Intralesional administration of formalin for treatment of epidermal inclusion cysts in five horses.

Five horses with unilateral epidermal inclusion cysts located in the nasal diverticula were sedated and treated with intralesional injection of neutral-buffered 10% formalin (volume range, 2 to 4.5 mL). After aspiration of the cyst, formalin was injected intralesionally until leakage of fluid around the needle was observed. After several weeks, desiccation of the cyst was evident; it was excised 2 weeks after treatment in 3 horses, digitally removed by the owner of 1 horse, and never removed in 1 horse, because the owner declined further treatment after resolution of the original swelling of the nasal diverticulum. Swelling of the cyst after treatment was observed in all horses; nasal discharge (2 horses) and a mild episode of epistaxis (1 horse) were the only other complications of the treatment. Intralesional administration of formalin appears to be a simple and effective treatment for epidermal inclusion cysts in the nasal diverticula of horses.