A hematoma is a localized collection of blood outside the blood vessels, usually in liquid form within the tissue. This distinguishes it from a bruise (ecchymosis), which is the spread of blood under the skin in a thin layer. Free blood in a body cavity (internal bleeding) is not the same because it is not trapped in a specific location.

Hematomas can occur within a muscle or within or between regions of connective tissue. Some hematomas form into hard masses under the surface of the skin. This is caused by the blood being trapped in planes of connective tissue (fascial planes). The pressure that develops in these closed areas is what limits blood loss in these injuries.

In most cases the sac of blood or hematoma eventually dissolves. However, in some cases they may continue to grow or show no change. If the blood accumulation does not self-resolve, then it may need to be surgically removed.

Eventually, most hematomas turn into seromas (an accumulation of clear, yellow fluid without many blood cells) Once bleeding into the hematoma has stopped, the blood within it clots and the clot separates from the serum (clear part of the blood). It is at this point that these are safe to open and drain the swelling.

Hematomas can also occur when the natural clotting system is disrupted by medications or by other illness. In horses, hematomas involving large vessels may continue to enlarge over weeks to months. In most cases, patience is indicated. Most eventually resolve.

Diagnosis of hematoma versus seroma or abscess requires needle sampling of the fluid contained within (called a needle aspirate). If it is pure blood, it is a hematoma, if it is yellow fluid it is a seroma. If it is pus, it is an abscess. Hematomas around joints cause swelling, decreased movement and pain and must be distinguished from fractures and ligament injury. Use of ultrasound and needle aspirate is common to make this distinction.

Specific hematomas deserve mention. Aural hematoma is a collection of blood within the tissue planes of the ear. Constant pressure on the tissues of the ear may cause damage to the ear cartilage and deformity of the ear. Hematomas in the wall of the vagina are fairly common after foaling and usually resolve within a few days post foaling.

Treatment: Draining hematomas in locations where bandaging (pressure) cannot be applied is often not advised, because once they are opened and the pressure is off, they will just continue to bleed.

The slow process of healing and reabsorption of hematomas can allow the broken down blood cells and hemoglobin pigment to gradually move out of where it is trapped. Gravity determines the direction of travel of the swelling as it heals, changing its appearance over time.