Sinus tympani: anatomic considerations, computed tomography, and a discussion of the retrofacial approach for removal of disease.

Surgical access to the sinus tympani remains a challenge for otologic surgeons. Usually, the retrotympanum is approached through the middle ear in an anterior to posterior direction during chronic ear surgery. Whether this is via a posterior tympanotomy or after canal wall down tympanomastoidectomy, visualization of the most posterior recess of the sinus tympani is often inadequate. The purpose of this investigation is two fold : (1) to describe the highly variable anatomy of the posterior tympanic cavity and (2) to review the retrofacial approach to the sinus tympani. Histologic sections, cadaver dissections, and diagrammatic illustrations are combined with computed tomographic (CT) imaging to provide a three-dimensional understanding of the sinus tympani and adjacent labyrinthine structures. Viewed from the mastoid, the anatomic structures that define the boundaries of the retrofacial approach include the facial nerve and stapedius muscle laterally, the lateral semicircular canal superiorly, the posterior semicircular canal posteromedially, the vestibule anteromedially, and the jugular bulb inferiorly. When the sinus tympani is well developed, saucerization within these boundaries gives wide access into the sinus and round window niche. The authors suggest that preoperative imaging can select patients who are candidates for a retrofacial approach to expose and remove disease in the sinus tympani. Contraindications to this approach include axial CT image measurements showing a contracted space between the posterior semicircular canal and the medial aspect of the facial nerve, lack of posterior expansion of the sinus tympani, and in cases where these measurements are marginal, the presence of a high jugular bulb or anteriorly positioned sigmoid sinus.