The maxillary sinus is the largest paranasal sinus and is divided by a thin septum into caudal and rostral parts. The frontal sinus has a large communication with the dorsal conchal sinus at its rostral end, thereby forming the conchofrontal sinus. The conchae or turbinates are delicate scrolls of bone that are attached laterally in the nasal passage and contain the conchal sinuses. The caudal and rostral maxillary sinuses have separate openings into the middle nasal meatus, and the caudal maxillary sinus communicates with the frontal sinus through the large frontomaxillary opening. Diseases that originate in one sinus cavity may extend to and involve others.
Most diseases of the paranasal sinuses cause mucopurulent or bloody nasal discharge. Drainage is unilateral, in contrast to disease of the lungs, pharynx, and guttural pouches, because the source of discharge is rostral to the caudal border of the nasal septum. Unilateral facial swelling, epiphora, dull percussion of the sinuses, and inspiratory noise are common manifestations of disorders of the sinuses.
On endoscopy, purulent material, a mass, or blood can be seen in the nasal passage originating from the nasomaxillary opening. Lateral and dorsoventral radiographs of the skull may reveal fluid lines, sinus cysts, solid masses, or lytic/proliferative changes associated with dental disease and neoplasia. Oblique projections in a dorsal to ventral direction may be required to improve views of the tooth roots. CT is useful, particularly for ventral conchal sinus disease. Centesis of the maxillary or frontal sinuses is performed to obtain fluid for bacterial culture, sensitivity testing, and cytologic examination. With sedation and local anesthesia, the sinuses can be examined in the standing horse by insertion of an arthroscope (4 mm). A second portal could be used to insert an instrument into the sinus to obtain specimens, debride tissue, and lavage the sinus cavity.
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.