Epilepsy occurs when networks of brain cells become abnormally excitable, firing electrical signals in uncontrolled bursts instead of the steady, coordinated patterns that normally govern brain activity. Around 4 to 10 out of every 1,000 people worldwide have active epilepsy, and roughly 5 million new cases are diagnosed each year. The underlying cause varies widely, from genetic differences present at birth to brain injuries acquired later in life, but the core mechanism is always the same: something shifts the balance between electrical excitation and inhibition in the brain, making seizures possible.
What Happens in the Brain During a Seizure
Your brain cells communicate through tiny electrical impulses. Each cell fires, sends a signal to its neighbors, then quiets down so the next cell can take over. This on-off rhythm is tightly controlled by channels on the surface of each cell that let charged particles (ions) flow in and out. Sodium channels trigger a cell to fire. Potassium and chloride channels help it settle back down.
In epilepsy, that balance breaks. A group of neurons begins firing together, too fast and too intensely, overwhelming the brain’s normal braking systems. If the burst stays in one area, it produces a focal seizure, which might cause a twitch in one hand, a strange taste, or a brief zone-out. If the electrical storm spreads across both sides of the brain, it becomes a generalized seizure, potentially causing loss of consciousness, full-body stiffening, or rhythmic jerking. In some cases, doctors can’t determine where the seizure starts, and it’s classified as unknown onset.
Genetic Causes
About 25% of the genes linked to epilepsy encode ion channels, the very structures that control whether a brain cell fires or stays quiet. When one of these genes carries a mutation, the channels it builds may not work correctly. A sodium channel that stays open too long, for example, lets too much electrical current flood the cell, making it fire repeatedly when it should stop. A faulty potassium channel can’t pull the cell back to its resting state quickly enough, producing the same result.
One well-studied example is mutations in the SCN1A gene, which provides instructions for building a key sodium channel in the brain. Certain spontaneous mutations in SCN1A cause a severe form of epilepsy that begins in infancy. But genetic epilepsy isn’t always this dramatic. Some people inherit a subtle combination of gene variants from both parents that slightly lowers their seizure threshold without causing any obvious structural damage to the brain. In these cases, brain scans look perfectly normal, yet the electrical wiring is inherently more prone to misfiring.
Brain Injuries and Structural Causes
Any event that physically damages brain tissue can create a focus for seizures. When neurons are destroyed by injury or disease, the surviving cells around the damaged area often rewire themselves in disorganized ways. This rewiring can form circuits that loop electrical signals back on themselves, generating the runaway firing that defines a seizure. The process isn’t always immediate. After a stroke or head injury, months or even years can pass before the first seizure appears, a delay known as epileptogenesis.
Stroke is the leading cause of new-onset epilepsy in people over 65. One study found that among older adults diagnosed with epilepsy, roughly 74% had a vascular cause, meaning their seizures traced back to disrupted blood flow in the brain. In younger adults, traumatic brain injury and brain tumors are more common structural triggers. In children, developmental abnormalities that form before birth, where a patch of brain tissue didn’t migrate to its correct location during fetal development, account for a significant share of cases.
Infections That Lead to Epilepsy
Certain infections can inflame or scar brain tissue enough to cause lasting epilepsy. Bacterial meningitis, an infection of the membranes surrounding the brain, frequently triggers seizures during the acute illness, though these often stop once the infection is treated. Viral encephalitis, which attacks the brain tissue itself, carries a higher risk of permanent epilepsy because it tends to leave behind more structural damage.
Globally, one of the most common infectious causes is neurocysticercosis, a parasitic infection where tapeworm larvae form cysts in the brain. It’s a leading cause of epilepsy in many low- and middle-income countries, which partly explains why the rate of new epilepsy diagnoses in those regions (up to 139 per 100,000 people annually) is nearly triple the rate in high-income countries (49 per 100,000). Other infections linked to epilepsy include tuberculosis affecting the brain, HIV, cerebral malaria, and a group of infections passed from mother to baby during pregnancy that can cause seizures, developmental delays, and vision problems in newborns.
Why Certain Ages Are More Vulnerable
Epilepsy follows a U-shaped curve across the lifespan, peaking in early childhood and again after age 65. In children, the brain is still developing rapidly, and immature neural circuits are inherently more excitable. Genetic epilepsies and developmental brain abnormalities account for most childhood cases. Many childhood epilepsy syndromes, particularly absence epilepsy (which causes brief staring spells accompanied by a distinctive 3-per-second electrical pattern on brain recordings), improve or resolve entirely by adolescence as the brain matures.
The second peak comes in older adults, driven largely by stroke, small-vessel disease, and neurodegenerative conditions. Cardiovascular risk factors like high blood pressure, diabetes, and atrial fibrillation are closely linked to late-onset epilepsy. Between these two peaks, teenagers and working-age adults develop epilepsy less frequently, and when they do, the cause is more often traumatic brain injury, a brain tumor, or unknown.
Triggers vs. Causes
There’s an important distinction between what causes epilepsy and what triggers individual seizures in someone who already has it. The cause is the underlying reason the brain is seizure-prone: a gene mutation, a scar from a stroke, a developmental abnormality. A trigger is a temporary condition that pushes an already-vulnerable brain past its threshold.
The most consistently documented triggers include sleep deprivation, psychological stress, alcohol withdrawal, and missed medication doses. Some people are also sensitive to flickering or flashing lights, a phenomenon called photosensitivity. Hyperventilation, which changes the blood’s carbon dioxide levels, can provoke certain seizure types. Not everyone with epilepsy has identifiable triggers, and having a trigger doesn’t mean the person caused their seizure. It simply means that certain physiological states make the brain’s existing vulnerability more likely to express itself.
How Epilepsy Is Identified
The primary tool for confirming epilepsy is an electroencephalogram, or EEG, which records the brain’s electrical activity through sensors placed on the scalp. In someone with epilepsy, the EEG often picks up characteristic patterns even between seizures. Focal epilepsies typically show sharp spikes originating from one brain region. Generalized epilepsies produce spike-and-wave patterns that appear simultaneously across both hemispheres. Absence epilepsy has a particularly recognizable signature: synchronized spike-and-wave bursts cycling at 3 per second.
Brain imaging, usually an MRI, helps identify structural causes like scars, tumors, or developmental abnormalities. When the EEG and MRI are combined with a detailed description of what happens during seizures, doctors can often pinpoint not just whether someone has epilepsy, but what type and where in the brain it originates. That matters because different epilepsy types respond to different treatments, and in some cases, the seizure focus can be surgically removed if medication doesn’t control it.
When the Cause Stays Unknown
Despite thorough testing, the cause of epilepsy remains unidentified in a substantial number of cases. Brain scans look normal, genetic testing doesn’t reveal a clear mutation, and there’s no history of injury or infection. These cases, sometimes called cryptogenic epilepsy, likely involve subtle genetic or microstructural factors that current technology can’t detect. In one study of adults diagnosed before age 65, cryptogenic epilepsy was the most common classification, accounting for over 44% of cases. Having no identifiable cause doesn’t change the diagnosis or the treatment options. It simply means the specific mechanism that tipped the brain toward seizures hasn’t been found yet.