The four pairs of paranasal sinuses are the maxillary, ethmoid, frontal, and sphenoid.
A short video demonstrating the function of the sinuses is available.
Anatomical and Developmental Facts
• The sinuses are normally air filled, lined by ciliated, pseudostratified, columnar epithelium with interspersed, goblet-type, mucus-secreting cells.
• The sinuses develop as outpouchings of the nasal chamber into the corresponding bony vaults.
• The ethmoid and maxillary sinuses are the only ones present at birth.
• The frontal and sphenoid sinuses start their growth after 3 years.
• The sinuses reach full development during adolescence.
The development of sinuses
Gestational month when development starts
Present in clinically significant size
The Nasal Turbinates
• There are three nasal turbinates—the superior, middle, and inferior, and, rarely, a fourth—the supreme.
• The role of the turbinates is to
a) regulate the intranasal pressure
b) provide olfaction
c) provide humidification
d) facilitate mucus secretion
e) absorb shocks
f) resonance the voice
g) diminish the skull weight.
Maxillary Sinus Facts
• The first to develop in the fetus in the second gestational month.
• At delivery, it is small (5–8 mm), and filled with fluid.
• It grows rapidly between birth and 3 years and from 7 to 18 years and reaches a volume of approximately 12 to 15 ml.
• Septa can rarely separate it into compartments.
• The first and second molar and second bicuspid teeth may project into its floor. Infection originating from these roots can spread into the sinuses.
• It communicates with the infundibulum in the middle meatus through the ostia, located in the superior and anterior portion of the medial wall.
• The outflow tract is against gravity and is upward and high on the medial wall.
• Drainage depends on ciliary action that moves secretions through the ostium toward the nose.
Ethmoid Sinus Facts
• Three to four ethmoid cells are generally present at birth.
• When fully developed, it is made of two groups of air cells, the anterior (smaller) and posterior (larger).
• There are 3 to 15 individual air cells, each of which open through their own ostium.
• The lamina papyracea forms the lateral wall. This paper-thin bone forms the orbital medial wall.
• The sinus roof forms the base of the skull.
• The cribriform plate is medial.
• Because of the thinness of the walls, infection can extend directly through the bone to the orbit.
Frontal Sinus Facts
• The sinuses are not seen in plain radiographs before 2 years of age.
• They are difficult to differentiate radiographically from the ethmoids until 6 to 8 years of age.
• They drain into the middle meatus via the frontal recess.
• The ostium between the sinus and nasal cavity can be short or wide.
• Agenesis occurs in approximately 5% of the population.
• Asymmetry is very common.
Sphenoid Sinus Facts
• The sinus occupies a strategic position in the base of the skull and borders several vital structures:
a) superiorly the optic nerve and pituitary gland
b) posteriorly the pons
c) anteriorly and laterally the superior orbital fissure, the cavernous sinus, and the internal carotid artery.
• A septum separates the right sinus from the left.
• Asymmetry is very common.
• Drainage into the nasal cavity is through the sphenoid ostium into the sphenoethmoid recess.
• It is relatively isolated, and therefore infrequently infected.
• Turbinates are scrolls of bone, covered by mucous membrane made of ciliated, pseudostratified, columnar epithelium.
• The turbinates project from the nasal cavity lateral wall.
• Their shape maximizes the nasal surface area mucous lining, which enables inspired air filtration, warming, and humidification.
• Each nasal side usually has three turbinates:
d) rarely, a fourth supreme.
• A smaller bony prominence anterior to the middle turbinate overlies the lacrimal gland and is called agger nasi.
• Meati are the spaces created by the turbinates.
• Below each turbinate is a corresponding meatus:
• The nasolacrimal duct opens into the inferior meatus at its anterior portion.
• The posterior ethmoid cells and sphenoid sinus open high in the nasal vault into the superior meatus.
• The frontal, maxillary, and anterior ethmoid sinuses drain to a common channel that drains into the middle meatus (the anterior sinus drainage system).
• The infundibulum that drains the maxillary sinus is approximately 6 mm long and 25 mm in diameter.
Osteomeatal Complex Facts
• The osteomeatal complex (OMC) is where the ostia and drainage channels of the maxillary and frontal sinuses are anatomically related to the anterior ethmoids.
• The complex consists of
a) anterior ethmoid sinuses
b) ostia of the frontal sinuses
c) ostia and infundibulum of the maxillary sinuses
d) middle meatus of the nasal cavity.
• Blockage or inflammation at the OMC is responsible for the de velopment of bacterial sinusitis, as it interferes with effective clearance.