Paranasal sinuses

Sinus paranasales

Also known as: sinuses, frontal sinus, maxillary sinus, ethmoidal cells, sphenoid sinus, maxillary antrum

Four paired air-filled extensions of the nasal cavity within the frontal, ethmoid, sphenoid and maxillary bones, lined with respiratory mucosa and draining into the nasal cavity.

Show on body map Respiratory Tract

Overview

Present at birthMaxillary and ethmoidal (rudimentary)

Frontal sinus appears≈6–8 years

Maxillary sinus volume (adult)≈15 mL

The sinuses develop as outgrowths of the nasal mucosa into the surrounding bones and are named after them. They lighten the skull, add resonance to the voice, contribute to humidifying air and act as a crumple zone protecting the orbits and brain. They are lined by ciliated respiratory epithelium whose cilia beat towards the natural ostia — not necessarily downward, so drainage depends on mucociliary clearance rather than gravity.

At birth only the maxillary sinuses and ethmoidal cells are present (small); the sphenoid sinus pneumatises in early childhood and the frontal sinus appears around 6–8 years and is not radiologically obvious until ≈7–10 years. Growth continues through adolescence, contributing to the adult shape of the face.

Location: Surrounding the nasal cavity: above (frontal), between the orbits (ethmoidal), behind (sphenoid) and lateral (maxillary).

Parts & subdivisions

Frontal sinus
In the frontal bone above the orbits, often asymmetrical; drains via the frontonasal duct into the middle meatus (ethmoidal infundibulum). Supplied by the supra-orbital nerve (V1).
Ethmoidal cells (air cells)
Multiple small cells in the ethmoidal labyrinth between orbit and nasal cavity: anterior and middle cells → middle meatus; posterior cells → superior meatus. The thin lamina papyracea separates them from the orbit.
Sphenoid sinus
In the body of the sphenoid, below the pituitary gland (sella turcica) and medial to the cavernous sinuses; drains into the spheno-ethmoidal recess. Route of the transsphenoidal approach to pituitary tumours.
Maxillary sinus (antrum of Highmore)
Largest sinus (≈15 mL), pyramidal, in the body of the maxilla; roof = orbital floor, floor = alveolar process (roots of upper molars and premolars). Drains via an ostium high on its medial wall into the semilunar hiatus of the middle meatus. Supplied by the infra-orbital and superior alveolar nerves (V2).

Anatomy

Innervation: Frontal and ethmoidal sinuses: CN V1 (supra-orbital, anterior and posterior ethmoidal nerves). Maxillary sinus: CN V2 (infra-orbital and superior alveolar nerves). Sphenoid sinus: posterior ethmoidal nerve (V1) and orbital branches of V2.
Arterial supply: Branches of the maxillary artery (infra-orbital, superior alveolar, sphenopalatine) and ophthalmic artery (supra-orbital, anterior and posterior ethmoidal).
Relations: Maxillary sinus — orbital floor above, upper molar roots below. Sphenoid — pituitary above, cavernous sinuses (internal carotid artery, CN III, IV, V1, V2, VI) laterally, optic canal superolaterally. Ethmoidal cells — orbit laterally, anterior cranial fossa above.

Function

Lighten the skull.
Add resonance to the voice.
Humidify and warm inspired air; produce mucus that drains into the nose.
Provide a crumple zone protecting the orbits and cranial cavity.

Clinical significance

Acute and chronic rhinosinusitis

Usually follows a viral URTI; obstruction of the ostiomeatal complex traps secretions. The maxillary sinus is most often involved because its ostium is high on the medial wall. Pain over the cheek may be referred to the upper teeth (shared superior alveolar nerves), and dental root infections can cause maxillary sinusitis.

Orbital cellulitis

Ethmoidal sinusitis can spread through the thin lamina papyracea into the orbit, causing proptosis, painful eye movements and reduced vision — an emergency (risk of cavernous sinus thrombosis and abscess).

Blow-out fracture

A blow to the orbit can fracture its thin floor into the maxillary sinus, trapping inferior rectus (impaired upward gaze, diplopia) and injuring the infra-orbital nerve (numb cheek).

Test yourself

Into which part of the nasal cavity does the maxillary sinus drain?

Middle meatus via the semilunar hiatus
Inferior meatus
Superior meatus
Sphenoethmoidal recess

Answer: Middle meatus via the semilunar hiatus

The maxillary ostium opens high on the medial wall of the sinus into the semilunar hiatus of the middle meatus — a poor position for gravitational drainage, which helps explain why maxillary sinusitis is common. The inferior meatus receives the nasolacrimal duct.