Nasal Cavity & Paranasal Sinuses

The nose is a perennial NEET PG favourite from Head & Neck Anatomy — the lateral wall meatuses, their sinus drainage openings, the arterial anastomoses behind epistaxis, and the olfactory pathway. Master the drainage map and the blood supply, and you have cleared the bulk of the marks here.

Boundaries & Subdivisions of the Nasal Cavity

The nasal cavity extends from the nares (nostrils) anteriorly to the choanae (posterior nasal apertures) posteriorly, where it communicates with the nasopharynx. A median nasal septum divides it into right and left halves. Each half is further divided into:

• Vestibule — the anterior dilated part, lined by skin bearing hairs (vibrissae) and sebaceous glands. The skin-mucosa junction is the limen nasi (a ridge formed by the upper border of the lower nasal cartilage).
• Respiratory region — the lower, larger part lined by pseudostratified ciliated columnar (respiratory) epithelium; richly vascular.
• Olfactory region — the upper part, opposite the superior concha and corresponding part of septum, lined by olfactory epithelium.

The Four Walls of Each Nasal Cavity

Wall	Major contributors
Roof	Nasal & frontal bones (frontonasal), cribriform plate of ethmoid (middle/horizontal part), body of sphenoid (posterior)
Floor	Palatine process of maxilla (anterior ¾) + horizontal plate of palatine bone (posterior ¼)
Medial wall (septum)	Perpendicular plate of ethmoid (above) + vomer (below) + septal cartilage (antero-inferior)
Lateral wall	Maxilla, lacrimal, ethmoidal labyrinth (conchae), perpendicular plate of palatine, medial pterygoid plate; conchae project from it

High-yield: The bony nasal septum = perpendicular plate of ethmoid (postero-superior) + vomer (postero-inferior). The anterior part is the septal (quadrangular) cartilage. The commonest site of septal deviation is the junction of septal cartilage and vomer.

The Lateral Wall — Conchae & Meatuses (Most Tested)

Three (sometimes four) curved bony shelves — the conchae (turbinates) — project from the lateral wall, each overhanging a meatus (a passage beneath it). The superior and middle conchae are parts of the ethmoid bone; the inferior concha is a separate independent bone.

The space above and behind the superior concha is the spheno-ethmoidal recess.

Stepwise drainage logic (the exam map)

Spheno-ethmoidal recess → sphenoid sinus → Superior meatus → posterior ethmoidal air cells → Middle meatus → frontal sinus, maxillary sinus, anterior + middle ethmoidal cells → Inferior meatus → nasolacrimal duct → Below inferior meatus / sphenopalatine foramen region.

Meatus / recess	Structure(s) that open into it
Spheno-ethmoidal recess	Sphenoid air sinus
Superior meatus	Posterior ethmoidal air cells
Middle meatus	Frontal sinus, anterior + middle ethmoidal cells, maxillary sinus (all via the hiatus semilunaris / infundibulum)
Inferior meatus	Nasolacrimal duct (guarded by valve of Hasner)

High-yield: Mnemonic for sinus drainage into the meatuses — "Sphenoid Sits in the Spheno-ethmoidal recess; Superior gets Posterior ethmoids; Middle gets the rest (Frontal, Maxillary, Anterior + Middle ethmoids); Inferior gets the Nasolacrimal duct." Or recall: all the anterior group of sinuses drain into the middle meatus; only the posterior ethmoids drain into the superior meatus; only the sphenoid drains into the spheno-ethmoidal recess.

Features of the Middle Meatus (high-yield surgical anatomy)

The middle meatus carries the busiest drainage and is the heart of functional endoscopic sinus surgery (FESS):

• Bulla ethmoidalis — a rounded swelling produced by the middle ethmoidal air cells; the middle ethmoids open on or above it.
• Hiatus semilunaris — a curved cleft below the bulla ethmoidalis.
• Infundibulum — the channel at the anterior end of the hiatus; the frontal sinus and anterior ethmoidal cells open here.
• Maxillary sinus ostium — opens into the hiatus semilunaris.
• Agger nasi — a small ridge anterosuperior to the middle concha, in front of its attachment; remnant of a nasoturbinal; an agger nasi air cell is clinically important in FESS as it sits in the frontal recess and can obstruct frontal sinus drainage.

High-yield: The maxillary sinus ostium opens high on its medial wall into the hiatus semilunaris — gravity does not aid drainage, which is why the antrum is prone to chronic infection and why antral lavage / FESS is required. The osteomeatal complex (OMC) — the region of the middle meatus draining the frontal, maxillary and anterior ethmoid sinuses — is the key target of FESS.

Blood Supply & Epistaxis (Examiner's Delight)

Both internal and external carotid systems supply the nose.

Arteries of the lateral wall and septum

Source	Branches to nose
Internal carotid → Ophthalmic artery	Anterior & posterior ethmoidal arteries (supply roof and upper part of lateral wall/septum)
External carotid → Maxillary artery	Sphenopalatine artery (chief artery of the nose), greater palatine, posterior lateral nasal, posterior septal
External carotid → Facial artery	Superior labial artery (septal branch)

High-yield: The sphenopalatine artery is the principal artery of the nasal cavity and the main culprit in posterior epistaxis. It enters via the sphenopalatine foramen (at the posterior end of the middle meatus). Ligation/cauterisation of the sphenopalatine artery is done for refractory posterior bleeds.

