General Medical Information

Sphenoidal Sinus

Ronald G. Amedee
Andrew J. Miller

The sphenoidal sinus usually is an asymmetrically paired structure divided by a deviated intersinus septum. Congdon classified the extent of pneumatization of the sinuses into three types. The first type, described as conchal, exists when the posterior extent of the sphenoidal sinus is well anterior to the sella turcica. Presellar pneumatization occurs when the posterior wall of the sphenoidal sinus reaches the anterior face of the sella turcica. Postsellar pneumatization is present when the sphenoidal sinus extends past the level of the sella turcica to approach the pons posteriorly and allows the sella to make a superior indentation in the sinus. These three types are present in 5%, 23.5%, and 67% of the population respectively, the other 4.5% manifest as intermediates between the conchal and presellar types.

The sphenoidal sinus can undergo extensive pneumatization with possible extension into the vomer, palatine, and maxillary bones as well as the greater wing of the sphenoid along with the pterygoid plates. The bony tubercle surrounding the optic nerve commonly becomes pneumatized, producing anterosuperior indentation in the roof of almost every sphenoidal sinus. The bony covering usually is less than 0.5 mm and is dehiscent in about 4% of the population. External and lateral to the sphenoidal sinus is the cavernous sinus, through which run many vital structures. Growth of the sphenoidal sinus can place many of these structures in proximity. For example, the internal carotid artery forms a prominence covered by thin bone in the lateral wall of the sphenoidal sinus in about 90% of the population, and 7% of these vessels are dehiscent. The maxillary branch of the trigeminal nerve and the vidian nerve produce bulges in the sphenoidal sinus in 30% of the population. Extreme caution must be taken when dissecting within the sphenoidal sinus.

Each sphenoidal sinus empties into the sphenoethmoidal recess through a small ostium. This ostium, which measures 0.5 to 4 mm, is located 10 mm above the sinus floor, or 30 degrees above the floor of the nasal cavity. The ostium is almost always membranous, the bony circumference being considerably larger than the orifice, as is common with the maxillary sinus ostium.