Background and aim 9 patients with 10 arteriovenous fistulas were treated with Onyx at our institution over a period of 19 months: 4 direct and indirect carotid–cavernous fistulas (CCFs) and 6 dural arteriovenous fistula (DAVFs). Complete occlusion was achieved with no recurrences or permanent complications in our small series. We report hemodynamic instability, including severe bradycardia and asystole, during embolization of DAVF and CCF with Onyx in several patients. These changes were reversible with interruption of Onyx injection and administration of atropine. No recurrence of symptoms after atropine administration was noted as Onyx embolization continued during the same session or during postprocedural 24 h monitoring. No adverse clinical consequences were noted.

Results Bradycardia was observed in 4 cases, with a brief asystole in 2 of these patients during transarterial and transvenous Onyx delivery at cavernous sinus and orbital levels. Based on our observation, hemodynamic instability was demonstrated during Onyx injection into the vessels that were in close proximity to the trigeminal nerve or its branches, especially in low-flow/low-volume compartment and may represent a direct effect of dimethyl sulfoxide/Onyx on the trigeminal nerve, resulting in vagal response from trigeminocardiac reflex.

Conclusion Proposed measures to prevent this complication include pretreatment with atropine or prophylactic placement of transvenous pacemakers in patients with underlying heart block and patients with contraindications to atropine use.