# Paranasal Sinuses > Paranasal sinuses for MBBS Anatomy: four paired sinuses, drainage sites, development timetable, sinusitis pain patterns and trans-sphenoidal surgery notes. - Canonical URL: https://prepelephant.com/topics/mbbs/anatomy/paranasal-sinuses - Exam / course: MBBS · Subject: Anatomy - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Paranasal Sinuses", PrepElephant, https://prepelephant.com/topics/mbbs/anatomy/paranasal-sinuses ## Direct answer Paired air-filled mucosal sacs pneumatise four bones around the nasal cavity — maxillary, ethmoid, frontal and sphenoid — developing as outgrowths of the nasal mucosa, present in rudimentary form at birth in the maxilla and ethmoid only. Their drainage is the examinable core: the maxillary, frontal and anterior ethmoid cells drain through the middle meatus, the posterior ethmoid cells into the superior meatus, and the sphenoid sinus into the sphenoethmoidal recess. Each sinus's neighbours explain its complications — maxillary to the teeth, ethmoid to the orbit through lamina papyracea, frontal to the anterior cranial fossa, sphenoid to the sella and cavernous sinus. ## What you must remember - The maxillary sinus is the largest, a pyramid with its base on the lateral nasal wall and roof formed by the orbital floor; its floor lies over the roots of the first and second premolars and molars. - Its ostium opens high on the superomedial wall into the hiatus semilunaris, so the sinus drains against gravity and relies on mucociliary clearance — a classic explanation for its frequent infection. - The frontal sinus drains through the frontonasal duct or recess into the middle meatus; it is absent at birth and develops from about the seventh year onward. - The ethmoid sinuses consist of thin-walled air cells between the orbit and nose, the lateral wall being the lamina papyracea; anterior cells drain to the middle meatus, posterior cells to the superior meatus. - The sphenoid sinus, pneumatising in early childhood, lies below the sella turcica, with the optic nerve and cavernous sinus in its lateral walls. - Pain referral patterns: maxillary sinusitis to the upper teeth and cheek, frontal sinusitis to the forehead worst on bending forward, ethmoiditis to the root of the nose and orbit, sphenoiditis to the vertex or retro-orbital region. - Mucociliary flow moves secretions toward the ostia, so ciliary function matters more than gravity — the principle behind ventilation and drainage in functional endoscopic sinus surgery. ## A typical case that uses all four sinuses A twenty-year-old with a heavy cold develops aching in the right cheek and upper teeth, pain on chewing, and fever. The anatomy localises it at once to the maxillary sinus, whose floor shares bone with the tooth roots — percussion of the teeth aches, and transillumination of the sinus is dull on the affected side. Because the ostium sits high on the medial wall near the roof, swollen mucosa at the hiatus semilunaris seals it, trapping secretions; the negative pressure and pus against a closed cavity explain the deep, boring ache made worse by bending forward. Extend the case in the direction examiners take it. If infection spreads through the paper-thin roof of the sinus — the orbital floor — the patient develops orbital cellulitis: proptosis, painful eye movements and lid swelling, an emergency treated with intravenous antibiotics and drainage of any subperiosteal collection per current practice. If the same patient had frontal sinusitis, the posterior wall borders the anterior cranial fossa and the diploic veins offer a route to meningitis or frontal lobe abscess — Pott's puffy tumour of the forehead marking frontal osteomyelitis. The sphenoid sinus, isolated and deep, presents with vertex or retro-orbital headache and threatens the optic nerve laterally; it is also the corridor for trans-sphenoidal hypophysectomy, passing through the nostril and sphenoid ostium directly to the sella floor. ## Where students slip The developmental timetable is the trap most fallen into: only the maxillary and ethmoid sinuses exist (in small form) at birth, the sphenoid pneumatises in early childhood and the frontal becomes radiologically evident around seven years — "all sinuses present at birth" is a failing statement. Second, the maxillary ostium is described as "low and anterior"; it is high on the superomedial wall near the sinus roof, which is precisely why drainage is inefficient and the middle meatus is the surgical target. Third, candidates forget the maxillary sinus's relation to the teeth in both directions: sinusitis mimics toothache, and dental infection of a molar root can seed the sinus secondarily — the dental origin must be sought in unilateral, treatment-resistant maxillary sinusitis. ## Frequently asked questions ### Which paranasal sinuses are present at birth? The maxillary and ethmoid sinuses, both small; the sphenoid and frontal sinuses pneumatise after birth, the frontal becoming evident around seven years. ### Where does the maxillary sinus drain? Through its ostium high on the superomedial wall into the hiatus semilunaris of the middle meatus, aided by mucociliary action rather than gravity. ### Which structure forms the lateral wall of the ethmoid sinuses? The lamina papyracea, a paper-thin plate separating the air cells from the orbit and the usual route of orbital complications. ### Which important structures lie in the lateral wall of the sphenoid sinus? The optic nerve above and the cavernous sinus with the internal carotid artery, making the sphenoid the most surgically delicate sinus. ### Why does maxillary sinusitis cause toothache? The sinus floor is the alveolar process housing the roots of the upper premolars and molars, so the shared nerves carry the pain to the teeth.