Nasal Cavity and Paranasal Sinuses

Paired cavities, divided by the nasal septum, extending from the nostrils (anterior nasal apertures) to the choanae (posterior nasal apertures, opening into the nasopharynx).

Roof:

• Formed by the nasal bone (anteriorly), the cribriform plate of the ethmoid (centrally, transmitting olfactory nerve filaments), and the sphenoid body (posteriorly).

Floor:

• Formed by the palatine process of the maxilla and the horizontal plate of the palatine bone (together forming the hard palate), separating the nasal cavity from the oral cavity.

Medial wall:

• The nasal septum: shared, dividing the two nasal cavities.

Lateral wall:

• The most anatomically complex wall, bearing the three conchae (superior, middle, inferior) and their corresponding meatuses, and the drainage openings of the paranasal sinuses and nasolacrimal duct.

Septal cartilage:

• The anterior, flexible cartilaginous portion.

Perpendicular plate of ethmoid:

• The superior/posterior bony portion, continuous with the cribriform plate above.

Vomer:

• The posteroinferior bony portion, articulating with the sphenoid (posterosuperiorly) and the palatine/maxillary bones (inferiorly).

• The septum is rarely perfectly midline (a deviated septum is a common normal anatomical variant, occasionally significant enough to cause nasal obstruction symptoms).

• Each concha (turbinate) is a curved bony shelf projecting from the lateral wall, increasing the surface area for warming/humidifying inspired air and creating turbulent airflow that improves particulate filtration; the space beneath each concha is termed a meatus.

Superior concha:

• The smallest, most posterior/superior, part of the ethmoid bone.

Middle concha:

• Part of the ethmoid bone, a key surgical landmark for accessing the middle meatus in endoscopic sinus surgery.

Inferior concha:

• The largest, an independent bone (not part of the ethmoid, unlike the other two).

Superior meatus:

• Beneath the superior concha; receives drainage from the posterior ethmoidal air cells.

Middle meatus:

• Beneath the middle concha; receives drainage from the frontal sinus, maxillary sinus, and anterior/middle ethmoidal air cells: the single most clinically important meatus, given the convergence of drainage from most of the paranasal sinus system here.

Inferior meatus:

• Beneath the inferior concha; receives the opening of the nasolacrimal duct.

Sphenoethmoidal recess:

• A small space superior/posterior to the superior concha (not a true "meatus" beneath a concha, but grouped functionally with them); receives drainage from the sphenoidal sinus.

• Frontal sinus → drains via the frontonasal duct into the infundibulum, then the middle meatus.
• Maxillary sinus → drains via its ostium (located high on the medial wall, an anatomically unfavourable, gravity-independent position for drainage; see Applied Anatomy) into the middle meatus (via the infundibulum, often through the same general region as the frontal sinus drainage, the osteomeatal complex, a critical anatomical/surgical concept representing the common final drainage pathway for the frontal, maxillary, and anterior ethmoidal sinuses, and therefore a key target/bottleneck in the pathophysiology and endoscopic surgical treatment of chronic sinusitis).
• Anterior ethmoidal cells → drain into the middle meatus (via the infundibulum/ethmoidal bulla region).
• Middle ethmoidal cells → drain into the middle meatus, characteristically opening directly onto (or bulging as) the ethmoidal bulla, a rounded prominence on the lateral wall of the middle meatus, a useful surgical landmark.
• Posterior ethmoidal cells → drain into the superior meatus.
• Sphenoidal sinus → drains into the sphenoethmoidal recess.
• Nasolacrimal duct → drains into the inferior meatus.

Summary principle: anterior/middle sinus groups (frontal, maxillary, anterior/middle ethmoidal) all converge on the middle meatus (the osteomeatal complex); posterior structures (posterior ethmoidal, sphenoidal) drain more posteriorly/superiorly (superior meatus, sphenoethmoidal recess).

• The nasal cavity receives a rich blood supply from both the internal and external carotid arterial systems, converging particularly at the anteroinferior septum (see Kiesselbach's area below).

Sphenopalatine artery:

• The terminal branch of the maxillary artery (external carotid system), entering the nasal cavity via the sphenopalatine foramen, the dominant arterial supply to the posterior/lateral nasal cavity and septum, the single largest contributor to nasal blood supply, and the source of most severe posterior epistaxis.

Anterior ethmoidal artery:

• A branch of the ophthalmic artery (internal carotid system), entering the nasal cavity via the anterior ethmoidal foramen, supplying the anterosuperior nasal cavity/septum.

Posterior ethmoidal artery:

• Also a branch of the ophthalmic artery, entering via the posterior ethmoidal foramen, supplying the posterosuperior nasal cavity.

