Equine paranasal sinus disease is a complex clinical challenge, primarily due to the intricate anatomy and their proximity to critical dental and neurological structures. Understanding the etiopathogenesis is essential for effective clinical management. Current scientific literature classifies these disorders into primary and secondary categories2, each with distinct pathophysiological pathways.

Anatomy and Mucosal Physiology
The equine paranasal sinuses consist of the rostral and caudal groups of sinuses, which are separated by a bony septum1,2. The rostral sinus compartment comprises the rostral maxillary and ventral conchal sinuses. It drains via the rostral sinonasal canal. The caudal sinus compartment comprises the caudal maxillary, conchofrontal, and sphenopalatine sinuses and drains via the caudal sinonasal canal. The nasomaxillary aperture (Apertura nasomaxillaris) is located in the middle nasal meatus, this slit-like opening leads into a common sinonasal canal which bifurcates into the rostral and caudal sinonasal canals.

Mucosal Characteristics and Defense
The sinuses are lined with pseudostratified columnar ciliated epithelium and mucus-producing goblet cells. Under normal conditions, mucociliary clearance is the primary defense mechanism, with cilia beating in a coordinated fashion toward the drainage ostia into the nasal cavity.
Histological studies show that the nasomaxillary aperture, the critical drainage point, has a significantly thicker mucosa (~820 μm) compared to the general sinus mucosa (~75 μm) and contains five times more goblet cells3. This makes the aperture highly prone to obstructive swelling during inflammation.

Age-Related Changes: In younger horses (under 5 years), the maxillary sinuses are largely occupied by the massive reserve crowns of the cheek teeth. As the horse ages, these teeth erupt and shorten, leading to a progressive increase in sinus volume and a decrease in mucosal thickness.

Common Pathological Processes
Sinus disease is primarily classified into two categories based on its origin.
1. Primary sinusitis: Often follows an upper respiratory tract infection. Viral or bacterial (e.g.,Streptococcus equi) insults cause mucosal edema and hypersecretion. If the swelling obstructs the narrow nasomaxillary opening, trapped exudate can become inspissated (thickened), leading to chronic, non-resolving infection.
2. Secondary sinusitis: The most frequent cause is dental disease (60–80% of cases), specifically periapical infections of the distal maxillary cheek teeth. Bacteria penetrate the thin alveolar bone to inoculate the sinus, often causing malodorous empyema due to anaerobic organisms.

Space-occupying lesions
Non-infectious pathologies also significantly alter sinus function: Paranasal sinus cysts: Benign, fluid-filled cavities that grow progressively, exerting pressure that may thin or distort facial bones can compromise the infraorbital nerve and nasolacrimal duct.

Progressive ethmoid hematomas (PEH): Locally destructive, non-neoplastic hemorrhagic polyps. They typically arise from the ethmoid labyrinth and expand into the sinuses, causing intermittent, low-grade epistaxis.

Neoplasia: Neoplastic sinus disease is rare with squamous cell carcinoma being the most frequently diagnosed malignant tumor in the paranasal sinuses. These are aggressive, invading bone and causing secondary bacterial infections due to tissue necrosis.

References
1. Brinkschulte M, Bienert-Zeit A, Lüpke M, Hellige M, Staszyk C, Ohnesorge B. Using semi-automated segmentation of computed tomography datasets for three-dimensional visualization and volume measurements of equine paranasal sinuses. Vet Radiol Ultrasound. 2013 Nov-Dec;54(6):582-90. doi: 10.1111/vru.12080.
2. O’Leary JM, Dixon PM. A review of equine paranasal sinusitis. Aetiopathogenesis, clinical signs and ancillary diagnostic techniques. Equine Veterinary Education. 2011;23(3):148-159. doi: 10.1111/j.2042-3292.2010.00176.x
3. Schwieder A, Ley S, Bienert-Zeit A, Ohnesorge B, Staszyk C. The equine paranasal sinus mucosa and the nasomaxillary aperture: A histological study. Vet J. 2020 Jul;261:105490. doi: 10.1016/j.tvjl.2020.105490.