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.
Fig 1 – Location of the 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.
Clinical Relevance – Transsphenoidal Surgery
The sphenoid bone shares a close anatomical relationship with the pituitary gland. Indeed, the pituitary can be accessed surgically by passing instruments through the sphenoid bone and sinus.
This type of surgery is known as endoscopic trans-sphenoidal surgery (ETSS), and is the usual treatment of choice for pituitary adenomas. It allows for the surgical management of pituitary pathology without the need for a more extensive craniotomy.
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.
Fig 3 – The conchae have been removed, showing the various openings on the lateral wall of the nasal cavity.
Clinical Relevance: Sinusitis
As the paranasal sinuses are continuous with the nasal cavity, an upper respiratory tract infection can spread to the sinuses. Infection of the sinuses causes inflammation (particularly pain and swelling) of the mucosa, and is known as sinusitis. If more than one sinus is affected, it is called pansinusitis.
The maxillary nerve supplies both the maxillary sinus and maxillary teeth, and so inflammation of that sinus can present with toothache.
