Onyx Migration to the Middle Cerebral Artery Treated by Mechanical Thrombectomy with Subsequent Delayed Migration of Residual Onyx: A Case Report.

Onyx is a non-adhesive liquid embolic agent used for intracranial dural arteriovenous fistulas. We report delayed distal migration of residual Onyx after successful mechanical thrombectomy. An 81-year-old woman developed acute middle cerebral artery occlusion following Onyx embolization, which was recanalized with contact aspiration. Residual Onyx, not clearly visible on angiography, was later identified on computed tomography and progressively migrated distally. One week later, further migration caused embolic infarction in the superior division of the middle cerebral artery. Residual Onyx may be difficult to detect and can migrate because of its non-adhesive nature, requiring careful postoperative evaluation and follow-up.