The Eustachian Tube as a Landmark for Early Identification of the Abducens Nerve During Endonasal Transclival Approaches.

Background: Expanded endonasal approaches have the potential to injure the abducens nerve (cranial nerve [CN] VI). The nerve's root entry zone (REZ) and cisternal segment (CS) are particularly prone to injury during the clivus resection and dural incision of transclival approaches.

Objective: To investigate the role of the eustachian tube (ET) as a surgical landmark for the REZ and CS of CN VI.

Methods: Transclival expanded endonasal approaches were performed bilaterally in 6 fresh-frozen cadaveric specimens (12 sides). Anatomic relationships between ET and CN VI were documented with neuronavigation.

Results: The mean vertical distance from the inferior brainstem point to the horizontal projection of CN VI REZ, CS midpoint, and interdural segment (ID) were 26.38 mm (95% confidence interval [CI] 17.36-35.4), 38.61 mm (95% CI 25.61-51.61), and 42.68 mm (95% CI 30.14-55.22), respectively. The relative vertical distance from the ET to the horizontal projections of the REZ, CS midpoint, and its ID were 6.43 mm (95% CI 3.25-9.61), 18.66 mm (95% CI 11.52-25.8), and 22.72 mm (95% CI 16.02-29.42), respectively. In the axial plane the angles between the ET and (1) the REZ and its midline horizontal projection point, (2) the midpoint and its midline horizontal projection point, and (3) ID and its midline horizontal projection point were 9.81 ± SD 5.20°, 18.50 ± SD 4.87°, and 24.71 ± SD 6.21°, respectively.

Conclusion: The ET may serve as a constant landmark to reliably predict the position of the REZ and CS of CN VI.