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Keros classification (Lateral Lamella)

Assessment of olfactory fossa depth on imaging prior to endoscopic sinus surgery, orbital decompression, or in patients with CSF rhinorrhea to predict surgical risk.

Key Images

Image

Look For First
  • Height of the lateral lamella of the cribriform plate on coronal CT or MRI
  • Depth measurement of the olfactory fossa in millimeters
  • Presence of asymmetry between right and left olfactory fossae
Key Image Findings
  • On coronal CT, measure the vertical distance from the fovea ethmoidalis to the level of the cribriform plate at the lateral lamella to determine fossa depth.
  • Type 1 Keros: olfactory fossa depth 1-3 mm, seen in 26.3% of population, represents shallow fossa with less risk of intracranial injury.
  • Type 2 Keros: olfactory fossa depth 4-7 mm, seen in 73.3% of population, represents intermediate depth and most common variant.
  • Type 3 Keros: olfactory fossa depth 8-16 mm, seen in only 0.5% of population, represents deep fossa with significantly increased surgical risk.
  • The lateral lamella is a thin vertical bony structure that extends from the fovea ethmoidalis superiorly to the cribriform plate inferiorly, forming the lateral boundary of the olfactory groove.
  • The cribriform plate (lamina cribrosa) is an extremely thin horizontal bony structure separating the olfactory fossa in the anterior cranial fossa from the olfactory cleft in the nasal cavity below.
  • Type 3 fossa depth or asymmetry exposes the cribriform plate to increased risk from direct trauma, tumor erosion, CSF leak erosion in benign intracranial hypertension, and iatrogenic injury during nasal or orbital surgery.
  • CSF rhinorrhea risk is increased in Type 3 anatomy due to proximity of the olfactory bulb and anterior ethmoidal artery to a thin and exposed cribriform plate.
Differential Diagnosis
  • Gera classification: alternative classification system proposed to address limitations of Keros system simplicity.
  • Thailand-Malaysia-Singapore (TMS) classification: alternative classification system for olfactory fossa depth.
  • Asymmetric olfactory fossae: may represent Type 3 anatomy on one side with increased risk even if absolute depth is within Type 2 range.
Discussion

Keros classification is based solely on the vertical height of the lateral lamella and does not account for other anatomic risk factors such as cribriform plate thickness, slope, or asymmetry.

Type 3 anatomy places the olfactory bulb and anterior ethmoidal artery at direct risk during endoscopic sinus surgery and pituitary surgery, necessitating careful surgical planning and technique.

CSF rhinorrhea can result from multiple etiologies including post-traumatic injury, tumor erosion, chronic raised intracranial pressure, and iatrogenic violation of the cribriform plate during surgery.

Asymmetry of olfactory fossa depth between sides may confer additional risk beyond the depth classification itself and should be noted in surgical planning.

The Keros classification remains the most widely used and simplest method for assessing olfactory fossa anatomy despite criticisms and the proposal of alternative more comprehensive systems.

Pre-operative imaging assessment and communication to the surgical team is essential for high-risk Type 3 anatomy to allow modification of surgical approach.

Reporting Pearls

Clearly specify the Keros classification type for each side (Type 1, 2, or 3) with the measured depth in millimeters, and comment on any asymmetry or anatomic variants that would increase surgical risk, such as: "Bilateral symmetric Type 2 Keros fossa (left 6 mm, right 5 mm)" or "Right Type 3 Keros fossa (10 mm) with prominent olfactory bulb—high risk anatomy for ESS."

Pitfalls
  • Failing to measure and classify both sides separately, as asymmetric anatomy can confer additional risk even if bilateral depths appear symmetric on casual inspection.
  • Relying on Keros classification alone without assessing other factors such as cribriform plate thickness, angle of slope, or presence of sphenoid sinus paradoxical septa.
  • Confusing the measurement endpoint: the depth must be measured from the fovea ethmoidalis to the cribriform plate along the lateral lamella, not from other landmarks.
  • Overlooking Type 3 anatomy in surgical planning, which should prompt communication with the operative surgeon regarding modified approach and increased risk of CSF rhinorrhea or intracranial injury.
Source: Reference source

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