Prevalence of Sphenoid Sinus Anatomical Variants in Endoscopic Skull-Base Surgery

The sphenoid sinus is one of the most variable cavities in the skull base, with pneumatization ranging from rudimentary conchal types to extensive postsellar and lateral recess patterns that extend into the clivus, pterygoid processes and greater wings.^{1, 2} Because the sinus forms a corridor to the sella and parasellar region, variation in its walls and septa directly alters the safety margin around the internal carotid arteries (ICAs), optic nerves and vidian canals during endoscopic skull-base surgery.^{2, 4}

Modern CT-based work has popularised Friedman/Yonsei-type classifications that distinguish conchal (type I), presellar (type II), sellar (type III) and postsellar or extensive lateral (type IV) pneumatization, allowing surgical series to correlate sinus type with ICA and optic nerve protrusion.^{3, 6} In parallel, Onodi cells, posterior ethmoid cells pneumatized superolateral to the sphenoid sinus, intersphenoid septal insertion on the ICA, carotid canal dehiscence, vidian canal variants and Delano optic nerve types have emerged as critical radiological markers of risk in transsphenoidal and extended endonasal approaches.^{5, 8}

This article provides a meta-analysis style synthesis of the best available radiological and anatomical data on sphenoid sinus variants relevant to endoscopic skull-base surgery, focusing on prevalence patterns that are robust enough to guide pre-operative planning.

This synthesis draws on high-quality CT and CBCT cohorts and key anatomical series that report sphenoid sinus type and surgically relevant variants in adults, with slice thickness typically at or below 1 mm.^{1, 3} Priority was given to studies using Friedman/Yonsei or compatible sagittal classifications of pneumatization and to work that quantified Onodi cells, intersphenoid septa, ICA dehiscence or protrusion, vidian canal morphology and optic nerve course.

Representative large cohorts from North Africa, the Middle East, Europe and South Asia were selected to illustrate regional patterns rather than to generate a single pooled estimate, because imaging protocols and classification schemes remain heterogeneous.^{2, 6} Where possible, percentages reported in the charts reflect rounded values from individual series rather than de novo statistical pooling.

Global Patterns of Sphenoid Pneumatization

Across CT cohorts using sagittal classifications compatible with Friedman/Yonsei, sellar (type III) pneumatization is consistently the dominant pattern, usually accounting for roughly two-thirds of sinuses, while postsellar or extensive lateral (type IV) patterns add a further fifth.^{1, 3} In the Libyan CT study of 225 adults, conchal, presellar, sellar and combined postsellar subtypes measured 1 %, 11.5 %, 35.5 % and 52 % respectively, with greater pneumatization strongly associated with ICA and optic nerve protrusion into the sinus.^{1, 7}

Multidetector CT work from India and Croatia has reported broadly similar distributions, with conchal and presellar forms each below 15 % and a combined sellar/postsellar prevalence between 70 % and 85 %, underscoring that a well-pneumatized sinus is the rule rather than the exception in surgical populations.^{3, 6}

Regional Variation in Pneumatization Patterns

Regional comparisons suggest that conchal and presellar types are slightly more frequent in Middle Eastern and some South Asian cohorts, whereas extensive sellar/postsellar pneumatization is particularly common in European and East Asian series.^{2, 6} Even so, in all regions reviewed here, at least 70 % of patients demonstrated a sellar or postsellar sinus, ensuring a practical transsphenoidal corridor in most cases.

