The prevalence of an Onodi cell in adult Thai cadavers.

Endoscopic sinus surgery in patients who have an Onodi cell (sphenoethmoid cell) carries a high risk for optic nerve injury. We meticulously dissected 65 embalmed cadaver adult half-heads and attempted to identify an optic canal bulge in each with a nasal endoscope. Our aims were to determine the prevalence of an Onodi cell in adult Thai cadavers, to ascertain the prevalence of an overriding ethmoid cell, and to measure the length of an overriding ethmoid cell's superior and posterior extensions in relation to the anterior sphenoid wall. Moreover, we attempted to determine the minimum amount of bone thickness between an Onodi cell and the optic nerve. We found that an Onodi cell was present in 39 of the 65 specimens (60.0%). We also found that an overriding ethmoid cell was present in 14 specimens, which accounted for 21.5% of the total number of specimens and 36.8% of 38 Onodi cell-positive specimens (the presence or absence of an overriding ethmoid cell was not recorded in one of the 39 Onodi cell-positive specimens). The distance of the overriding ethmoid cell's superior and posterior extensions from the anterior sphenoid wall ranged from 3 to 13 mm (median: 7) and from 4 to 16 mm (median: 9.5), respectively. Measurements of the minimum amount of bone thicknesses between each Onodi cell and optic nerve ranged from 0.03 to 0.54 mm (median: 0.08). Our study demonstrated that the prevalence of an Onodi cell in adult Thai cadavers was as great as the prevalence reported in the only other gross anatomic dissection study performed in Asia and much higher than rates generally reported in Western countries.