Diseases of the Paranasal Sinuses in Horses

Primary sinusitis occurs subsequent to an upper respiratory tract infection that has involved the paranasal sinuses. It usually involves all sinus cavities but can be confined to the ventral conchal sinus. This cavity is difficult to detect radiographically and access surgically. Secondary sinusitis can result from tooth root infection, fracture, or sinus cyst. The first molar, fourth premolar, and third premolar (in decreasing frequency) are the most likely to develop tooth root abscesses. Clinical signs of secondary sinusitis closely resemble those of primary sinusitis, including unilateral mucopurulent nasal discharge and facial deformity. Tooth root abscesses typically produce a fetid nasal discharge. Treatment of primary sinusitis involves lavage of the sinus cavity and systemic antimicrobial therapy based on culture and sensitivity results. Secondary sinusitis requires removal of affected cheek teeth or cystic material via sinusotomy.

Progressive ethmoid hematoma is a locally destructive mass of nasal passages and paranasal sinuses of uncertain etiology. The mass resembles a tumor in appearance and development but is not neoplastic. Large hematomas usually arise from the ethmoid labyrinth, and smaller masses arise from the floor of the sinuses. Masses originating in the sinus extend into the nasal passage. An expanding hematoma can cause pressure necrosis of surrounding bone but rarely causes facial distortion; it is primarily seen in horses >6 yr old. Low-grade, spontaneous, intermittent, unilateral epistaxis is the most common clinical sign. Horses with extensive masses may have reduced airflow through the affected nasal passage and fetid breath. In longstanding cases, the mass may protrude from the nares. In most instances, the lesion can be seen extending into the nasal passages on endoscopic examination, and the extent of the mass can be determined radiographically. Conservative management includes intralesional injection of the mass with 4% formaldehyde. Formalin is injected into the mass using a guarded endoscopic needle. The mass typically regresses rapidly, but recurrence is common. Neurologic signs have been reported after intralesional formalin injection, associated with communication of the hematoma into the calvarium. Surgical excision is achieved via frontonasal bone flap.

Sinus cysts are single or loculated fluid-filled cavities with an epithelial lining. They develop in the maxillary sinuses and ventral conchae and can extend into the frontal sinus. A congenital form has been described. Sinus cysts are typically found in horses <1 yr old but can also be seen in those >9 yr old. The primary clinical signs are facial deformity, nasal discharge, and partial airway obstruction. Radiographs are more likely to identify a sinus cyst than endoscopic examination. Multiloculated densities and fluid lines in the sinuses are observed radiographically; occasionally, dental distortion, flattening of tooth roots, soft-tissue mineralization, and deviation of the nasal septum are seen. Treatment involves radical surgical removal of the cyst and associated conchal lining. Prognosis for complete recovery is good, and the recurrence is low. Some horses may have a permanent, mild mucoid discharge after surgery.