal position prevented normal migration of the stapes, and since the oval window cannot form without the stimulus of stapes contact, there was no induction of the oval window and hence oval window atresia (2). A bifurcated facial nerve which lies across the oval fossa is a surgical hazard which precludes stapedotomy (3).