Little's Area & Kiesselbach's Plexus

The antero-inferior part of the nasal septum (Little's area) is the site of an arterial anastomosis called Kiesselbach's plexus, formed by FOUR arteries:

1. Anterior ethmoidal artery (from ophthalmic)
2. Sphenopalatine artery (septal branch, from maxillary)
3. Greater palatine artery (from maxillary)
4. Septal branch of superior labial artery (from facial)

High-yield mnemonic — Kiesselbach's plexus: "LEGS" → Labial (superior labial septal), Ethmoidal (anterior), Greater palatine, Sphenopalatine. Little's area is the commonest site of anterior epistaxis, especially in children and young adults.

Woodruff's plexus — a venous (and arterial) plexus on the posterior part of the lateral wall / posterior end of inferior meatus, a recognised source of posterior epistaxis in adults/elderly.

Epistaxis clinical correlation

• Anterior epistaxis → Little's area / Kiesselbach → common in children, trauma (nose picking), usually self-limiting; manage with pinching, anterior packing, silver-nitrate cautery.
• Posterior epistaxis → sphenopalatine artery / Woodruff's plexus → common in elderly, hypertensives; profuse, needs posterior packing or endoscopic sphenopalatine artery ligation.

Nerve Supply

Olfactory (Special Sensory)

The olfactory mucosa (upper concha + corresponding septum) bears bipolar olfactory receptor neurons. Their central processes form ~20 olfactory nerve filaments (CN I) that pierce the cribriform plate of the ethmoid to reach the olfactory bulb.

High-yield flow: Olfactory receptor neuron → unmyelinated axons (olfactory nerve filaments) → cribriform plate → olfactory bulb → mitral cells → olfactory tract → lateral & medial olfactory striae → primary olfactory cortex (piriform cortex, uncus, entorhinal area). Smell is the only special sense that does not relay in the thalamus before reaching cortex.

High-yield: A fracture of the cribriform plate (anterior cranial fossa fracture) tears the olfactory filaments and the meninges → anosmia + CSF rhinorrhoea (clear discharge that tests positive for glucose / β-2 transferrin). Bilateral anosmia also raises suspicion of an olfactory groove meningioma (Foster Kennedy syndrome: ipsilateral optic atrophy + contralateral papilloedema + anosmia).

General Sensory & Autonomic

Function	Nerve
General sensation, antero-superior	Anterior ethmoidal nerve (V1 via nasociliary)
General sensation, postero-inferior (most of lateral wall & septum)	Nasopalatine & posterior superior nasal nerves (V2, via pterygopalatine ganglion)
Secretomotor (parasympathetic) to nasal glands	Greater petrosal nerve → nerve of pterygoid canal → pterygopalatine ganglion → V2 branches
Vasomotor (sympathetic)	Deep petrosal nerve (from internal carotid plexus) → nerve of pterygoid canal

High-yield: The pterygopalatine (sphenopalatine) ganglion is the parasympathetic ganglion for the nose, palate and lacrimal gland. The nasopalatine nerve (long sphenopalatine nerve) runs on the septum, exits via the incisive foramen to supply anterior hard palate — site of nasopalatine duct cysts and dental block.

Paranasal Sinuses

Four paired, air-filled extensions of the nasal cavity within the skull bones, lined by respiratory mucosa, named after the bone containing them: frontal, maxillary, sphenoid, ethmoid. They lighten the skull, add resonance to voice, humidify/warm air, and act as a crumple zone for facial trauma. At birth only the maxillary and ethmoid sinuses are sizeable; the frontal and sphenoid develop after age ~2 and reach adult size after puberty.

The Maxillary Sinus (Antrum of Highmore) — most clinically tested

• Largest sinus; pyramidal, occupies the body of the maxilla.
• Roof = floor of orbit (contains infraorbital nerve) — infections may spread to orbit; Floor = alveolar process, related to roots of premolar & molar teeth (esp. 2nd premolar, 1st molar). Apical dental infection → odontogenic sinusitis; tooth extraction may create an oro-antral fistula.
• Ostium lies high on the medial wall → opens into the hiatus semilunaris of the middle meatus → poor gravitational drainage → predisposed to chronic maxillary sinusitis.

High-yield: The maxillary sinus is the first sinus to develop and the commonest to be infected. It is supplied by the maxillary nerve (V2) — hence pain referred to upper teeth and cheek. Carcinoma of the maxillary antrum may present late with cheek swelling, epistaxis, loosening of teeth, or proptosis.

