Case description: A 13-year-old female Miniature Horse was evaluated for progressive unilateral exophthalmia of the left globe of 3 weeks' duration.
Clinical findings: Results of a physical examination were unremarkable. Ophthalmic examination identified exophthalmus of the left globe with complete resistance to retropulsion and mild blepharoconjunctivitis. Computed tomography revealed a large, space-occupying mass within the left caudal maxillary and left conchofrontal sinuses. The mass extended into the left retrobulbar space and contacted the cribriform plate. Trephination yielded copious amounts of turbid yellow fluid. The diagnosis was a sinonasal cyst.
Treatment and outcome: Subtotal surgical excision of the cyst via a frontonasal osteoplastic flap was curative, with complete resolution of the exophthalmus. Histologic examination confirmed diagnosis of a sinonasal cyst. There was no evidence of cyst recurrence by 4 months after surgery.
Clinical relevance: Sinonasal cyst should be a differential diagnosis for retrobulbar disease in horses. Exophthalmia may be the only clinical finding in horses with a sinonasal cyst.