The posterior-inferior recess of the sinus tympani, an anatomical novelty

Background: The anatomy of the retrotympanum is highly variable, and surgical access is challenging. In the medial retrotympanum, a descending series of recesses are found: the posterior tympanic sinus, the sinus tympani (ST), and the subtympanic sinus (STS). Most of the previous anatomical studies of the ST evaluated it as a single depression of variable width and depth, without recesses, just on axial sections. Method: The ST was evaluated bilaterally in all the anatomical planes on a lot of 100 cases. Two sagittal anatomical types of the ST were defined and counted: type 0 (saccular ST), with absent postero-inferior recess (PIR) of the ST, and type 1, ST with PIR (bowl-shaped ST). Results: In 200 sides, 144 type 0 ST (72 %) and 56 types 1 (PIRs) of the ST were found (28 %). On the right/left sides the type 0 ST was found in 74 %/70 % and the type 1, with PIR, in 26 %/30 %. There were no significant correlations between sex and the ST types on both sides. In the general lot, bilaterally symmetrical types 0 were found in 68 %, bilaterally symmetrical types 1 in 24 %, and the bilaterally asymmetrical combination 0+1 +1 in just 8 %. Conclusions: The PIR of the ST is not a scarce anatomical variation in the retrotympanum. It is hidden deep to the ST and difficult to access through the middle ear. It also could retain residual cholesteatoma. A retrofacial approach could access the PIR of the ST if no other anatomical limitations occur.