Mycotic Sphenopalatine Sinusitis With Concurrent Compression of the Optic Nerves and Chiasm and Severe Visual Impairment in A Horse

A 15‐year‐old Swiss Warmblood gelding was presented to the ophthalmology service of Zurich University due to severe visual impairment. Ophthalmic and neurologic examinations were performed, raising suspicion of post‐retinal blindness. Standing contrast computed tomography (CT) of the head revealed a partially calcified, soft tissue attenuating mass in the sphenopalatine sinus with concurrent compression of the optic nerves and chiasm. Differential diagnosis included neoplasia and granulomatous disease. The horse was euthanized, and the head was subsequently examined by magnetic resonance imaging (MRI) followed by gross and histopathology. MRI showed compression of the optic nerves and chiasm. Histopathology revealed the formation of fibrous granulation tissue, osseous metaplasia, and pyogranulomatous inflammation in the sphenopalatine sinus. Periodic acid‐Schiff reaction and Grocott silver staining demonstrated branching septated filament hyphae and fungal spores. The optic nerves and chiasm were evident of mild neuronal atrophy, showing mild gliosis, vacuolation, and mild lympho‐plasmacytic inflammation. Mycotic sphenopalatine sinusitis should be considered as a more specific diagnosis for post‐retinal blindness in horses due to the compression of the optic nerve(s) and chiasm.