Horner's Syndrome Associated with Disseminated Thoracic Melanoma in a Lipizzaner Horse

A 20-year-old gelding Lipizzaner horse was referred for onset of profuse sweating and increased heat on the right side of the head, neck, and thoracic limb. On physical examination, the horse presented tachycardia, muffled heart sounds with a pansystolic murmur, and signs of heart failure such as ventral edema, jugular distention, and pulsation. Ultrasonography examination revealed a bilateral pleural effusion and a large intrathoracic mass extending into the anterior mediastinum, which shifted the heart dorsocaudally. Thoracocentesis revealed serosanguineous fluid indicative of a melanocytic tumor. Thoracoscopy revealed a large dark mass extending into and filling the cranioventral mediastinum. The mass appearance was consistent with a melanoma, and because prognosis was poor, the owner elected euthanasia. A necropsy supported the presence of disseminated melanomas with a greater thoracic mass involving the right cervicothoracic ganglion, leading to clinical signs of sympathetic denervation.