Relationship of the optic nerve to the medial rectus muscle during endonasal dissection of the medial intraconal orbital apex

BACKGROUND: Few studies have established surgical landmarks for endoscopic endonasal dissection of the intraconal orbital apex (OA). OBJECTIVE: We describe the optic nerve (ON) anatomy and its relationships, as seen during a fully endoscopic, endonasal approach to the medial intraconal OA. METHODS: The study question was approached through a cadaver dissection and a radiographic study. Four formalin-fixed, latex-injected cadaver heads were dissected using transnasal endoscopic techniques. Dissection was performed using 0 degree and 30 degree nasal endoscopes and standard endoscopic sinus surgical instrumentation. A bi-nostril 4-handed technique was used. The anatomy of 8 medial OAs was evaluated and recorded. As the radiographic portion, 100 consecutively enrolled patient magnetic resonance images were evaluated, with particular attention given to the relationship of the ON to the medial rectus muscle (MRM) in 200 orbits. RESULTS: Intraconally, the ON consistently coursed along the superior half of the MRM. Interestingly, the nerve wasmore easily identified froma superior approach after retracting the MRM inferiorly. With the identification of the nerve at the OA, carrying the dissection of themedialOA was easily accomplished with the ON as the guiding landmark. The radiographic portion of this study revealed a consistent relationship between the superior edge of the ON and the MRM. This relationshipwas maintained in the orbital apex in 98.4%-100% of the orbits examined. CONCLUSION: The superior edge of the optic nerve is consistently found coursing along the superior half of the MRM, facilitating facile identification and further dissection navigation.