The "en bloc" shift: ipsilateral and contralateral abducens nerve palsy following microvascular decompression for trigeminal neuralgia caused by vertebrobasilar dolichoectasia. Illustrative cases.

BACKGROUND

Microvascular decompression (MVD) for trigeminal neuralgia (TN) caused by vertebrobasilar dolichoectasia (VBD) is technically challenging due to the vessel's size and stiffness. While ipsilateral abducens nerve (cranial nerve [CN] VI) palsy is a known complication, contralateral palsy resulting from force transmission remains unreported.

OBSERVATIONS: The authors describe 2 cases of CN VI palsy following MVD for VBD-associated TN. Case 1 developed ipsilateral palsy after caudal transposition of the vertebral artery, resulting from a paradoxical medial shift of the proximal vessel segment. Case 2 presented with a rare contralateral CN VI palsy. Postoperative imaging indicated that mobilizing the rigid VBD caused an "en bloc" shift of the entire vertebrobasilar system, resulting in secondary compression of the contralateral CN VI. Both patients achieved immediate pain relief, and the palsies recovered with conservative management.

LESSONS: Mobilizing a dolichoectatic VBD transmits mechanical force to distant segments via an en bloc shift, potentially injuring CN VI on either the operative or contralateral side. Preoperative identification of risk factors-specifically severe calcification, preexisting contralateral deviation, and minimal contralateral CSF space-is crucial. Surgeons should recognize that vascular transposition can transmit force bilaterally and must counsel patients regarding these risks. https://thejns.org/doi/10.3171/CASE2634.