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Onodi Cells and Sphenoid Sinus

Onodi cells are the most hazardous variation since, if not reported before surgery, they may be a potential cause of severe complications. There are two definitions of Onodi cells in the literature. In the first they are defined as the most posterior ethmoid cells, superolateral to the sphenoid sinus and associated with a bulging, even if minimal, of the bony wall of the optic nerve (Kainz and Stammberger 1992) (Fig. 2.16). The second definition describes Onodi cells as posterior ethmoid cells extending into the sphenoid bone, either close to or impressed by the optic nerve bony wall (Stammberger and Kennedy 1995) (Fig. 2.17). The prevalence has been reported to range between 3.4% to 51%, this differ

Onodi Cell

Fig. 2.16a-c. a On the axial plane the Onodi cell (OnC) arises on the right side from the posterior ethmoid and extends posteriorly and medially (arrows). b On the coronal plane the Onodi cell runs over the right and left sphenoid sinus roof (arrowheads indicate the thin left sphenoid sinus roof). The left sinus extends laterally into the pterygoid process (arrows). c On the sagittal plane a single large Onodi cell occupies the whole right posterior ethmoid sinus. Opposite arrows point to the ground lamella of the middle turbinate. Left posterior ethmoid cell (PEC), sphenoid sinus (SS), ethmoid bulla (B), frontal sinus (FR), agger nasi cell (A), middle (MT) and inferior (IT) turbinates, optic nerve and canals (ON), foramen rotundum (FS), vidian canal (VC), medial (MPP) and lateral (LPP) pterygoid plates, hamulus of medial pterygoid plate (h)

Fig. 2.16a-c. a On the axial plane the Onodi cell (OnC) arises on the right side from the posterior ethmoid and extends posteriorly and medially (arrows). b On the coronal plane the Onodi cell runs over the right and left sphenoid sinus roof (arrowheads indicate the thin left sphenoid sinus roof). The left sinus extends laterally into the pterygoid process (arrows). c On the sagittal plane a single large Onodi cell occupies the whole right posterior ethmoid sinus. Opposite arrows point to the ground lamella of the middle turbinate. Left posterior ethmoid cell (PEC), sphenoid sinus (SS), ethmoid bulla (B), frontal sinus (FR), agger nasi cell (A), middle (MT) and inferior (IT) turbinates, optic nerve and canals (ON), foramen rotundum (FS), vidian canal (VC), medial (MPP) and lateral (LPP) pterygoid plates, hamulus of medial pterygoid plate (h)

ence probably being related to the criteria used to classify this variant.

The sphenoid sinus itself displays highly variable degrees of pneumatization, including the anterior clinoid and pterygoid processes (Fig. 2.18). These variants may jeopardize endoscopic surgery, particularly when associated with focal areas of de-hiscence of the bony walls of the sphenoid sinus. While a complete dehiscence of the bone covering the optic nerve (12%-22%) or the internal carotid artery (8%) has been infrequently found, a paper-thin bony lamina covering these bulgings has been observed more often (KaiNz and StammbERgER 1992; DeLano et al. 1996; ).

It is important for the radiologist not only to detect thinning or dehiscences of the lateral sphenoid sinus walls but also to precisely assess the relationship of the sphenoid sinus and posterior ethmoid

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