THE RELIABILITY OF COMPUTERIZED TOMOGRAPHIC DETECTION OF THE ONODI (SPHENOETHMOID) CELL

Optic nerve injury is a devastating potential complication of endoscop ic sinus surgery. Anatomic variations of the posterior ethmoid sinus a re certainly contributing factors. In the most well described posterio r ethmoid anatomical variant, the sphenoethmoid or Onodi cell, the opt ic nerve is placed at risk during sinus surgery. Improving preoperativ e and intraoperative identification of the sphenoethmoid (Onodi) cell could decrease the risk of optic nerve injury. The purpose of this inv estigation was to assess the reliability of computerized tomography (C T) in detecting the sphe noethmoid (Onodi) cell, and further our under standing of this clinically relevant anatomic variant. A total of 41 s inonasal complexes from 21 human adult cadaveric heads were studied wi th a standard coronal and axial plane CT, and subsequent endoscopic di ssection. The prevalence of the sphenoethmoid (Onodi) cell was determi ned by CT and endoscopic dissection, as were other anatomic characteri stics of the posterior ethmoid anatomy. In our study, CT identified a sphenoethmoid (Onodi) cell in 3/41 (7%) of the sphenoethmoid complexes . However, anatomic dissection identified a sphenoethmoid (Onodi) cell in 16/41 (39%) complexes. Coronal orientation of the anterior sphenoi d wall was never associated with a sphenoethmoid (Onodi) cell. Convers ely, oblique or horizontal orientations were present in all cases of s phenoethmoid (Onodi) cells. Current CT scanning protocols for the para nasal sinuses did not reliably detect the Onodi cell. Endoscopic disse ction indicates that the sphenoethmoid (Onodi) cell is a more frequent anatomic variant than previously appreciated. Awareness of anterior s phenoid wall orientation may assist surgeons in identifying the Onodi cell, thereby reducing the risk of optic nerve trauma.