The optic nerve and the internal carotid artery in the sphenoid sinus. (Rhinoscopic Clinic).

Several structures can bulge into the sphenoid sinus. These structures include (1) the optic nerve and the internal carotid artery in the superolateral wall, (2) the posterior ethmoid cells in the anterosuperior wall (the Onodi cell), (3) the maxillary nerve in the lateral wall, and (4) the canal of the vidian nerve in the floor.

The bulges produced by the optic nerve and the internal carotid artery are of considerable clinical importance (Figure). Depending on the degree of pneumatization of the sphenoid sinus, these bulges can be either barely noticeable or quite obvious. If the anterior clinoid process of the lesser wing of the sphenoid bone is pneumatized, there might be a deep recess (infraoptic recess) in the superior and lateral corner of the sphenoid sinus (Figure, A & B). (1,2) Such a recess can clearly separate the optic nerve from the internal carotid artery (Figure, A & B). If the anterior clinoid process is not pneumatized, the optic nerve is sometimes difficult to identify.

The superior bulge of the optic nerve extends near the roof horizontally from posterior to anterior in the superolateral part of the sphenoid sinus and usually disapears gradually toward the anterior wall (Figure, B & C). The bony covering of the optic nerve is usually seen and it is dehiscent in 4 to 6% of cases. (2,3)

The internal carotid artery originates in the common carotid artery in the neck and ascends toward the brain. It has cervical, petrous, cavernous, and cerebral portions. The internal carotid artery enters the carotid canal in the petrous portion of the temporal bone. The cavernous portion of the internal carotid artery lies within the cavernous sinus. The artery lies adjacent to the sphenoid sinus during its passage through the cavernous sinus and produces a variable bulge in the lateral wall of the sphenoid sinus (Figure, B-D). (2) These variations can cause different patterns of bulges in the internal carotid arteries in the sphenoid sinus. (2) The bulge usually runs obliquely from inferior to superior in the lateral wall of the sphenoid sinus (Figure, B). In extreme cases, the carotid bulge is prominent, and the bulges in both carotid arteries almost make contact in the midline (Figure, D). (2) The bulges can occupy most of the sphenoid sinus.

The bony covering of the internal carotid artery can be partially dehiscent (25%). (2,4) The artery can be entirely exposed without a bony covering (Figure, B).

The endoscopic sinus surgeon should remember the anatomic relationship of the optic nerve and the internal carotid artery in the sphenoid sinus. Palpation of the superolateral wall of the sphenoid sinus with a sharp, pointed instrument during surgery and during the postoperative period, when a sphenoidotomy is done, should be avoided to prevent injury to the exposed optic nerve or internal carotid artery. Inadvertent injury to these structures can result in blindness and catastrophic hemorrhage.

References

(1.) Yanagisawa E. Endoscopic view of sphenoid sinus cavity. Ear Nose Throat J 1993;72:393-4.

(2.) Stammberger H. Functional Endoscopic Sinus Surgery: The Messerklinger Technique. Philadelphia: B.C. Decker, 1991.

(3.) Wigand ME. Endoscopic Surgery of the Paranasal Sinuses and Anterior Skull Base. New York: Thieme Medical Publishers, 1990.

(4.) Kennedy DW, Bolger WE, Zinreich SJ. Diseases of the Sinuses: Diagnosis and Management. Hamilton, Ont.: B.C. Decker, 2001.

From the Southern New England Ear, Nose, Throat, and Facial Plastic Surgery Group, New Haven, Coon.; the Section of Otolaryngology, Yale University School of Medicine, New Haven; and the Hospital of St. Raphael, New Haven.