What is the anatomy of the sphenoid sinus?

Sphenoid Sinus Anatomy

Location and Basic Structure

The sphenoid sinus is a paired pneumatized cavity located within the body of the sphenoid bone, positioned centrally at the skull base with critical neurovascular structures in immediate proximity. 1, 2

Developmental Timeline

• The sphenoid sinus begins development around age 3-4 years and reaches near-adult size by age 12-14 years. 3
• Pneumatization occurs later than the maxillary and ethmoid sinuses, which are present at birth. 3
• In young children, absent or minimal sphenoid sinuses are developmentally normal and should not be interpreted as pathologic on imaging studies. 3

Pneumatization Patterns and Classification

The degree of sphenoid sinus pneumatization is highly variable and can be classified into distinct types based on the extent of air cell development 4:

• Type I (0.3%): Completely absent or minimal pneumatization 4
• Type II (6.6%): Posterior wall lies anterior to the anterior sellar wall 4
• Type III (57.2%): Posterior wall extends between the anterior and posterior sellar walls—the most common pattern 4
• Type IVa (17.9%): Posterior wall extends beyond the posterior sellar wall without air posterior to the sella 4
• Type IVb (17.9%): Similar to IVa but with pneumatization extending posterior to the sella 4

Pneumatization Extensions

Beyond the basic sinus body, pneumatization can extend into multiple directions, creating surgical "windows" for transsphenoidal approaches 5:

• Lateral extensions toward the cavernous sinus and Meckel's cave 5
• Clival extensions posteriorly 5
• Lesser wing extensions laterally and superiorly 5
• Anterior extensions 5
• Pterygoid process pneumatization occurs in 39.7% of patients 6
• Anterior clinoid process pneumatization occurs in 17.2% of patients 6

Critical Neurovascular Relationships

Internal Carotid Artery

• The internal carotid artery courses through the lateral wall of the sphenoid sinus within the cavernous sinus. 7
• A smooth, covered course occurs in 89.5% of cases, while a free (dehiscent) course projecting into the sinus lumen occurs in 10.5%. 4
• Bony dehiscence over the carotid canal occurs in 2.7% of cases and must be identified preoperatively to prevent catastrophic arterial injury. 7, 4

Optic Nerve

• The optic nerve courses through the optic canal in the superolateral wall of the sphenoid sinus. 7
• A smooth, covered course occurs in 87.0% of cases, while a free course projecting into the sinus occurs in 13.7%. 4
• Bony dehiscence over the optic canal occurs in 16.7% of cases and must be identified preoperatively to prevent visual loss. 7, 4
• Optic canal protrusion is significantly associated with anterior clinoid pneumatization. 6

Cavernous Sinus

• The cavernous sinuses flank the sella turcica laterally and contain cranial nerves III, IV, V1, V2, and VI along with the internal carotid artery. 7
• Tumor invasion into the cavernous sinus affects surgical approach planning. 7

Vidian Canal and Foramen Rotundum

• Vidian canal protrusion into the sphenoid sinus occurs in approximately 30% of cases (158 of 534 sides examined), with 60 being bilateral. 6
• Vidian canal protrusion is significantly associated with pterygoid process pneumatization. 6
• Foramen rotundum protrusion is also significantly associated with pterygoid pneumatization. 6

Ostium and Drainage

• The sphenoid sinus ostium opens into the sphenoethmoidal recess, located superior to the superior turbinate. 8
• The ostium can sometimes be visualized and cannulated during nasal endoscopy by deflecting the endoscope tip superiorly in the sphenoethmoidal recess. 8
• Pus may be visualized streaming from the ostium in cases of sphenoid sinusitis. 8

Septations

• The sphenoid sinus is typically divided by a midline septum, though this septum is frequently asymmetric and may insert laterally onto the carotid canal rather than in the midline. 7
• Accessory septations are common anatomical variants that can complicate surgical approaches. 1

Clinical Implications and Surgical Considerations

Preoperative Imaging Requirements

• Thin-section CT with multiplanar reformatting and MRI must be obtained before any sphenoid sinus surgery to identify bony dehiscence, variant anatomy, and septum insertion patterns. 7, 6
• CT provides superior osseous detail for identifying bony dehiscence and pneumatization patterns. 7
• In pediatric patients with incompletely pneumatized sinuses, transsphenoidal surgery remains feasible with appropriate image guidance. 7

Surgical Complications to Avoid

The sphenoid sinus's intimate relationship with critical structures creates significant surgical risk 8, 7:

• Cerebrospinal fluid leak from inadvertent dural violation 8, 7
• Internal carotid artery injury or pseudoaneurysm formation—potentially fatal 7
• Optic nerve injury causing visual loss 8, 7
• Meningitis from contamination 8, 7
• Cranial nerve deficits from cavernous sinus involvement 8
• Hemorrhage in the immediate postoperative period 8

Infection Complications

• Sphenoid sinusitis carries significant risk for intracranial complications due to direct extension into the cavernous sinuses, resulting in cranial nerve deficits and internal carotid artery thrombosis. 8, 3
• Infection may extend directly into veins draining into the cavernous sinuses. 8
• The proximity to the skull base increases the propensity for intracranial complications compared to other paranasal sinuses. 3

Common Pitfalls

• Failing to recognize that sphenoid sinuses are not fully developed in children under 6 years of age, leading to inappropriate imaging interpretation or misdiagnosis of frontal sinusitis. 3
• Assuming symmetric anatomy—the sphenoid septum frequently inserts asymmetrically onto the carotid canal. 7
• Underestimating the frequency of bony dehiscence over neurovascular structures (16.7% for optic nerve, 2.7% for carotid artery). 4
• Not obtaining adequate preoperative imaging before any surgical intervention in this region. 7, 6