A brain lesion is a generalized term describing any area of abnormal, damaged, or injured tissue within the brain. It is not a specific diagnosis but rather a finding on medical imaging, such as an MRI or CT scan, indicating an underlying pathology. Lesions can manifest as a bruise, a fluid-filled sac, a scar, or an area of dead tissue, reflecting the variety of conditions that cause these tissue changes. Understanding the cause is essential, as the nature of the lesion reflects the specific biological mechanism that led to its formation.
Lesions Caused by Abnormal Cell Growth
Lesions resulting from abnormal cell growth are characterized by the formation of a mass that physically occupies space within the skull. These growths, known as tumors or neoplasms, can originate from the brain tissue itself or spread from cancer elsewhere in the body. Primary brain tumors arise from various cell types, such as glial cells (gliomas) or the protective layers around the brain (meningiomas). These masses cause neurological symptoms by destroying surrounding tissue and through a phenomenon called “mass effect.”
Mass effect occurs when the growing tumor displaces or compresses adjacent healthy brain tissue, increasing pressure within the skull. An aggressive tumor, such as a glioblastoma, often appears as a contrast-enhancing mass surrounded by swelling (edema), which contributes to this pressure. Metastatic lesions, or secondary brain tumors, are clusters of cancer cells that have traveled through the bloodstream, commonly originating from lung, breast, or skin cancers. These secondary lesions are frequently multiple and cause damage through compression and tissue invasion.
Not all mass lesions are cancerous; some are non-neoplastic cysts that still exert damaging pressure. Arachnoid cysts are fluid-filled sacs located between the brain and its protective membranes, often present from birth. Colloid cysts, which form in the brain’s ventricles, can block the flow of cerebrospinal fluid, leading to hydrocephalus. In all these cases, the lesion is a physical mass that disrupts normal brain function by displacing neural structures.
Lesions Caused by Vascular Events
Vascular lesions occur when blood flow to or from a part of the brain is compromised, leading to tissue death known as an infarct. Most of these events are ischemic, caused by a blockage that starves brain cells of oxygen and glucose. This blockage is often a thrombus (a clot formed locally in a narrowed artery due to atherosclerosis) or an embolus (a clot that travels from another part of the body before lodging in a cerebral vessel). When brain cells are deprived of oxygenated blood for more than a few minutes, they undergo necrosis, resulting in a permanent lesion.
Hemorrhagic lesions are caused by the rupture of a blood vessel, leading to bleeding directly into the brain tissue. This event, known as an intracerebral hemorrhage, causes immediate tissue damage through two mechanisms. First, the physical volume of the accumulating blood (hematoma) creates a rapid mass effect, compressing and displacing brain matter. Second, the extravasated blood is chemically toxic to neurons and glia. Components like hemoglobin and iron trigger inflammation and oxidative stress that expands the lesion over several days.
A ruptured aneurysm, a bulge on a weakened artery wall, is a specific cause of hemorrhagic lesion, often resulting in a subarachnoid hemorrhage. The sudden release of blood into the space surrounding the brain rapidly increases intracranial pressure and can lead to secondary damage. This bleeding also causes chemical irritation to other blood vessels, potentially leading to vasospasm. Vasospasm is a narrowing of arteries that subsequently causes new ischemic lesions in previously healthy tissue.
Lesions Caused by Infection and Immune Response
Infectious lesions develop when pathogens penetrate the brain’s defenses, causing localized destruction or widespread inflammation. A brain abscess is a focal, contained infection, typically caused by bacteria, fungi, or parasites that have traveled from a distant site (e.g., a lung or dental abscess). The body attempts to wall off the infection by forming a capsule around the pus and necrotic tissue, creating a mass that damages the brain through tissue destruction and pressure effects.
A generalized infectious lesion is encephalitis, which is inflammation of the brain parenchyma, most commonly caused by viruses like Herpes Simplex Virus. This widespread inflammation and resulting cerebral edema destroy nerve cells across multiple brain areas, leading to diffuse lesions. Meningitis, an inflammation of the protective layers covering the brain and spinal cord, can also cause lesions if the infection spreads into the underlying brain tissue or if inflammation causes blood vessel damage leading to infarcts.
Immune-mediated lesions occur when the body’s defense system mistakenly attacks healthy brain components. Multiple Sclerosis (MS) is the most well-known example, where immune cells (T-cells and B-cells) cross the blood-brain barrier. These cells attack the myelin sheath, the fatty covering that insulates nerve fibers, stripping it away to form characteristic demyelinated plaques or lesions. This process, which involves releasing inflammatory cytokines, leads to slowed nerve signaling and eventual permanent axonal injury.
Other autoimmune conditions, such as neurosarcoidosis and central nervous system vasculitis, also create lesions through inflammation. Neurosarcoidosis can involve the formation of granulomas (small areas of inflammation that act as masses) or inflammation of the brain’s blood vessels. This vasculitis narrows the vessels, potentially leading to small ischemic strokes that present as multiple, scattered lesions on imaging. The common element in these conditions is an inappropriate inflammatory response that causes tissue damage and scarring.
Lesions Caused by Trauma and External Factors
Traumatic brain injury (TBI) is a direct physical cause of lesions, where external force results in immediate structural damage to brain tissue. A cerebral contusion is a bruise on the brain surface, caused when the brain strikes the inner skull during rapid acceleration or deceleration. These focal lesions are termed “coup” (beneath the impact site) or “contrecoup” (on the opposite side as the brain recoils). Contusions involve hemorrhagic necrosis; over time, the damaged area is reabsorbed, often leaving behind localized tissue loss.
Diffuse Axonal Injury (DAI) is a form of TBI resulting from powerful rotational or shearing forces, such as those sustained in high-speed car accidents. Because the brain’s gray and white matter have different densities, rapid movement causes the long connecting nerve fibers (axons) within the white matter to stretch and tear. This microscopic tearing disrupts communication between brain regions and is responsible for the prolonged unconsciousness often seen in severe TBI cases, even when initial imaging appears normal.
Beyond physical force, certain external exposures can also lead to localized tissue destruction. Radiation necrosis is a delayed complication that can occur months or years after a patient receives radiation therapy for cancer. The high-energy radiation damages the cells and small blood vessels of the brain, particularly in the white matter, leading to tissue death and chronic inflammation. Chronic exposure to heavy metals, such as lead or mercury, can cause neurotoxicity by crossing the blood-brain barrier and interfering with cellular processes, leading to diffuse or focal lesions.