Asymmetry of Ethmoid Roof
The morphology of the ethmoid roof is usually inconstant. Asymmetry of the cribriform plate may be seen, directly related to the length of the vertical lamella, that inserts on it (Fig. 2.15). It has been observed in about 10% of patients (Dessi et al. 1994; LEbowitz et al. 2001). During surgery, this anomaly entails an increased risk of iatrogenic CSF fistulization or of injury of the anterior ethmoid artery, which courses along the most lateral aspect of the cribriform plate (Fig. 2.12). The depth of the ethmoid roof has been classified by Keros (1962) in three different types. In type 1 the vertical lamella of the cribriform plate is very short, therefore the olfactory fossa is almost flat; in type 2 the vertical lamella is longer, the olfactory fossa deeper; in type 3 the vertical lamella is particularly long (more than 13 mm) and the roof of the ethmoid is noticeably higher than the cribriform plate. This variation has to be known, because the deeply located olfactory fossa places the thin vertical lamella at risk of penetration during endonasal surgery (Keros 1962).
Finally, dehiscences and medial deviations can also be found at the level of the lamina papyracea (Fig. 2.19). In most cases they occur close to the insertion of the ground lamella of the middle turbi-nate into the lamina papyracea. They may be either congenital or due to trauma. Whichever the case excessive medial deviation or dehiscence constitute essential information prior to endonasal surgery. If not known prior to surgery, the risk of intra-orbital penetration is very high.
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