A brain lesion is a descriptive term for any area of damaged or abnormal tissue within the brain. These areas can be detected through medical imaging, such as an MRI or CT scan. The presence of a lesion indicates that an underlying pathological process has disrupted the normal structure and function of the brain. Because the brain controls virtually every bodily function, the location and size of a lesion determine the specific symptoms a person experiences, ranging from subtle changes in thought to severe motor impairment. Determining the cause of a lesion is a detailed process, as the origins are highly diverse, spanning from issues with blood flow to the body’s immune response.
Vascular Disruptions
Lesions frequently arise from problems with the brain’s blood supply, which are broadly categorized as strokes. An ischemic stroke, the most common type, occurs when a blood vessel becomes blocked, often by a clot, leading to a profound lack of oxygen and nutrients. This deprivation causes the rapid death of brain cells, a process called infarction, which creates a visible area of necrotic tissue on a scan.
Conversely, a hemorrhagic stroke causes lesions by the rupture of a blood vessel, leading to bleeding directly into the brain tissue. This pooling of blood, known as an intracerebral hemorrhage, mechanically compresses and destroys the surrounding brain cells. The physical destruction from the hematoma is compounded by the toxic nature of the blood components themselves. The collected blood increases pressure within the skull, known as mass effect, which can further damage distant areas by restricting their blood flow. Both ischemic and hemorrhagic events initiate an inflammatory response that contributes to secondary tissue injury.
Infections and Inflammatory Conditions
Microorganisms like bacteria, viruses, and fungi cause brain lesions through direct invasion or inflammation. A bacterial infection can lead to a brain abscess, a localized, pus-filled lesion resulting from the body’s attempt to wall off the infection. These lesions destroy tissue by direct pressure and the release of inflammatory mediators. Other pathogens can cause widespread inflammation of the brain itself (encephalitis) or the surrounding membranes (meningitis), leading to diffuse tissue damage and neurological deficits.
The body’s own immune system can also mistakenly attack brain tissue, resulting in autoimmune-mediated lesions. Multiple Sclerosis (MS) is the most recognized example, characterized by the immune system targeting myelin, the fatty protective sheath around nerve fibers. This misguided attack strips the myelin away, leaving behind areas of scar tissue. The demyelination disrupts the ability of nerve cells to transmit electrical signals, which manifests as various neurological symptoms depending on the location of the plaque. Other conditions, such as Acute Disseminated Encephalomyelitis (ADEM), involve a similar demyelinating process, often triggered by a preceding viral or bacterial infection.
Abnormal Tissue Growth
Lesions can result from uncontrolled cell division, forming masses known as tumors or neoplasms. These abnormal growths are classified based on where they originate. Primary brain tumors start within the brain or its surrounding structures. These tumors cause damage in two principal ways: by directly invading and destroying healthy neural tissue, or by exerting a “mass effect.”
The mass effect is the pressure placed on adjacent brain structures as the tumor grows within the confined space of the skull. This pressure can distort brain anatomy, block the flow of cerebrospinal fluid, and compress vital centers, leading to tissue dysfunction. Alternatively, metastatic tumors, which are about four times more common than primary tumors, arise from cancer cells that have traveled from a primary site elsewhere in the body. These secondary lesions are composed of the original cancer cells and can appear as single or multiple distinct masses within the brain parenchyma.
Traumatic Brain Injury
External mechanical forces, often resulting from accidents or violence, can cause lesions. Traumatic Brain Injury (TBI) can produce contusions, which are bruises of the brain tissue. These lesions are caused when the brain forcefully impacts the interior of the skull due to the brain sloshing within the cranial vault.
Diffuse Axonal Injury (DAI) results from the rapid acceleration and deceleration forces experienced in high-impact events like car crashes. These forces create shearing stress that stretches and tears the long connecting fibers of the brain, the axons, particularly in the white matter. DAI causes widespread, microscopic damage across the brain, leading to functional disconnection and often a prolonged state of unconsciousness.