Introduction: Cholesterol granuloma (CG) represents a histological classification indicative of a foreign body reaction characterized by giant cells responding to cholesterol crystals and hemosiderin, typically resulting from erythrocyte rupture. These granulomas are rare, benign cystic lesions, primarily localized within the temporal bone, most commonly at the petrous apex of the skull.

Material: We present the case of a 69-year-old male patient, who sought medical attention at our department in 2021. He presented with a persistent one-year history of a right hearing loss, chronic retroauricular pain, and cephalgia, all following a suspected middle ear infection. The patient's medical history included a catheter embolization in 2009 to address a right temporal-occipital arteriovenous dural fistula, during which Onyx embolic material was employed. Subsequent computed tomography scans revealed partial opacity within the right mastoid and the presence of metal-dense circular structures.

Results: A surgical mastoidectomy procedure revealed scattered metal material resembling Onyx substance. The histological examination confirmed the presence of a cholesterol granuloma within the affected area. After a two-year follow-up, the patient exhibited no recurrence of the initial complaints, suggesting successful management and resolution of the cholesterol granuloma.

Discussion: While there are few reports in the literature where the migration of Onyx materials has resulted in peripheral facial palsy, our case is the first documented instance of a cholesterol granuloma emerging after Onyx embolization for an arteriovenous malformation. This unique complication underscores the need for further exploration in managing foreign material migration into the mastoid region.

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