Keros classification of olfactory fossa

Last revised by Yuranga Weerakkody on 11 Apr 2023

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

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 is bounded laterally by the vertical lateral lamella. The lateral lamella joins the cribriform plate to the fovea ethmoidalis.

In adults, the olfactory recess is a variable depression in the cribriform plate that medially is bounded by the perpendicular plate and laterally by the lateral lamella. It contains olfactory bulb and the anterior ethmoidal artery. 

The depth of the olfactory fossa is determined by the height of the lateral lamella of the cribriform plate, which can be classified into three categories:

  • type 1: has a depth of 1-3 mm (26.3% of population)
  • type 2: has a depth of 4-7 mm (73.3% of population)
  • type 3: has a depth of 8-16 mm (0.5% of population)

The type 3 essentially 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.

Thin bone in the skull base, especially the cribriform plate, is susceptible to erosion, encephalomeningocele herniation and CSF leaks

The most common sites of erosion or defects in the skull base are

  • cribriform plate (51%)
  • sphenoid lateral pterygoid recess (31%)
  • ethmoid roof (8%)
  • perisella
  • inferolateral or pterygoid recesses

History and etymology

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.

See also

ADVERTISEMENT: Supporters see fewer/no ads

Cases and figures

  • Figure 1: annotated image - olfactory fossa
    Drag here to reorder.
  • Figure 2: annotated image - olfactory fossa
    Drag here to reorder.
  •  Case 1: type 1
    Drag here to reorder.
  •  Case 2: type 2
    Drag here to reorder.