Keros classification of olfactory fossa

Last revised by Frank Gaillard on 10 Mar 2024

The Keros classification is a method of classifying the depth of the olfactory fossa.

The Keros classification remains widely used, although its simplicity has been criticized, and alternative classifications have been proposed (e.g. Gera classification and Thailand-Malaysia-Singapore (TMS) classification) 5,6

The ethmoid labyrinth is covered by the fovea ethmoidalis of the frontal bone and separates the ethmoidal cells from the anterior cranial fossa.

The very thin horizontal cribriform plate (lamina cribrosa) of the ethmoid bone separates the olfactory fossa (or groove) of the anterior cranial fossa above from the olfactory cleft of the nasal cavity below.

The olfactory groove is bounded laterally by the vertical lateral lamella, which extends from the fovea ethmoidalis to the cribriform plate and medially by the crista galli. It contains the olfactory bulb and the anterior ethmoidal artery 5

The depth of the olfactory fossa is determined by the height of the lateral lamella of the cribriform plate and is grouped into three categories:

  • type 1: has a depth of 1-3 mm (26.3% of the population)

  • type 2: has a depth of 4-7 mm (73.3% of the population)

  • type 3: has a depth of 8-16 mm (0.5% of the population)

Type 3, or asymmetry of the depth of the olfactory fossa, exposes more of the very thin cribriform plate to potential damage from trauma, tumor erosion, CSF erosion (in benign intracranial hypertension) and local nasal surgery/orbital decompression 2,5.

This, in turn, places the olfactory bulb and anterior ethmoidal artery at risk and predisposes to CSF rhinorrhea.

This classification was described in 1962 1 by Predrag Keros (1933-2018) 3, a Croatian physician and later Professor of Medicine at the University of Zagreb.

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