If a hematoma—a localized collection of blood outside of blood vessels—is left untreated, the consequences vary from minor discomfort to life-threatening complications depending on its location and size. For surface-level (subcutaneous) hematomas, the body usually reabsorbs the blood, but an untreated large one can become infected, leading to an abscess that requires surgical drainage and antibiotics. In muscles, an untreated hematoma can cause a rare condition called myositis ossificans, where the body mistakenly deposits calcium into the injured muscle, turning part of it into bone-like tissue and causing chronic pain or permanent loss of mobility. The most dangerous risks involve "internal" hematomas; for example, an intracranial hematoma (in the brain) can cause a rapid buildup of pressure within the skull, leading to permanent brain damage, seizures, or death if the pressure is not surgically relieved. In other areas, a large hematoma can cause compartment syndrome, where the pressure restricts blood flow to nearby nerves and tissues, potentially leading to tissue death (necrosis) and requiring emergency medical intervention.