Avoiding Injury to the Abducens Nerve in Expanded Endonasal Endoscopic Surgery: Anatomical and Clinical Case Studies.

Introduction: Understanding the course of the most medially located parasellar cranial nerve, the abducens, becomes critical when performing an expanded endonasal approach (EEA). We report an anatomoclinical study of the abducens nerve and describe relevant surgical nuances to avoid its injury acquired during 1300 + EEAs.

Material and Methods: Ten anatomical specimens were dissected using endoscopes attached to an HD camera. A series of anatomical measurements and relationships of the abducens nerve were noted. Illustrative clinical cases are described to translate those findings into practice.

Results: Cisternal, interdural, gulfar, and cavernous segments of the abducens were identified intracranially. Traditional anatomical concepts such as Dorello's canal and Gruber's ligament were challenged. The mean distance from the vertebrobasilar junction (VBJ) to the pontomedullary sulcus (PMS) was 4 mm; the horizontal distance between both abducens nerves at the PMS was 10 mm, and between both abducens at the interdural segment was 18.5 mm. The vertical distance from the lacerum segment of ICA to the abducens nerve was 13 mm. At the superior orbital fissure, the abducens nerve and V2 were at an average vertical distance of 11.5 mm. The upper limit of the lacerum segment of the ICA was noted to be at the same level of the interdural portion of the VI nerve posteriorly.

Conclusions: Anatomical landmarks to localize the abducens nerve intraoperatively, such as the VBJ for the transclival approach, the lacerum segment of the carotid for the medial petrous apex approach, and V2 for Meckel's cave approach, are reliable and complementary to the use of intraoperative electrophysiological monitoring.