Endoscopic Endonasal Transoculomotor Triangle Approach to the Parapeduncular Space: Surgical Anatomy, Technical Nuances, and Case Series

Introduction: Pituitary adenomas may extend into the parapeduncular space by invading through the roof of the cavernous sinus. Currently, a transcranial approach is the preferred choice, with or without the combination of an endonasal approach. Here we present a novel surgical approach that takes advantage of the natural corridor provided by the tumor to further open the oculomotor triangle and resect tumor extension into the parapeduncular space.

Materials and Methods: Five injected specimens were used to demonstrate in detail the surgical anatomy related to the approach. We retrospectively reviewed four cases in which the proposed approach was used.

Results: The oculomotor triangle is located in the superior compartment of the cavernous sinus, where it forms the posterior aspect of the cavernous sinus roof. The anterior petroclinoid, the posterior petroclinoid and the interclinoid ligaments form the boundaries of the oculomotor triangle. The oculomotor nerve travels behind the oculomotor triangle dura, just lateral and nearly parallel to the interclinoidal ligament, which represents a key surgical landmark. From a technical perspective, the first step involves accessing the superior compartment of the cavernous sinus; this is typically facilitated by resecting tumor invading this compartment. Once the dura forming the oculomotor triangle is exposed, the interclinoid ligament should be identified. Electrostimulation may be used aiming to identify the course of the oculomotor nerve behind the dura of the oculomotor triangle. The interclinoid ligament is transected just before its attachment to the posterior clinoid and the dural opening may be extended medially toward the sellar diaphragm and inferolaterally across the oculomotor triangle dura and toward the anterior petroclinoidal ligament. This key maneuver provides access to the parapeduncular space, with exposure of the oculomotor nerve, posterior communicating and posterior cerebral arteries, uncus of the temporal lobe, and cerebral peduncle. The main neurovascular structure at risk when opening the dura of the oculomotor triangle, other than the oculomotor nerve, is the posterior communicating artery. The pattern of oculomotor nerve displacement at its cisternal segment was variable in this series, with one case of superior displacement and three of medial or medial-inferior displacement. The superior limit of the approach is formed by the optic tract and crural cistern contents, including the anterior choroidal artery and the anterior segment of the basal vein. Uncal branches from the anterior choroidal artery may become adherent to the tumor capsule. The anterior petroclinoidal ligament limits the exposure laterally and forms the tentorial edge, where the trochlear nerve may be identified entering the cavernous sinus just inferiorly. In all 4 cases, the tumor invading the parapeduncular space was completely removed. One patient had a partial oculomotor nerve palsy that completely resolved in 2 weeks, and one patient had transient diabetes insipidus, which resolved in 48 hours.

Conclusions: The endoscopic endonasal transoculomotor approach is an original alternative for removal of tumor extension into the parapeduncular space in a single procedure. Meticulous knowledge of surgical anatomy and technical expertise in endoscopic endonasal surgery is required to safely and effectively apply the proposed approach.

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