Pneumatisation of the maxillary sinus (MS) is variable. The archived cone-beam computed tomography file of a 54-year-old female was retrospectively evaluated anatomically. Nasal or retrobullar recesses of the MSs (NRMS) were found. The MSs were bicameral. NRMSs extended from the postero-lateral chambers of the MSs into the lateral nasal walls. The right NRMS was reached superior to the middle turbinate and the ethmoidal bulla was applied on its anterior side. The left NRMS had two medial pouch-like ends, one beneath the ethmoidal bulla and the other on the anterior side of the basal lamella of the middle turbinate. Additional anatomical findings were the uncinate bulla, infraorbital recesses of the MS, maxillary recess of the sphenoidal sinus, and atypical posterior insertions of the superior nasal turbinates, maxillo-ethmoido-sphenoidal and ethmoido-sphenoidal. The NRMS is a novel finding and could lead to erroneous endoscopic corridors if not documented before the interventions.

Keywords: Nasal cavity, Turbinates, Paranasal sinuses, Maxilla, Ethmoid bone