Sphenoid sinus

The sphenoid sinus is a paired paranasal sinus occurring within the body of the sphenoid bone. It represents one pair of the four paired paranasal sinuses. The pair of sphenoid sinuses are separated in the middle by a septum of sphenoid sinuses. Each sphenoid sinus communicates with the nasal cavity via the opening of sphenoidal sinus. The two sphenoid sinuses vary in size and shape, and are usually asymmetrical.

Structure
On average, a sphenoid sinus measures 2.2 cm vertical height, 2 cm in transverse breadth; and 2.2 cm antero-posterior depth.

Each spehoid sinus is contained within the body of sphenoid bone, being situated just inferior to the sella turcica. The two sphenoid sinuses are separated medially by the septum of sphenoidal sinuses (which is usually asymmetrical).

An opening of sphenoidal sinus forms a passage between each sphenoidal sinus, and the nasal cavity. Posteriorly, an opening of sphenoidal sinus opens into the sphenoidal sinus by an aperture high on the anterior wall the sinus; anteriorly, an opening of sphenoidal sinus opens into the roof of the nasal cavity via an aperture on the posterior wall of the sphenoethmoidal recess (occurring just superior the choana).

Innervation
The mucous membrane receives sensory innervation from the posterior ethmoidal nerve (branch of the ophthalmic nerve (CN V1)), and branches of the maxillary nerve (CN V2).

Postganglionic parasympathetic fibers of the facial nerve that synapsed at the pterygopalatine ganglion control mucus secretion.

Anatomical relations
Proximal structures include: the optic canal and optic nerve, internal carotid artery, cavernous sinus, trigeminal nerve, pituitary gland, and the anterior ethmoidal cells. One study found that carotid canal protrudation into the sphenoid sinus wall was present 23.9-32.1% in males and 35.5-36.2% in females, dehiscence in carotid canal was detected more in females (34%) compared to males (22%), optic canal protrudation was 33.3 and 30.5% in males and females, and optic canal dehiscence was detected 11.3 and 9.9% in males and females.

Anatomical variation
The sphenoid sinuses vary in size and shape, and, owing to the lateral displacement of the intervening septum of sphenoid sinuses, are rarely symmetrical.

When exceptionally large, the sphenoid sinuses may extend into the roots of the pterygoid processes or greater wings of sphenoid bone, and may invade the basilar part of the occipital bone.

The septum of the sphenoidal sinuses may be partially or completely absent. Additional incomplete septa may also be present.

Development
The sphenoidal sinuses are minute at birth; their main development takes place after puberty.

Clinical significance
The sphenoid sinuses cannot be palpated on physical examination. However, patients with isolated sphenoid sinusitis may complain of occipital or vertex headache, retro-orbital pain, otalgia, drowsiness, or meningitis-like symptoms.

A potential complication of sphenoidal sinusitis is cavernous sinus thrombosis.

If a fast-growing tumor erodes the floor of the sphenoidal sinus, the vidian nerve could be in danger. If the tumor spreads laterally, the cavernous sinus and all its constituent nerves could be in danger.

An endonasal surgical procedure called a sphenoidotomy may be carried out to enlarge the sphenoid sinus, usually in order to drain it.

Of note, the sphenoid sinus should be distinguished from an Onodi cell, an anatomic variant that is the posterior-most ethmoidal air cell. Onodi cells typically extend posteriorly to lie superolateral to the sphenoid sinus and thus in close proximity to the optic nerve and internal carotid artery. Failure to recognize an Onodi cell on CT scan prior to surgery may put these structures at risk. One study found that an Onodi cell was present 26.6% and 19.1% in males and females, respectively.

Transsphenoidal surgery
Because only thin shelves of bone separate the sphenoidal sinuses from the nasal cavities below and hypophyseal fossa above, the pituitary gland can be surgically approached through the roof of the nasal cavities by first passing through the anterioinferior aspect of the sphenoid bone and into the sinuses, followed by entry through the top of the sphenoid bone into the hypophyseal fossa.