The Radiological Anatomy of the Olfactory Fossa

Introduction: Computed tomography (CT) is a powerful tool for delineating the anatomy of the anterior skull base and paranasal sinuses. The endoscopic sinus surgeon must be well acquainted with anatomical variation to avoid dangerous iatrogenic complications. The Keros classification is a useful system for identifying potentially dangerous surgical anatomy, but it is incomplete in its description of the olfactory fossa. The goal of this study is to further characterize the relevant anatomical features of this area, along with other parameters important for endoscopic sinus surgery.

Methods: CT scans of the paranasal sinuses from a database of 30 patients with various sinonasal diagnoses were randomly selected and analyzed for multiple radioanatomic features. Patients with underlying conditions that affect the integrity of the skull base were excluded. The following features were assessed using image analysis software: olfactory fossa depth, the length of the lateral lamella and the angle at which it meets the cribriform plate, fovea ethmoidal is length and shape, ethmoid roof height and slope, and the position and course of the anterior ethmoid artery. The anterior skull base was defined as the point of exit of the anterior ethmoid artery, whereas the posterior skull base was defined as the point of the sphenoethmoidal junction. Statistical analysis was performed assessing for differences in the above parameters based on skull base position and laterality, and patient demographics.

Results: The 30 scans randomly selected included 17 males and 13 females with various sinonasal inflammatory conditions. The mean olfactory fossa depth of the anterior and posterior skull base was 3.4 ± 1.1 mm and 2.4 ± 0.9 mm, respectively (p < 0.05). The mean lateral lamella length was 3.6 ± 0.9 mm, which did not significantly vary based on patient characteristics, skull base position or laterality. The angle of the lateral lamella varied significantly by skull base position, measuring 63.1 ±17.8 degrees anteriorly, and 39.1 ±17.9 degrees posteriorly (p < 0.05). In scans where the depth of the olfactory fossa was classified as a Keros type I, 25.3% had lateral lamella were longer than 4 mm. Furthermore, 43.7% had lateral lamellae with angles less than 45 degrees. Skull base height, measured at various points, was significantly lower in females compared with males. The angle of the skull base was 19.1 ± 9.5 degrees anteriorly and only 2.4 ± 9.1 degrees posteriorly (p < 0.05). Most importantly, moving anteriorly, the posterior skull base sloped downward in 46.7% of patients.

Conclusion: Thorough preoperative assessment of CT scans is crucial to understanding the inherent variability of skull base anatomy. The skull base surgeon must appreciate that even “safe” anatomy can still contain features such as long and acutely angled lateral lamella, which may predispose patients to iatrogenic injury. Finally, despite the commonly taught surgical pearl that the skull base slopes upward in a posterior to anterior direction, this study demonstrates that a significant portion of patients can have the opposite occurring, especially in the posterior ethmoid cavity.