Canal Wall Down Mastoidectomy w Lateral Semicircular Canal Fistula

Findings:

CT temporal bones in axial and coronal planes with inclusion of an oblique modified reconstruction demonstrate a large well aerated bone defect in the right mastoid region consistent with canal wall down mastoidectomy. The lateral margin of the lateral semicircular canal is truncated.

Discussion:

Violation of the otic capsule may occur less than 10% of the time during choleteatoma resection and other middle ear surgery, causing a lateral semicircular canal fistula. It is considered a "third window" along with superior semicircular canal dehiscence. Symptoms are nonspecific, including otorrhea, dizziness, and hearing loss. The severity of the defect has a classifcation by Dornhoffer and Milewski that cannot be distinguished on imaging and only confirmed intraoperatively.

Type 1 is a defect with intact endosteum, Type 2a is endosteum violated w preserved perilymph, Type 2b perilymph is violated, Type 3 is disruption of membranous labyrinth and endolymph. This classification is based on the depth of involvement rather than the size of the defect, and as expected worse outcomes are associated with higher grade lesions.

Reference: Dornhoffer JL and Milewski, C. Management of the open labyrinth. Otolaryngol Head Neck Surg 1995;112:410-14.

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