Left facial swelling with bone deformation due to an occlusive empyema of the maxillary sinus in a young horse

A one year old female Haflinger foal developed left facial swelling. The clinical and radiological examinations indicated a neoplastic lesion. Computed tomography showed enlargement of the left maxillary sinus, some fluid in the lumen, and marked thickening of its inner layer. The nasal septum was deviated to the right, and the right maxillary sinus was reduced in size. Histopathologically a chronic, demarcated empyema of the left maxillary sinus was found. The distension of the maxillary sinus was caused by the closure of the nasomaxillary aperture and retention of air and pus. Similar lesions are discussed as differential diagnoses.