Greater palatine artery:

• A branch of the maxillary artery, primarily supplying the hard palate, but sending a small branch through the incisive canal to contribute to the anteroinferior septal blood supply.

Superior labial artery:

• A branch of the facial artery (external carotid system), contributing to the anterior nasal septum's blood supply via a small septal branch.

• A vascular plexus on the anteroinferior nasal septum, formed by the anastomosis of four arteries (representing both major arterial systems of the head): the sphenopalatine artery (posterior contribution), the anterior ethmoidal artery (superior contribution), the greater palatine artery (inferior contribution, via the incisive canal), and the superior labial artery (anterior contribution, a branch of the facial artery).
• This richly anastomotic, superficially located plexus is the site of the vast majority (approximately 90%) of epistaxis (nosebleeds), particularly in younger patients, given its exposed anterior position (easily traumatised by nose-picking or minor injury) and the convergence of multiple vessels in a relatively small, superficial area.

Olfactory innervation:

• The olfactory epithelium, located in the roof of the nasal cavity (superior concha/septum region), gives rise to CN I fibres, passing through the cribriform plate.

General sensory innervation:

• Anterosuperior nasal cavity: Anterior ethmoidal nerve (branch of the nasociliary nerve, ophthalmic division/V1).
• Posteroinferior nasal cavity (the majority): Branches of the maxillary division (V2), via the pterygopalatine ganglion's sensory pass-through fibres (nasopalatine and posterior nasal nerves).

Frontal sinus:

• Location: Within the frontal bone, above the medial orbital margins, variable in size (absent or minimal in young children, developing through adolescence).
• Drainage: Frontonasal duct → middle meatus (osteomeatal complex).
• Relations: Orbit (inferiorly/laterally), anterior cranial fossa (posteriorly), a thin posterior wall makes frontal sinusitis a potential source of intracranial spread.

Maxillary sinus:

• Location: Within the maxilla, the largest paranasal sinus, forming part of the orbital floor (roof of the sinus).
• Drainage: Via its ostium (high on the medial wall) → middle meatus.
• Relations: Orbital floor (roof of sinus), upper posterior teeth (premolars/molars, their roots often project into or closely approximate the sinus floor, see Applied Anatomy), nasal cavity (medially).

Ethmoidal air cells (anterior, middle, posterior):

• Location: Within the ethmoid bone, between the orbits, a honeycomb of small air cells rather than a single large cavity.
• Drainage: Anterior/middle cells → middle meatus; posterior cells → superior meatus.
• Relations: Orbit (laterally, separated by the thin lamina papyracea, a common route for orbital cellulitis spread), anterior cranial fossa (superiorly, via the cribriform plate region).

Sphenoidal sinus:

• Location: Within the sphenoid body, the most posterior sinus.
• Drainage: Sphenoethmoidal recess.
• Relations: Pituitary gland (superiorly, separated by only a thin bony floor of the sella turcica, the anatomical basis for the transsphenoidal approach to pituitary surgery, accessing the gland via the nasal cavity and through the sphenoidal sinus), cavernous sinus and internal carotid artery (laterally), optic nerve/chiasm (superiorly).

Epistaxis:

• The vast majority arises anteriorly from Kiesselbach's area, typically manageable with direct pressure/local measures; severe posterior epistaxis (less common but more dangerous, given the larger vessel calibre and difficulty achieving direct pressure) most often originates from the sphenopalatine artery, sometimes requiring surgical/endoscopic arterial ligation or embolisation for control.

Sinusitis:

• Inflammation/infection of one or more paranasal sinuses, most commonly related to obstruction of the osteomeatal complex (the shared drainage pathway for the frontal, maxillary, and anterior ethmoidal sinuses), this anatomical bottleneck explains why sinusitis often involves multiple sinuses simultaneously (given their shared drainage route) and is the key target of functional endoscopic sinus surgery (FESS), which aims to restore normal drainage through this region rather than simply removing diseased tissue.

Maxillary sinus dental relationships:

• The roots of the upper premolar and molar teeth often lie in very close proximity to (or occasionally protrude into) the maxillary sinus floor, separated by only a thin layer of bone (or, in some individuals, no bone at all), explaining why dental infections/extractions in this region can cause or be complicated by maxillary sinusitis, and conversely why maxillary sinus pathology can sometimes present with dental pain (a diagnostic pitfall worth recognising).

Spread to orbit:

• The thin lamina papyracea (medial orbital wall, formed by the ethmoid bone) provides minimal barrier between the ethmoidal air cells and the orbit, making ethmoidal sinusitis the most common source of orbital cellulitis/abscess via direct spread through this thin bony wall, a serious complication given the orbit's further connections to the cavernous sinus, potentially progressing to vision-threatening or intracranial complications if not promptly treated.