A hematoma is a localized collection of blood that accumulates within soft tissue following the rupture of blood vessels, typically caused by blunt trauma. In horses, hematomas most often appear beneath the skin as a firm, fluctuant swelling that develops within hours of an injury. Common locations include the neck and shoulder (from trailer loading collisions or fence impact), the flank, and the girth area. Internal hematomas — collections in deeper tissue planes or body cavities — are more serious and less immediately visible.
The external presentation of a hematoma resembles an abscess or edematous swelling, but on palpation it is typically cool rather than warm, and aspirating the fluid yields blood rather than exudate. A fresh hematoma contains liquid or clotted blood; as the clot organizes over days to weeks, it becomes firmer and may calcify if large. The local blood supply to adjacent tissues can be compromised if the hematoma is large enough to create pressure.
Small hematomas often resorb without treatment. Large hematomas may require veterinary management: drainage under aseptic conditions, compression bandaging to prevent refilling, and monitoring for secondary infection. Premature drainage of an organizing hematoma can disrupt clot formation and cause recurrent bleeding. A hematoma that persists, grows, or becomes warm and painful warrants prompt veterinary assessment to rule out infected pocket versus sterile blood collection formation or underlying vessel injury. Internal hematomas involving the spleen or liver, though rare, are surgical emergencies. Monitoring pulse and mucous membrane checks for internal bleeding and mucous membranes in a horse with suspected internal hemorrhage is standard first-response practice.