A patient with chronic suppurative otitis media (CSOM) with cholesteatoma undergoes mastoidectomy. Intraoperatively, the surgeon identifies the second genu (bend) of the facial nerve in the mastoid segment. Which structure lies immediately lateral to the facial nerve at the level of the second genu?

• A Lateral semicircular canal (horizontal semicircular canal) ✓
• B Posterior semicircular canal at the mastoid antrum
• C External auditory canal bony floor
• D Cochlea at the basal turn

Explanation

The facial nerve takes a second genu (bend) at the junction of its horizontal (tympanic) segment and its vertical (mastoid) segment, just posterior and medial to the short process of the incus and the lateral semicircular canal. The lateral (horizontal) semicircular canal is the most important landmark for the facial nerve in ear surgery — the facial nerve lies just inferior (medial) to the lateral SCC at its second genu. When drilling in the mastoid, surgeons use the lateral SCC as a landmark to prevent inadvertent facial nerve injury. The bulge of the horizontal SCC is visible in the aditus ad antrum region of the middle ear.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.