Syndrome of Inappropriate Antidiuresis and Symptomatic Hyponatremia After Onyx Embolization of a Carotid-Cavernous Fistula: A Case Report.

BACKGROUND Carotid-cavernous fistulas (CCFs) are abnormal arteriovenous communications between the carotid artery and cavernous sinus. They may be direct or indirect and commonly present with both ocular and neurovascular symptoms. Endovascular embolization is the main treatment, with high rates of fistula obliteration and clinical improvement. Although generally safe, complications can occur, including cranial nerve dysfunction and vascular injury. Endocrine complications following carotid-cavernous fistula embolization are exceedingly rare. To our knowledge, only 1 case of syndrome of inappropriate antidiuresis (SIAD) after Onyx embolization has been reported in the literature. CASE REPORT A 72-year-old woman developed severe symptomatic SIAD 9 days after transarterial Onyx embolization of an indirect CCF. The procedure was successful but associated with worsening of preexisting left abducens nerve paresis. She subsequently re-presented with dizziness, truncal ataxia, and confusion, which resulted in a fall involving wrist fracture but no head strike. Investigations demonstrated profound hypotonic euvolemic hyponatremia with elevated urine osmolality and urine sodium, suggestive of SIAD. The absence of hypovolemia, polyuria, and a negative fluid balance argued against cerebral salt wasting. Adrenal insufficiency, hypothyroidism, and other causes were excluded. Treatment with hypertonic saline and fluid restriction resulted in gradual sodium correction and complete neurological recovery. CONCLUSIONS Transient hypothalamic-pituitary dysfunction related to Onyx-associated local mass effect and hemodynamic changes may represent a plausible mechanism. This case highlights delayed-onset SIAD as a rare but clinically significant complication temporally associated with CCF embolization. The findings support consideration of both early and delayed electrolyte monitoring, particularly within the first 1 to 2 weeks after the procedure.