A hematoma is a localized collection of blood that has escaped from a damaged blood vessel and pooled within the surrounding tissues. This collection usually clots or partially clots, forming a mass or lump. While often mistaken for a common bruise (contusion), a bruise involves only minor leakage from small capillaries and appears flat. A hematoma involves larger vessels and a greater volume of blood, creating a measurable accumulation beneath the skin or deep inside the body.
The Mechanism of Blood Vessel Damage and Leakage
Hematoma formation begins with a breach in the integrity of the vascular system, involving arteries, veins, or capillaries. Blunt force trauma is the most common cause, where a sudden impact compresses and tears vessel walls. This trauma overwhelms the vessel’s ability to seal the rupture, causing blood to flow rapidly into adjacent tissue spaces. Hematomas are often associated with falls, motor vehicle accidents, or direct blows.
The body’s immediate response is to initiate the clotting cascade to halt the hemorrhage. Platelets aggregate at the injury site, and a sequence of protein activations culminates in a stable fibrin mesh. This mesh traps red blood cells and plasma, forming the characteristic semi-solid clot. If the pressure of the escaping blood is too high, especially from an arterial source, clotting may be delayed or ineffective, allowing a large pool of blood to gather.
Underlying medical conditions can cause vessel rupture without external trauma. For instance, an aneurysm—a weakened section of an artery wall—can burst and lead to a spontaneous hematoma. Individuals taking anticoagulant medications are at increased risk because these drugs interfere with the natural clotting process, allowing minor vessel damage to result in a larger bleed. Similarly, inherited or acquired clotting disorders impair the body’s ability to create a firm clot, contributing to hematoma formation.
Where Blood Collects: Types of Hematomas by Location
The location where blood collects dictates the name, appearance, and severity of the hematoma. Subcutaneous hematomas are the most visible, forming just beneath the skin as a raised, firm lump. Intramuscular hematomas occur deep within muscle tissue and are often painful because the bleeding is contained by tight fascia. These are common in athletes and can lead to swelling that restricts local blood flow.
Hematomas occurring within the skull, known as intracranial hematomas, are a medical concern due to the fixed, non-expanding nature of the cranium. An epidural hematoma forms between the skull and the dura mater (the brain’s outer protective lining) and is often associated with an arterial tear. A subdural hematoma develops beneath the dura mater, typically from a tear in the bridging veins. The skull’s confined space means a small volume of blood can exert significant pressure on brain tissue, leading to neurological symptoms.
How the Body Clears the Clot: The Healing Process
Once bleeding stops and the hematoma stabilizes, the body initiates a systematic process of reabsorption to clear the clotted blood. This cleanup is managed by specialized immune cells called macrophages, which migrate to the clot site. Macrophages act as scavengers, engulfing and digesting red blood cells and the fibrin mesh through phagocytosis. The breakdown of this material gradually reduces the size and tension of the hematoma.
Resolution involves the chemical degradation of hemoglobin, the oxygen-carrying protein within red blood cells. As macrophages break down the trapped cells, hemoglobin is released and metabolized, causing the familiar color changes seen in superficial hematomas. Initially, the dark red color transitions to blue or purple. The enzymatic breakdown of hemoglobin then produces biliverdin, a green pigment, causing the hematoma to take on a greenish hue.
Biliverdin is further processed by the enzyme biliverdin reductase, converting it into bilirubin, a yellow-brown pigment. This final stage explains the yellowish coloration that persists until the hematoma is fully resolved. The entire process, from initial pooling to complete reabsorption, depends on the size and location of the hematoma, often taking several weeks for cleanup.
Indicators of Severity and Urgent Concern
While many hematomas resolve without intervention, certain signs indicate dangerous pressure or complications requiring immediate medical evaluation. Any hematoma following a head injury accompanied by neurological changes is a serious concern. This includes a severe, persistent headache, confusion, nausea, repeated vomiting, slurred speech, or sudden weakness on one side of the body. These symptoms signal that an intracranial hematoma is rapidly expanding and compressing the brain.
For hematomas in the limbs, signs of excessive pressure or potential infection require urgent care. Intensifying pain, numbness, tingling, or paleness in the extremity may indicate restricted blood flow and nerve function. Additionally, warmth, spreading redness, or a fever suggests a secondary bacterial infection within the pooled blood, necessitating prompt medical treatment.