The Frontal, Sphenoid & Ethmoid Sinuses

Sinus	Location	Drains into	Key relations / clinical
Frontal	Frontal bone, above orbit	Middle meatus (frontonasal duct / infundibulum)	Supplied by supraorbital nerve (V1); infection → frontal headache, Pott's puffy tumour (osteomyelitis)
Sphenoid	Body of sphenoid	Spheno-ethmoidal recess	Related to optic nerve, cavernous sinus, pituitary, ICA; route for trans-sphenoidal hypophysectomy
Ethmoidal (labyrinth)	Between orbit & nasal cavity (lamina papyracea laterally)	Anterior + middle → middle meatus; posterior → superior meatus	Thin lamina papyracea → ethmoiditis spreads to orbit → orbital cellulitis (commonest sinus cause in children)

High-yield: The ethmoidal sinuses are the commonest source of orbital complications of sinusitis in children because the lamina papyracea (paper-thin medial orbital wall) is easily breached. The sphenoid sinus has dangerous relations — internal carotid artery and optic nerve lie in its lateral wall.

Development & Histology Pearls

• Olfactory epithelium: pseudostratified columnar with bipolar olfactory receptor neurons (only neurons that regenerate from basal stem cells), supporting (sustentacular) cells, and Bowman's glands (serous, dissolve odorants).
• Respiratory epithelium: pseudostratified ciliated columnar with goblet cells; cilia beat toward the nasopharynx (mucociliary clearance).
• Swell bodies / cavernous tissue over the inferior concha and lower septum cause the nasal cycle (alternating congestion every few hours).

Complications & Clinical Correlations

• Sinusitis — obstruction of the osteomeatal complex → impaired drainage → infection. Maxillary commonest; pain worse on bending forward.
• Orbital cellulitis — most often from ethmoid sinusitis via lamina papyracea (Chandler classification).
• Cavernous sinus thrombosis — spread from "danger area of face" / sphenoid sinusitis via ophthalmic veins (valveless).
• CSF rhinorrhoea — cribriform plate fracture; "tram-track"/halo sign, β-2 transferrin positive.
• Epistaxis — anterior (Little's area) vs posterior (sphenopalatine/Woodruff).
• Choanal atresia — congenital failure of canalisation of choanae; bilateral → neonatal respiratory distress relieved by crying (cyclical cyanosis) since neonates are obligate nasal breathers.

Key Differentials / Discriminators

Confusing pair	Discriminator
Superior vs middle meatus drainage	Superior = posterior ethmoids only; Middle = frontal + maxillary + anterior & middle ethmoids
Sphenoid vs ethmoid drainage	Sphenoid → spheno-ethmoidal recess; ethmoids split between superior & middle meatus
Anterior vs posterior epistaxis	Anterior = Little's/Kiesselbach (young); Posterior = sphenopalatine/Woodruff (elderly)
Bulla ethmoidalis vs agger nasi	Bulla = middle ethmoid cells (in middle meatus); Agger nasi = anterior ridge, frontal recess cell
Nasolacrimal duct vs sinuses	NLD opens only into the inferior meatus (no sinus does)

Recently asked / exam angle

• "Which structure opens into the inferior meatus?" → Nasolacrimal duct (single best answer; no paranasal sinus drains there).
• "All of the following drain into the middle meatus EXCEPT…" → odd-one-out is usually posterior ethmoidal sinus or sphenoid sinus.
• "Kiesselbach's plexus is formed by all EXCEPT…" → distractor is usually posterior ethmoidal artery (it is the anterior ethmoidal that contributes) or middle meningeal.
• "Chief/principal artery of the nose" → Sphenopalatine artery.
• "Olfactory nerve passes through which foramen/plate?" → Cribriform plate of ethmoid.
• "Sinus draining into spheno-ethmoidal recess?" → Sphenoid sinus.
• "Commonest paranasal sinus involved in orbital cellulitis in children?" → Ethmoid (via lamina papyracea).
• "First paranasal sinus to develop / commonest infected?" → Maxillary.
• Image-based MCQs of the lateral wall labelling bulla ethmoidalis, hiatus semilunaris, conchae are increasingly common.

Rapid revision

1. Bony septum = perpendicular plate of ethmoid + vomer; antero-inferior part is septal cartilage.
2. Sphenoid sinus → spheno-ethmoidal recess; posterior ethmoids → superior meatus.
3. Frontal + maxillary + anterior & middle ethmoids → middle meatus.
4. Nasolacrimal duct → inferior meatus (valve of Hasner).
5. Maxillary sinus: largest, first to develop, commonest infected, ostium high on medial wall → poor drainage.
6. Sphenopalatine artery = chief artery of nose & main posterior epistaxis source; enters via sphenopalatine foramen.
7. Little's area / Kiesselbach's plexus ("LEGS" — Labial, Ethmoidal anterior, Greater palatine, Sphenopalatine) = anterior epistaxis.
8. Woodruff's plexus (posterior inferior meatus) = posterior epistaxis in elderly.
9. Olfactory nerve (CN I) pierces cribriform plate → olfactory bulb; smell bypasses the thalamus.
10. Cribriform plate fracture → anosmia + CSF rhinorrhoea (β-2 transferrin positive).
11. Pterygopalatine ganglion = parasympathetic secretomotor supply to nasal glands (via greater petrosal nerve).
12. Lamina papyracea (ethmoid) is the weak link → ethmoiditis is the commonest cause of paediatric orbital cellulitis.