Radiographic and CT features of zygomatic arch pneumatization in a Thoroughbred with a recurrent progressive ethmoid hematoma.

A 7-year-old Thoroughbred gelding presented with a history of mild unilateral right-sided epistaxis and facial swelling over the right caudal maxillary sinus and zygomatic arch. Eleven months previously a progressive ethmoid hematoma had been surgically removed from the right caudal maxillary and conchofrontal sinus. Computed tomography identified a large expansile soft-tissue attenuating mass in the right caudal maxillary sinus, with protrusion into the conchofrontal sinus, that extended into the zygomatic arch. Lytic expansion and thinning of the cortex with pneumatization of the zygomatic arch was present. The mass was surgically excised after a biopsy had confirmed the recurring progressive ethmoid hematoma.