The primary diagnosis involving an abnormal collection of blood is a hematoma, a condition where blood pools outside of blood vessels and fills a space it doesn’t belong. Hematomas can form almost anywhere in the body, from just beneath the skin to deep inside the skull, chest, abdomen, or joints. The specific diagnosis depends on where the blood collects and what caused it.
What a Hematoma Is
A hematoma forms when blood escapes from damaged blood vessels but has nowhere to drain. Instead of flowing normally, it pools and clots in surrounding tissue or inside a body cavity, creating a contained mass. Small hematomas near the surface often look like deep, firm bruises. Deeper ones may not be visible at all but can cause pain, pressure, and swelling in the affected area.
Most hematomas result from some form of trauma: a fall, a blow, surgery, or even a minor injury in someone whose blood doesn’t clot well. They can also develop spontaneously in people taking blood-thinning medications. The incidence of spontaneous muscle hematomas runs about 0.5 to 1.5% among patients on older blood thinners like warfarin, with risk climbing when anticoagulation intensity is too high, kidney function is impaired, or antiplatelet drugs are used at the same time.
Hematomas Inside the Skull
Blood collections inside the skull are among the most dangerous types, and they carry distinct names based on exactly where the blood sits relative to the brain and its protective layers.
An epidural hematoma forms between the skull bone and the tough outer membrane covering the brain. It typically follows blunt trauma to the side of the head, often with a skull fracture that tears an artery running along the inside of the skull. Because the bleeding source is arterial, pressure builds rapidly.
A subdural hematoma develops in the space just beneath that outer membrane. It happens when the brain shifts inside the skull during an impact, stretching and tearing small veins that bridge the gap between the brain surface and the skull. Subdural hematomas are more common in older adults and can sometimes develop slowly over days or weeks after a relatively minor bump.
A subarachnoid hemorrhage involves bleeding into the fluid-filled space that cushions the brain. While trauma is the most common cause overall, non-traumatic cases often stem from a ruptured aneurysm (a weak, ballooning spot on a blood vessel). An intraparenchymal hemorrhage is bleeding directly into the brain tissue itself, often linked to uncontrolled high blood pressure or penetrating injuries.
For brain hematomas, size and the patient’s level of consciousness determine whether surgery is needed. French neurosurgical guidelines, for example, recommend surgical removal of cerebellar hematomas larger than 3 centimeters when consciousness is significantly impaired. Smaller collections may be monitored or managed with a drain to relieve pressure from fluid buildup.
Blood in Body Cavities
When blood collects inside the major body cavities rather than within tissue, it gets a name specific to that location. A hemothorax is blood in the chest cavity around the lungs. A hemoperitoneum is blood pooling in the abdominal cavity, often from a ruptured organ or a ruptured ectopic pregnancy. A hemopericardium is blood in the sac surrounding the heart.
Hemopericardium is particularly dangerous because the pericardial sac is a tight, relatively inflexible space. Normally it holds only about 15 to 50 milliliters of fluid. When blood fills this space, it compresses the heart and prevents it from expanding fully between beats. This progressively reduces the heart’s output and can lead to cardiac tamponade, a life-threatening condition marked by low blood pressure, bulging neck veins, and muffled heart sounds. Ultrasound of the heart (echocardiography) is the most reliable way to confirm fluid in the pericardial sac and measure how much has accumulated.
Blood Inside a Joint
Hemarthrosis is the medical term for blood collecting inside a joint cavity. It causes acute pain, noticeable swelling, warmth, and a significant drop in range of motion. The knee is the most commonly affected joint.
Trauma is the leading cause. If blood and fat are both found in the joint (a finding called lipohemarthrosis), it strongly suggests a fracture has extended into the joint space or that a ligament or cartilage structure inside the joint has been seriously damaged.
When hemarthrosis occurs spontaneously or after only minor contact, it raises suspicion for a bleeding disorder like hemophilia. Hemarthrosis is actually the most common musculoskeletal problem in hemophilia. In severe cases, where clotting factor activity is below 1% of normal, 75 to 90% of patients experience joint bleeding, with the first episode typically occurring between ages 2 and 3. Diagnosis is confirmed by withdrawing fluid from the joint with a needle. True hemarthrosis has a distinctive feature: the blood doesn’t clot in the syringe because the body has already broken down clotting proteins within the joint.
Other Common Locations
Hematomas can form in many other places, each with its own typical scenario:
- Subungual hematoma: Blood trapped under a fingernail or toenail, usually after something crushes or strikes it. The pressure of trapped blood under the nail causes intense throbbing pain.
- Rectus sheath hematoma: Blood collecting within the abdominal wall muscles, sometimes from vigorous coughing or straining, especially in people on blood thinners.
- Subchorionic hematoma: A blood collection between the placenta and the uterine wall during pregnancy, often discovered on ultrasound after vaginal bleeding in the first trimester.
- Cephalohematoma: A blood collection on a newborn’s skull, beneath the membrane covering the bone, usually caused by pressure during delivery. It feels like a raised, soft lump and typically resolves on its own over weeks.
How Hematomas Are Found on Imaging
CT scans are the primary tool for detecting hematomas, especially in emergencies involving the brain or abdomen. Fresh blood appears bright white on a CT scan because clotted blood is denser than surrounding tissue. When whole blood clots, its density rises significantly compared to liquid blood still circulating in vessels. Over the following days and weeks, the clot gradually breaks down and becomes less dense on imaging, shifting from bright white toward gray. This predictable change lets radiologists estimate how old a hematoma is, which is critical for deciding on treatment.
Ultrasound is preferred for certain locations, including the heart, abdomen, and joints, particularly when speed matters or radiation exposure is a concern (as in pregnancy). MRI provides the most detailed view for hematomas in muscles, soft tissues, and certain brain locations where CT may miss subtle collections.
Recovery and Healing
Small, superficial hematomas typically resolve on their own within 1 to 4 weeks as the body gradually reabsorbs the pooled blood. The familiar color changes of a bruise, from deep purple to green to yellow, reflect this breakdown process. Rest, ice, compression, and elevation can help speed healing and reduce discomfort for minor cases.
Larger or deeper hematomas may need to be drained, especially if they’re causing pressure on surrounding structures, showing signs of infection, or failing to shrink on their own. Hematomas inside the skull, chest, or abdomen almost always require close monitoring and often surgical intervention. The timeline for recovery in these cases depends heavily on the size, location, and whether the underlying cause (such as a bleeding disorder or anticoagulant therapy) has been addressed.
Hematomas that keep growing deserve immediate attention. Increasing pain, expanding swelling, numbness or tingling near the site, and signs of shock (rapid pulse, lightheadedness, pale skin) all suggest active bleeding that hasn’t stopped. In the brain, a worsening headache, confusion, weakness on one side of the body, or decreasing alertness signals rising pressure that can become life-threatening within hours.