Idiopathic mediastinal chyloma in a cat: a case report of a novel manifestation of chyle leakage

Chyloma is a rare pseudocystic collection of chyle resulting from leakage of the thoracic duct or its branches. While sporadically reported in humans, it has not been previously described in veterinary medicine. This report describes the first case of an idiopathic mediastinal chyloma in a cat. A three-year-old, castrated male, domestic shorthair cat presented with acute dyspnea and lethargy. Thoracic computed tomography (CT) revealed a well-circumscribed cystic mass in the right caudal mediastinum causing compressed the adjacent lungs and heart. The lesion was surgically excised and contained chylous fluid. Histopathological examination confirmed a fibrous-walled pseudocyst lacking an epithelial lining, supporting the diagnosis of chyloma. Clinical signs resolved after surgery. Five months later, chylothorax developed at a different site, with CT lymphangiography demonstrating thoracic duct leakage. No underlying cause was identified, and both conditions were considered idiopathic. The cat was managed conservatively with no recurrence during the six-month follow-up period. Chyloma should be considered in the differential diagnosis of mediastinal cystic lesions in cats. Surgical excision may be effective for relieving mass effect, and long-term monitoring is recommended given the potential for recurrent chyle leakage.