A hematoma is a collection of blood outside of blood vessels, which usually occurs when an injury causes blood to collect and pool under the skin. Hematomas can be seen under the skin or nails as purplish bruises of different sizes, and they can also happen deep inside the body where they may not be visible. Hematomas are classified based on their location, such as subdural, spinal, subungual (under the finger or toenail bed), hepatic (liver), or ear or aural hematoma. The most common cause of a hematoma is injury or trauma to blood vessels, which can happen as a result of any damage to blood vessels. Hematomas usually resolve spontaneously over time as the blood debris is removed and the blood vessel wall is repaired by the bodys repair mechanisms. However, if the hematoma symptoms are severe or if it continues to expand over the course of a few days, it is important to visit a doctor right away.