Brain lesions are areas of damaged or abnormal tissue in the brain, and they have a wide range of causes, from head injuries and strokes to autoimmune diseases and tumors. Some are harmless and discovered by accident on a scan. Others signal a serious condition that needs treatment. Understanding the most common causes can help you make sense of a diagnosis or an unexpected finding on an MRI.
White Matter Lesions and Normal Aging
If you’re over 60 and had a brain MRI, there’s a very good chance the report mentions white matter lesions. In a population-based study of over 1,000 adults aged 60 to 90, only 5% had no white matter lesions at all. That means roughly 95% of older adults have at least some. The number and size of these lesions increase with age, and women tend to have slightly more than men, particularly in the frontal region of the brain.
These age-related lesions are typically linked to small vessel disease, where tiny blood vessels in the brain narrow or stiffen over time. High blood pressure, diabetes, and smoking accelerate the process. A few scattered white matter lesions in an older adult are usually not cause for alarm, but a heavy burden of them is associated with higher risks of cognitive decline, stroke, and balance problems.
Stroke and Vascular Disease
Stroke is one of the most common causes of brain lesions. An ischemic stroke occurs when a blood vessel supplying the brain is blocked, cutting off oxygen and glucose to the tissue downstream. White matter is especially vulnerable because it receives less blood flow and has fewer backup blood vessels compared to the brain’s gray matter. When oxygen drops, a cascade of damage begins: toxic levels of certain brain chemicals build up, triggering inflammation and oxidative stress that kill the cells responsible for insulating nerve fibers. The result is permanent structural damage.
A hemorrhagic stroke, where a blood vessel bursts and bleeds into or around the brain, creates a different kind of lesion. The pooling blood puts direct pressure on surrounding tissue and disrupts normal function. Even small “silent” strokes that go unnoticed can leave lesions visible on later imaging.
Traumatic Brain Injury
Physical trauma to the head produces several distinct types of brain lesions, depending on the force and location of impact.
- Concussion: A mild traumatic brain injury caused by a blow to the head or a sudden, violent movement of the brain inside the skull. It may not always show up on standard imaging, but it can take minutes to several months to fully heal.
- Contusions: Bruises on the brain itself, caused when small blood vessels bleed into brain tissue. These often form on the opposite side of the brain from where the impact occurred.
- Diffuse axonal injury: One of the most common and serious types of traumatic brain injury. It involves widespread damage to the brain’s white matter, typically from car accidents, falls, or sports injuries. The force shears and stretches nerve fibers throughout the brain, disrupting communication between neurons.
- Hematomas: Collections of blood that form when a vessel bursts. They can develop between the skull and the brain’s protective membranes, or within the brain tissue itself. All types create dangerous pressure on surrounding structures.
Damage from trauma doesn’t always stop at the moment of injury. Secondary damage can develop over hours or days as contusions continue to bleed and expand, or as the protective blood-brain barrier breaks down and allows harmful substances to leak into brain tissue. Repeated head injuries over time can also lead to chronic traumatic encephalopathy (CTE), a progressive condition most associated with contact sports.
Multiple Sclerosis
Multiple sclerosis (MS) is one of the most well-known causes of brain lesions in younger adults. It’s an autoimmune disease in which the body’s immune system attacks the protective coating around nerve fibers, called myelin. The process starts when certain immune cells cross the blood-brain barrier and trigger inflammation. This draws in additional immune cells, particularly macrophages, which make direct contact with myelin sheaths and actively strip them away.
The lesions MS produces, sometimes called plaques, have a characteristic appearance on MRI. They tend to cluster around the fluid-filled spaces deep in the brain (periventricular areas) and have sharp, well-defined borders. As myelin is destroyed, the nerve fibers underneath lose their ability to transmit signals efficiently. This is what produces MS symptoms like numbness, vision problems, fatigue, and difficulty with coordination. Lesions can come and go during relapses and remissions, but over time they may cause permanent damage.
Brain Tumors
Tumors are another major category of brain lesion. About half of all brain tumors originate in the brain itself (primary tumors), while the other half are metastatic, meaning cancer from elsewhere in the body has spread to the brain. Metastatic brain tumors tend to appear as multiple lesions, though certain cancers, like kidney cancer, more often produce a single lesion.
Whether primary or metastatic, brain tumors cause damage in a few ways. They compress and displace surrounding tissue as they grow. They can disrupt blood supply to nearby areas. And they may trigger swelling that increases pressure inside the skull. Symptoms from both types tend to overlap, making imaging and biopsy essential for telling them apart.
Autoimmune Encephalitis
Beyond MS, several other autoimmune conditions cause brain lesions by producing antibodies that attack brain tissue. In autoimmune encephalitis, these antibodies target either proteins inside neurons or receptors on their surface, and the type of antibody determines where lesions appear and how they look on imaging.
Some subtypes produce classic bright spots in the brain’s temporal lobes on MRI. Others are harder to detect. One common form targets a receptor involved in memory and behavior, and brain scans look completely normal in up to 89% of cases at initial presentation. Another subtype, which affects potassium channels, shows temporal lobe lesions in about 69% of patients and can eventually cause permanent scarring in that region. A rarer form produces unusual “migratory” lesions that shift location over time. These conditions are increasingly recognized as treatable causes of confusion, seizures, memory loss, and personality changes, particularly in younger patients.
Infections
Bacterial, viral, fungal, and parasitic infections can all produce brain lesions. A brain abscess forms when bacteria or fungi create a walled-off pocket of infection within the brain tissue. Viral infections like herpes simplex encephalitis tend to damage the temporal lobes. HIV can cause progressive white matter damage. Parasitic infections, though less common in developed countries, can create cyst-like lesions. Infections typically reach the brain through the bloodstream, from a nearby infection in the sinuses or ears, or through an opening created by surgery or a skull fracture.
How Location Shapes Symptoms
The symptoms a brain lesion produces depend far more on where it sits than on what caused it. A lesion in the back part of the frontal lobe, which controls voluntary movement, causes weakness or paralysis on the opposite side of the body. Damage to the left side of this area means weakness on the right side, and vice versa. A lesion in the front part of the parietal lobe impairs sensation on the opposite side of the body, making it hard to identify whether you’re feeling pain, heat, cold, or vibration.
Lesions in the temporal lobes often affect memory, language comprehension, or emotional processing. Lesions near the brainstem can disrupt breathing, heart rate, and consciousness. And lesions in the cerebellum tend to cause problems with balance and coordination. This is why two people with very different diagnoses (one with MS, another with a small stroke) can have nearly identical symptoms if their lesions happen to be in the same spot. It’s also why a single disease like MS can look so different from one person to the next, depending on where each new lesion forms.