Seizures in adults are caused by anything that disrupts the normal electrical activity in the brain, from structural damage like stroke and head injuries to temporary metabolic imbalances like dangerously low blood sugar. In many cases, especially in older adults, no clear cause is ever identified. Understanding the difference between a one-time provoked seizure and the beginning of epilepsy is key to knowing what comes next.
Provoked vs. Unprovoked Seizures
Not all seizures mean you have epilepsy. A provoked seizure happens in response to a temporary, reversible trigger: a metabolic imbalance, a toxic exposure, or withdrawal from alcohol or other substances. Remove the trigger, and the seizures stop. An unprovoked seizure, on the other hand, occurs without an obvious immediate cause and may signal an underlying condition in the brain. Epilepsy is diagnosed when someone has two or more unprovoked seizures, or when one unprovoked seizure occurs alongside evidence of a high risk for more.
This distinction matters because it changes how the seizure is treated. A provoked seizure often requires treating the underlying problem rather than starting long-term seizure medication.
Stroke and Cerebrovascular Disease
Stroke is the single most common identified cause of new-onset seizures in adults, and it dominates in people over 65, accounting for up to half of new epilepsy cases in that age group. About 10% of all stroke patients eventually develop post-stroke epilepsy. The seizures can start within the first week after a stroke, but 40% to 80% of cases emerge within the first year.
A stroke damages brain tissue either by cutting off blood flow (ischemic stroke) or by bleeding into the brain (hemorrhagic stroke). That damaged tissue can form scar-like areas that generate abnormal electrical signals. Even small strokes that go unnoticed, sometimes called “silent strokes,” can create enough disruption to trigger seizures months or years later.
Traumatic Brain Injury
Head injuries are a leading cause of seizures in younger and middle-aged adults, and the risk scales directly with how severe the injury is. Among people who experience a post-traumatic seizure, the 10-year risk of developing epilepsy is about 41%. That risk climbs sharply with injury severity: roughly 34% after a mild injury, but 62% after a focal brain injury where a specific area of the brain is damaged.
Timing also matters. If a first seizure happens within two years of the head trauma, the risk of epilepsy is significantly higher (about 49%) compared to seizures that appear later (about 36%). Early seizures suggest the injury created conditions in the brain that are actively prone to generating abnormal electrical activity, rather than a single isolated event.
Brain Tumors
Seizures are often the first symptom of a brain tumor, and certain tumor types are far more likely to cause them than others. Slow-growing, low-grade tumors tend to provoke more seizures than aggressive ones, likely because they sit in the brain long enough to irritate surrounding tissue without destroying it outright.
The numbers vary dramatically by tumor type. About 63% of adults with low-grade astrocytomas experience seizures. That figure rises to 65% for oligodendrogliomas, 72% for mixed oligoastrocytomas, 81% for gangliogliomas, and as high as 94% for a rare type called DNET. Higher-grade, more aggressive tumors still cause seizures, but at lower rates. Location matters too: tumors near the surface of the brain, particularly in the temporal and frontal lobes, are more likely to trigger seizure activity than those deep inside the brain.
Brain Infections
Infections that reach the brain or its surrounding membranes can provoke seizures both during the acute illness and for years afterward. The long-term epilepsy risk depends heavily on which infection is involved. A large study tracking patients for a decade found that herpes simplex encephalitis carried a 26% risk of developing epilepsy, and brain abscesses carried a 30% risk. Bacterial meningitis posed a lower but still meaningful 4% risk, while tick-borne encephalitis came in around 2%.
These infections cause seizures through a combination of direct tissue damage, swelling, and inflammation that disrupts the brain’s normal signaling. Even after the infection clears, the scarring and structural changes it leaves behind can serve as a long-term source of abnormal electrical activity.
Metabolic and Chemical Imbalances
The brain is exquisitely sensitive to changes in blood chemistry. When certain levels swing far enough from normal, seizures can result. The most common metabolic triggers include low sodium (hyponatremia, below 136 mEq/L), low blood sugar (below 50 mg/dL), very high blood sugar, low calcium, and low magnesium. Kidney failure and liver failure can also cause seizures by allowing toxins to build up in the blood that the brain can’t tolerate.
These seizures are considered provoked. They’re the brain’s reaction to an abnormal internal environment, not a sign of a structural brain problem. Correcting the imbalance usually stops the seizures. This is why blood tests are one of the first things ordered when an adult has a seizure for the first time.
Alcohol and Substance Withdrawal
Alcohol withdrawal is one of the most common causes of provoked seizures in adults. The brain adapts to chronic alcohol exposure by increasing its baseline level of excitability to counteract alcohol’s sedating effects. When alcohol is suddenly removed, that heightened excitability goes unchecked, and seizures can result.
The highest-risk window for withdrawal seizures is 24 to 48 hours after the last drink, though they can occur earlier. These seizures are typically generalized, meaning they involve the whole brain rather than starting in one spot. Other substances can provoke seizures too, including withdrawal from benzodiazepines and barbiturates, as well as overdoses of stimulants like cocaine and amphetamines.
Neurodegenerative Disease
Conditions like Alzheimer’s disease and other forms of dementia increase seizure risk as they progressively damage brain tissue. This is a growing cause of new-onset epilepsy in older adults, alongside stroke and tumors. The seizures can be subtle in people with dementia, sometimes appearing as episodes of confusion, staring, or sudden worsening of cognitive function rather than the dramatic convulsions most people picture. This makes them easy to miss or attribute to the dementia itself.
Triggers That Lower the Seizure Threshold
Beyond the structural and metabolic causes, certain lifestyle and physiological factors don’t directly cause seizures but lower the threshold at which the brain is willing to produce one. In someone who already has a seizure-prone brain, these triggers can be the final push.
- Sleep deprivation is one of the most consistently reported triggers. It increases brain excitability in ways that are measurable on an EEG.
- Stress is the trigger patients self-report most often, though the biological mechanism is less clearly defined.
- Hormonal changes during the menstrual cycle can influence seizure frequency in women, a pattern called catamenial epilepsy.
- Fever lowers the seizure threshold generally, which is why infections with high fevers can provoke seizures even when the infection hasn’t reached the brain.
- Flashing or flickering lights trigger seizures in a small percentage of people with photosensitive epilepsy.
These factors matter most for people who already have epilepsy or an underlying vulnerability. A healthy brain can usually tolerate a bad night of sleep without seizing, but a brain with existing damage or a genetic predisposition may not.
When No Cause Is Found
A significant number of adults who develop seizures never receive a clear explanation. This is especially true in older adults, where extensive testing with brain imaging, blood work, and EEGs may all come back unremarkable. In these cases, the diagnosis is often classified as epilepsy of unknown cause. Some of these cases likely involve tiny areas of brain damage too small for current imaging to detect, genetic factors that haven’t been identified yet, or autoimmune processes where the body’s immune system attacks brain tissue. The absence of a clear cause doesn’t change the need for treatment if seizures recur, but it can make the experience more frustrating for patients looking for answers.