Paranasal sinuses and their syntopy, their development

The paranasal sinuses are air-filled extensions of the nasal cavity. There are four paired sinuses – named according to the bone in which they are located – maxillary, frontal, sphenoid and ethmoid. Each sinus is lined by a ciliated pseudostratified epithelium, interspersed with mucus-secreting goblet cells.

The function of the paranasal sinuses is a topic of much debate. Various roles have been suggested:

• Lightening the weight of the head
• Supporting immune defence of the nasal cavity
• Humidifying inspired air
• Increasing resonance of the voice

The paranasal sinuses are formed during development by the nasal cavity eroding into the surrounding bones. All the sinuses therefore drain back into the nasal cavity – openings to the paranasal sinuses can be found on the roof and lateral nasal walls.

In this article, we shall look at the anatomy of the paranasal sinuses – their structure, anatomical relations and innervation.

Paranasal Sinuses

Frontal Sinuses

There are two frontal sinuses located within the frontal bone of the skull. They are the most superior of the paranasal sinuses, and are triangular in shape.

Drainage is via the frontonasal duct. It opens out at the hiatus semilunaris, within the middle meatus of the nasal cavity.

Sensation is supplied by the supraorbital nerve (a branch of the ophthalmic nerve), and arterial supply is via the anterior ethmoidal artery (a branch of the internal carotid).

Sphenoid Sinuses

The sphenoid sinuses are situated within the body of the sphenoid bone. They open out into the nasal cavity in an area supero-posterior to the superior cocha – known as the spheno-ethmoidal recess.

They are innervated by the posterior ethmoidal nerve (a branch of the ophthalmic nerve), and branches of the maxillary nerve. They recieve blood supply from pharyngeal branches of the maxillary arteries.

Ethmoidal Sinuses

There are three ethmoidal sinuses located within the ethmoid bone:

• Anterior – Opens onto the hiatus semilunaris (middle meatus)
• Middle – Opens onto the lateral wall of the middle meatus
• Posterior – Opens onto the lateral wall of the superior meatus

They are innervated by the anterior and posterior ethmoidal branches of the nasociliary nerve and the maxillary nerve. The anterior and posterior ethmoidal arteries are responsible for arterial supply.

Maxillary Sinuses

The maxillary sinuses are the largest of the sinuses. They are located laterally and slightly inferiorly to the nasal cavities.

They drain into the nasal cavity at the hiatus semilunaris, underneath the frontal sinus opening. This is a potential pathway for spread of infection – fluid draining from the frontal sinus can enter the maxillary sinus.

Development of the Paranasal Sinuses

The paranasal sinuses develop as extensions of the nasal cavity into adjacent bones, with each sinus following a distinct timeline:

• Maxillary Sinuses: These are the first to develop, beginning as outpouchings from the lateral wall of the nasal cavity during the third month of gestation. They continue to grow postnatally, with significant expansion occurring during periods of dental development.
• Ethmoid Sinuses: Development starts around the fifth month of gestation as multiple invaginations from the lateral wall of the nasal capsule. At birth, the ethmoid sinuses are the most developed among the paranasal sinuses, containing a complete number of cells in varying stages of development. Rapid progression in ethmoid sinus development occurs during the first two years of life, with a second phase of rapid development just before puberty. Complete development is typically achieved by puberty.
• Sphenoid Sinuses: These originate from the posterior part of the nasal capsule during the third month of gestation. Although minimal in size at birth, the sphenoid sinuses undergo significant pneumatization during childhood, reaching their adult size between the ages of 9 and 12.
• Frontal Sinuses: Development begins with the anterior pneumatization of the frontal recess into the frontal bone around the 16th week of gestation. However, the frontal sinuses are not fully developed until late adolescence or early adulthood.

Mechanisms of Sinus Development

The development of the paranasal sinuses involves complex interactions between epithelial and mesenchymal tissues. The lateral nasal wall exhibits a series of folds called ethmoturbinals during the eighth week of gestation. Initially, five to seven folds appear, but through processes of fusion and regression, three or four folds remain by the 15th week. The primary furrows formed between these ethmoturbinals give rise to various meatuses and recesses within the nasal cavity. Additionally, the cartilaginous nasal capsule surrounding the nasal cavity contributes to the formation of the paranasal sinuses and lateral nasal wall structures. Some researchers propose that this cartilaginous capsule plays a primary role in sinus development, with the formation of ridges and furrows being a secondary phenomenon.

Postnatal Development and Pneumatization

After birth, the paranasal sinuses continue to grow and pneumatize, a process influenced by genetic and environmental factors. The maxillary and ethmoid sinuses are present at birth but undergo significant expansion during early childhood. The sphenoid and frontal sinuses develop later, with the sphenoid sinuses reaching adult size by early adolescence and the frontal sinuses continuing to develop into late adolescence or early adulthood. Pneumatization involves the resorption of bone and the expansion of the sinus cavities, contributing to the overall growth and morphology of the facial skeleton.

https://teachmeanatomy.info/head/organs/the-nose/paranasal-sinuses/

Elwany, S., Askar, M. (2023). Development of the Nose, Paranasal Sinuses. In: Rhinology Review. Springer, Cham. https://doi.org/10.1007/978-3-031-08794-3_1

The Paranasal Sinuses. In: Wells RG. eds. Diagnostic Imaging of Infants and Children. McGraw-Hill Education; 2013. Accessed December 23, 2024. https://accesspediatrics.mhmedical.com/content.aspx?bookid=1429&sectionid=84706028