Hematoma can be defined as localized collection of blood outside of blood vessels. It is a circumscribed collection of blood, usually clotted, in a tissue or organ, caused by a break in a blood vessel. The breakage may be spontanous or caused by a trauma. Most commonly, hematomas are caused by an injury to the wall of a blood vessel, prompting blood to seep out of the blood vessel into the surrounding tissues. A hematoma can result from an injury to any type of blood vessel (artery, vein, or small capillary). A hematoma usually describes bleeding which has more or less clotted as opposed to hemorrhage which is characterized by an active, ongoing bleeding.
The blood vessels in the body are under constant repair. Minor injuries occur routinely and the body is usually able to repair the damaged vessel wall by activating the blood clotting cascade and forming fibrin patches. Sometimes the repair fails if the damage is extensive and the large defect allows for continued bleeding. As well, if there is great pressure within the blood vessel, for example a major artery, the blood will continue to leak and the hematoma will expand.
Seroma is also the same except that it is a collection of only serum without red blood cells being present. It occurs when blood vessels of the dog are ruptured and the blood plasma starts to seep out and it becomes inflamed by the dying and injured cells. These injured cells also contribute to this fluid.
Hematomas and seromas can occur anywhere in the body but the most common type is subdermal which occurs under the skin. Hematomas and seromas can also occur in head, brain, within other organs of the body and even on the ear (aural hematoma). They are named according to their location such as subdural hematoma (occurs between brain tissue and inside lining of the brain), Spinal epidural hematoma (occurs between spinal vertebrae and outside lining of spinal cord) etc.
Symptoms of hematomas and seromas depend on the size, location and whether they cause swelling or edema.
Subdermal hematomas and seromas are characterized by fluctuant swelling under the skin. Seizures, coma and other nuerological disorders characterize hematomas/seromas in the head and/or brain. If hematoma or seroma affect a particular organ, such as liver or spleen, the dog mostly remains asymptomatic or shows signs related to dysfunction of that particular organ.