Resection of a posterior fossa arteriovenous malformation complicated by leaked Onyx: a case report and review of literature

Extravasation of Onyx is a rare complication during embolization of arteriovenous malformations (AVM). We present a case of embolization that was complicated by leakage of Onyx into the cerebellum which was later encountered during surgical excision of the AVM. Our goal is to report this rare event and to outline successful treatment of this complication. The patient's records were reviewed for medical history, laboratory and radiologic workup, and outpatient clinical follow-up. A 62-year-old female presented with Hunt Hess grade 2 and modified Fisher grade 2 subarachnoid hemorrhage (SAH) secondary to ruptured left posterior inferior cerebellar artery (PICA) aneurysm associated with a superior cerebellar vermian AVM. Following endovascular intervention, the aneurysm was completely embolized; however, only 75% of the AVM could be safely obliterated. Extravasation of Onyx from the ruptured aneurysm was noted on her initial angiogram. Elective suboccipital craniectomy was subsequently planned for resection of the residual AVM where the extravasated Onyx posed an operative nuisance during resection. Post-op angiogram confirmed complete resection of the AVM, as well as the bulk of the extravasated Onyx. Patient did well post-operatively, remaining neurologically intact throughout her hospital course. Although infrequently reported in the literature, Onyx extravasation is a potential complication that neurosurgeons should be ready to face. Adherence of Onyx to surrounding parenchyma could hinder optimal surgical resection of AVM and increase complications. Therefore, careful surgical dissection should be performed with special care to delicate neurovasculature. In this case, complete resection of the AVM and Onyx mass was safely achieved.