During mastoid surgery for cholesteatoma, the surgeon notes absence of bone over the horizontal (tympanic) segment of the facial nerve near the oval window. In temporal bone studies, what is the most frequent site of congenital dehiscence of the facial canal?

• A Mastoid segment just distal to the second genu
• B Labyrinthine segment near the geniculate ganglion
• C Tympanic segment adjacent to the oval window ✓
• D Intrameatal segment within the internal acoustic meatus

Explanation

The tympanic (horizontal) segment of the facial canal, particularly the part adjoining the oval window niche, is the most commonly dehiscent portion, reported in up to half of temporal bones in some series. The nerve may prolapse over the promontory or stapes, placing it at risk during curettage of the posterior annulus and disease clearance near the oval window. Dehiscence of the labyrinthine segment is rare because that part of the canal is thickest.

Reference: Gray's Anatomy, 42nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.