Use of the Trochlear Nerve in the Ambient Cistern as a Key Surgical Landmark in a Microsurgical Resection of a Subthalamic Mesencephalic Cavernous Malformation.

Abstract

Brainstem cavernous malformations (CMs) represent a significant surgical challenge due to their deep location and high risk of rebleeding. Subthalamic-mesencephalic (SM) lesions account for fewer than 1% of cases.^1-4 Surgical treatment should be considered in symptomatic CM with documented hemorrhage and progression, particularly in the brainstem, where the risk of recurrent bleeding and neurologic deterioration is high.⁴^,⁵ The trochlear nerve (TN) is an early critical surgical anatomical landmark that enables identification of adjacent cerebellomesencephalic fissure structures.^6-10 This case highlights the utility of the TN as a reliable microsurgical landmark for approaching lesions in the SM region. Careful anatomical knowledge and planning allow safe and effective resection while minimizing morbidity.¹^,³^,⁵ A 47-year-old woman with an SMCM developed progressive neurologic deficits, including hemiparesis, hypoesthesia, and dysarthria, despite previous stereotactic radiosurgery. Serial neuroimaging demonstrated lesion growth and recurrent hemorrhage. Given the failure of radiosurgery and progressive clinical decline, microsurgical resection was indicated. A lateral supracerebellar infratentorial approach with tentorial resection was selected to access the posterolateral mesencephalic tegmentum. Circumferential dissection was achieved under continuous neurophysiological monitoring and indocyanine green angiography, preserving the developmental venous anomaly. Postoperatively, the patient was extubated without new deficits and discharged on the fifth day. Follow-up magnetic resonance imaging confirmed complete resection (see Video 1). Although surgery for SMCM is challenging due to critical neural pathways, identifying reliable anatomical landmarks such as TN can improve safety. Confidence with the regional anatomy makes surgical approach both safer and potentially curative for these vascular lesions.