Progressive ethmoidal hematoma (PEH) is a locally destructive, polyp-like mass which forms in the horse's nose or sinus. The mucosal capsule splits intermittently, resulting in bloody discharge from one or both nostrils. Hematomas can grow quite large, often to the point where they cause decreased airflow through the affected nostrils, resulting in respiratory difficulty and facial deformity.

Most horses present with repeated low-grade bleeding from one nostril. Nasal bleeding is not related to exercise and the discharged blood is not fresh. Between nose bleed episodes, dirty nasal discharge is often seen. Performance horses tend to develop noisy breathing.