Radioanatomical assessment of the subtympanic sinus and its relationship to facial nerve position and temporal bone pneumatization in adults: CT study

BACKGROUND: The subtympanic sinus (STS) is a recess of the inferior retrotympanum of various morphology and depth. Its shape and dimensions may depend on the configuration of surrounding bony ridges and the degree of temporal bone pneumatization. Despite increasing interest in retrotympanic anatomy, quantitative radiological descriptions of STS variability in adults remain limited. Our aim was to describe the morphology, dimensions, and distribution of the subtympanic sinus in adults and to analyze its anatomical relationships with the facial nerve, the subcochlear canaliculus, and pneumatization of the temporal bone. MATERIALS AND METHODS: Computed tomography (HRCT and CBCT) scans of 82 adults (164 temporal bones) without otologic disease were retrospectively analyzed. The STS was classified according to its extension relative to the mastoid segment of the facial nerve. Sinus depth and width were measured, and the presence of facial nerve-adjacent pneumatization, subcochlear canaliculus, and pneumatization within the vascular compartment of the petrous bone were assessed. RESULTS: Type A STS configuration was most prevalent (80.5%), followed by type B (16.5%) and type C (3.0%). STS depth increased progressively from type A to type C, with statistically significant differences among groups (p < 0.001), whereas sinus width did not differ significantly. A connection with the subcochlear canaliculus was present in 62.8% of temporal bones and was significantly associated with greater STS depth. Facial nerve-adjacent pneumatization was frequent (85.4%) and correlated with increased STS depth. CONCLUSIONS: The subtympanic sinus exhibits substantial morphological variability in adults, primarily expressed through differences in depth. Its configuration may reflect broader patterns of temporal bone pneumatization and may support the concept of an integrated retrotympanic and infracochlear anatomical system.