Salvage embolization for accidental arterial occlusion of a high-grade AVM resulted from excessive Onyx reflux.

Pre-operative stage embolization is a valuable strategy for managing large arteriovenous malformations (AVMs). However, reflux of Onyx may be out of control and cause accidental embolization at the feeding artery's opening. We report a case of 27-year-old male suffering from right occipital AVM bleeding with left hemianopia. The AVM was mainly supplied by two right posterior cerebral artery (PCA) branches. According to Spetzler-Martin Grade V AVM with acute bleeding, stage embolization followed by surgical excision was performed. However, during the first-stage embolization, which was planned to control superior PCA branch only, excessive Onyx reflux was noted with occlusion of inferior PCA branch opening. Right internal carotid artery (ICA) angiography showed residual large nidus volume which was supplied by pial collateral vessel from anterior temporal artery to original inferior PCA branch territory. The residual large volume causes surgical difficulty, and the small size of pial collateral vessel is not suitable for microcatheterization. Second-stage embolization was still performed for attempting further decrease of nidus volume. According to the eccentric stacking nature of Onyx, we successfully navigate microcatheter through the Onyx cast in obstructed inferior PCA branch opening. Subsequently, pre-operative embolization can be finished as initial planning, followed by surgical excision without neurological deficits. In cases of accidental feeder embolization, our experience suggests that the pass of microcatheter through the previous Onyx cast can serve as a viable option to complete nidus embolization.