Seizures happen when groups of nerve cells in the brain fire electrical signals in an abnormal, synchronized burst. This can be triggered by dozens of different conditions, from a sudden drop in blood sugar to a structural problem in brain tissue that has been present since birth. Some seizures have a clear, identifiable cause. Others occur without any obvious explanation.
Understanding why seizures happen starts with what’s going on inside the brain, then branches into the many medical conditions, injuries, and everyday triggers that can set one off.
What Happens Inside the Brain
Your brain runs on a delicate balance between signals that excite nerve cells and signals that calm them down. The main “go” chemical is glutamate, and the main “stop” chemical is GABA. When that balance tips too far toward excitation, large clusters of neurons can start firing together in rapid, rhythmic bursts. This hypersynchronized activity is the electrical signature of a seizure.
Researchers have observed that these abnormal firing patterns produce high-frequency oscillations that can spread across centimeters of brain tissue. When the brain’s natural braking system (GABA signaling) is weakened, those oscillation clusters grow broader and more chaotic, eventually producing the visible symptoms of a seizure: muscle jerking, loss of awareness, staring spells, or full-body convulsions depending on which parts of the brain are involved.
Anything that disrupts this excitation-inhibition balance, whether it’s a scar in brain tissue, a chemical shift in the blood, or a genetic quirk in how ion channels work, can potentially cause a seizure.
Structural Problems in the Brain
About 3 out of 10 people with epilepsy have a structural change in the brain that drives their seizures. These changes fall into two broad categories: ones present from birth and ones acquired later in life.
Problems Present From Birth
During fetal development, brain cells migrate outward to form the cortex, the brain’s outer layer. If this migration goes wrong, patches of misplaced neurons called cortical dysplasia can form. These misplaced cells signal each other abnormally and produce recurring seizures. In rare cases, one entire hemisphere of the brain develops larger than the other, a condition called hemimegalencephaly.
Problems Acquired Later
Brain tumors, strokes, and infections can all leave behind damaged or scarred tissue that becomes a seizure focus. One well-known example is mesial temporal sclerosis, where scarring forms in the hippocampus (a memory-related structure deep in the temporal lobe). This is one of the most common findings in adults with drug-resistant epilepsy.
Traumatic Brain Injury
Head injuries are a significant cause of seizures, and the risk scales directly with how severe the injury is. In the general population of people who suffer a head injury, roughly 2 to 2.5% develop post-traumatic seizures. That number climbs to about 5% for those hospitalized and 10 to 15% for those with severe injuries. Children with severe traumatic brain injury face the highest risk, with seizure rates reaching 30 to 35%.
Post-traumatic seizures can appear within hours of the injury or emerge months to years later. Early seizures (within the first week) don’t always lead to ongoing epilepsy, but late-onset seizures are more likely to recur.
Metabolic and Chemical Imbalances
Your brain is extremely sensitive to the chemical composition of your blood. When key electrolytes or blood sugar drop below certain thresholds, seizures can follow.
- Low sodium (hyponatremia): Seizures typically occur when blood sodium falls below 115 mEq/L, though a rapid drop can trigger them at higher levels.
- Low calcium (hypocalcemia): Seizures tend to occur at calcium levels between 5 and 6 mg/dL, roughly half the normal value.
- Low blood sugar (hypoglycemia): Seizures usually strike when glucose falls below 40 mg/dL. The brain depends almost entirely on glucose for fuel, so a steep drop can trigger a crisis quickly.
Kidney failure and liver failure can also provoke seizures by allowing toxins to build up in the blood that would normally be filtered out. Hormonal shifts during the menstrual cycle are another recognized trigger, a pattern sometimes called catamenial epilepsy.
Infections That Affect the Brain
Infection is likely the most common cause of epilepsy worldwide, though it disproportionately affects people in lower-income regions. The infections most closely linked to seizures include:
- Neurocysticercosis: A parasitic infection from pork tapeworm larvae that form cysts in the brain. It is one of the leading causes of epilepsy in Latin America, sub-Saharan Africa, and parts of Asia.
