A hematomaorhaematoma is a localized collection of blood outside the blood vessels, usually in liquid form within the tissue. An ecchymosis, commonly (although erroneously) called a bruise, is a hematoma of the skin larger than 10mm.
It is not to be confused with hemangioma, which is an abnormal buildup of blood vessels in the skin or internal organs.
The word "haematoma" came into usage around 1850. The word derives from the Greek roots "heme-" (blood) and -oma, from soma, meaning body = a body of blood. Another etymological derivation would be from "haemat-" and "-oma" = "-ing", thus simply "bleeding".
Hematomas can occur within a muscle. Some hematomas form into hard masses under the surface of the skin. This is caused by the limitation of the blood to a subcutaneous or intramuscular tissue space isolated by fascial planes. This is a key anatomical feature that prevents such injuries from causing massive blood loss. 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 sac of blood does not disappear, then it may need to be surgically removed. Hematomas can occur when heparin is given via an intramuscular route; to avoid this, heparin must be given intravenously or subcutaneously.
The slow process of reabsorption of hematomas can allow the broken down blood cells and hemoglobin pigment to move in the connective tissue. For example, a patient who injures the base of his thumb might cause a hematoma, which will slowly move all through the finger within a week. Gravity is the main determinant of this process.
Hematomas on articulations can reduce mobility of a member and present roughly the same symptoms as a fracture.
In most cases, movement and exercise of the affected muscle is the best way to introduce the collection back into the blood stream.
A mis-diagnosis of a hematoma in the vertebra can sometimes occur; this is correctly called a hemangioma (buildup of cells) or a benign tumor.