Objective The aim of this study is to describe the endoscopic anatomy of the subtympanic sinus (STS), establish a classification according to its extension regarding the level of the facial nerve (FN), and assess the feasibility of the transcanal endoscopic approach to the STS. Study Design Experimental anatomic research. Setting Temporal bone laboratory. Methods We performed endoscopic dissection of 34 human whole head and ear block specimens. Of those, 29 underwent high-resolution computed tomography. The STS was classified according to its extension regarding the level of the FN: type A, no extension medial to the FN; type B, extension to the medial limit of the FN; type C, extension of the sinus medially and posteriorly from the FN into the mastoid cavity. Results The majority of cases (n = 21, 72%) showed a shallow type A STS. We observed a deep type B configuration in 6 cases (21%) and a type C in 2 cases (7%). The STS was completely exposable with a 0° endoscope in 44% of the specimens. Using a 45° endoscope, we gained complete insight in 79%. However, in 21% of the cases, the posteromedial extension of the STS was too deep to be completely explored by an endoscopic transcanal approach. Conclusion The majority of the STS is shallow and does not extend medially from the FN. This morphologic variant allows complete transcanal endoscopic visualization. In more excavated STS, a complete endoscopic exploration is not achievable, and a retrofacial approach may be adopted to completely access the STS.
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