- Bacterial meningitis: Inflammation of the membranes around the brain and spinal cord. Seizures often occur during the acute infection but frequently stop once the infection is treated.
- Viral encephalitis: Direct infection of brain tissue by viruses like herpes simplex can cause both acute seizures and long-term epilepsy.
- Cerebral malaria: A severe complication of malaria that can cause seizures, particularly in young children in endemic areas.
- HIV and tuberculosis: Both can affect the central nervous system and raise seizure risk, especially in advanced disease.
Infections acquired during pregnancy (collectively called TORCH infections, including toxoplasmosis, rubella, cytomegalovirus, and herpes simplex) can cause seizures in newborns along with developmental delays and other neurological problems.
How Causes Differ by Age
The most likely cause of a seizure shifts dramatically across the lifespan.
In newborns and infants, the leading causes are birth injuries, oxygen deprivation during delivery, infections, and congenital brain malformations. Febrile seizures (triggered by fever) are the most common seizure type in early childhood, affecting children between six months and five years old. They occur with fevers of 100.4°F (38°C) or higher. Simple febrile seizures last less than 15 minutes, involve the whole body, and happen only once in a 24-hour period. Complex febrile seizures last 15 minutes or longer, may affect only one side of the body, or recur within the same day. Most children who have a simple febrile seizure will not develop epilepsy.
In young adults, head injuries, genetic epilepsy syndromes, and substance use are more prominent causes. After age 35, stroke becomes one of the leading causes. In older adults, new-onset seizures are frequently linked to stroke, brain tumors, or dementia. Alzheimer’s disease and other forms of neurodegeneration raise seizure risk as the disease progresses.
Alcohol and Drug Withdrawal
Withdrawal from alcohol is one of the most common causes of provoked seizures in adults. The brain adapts to chronic alcohol exposure by ramping up its excitatory signaling to compensate for alcohol’s sedating effects. When alcohol is suddenly removed, excitation goes unchecked and seizures can result.
Alcohol withdrawal seizures typically appear 6 to 48 hours after the last drink, with peak risk between 12 and 48 hours. About 95% of these seizures occur within the 7 to 38 hour window. They affect roughly 1% of people going through alcohol withdrawal, though the risk is much higher in those with a history of heavy, long-term drinking or prior withdrawal seizures.
Benzodiazepines and barbiturates work on the same brain calming system as alcohol, and abrupt cessation after prolonged use carries a similar seizure risk. Certain recreational drugs, particularly cocaine and synthetic stimulants, can also provoke seizures during active use by flooding the brain with excitatory activity.
Everyday Triggers in People With Epilepsy
For people who already have a seizure disorder, certain everyday factors can lower the threshold and provoke a breakthrough seizure. Emotional stress is the most frequently self-reported trigger. Sleep deprivation is another major one: in a study of over 1,000 people who had their first unprovoked seizure, 20% of those seizures were associated with sleep deprivation.
Other commonly reported triggers include missed doses of seizure medication, excessive alcohol use, illness or fever, flashing or flickering lights (in those with photosensitive epilepsy), and hormonal changes tied to menstruation. Exposure to certain toxins, including heavy metals and carbon monoxide, can also provoke seizures in susceptible individuals.
It’s worth noting that many of these triggers don’t cause seizures on their own in someone without an underlying predisposition. They act more like a final push in a brain that is already closer to the seizure threshold than normal.
When No Cause Is Found
In a substantial number of cases, no specific cause can be identified even after thorough testing. These are classified as seizures of unknown etiology. Some likely involve genetic factors that subtly alter how ion channels or neurotransmitter receptors function, making certain neurons more excitable without producing any visible abnormality on brain imaging. Genetic testing is increasingly able to identify these mutations, but for many people, the cause remains genuinely uncertain. Having no identifiable cause does not change the fact that the seizures are real or that they respond to treatment.