Horner's syndrome associated to disseminated thoracic melanoma in a lipizzaner horse

A20-year-oldgeldingLipizzanerhorsewas referredforonsetofprofusesweating andincreased 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 intrathoracicmass 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 cranioventralmediastinum. Themass appearancewas consistent with amelanoma, and because prognosis was poor, the owner elected euthanasia. A necropsy supported the presence of disseminatedmelanomaswith a greater thoracicmass involving the right cervicothoracic ganglion, leading to clinical signs of sympathetic denervation.