Which of the following is the most common site of bone erosion in cholesteatoma?

Explanation

## Sites of Bone Erosion in Cholesteatoma **Key Point:** The **lateral semicircular canal (lateral SCC)** is the most commonly eroded bony structure in cholesteatoma, as consistently cited in major ENT textbooks (Scott-Brown's Otorhinolaryngology, Dhingra's Diseases of Ear, Nose and Throat). ### Why the Lateral Semicircular Canal? - **Anatomical proximity:** Cholesteatoma arising in the attic/epitympanum extends posteriorly and superiorly, placing it in direct contact with the lateral wall of the lateral semicircular canal. - **Thin bony covering:** The bony shell overlying the lateral SCC is naturally thin and susceptible to enzymatic erosion by cholesteatoma matrix. - **Enzymatic osteolysis:** Cholesteatoma produces collagenases and other proteolytic enzymes that preferentially erode the thin bone of the lateral SCC. ### Hierarchy of Bone Erosion (by frequency) 1. **Lateral semicircular canal** *(most common)* — labyrinthine fistula → vertigo, positive fistula test 2. **Ossicular chain** — incus long process most vulnerable → conductive hearing loss 3. **Facial nerve canal** — facial paralysis (rare but serious) 4. **Tegmental bone** — intracranial extension (epidural abscess, meningitis) 5. **Mastoid cortex** — postauricular abscess, Bezold's abscess ### Clinical Significance of Lateral SCC Erosion - **Labyrinthine fistula** is the most feared complication of lateral SCC erosion - Presents with: **vertigo**, **nystagmus**, and a **positive fistula test** (Hennebert's sign) - CT temporal bone: demonstrates bony defect over the lateral SCC - Surgical management requires careful preservation of the membranous labyrinth **High-Yield:** Fistula test (positive pressure in EAC → nystagmus/vertigo) is the bedside test for lateral SCC fistula from cholesteatoma. **Clinical Pearl:** In a patient with chronic ear disease, new-onset vertigo should raise suspicion for lateral SCC erosion by cholesteatoma — CT temporal bone is mandatory before surgery. ### Complications Based on Site Eroded | Site Eroded | Complication | |-------------|-------------| | **Lateral SCC** | Labyrinthine fistula, vertigo (most common) | | **Ossicular chain (incus)** | Conductive hearing loss | | **Facial nerve canal** | Facial paralysis | | **Tegmental bone** | Epidural abscess, meningitis, brain abscess | | **Mastoid cortex** | Postauricular abscess, Bezold's abscess | **Reference:** Dhingra PL, Dhingra S. *Diseases of Ear, Nose and Throat & Head and Neck Surgery*, 7th ed. — Lateral semicircular canal is cited as the most common site of bony erosion in cholesteatoma. Scott-Brown's Otorhinolaryngology, 8th ed., concurs.