A brain lesion is an area of abnormal tissue identified within the brain on diagnostic imaging. These irregularities appear as distinct dark or light spots on scans, such as Magnetic Resonance Imaging (MRI) or Computed Tomography (CT), that do not resemble healthy brain matter. Pinpointing the exact underlying cause is necessary. The required medical management and treatment plan are entirely dependent on the nature of the abnormal tissue.
Lesions Caused by Vascular Events
Disruptions in the brain’s blood supply, known as strokes, are a frequent cause of sudden lesion formation. These events are categorized as either a blockage or a bleed.
The most common type is an ischemic lesion, resulting from a lack of blood flow when an artery is blocked by a clot or plaque. This interruption deprives the tissue of oxygen and nutrients, leading to rapid cell death. The damaged area eventually develops into a permanent scar known as gliosis, where specialized support cells called glia proliferate to fill the void.
Conversely, a hemorrhagic lesion occurs when a blood vessel ruptures, causing blood to leak into the surrounding tissue. This pooling of blood creates a localized mass that exerts pressure on adjacent brain structures, leading to damage.
Hemorrhagic lesions are often caused by conditions that weaken vessel walls, such as high blood pressure or an aneurysm. Damage results from the direct toxicity of blood components on neural tissue and the mechanical compression caused by the expanding hematoma.
Lesions from Autoimmune and Inflammatory Conditions
The body’s immune system can mistakenly target healthy brain components, leading to inflammatory lesions. Multiple Sclerosis (MS) is the most recognized example. In MS, the immune system attacks the myelin sheath, the fatty coating that insulates nerve fibers and allows for efficient signal transmission.
This attack causes localized areas of inflammation and demyelination, known as plaques or lesions. The resulting loss of insulation slows or blocks the transmission of nerve signals, causing neurological symptoms.
Other systemic conditions can also induce inflammatory brain lesions. Vasculitis involves inflammation of blood vessel walls, which can narrow or weaken vessels and lead to small ischemic or hemorrhagic lesions. Additionally, conditions like sarcoidosis can cause localized inflammatory cell clusters, called granulomas, that present as lesions on imaging.
Lesions Resulting from Infections
Pathogens that invade the central nervous system cause infectious lesions through direct tissue destruction and the body’s inflammatory response. One specific type is a brain abscess, a localized pocket of pus and necrotic tissue.
Abscesses typically begin as an area of non-encapsulated inflammation called cerebritis, often caused by bacteria that have spread from a nearby source. The body attempts to wall off the infection, forming a fibrous capsule around the dead tissue and pus. This process creates a distinct, often ring-enhancing lesion visible on imaging.
Viruses can also cause widespread lesions through encephalitis, which is the inflammation of the brain tissue. Viral particles directly invade and damage neurons and glial cells, triggering an immune response. This widespread inflammation causes tissue damage and swelling, appearing as diffuse lesions on imaging.
Lesions Due to Abnormal Growths
Abnormal cell proliferation, commonly known as a tumor, forms a distinct type of brain lesion. These growths are classified based on their origin, which dictates their behavior and mechanism of damage. A primary brain tumor, such as a glioma, begins within the brain tissue itself, arising from cells like glia or neurons.
Primary tumors cause destruction by infiltrating and destroying normal brain cells as they grow. They also cause damage through mass effect, physically compressing surrounding functional brain tissue and vital structures. This compression can block the flow of cerebrospinal fluid or increase pressure inside the skull.
In contrast, metastatic tumors, or secondary tumors, originate elsewhere in the body and spread to the brain via the bloodstream. These lesions are composed of non-brain cells and tend to be multiple. Like primary tumors, they create lesions through mass effect and localized tissue destruction.
Lesions Related to Physical Trauma
Lesions resulting from physical trauma are direct injuries caused by external mechanical forces acting on the head and brain during a Traumatic Brain Injury (TBI). One common form is a cerebral contusion, which is a bruise on the brain tissue caused when the brain strikes the inside of the skull during impact.
Bleeding from torn vessels can also lead to focal lesions called hematomas, which are collections of clotted blood, such as epidural or subdural hematomas. These lesions represent acute, mechanical damage rather than a disease process.
A more diffuse injury is the Diffuse Axonal Injury (DAI), which occurs from rapid rotational or acceleration-deceleration forces. This motion causes a widespread shearing or tearing of the brain’s axons. DAI lesions are often microscopic and spread across the white matter, disrupting the brain’s communication network.