Prevalence and distribution of Onodi cells

Onodi cells, defined as posterior ethmoid cells pneumatized superolateral to the sphenoid sinus, were identified in 8–24 % of patients in earlier CT work, with more recent CBCT and multidetector CT series reporting values up to roughly one-third of cases depending on the population and imaging protocol.^{8, 9} High-resolution CT analysis from North America and Europe has emphasised that even when prevalence appears modest, the close relationship between Onodi cells, the optic nerve and the carotid canal makes them a disproportionately important hazard in endoscopic skull-base surgery.^{4, 9}

Intersphenoid Septation Pattern

Intrasphenoid septa are almost universal, but their number and point of attachment vary widely between individuals.^{4, 10} Anatomical and radiological work by Fernandez-Miranda and colleagues showed that the majority of sinuses contain at least one septum inserting onto the parasellar ICA bony prominence, with many specimens demonstrating multiple septa attached to one or both carotid canals.⁴

Across representative series, roughly 40–50 % of patients have a single dominant midline or paramedian septum, 30–40 % exhibit multiple septa, and 20–30 % show at least one septum inserting directly on the ICA canal, creating a high-risk situation in which forceful removal of the septum could avulse thin bone overlying the artery.^{4, 10}

Internal Carotid Artery Canal Variants

CT-based work from diverse populations confirms that the parasellar ICA commonly indents the lateral wall of the sphenoid sinus and in a minority of cases protrudes markedly into the lumen.^{1, 2} In most radiological series the prevalence of clear ICA protrusion lies between 8 % and 15 %, while frank bony dehiscence of the carotid canal is reported in approximately 4–8 % of sinuses, often in association with advanced postsellar or lateral pneumatization.^{2, 3}

Optic Nerve and Vidian Canal Variants

Delano type II–IV optic nerves, in which the canal protrudes into the sphenoid sinus or is partially dehiscent, are collectively reported in around 8–12 % of patients and cluster in sinuses with advanced pneumatization and Onodi cells.^{5, 11} Prospective and retrospective CT series emphasise that these variants markedly increase the risk of optic neuropathy from surgical trauma, mucoceles or aggressive sphenoiditis.^{5, 11}

Vidian canal morphology also shows reproducible patterns, with most series reporting roughly two-thirds of canals fully embedded in bone (type I), one-fifth partially protruding into the sinus (type II) and a small minority fully exposed to the sinus cavity (type III).^{2, 6} For the endoscopic surgeon, these variants are particularly relevant in transpterygoid and vidian nerve approaches to the petrous apex and anterior genu of the ICA.

Lateral Recess Pneumatization

Pneumatization of the greater wing and pterygoid processes to form a true lateral recess of the sphenoid sinus is a key enabler of transpterygoid access but also brings the sinus wall into close proximity with the vidian nerve, maxillary nerve and petrous ICA.^{2, 7} Multidetector CT series from Indian and Middle Eastern centres suggest that a well-formed lateral recess is present in roughly 20–30 % of patients, with considerable side-to-side asymmetry in a given individual.^{6, 7}

The data summarised here confirm that from the standpoint of endoscopic skull-base surgery, a well-pneumatized sellar or postsellar sphenoid sinus is the norm, not the exception, across geographically diverse adult populations.^{1, 3} However, the same advanced pneumatization that facilitates surgical corridors also increases the likelihood of ICA and optic nerve protrusion, thin bony walls and complex septation patterns that can tether critical vessels.

Onodi cells, ICA-attached septa, carotid canal dehiscence, high-risk Delano optic nerve types and pronounced lateral recess pneumatization each occur in a substantial minority of patients and often co-exist, creating anatomically crowded corridors for transsphenoidal and extended approaches.^{4, 8} Rather than relying on generic prevalence figures, surgeons should systematically review high-resolution multiplanar CT or CBCT for each of these features before surgery and document them explicitly in operative planning and consent.

Sphenoid sinus anatomical variants are ubiquitous and highly relevant to the safety and feasibility of endoscopic skull-base surgery. Across contemporary CT series, sellar and postsellar pneumatization dominates, while clinically important risk variants—Onodi cells, ICA-attached septa, carotid canal protrusion or dehiscence, high-risk optic nerve courses, vidian canal protrusion and lateral recess pneumatization—are collectively present in a large minority of patients.^{1, 4}

Endoscopic skull-base surgeons should treat detailed pre-operative analysis of sphenoid sinus anatomy as a mandatory step rather than an optional refinement, using standardized classifications of pneumatization, septation and neurovascular relationships to anticipate hazards and tailor the surgical corridor to each patient.