Microsurgical anatomy of the abducens nerve.

Objective: To clarify the oriented classification, relationships, and variations of the abducens nerve and provide a detailed description of its microsurgical anatomic features.

Methods: A microsurgical anatomic dissection of the abducens nerve was performed in 100 specimens obtained from 50 adult cadaveric heads fixed in formalin and two adult cadaveric heads stained with hematoxylin and eosin for histological examination. Important neurovascular and structural relationships of the abducens nerve were observed.

Results: The abducens nerve was divided into five segments (cisternal, petroclival, internal carotid artery, fissural, and intraconal). It coursed in the petroclival venous confluence and there was a complex anatomic relationship. Two new types of abducens nerve variations were found. In one type, the duplicated nerve is split into two branches for a limited length in the cavernous sinus (CS). The other is a complex type, which has a complex course and pattern. This type of duplicated abducens nerve has a communicating branch in the cistern and numerous fasciculi in the CS. In addition, the two branches do not accompany each other for the entire course in the CS.

Conclusion: The vulnerability of the abducens nerve results from diverse factors. The inferolateral trunk, which arises from the intracavernous segment of carotid artery (also called the artery of the inferior CS), is an important landmark for finding the abducens nerve and sympathetic nerve. Variations of the abducens nerve are not rare. Keeping variations of the nerve in mind is important during skull base operations and transvenous endovascular interventions. Understanding the relationship of the abducens nerve with adjacent structures will help us in preparing for safe surgery.