Synovial hemangioma in an adult horse.

A 15-year-old male horse was diagnosed with a rare condition called synovial hemangioma, causing lameness in his left forelimb. After excising the mass and conducting a histopathological evaluation, the horse showed no signs of lameness one year post-surgery.

Presenting Symptoms and Initial Evaluation

• The study describes the case of a 15-year-old gelding horse that displayed progressive lameness in the left forelimb over a period of 2.5 months.
• The clinical examination uncovered an enlargement of the deep flexor tendon sheath, which displayed a firm yet resilient consistency and noticeable sensitivity to touch.
• An ultrasound examination revealed an irregular density in the mesotendineum, the connective tissue between tendons, and a pronounced swelling in the flexor tendon sheath.

Tissue Examination and Diagnosis

• The researchers then removed a firm, white, nodular mass expanding the flexor tendon sheath to about 1 cm in diameter. This tissue, along with samples from the deep flexor tendon and corresponding tendon sheath, was sent for a detailed microscopic examination, known as a histopathological evaluation.
• Upon microscopic examination, the mass was found to be made up of gaps and an intricate network of vascular channels, or blood vessels, extending between the collagen fibers of the deep flexor tendon. Its notable characteristic was the lining of the channels by abnormal cells that contain a few erythrocytes or red blood cells.
• Using immunohistochemistry, the researchers found that these abnormal cells exhibited positive staining for vimentin and factor VIII-related antigen, indicating the presence of a specific type of tumor.
• Adjacent to these cells were pericytes, cells that wrap around the endothelial cells of capillaries and venules throughout the body. These pericytes expressed the α-smooth muscle actin antigen, a protein marker found in smooth muscle cells and involved in cell mobility and structure,
• Based on these histopathological and immunohistochemical findings, a diagnosis of synovial hemangioma was made. Synovial hemangiomas are rare, benign, but locally aggressive vascular tumors, typically found in humans’ joint cavities and less commonly in animals.

Post-Surgical Follow-Up

• Following the surgical excision of the mass, the horse had no signs of lameness one year later, indicating a successful treatment outcome.