Epilepsy develops when something changes the way brain cells communicate, causing them to fire in sudden, uncontrolled bursts. That “something” varies enormously: it can be a genetic trait you’re born with, a head injury, a stroke, an infection, or a structural abnormality in the brain. In roughly 50% of cases worldwide, no specific cause is ever identified.
What Happens in the Brain
Your brain cells communicate through a balance of electrical and chemical signals. Some signals excite neighboring cells to fire, while others inhibit them and keep activity in check. Epilepsy develops when that balance tips toward too much excitation or too little inhibition.
At the cellular level, this involves tiny channels in the surface of brain cells that control the flow of sodium, potassium, and calcium. These channels open and close in precise sequences to generate electrical signals. When inherited or acquired changes alter how these channels work, or when the chemical messengers that normally calm brain activity stop functioning properly, groups of neurons can begin firing together in synchronized bursts. That synchronized misfiring is a seizure. When the brain develops a tendency to produce seizures repeatedly, the condition is epilepsy.
This process, sometimes called epileptogenesis, doesn’t always happen instantly. After a brain injury or stroke, for example, the brain can undergo gradual changes over weeks or months before seizures begin. Scar tissue forms, neural connections rewire, and eventually a region of the brain becomes capable of generating spontaneous seizures.
Genetic Causes
Some people are born with gene variations that directly affect how their brain cells’ ion channels or chemical receptors function. These genetic forms of epilepsy often appear in childhood and can run in families, though having a relative with epilepsy doesn’t guarantee you’ll develop it.
The numbers help put the risk in perspective. The general population has about a 1.4% chance of developing epilepsy by age 40. If you have a sibling with epilepsy, your risk rises to roughly 4.6%, and if a parent has it, the risk for their children is about 3.9%. The type of epilepsy matters significantly: for generalized epilepsy (the kind that affects the whole brain at once), siblings and children of someone affected face an 8.2% risk. For focal epilepsy with no known structural cause, the increased risk is more modest.
Sex also plays a role in inheritance patterns. Siblings of women with epilepsy face a 5.8% risk, compared to 3.5% for siblings of men with the condition. Children of mothers with epilepsy have a 5.3% risk, while children of fathers with epilepsy have only a 1.9% risk, close to the general population rate.
Not all genetic epilepsy is inherited from a parent. Some cases result from new, spontaneous gene mutations that occur during fetal development. One well-studied example involves genes in the MTOR pathway, which controls how brain cells form and grow. Mutations in these genes can cause cells in the brain’s outer layer to develop abnormally, a condition called cortical dysplasia, which is a common cause of seizures in children.
Structural Brain Abnormalities
Anything that physically alters brain tissue can create a focus for seizures. This includes brain malformations that form before birth, tumors, abnormal blood vessels, and damage from injuries.
Cortical dysplasia is one of the most common structural causes. It occurs when groups of cells in the brain’s outer layer don’t form correctly during fetal development. The misformed cells can be oversized, arranged in the wrong pattern, or accompanied by other abnormalities like lesions or tumors. Because these cells don’t integrate normally into brain circuits, they can generate abnormal electrical activity.
Traumatic brain injury is another major structural cause. A severe blow to the head can damage brain tissue directly, and the resulting scarring and inflammation can eventually produce a seizure-prone area. The more severe the injury, the higher the risk.
Stroke and Aging
Stroke is the leading cause of new-onset epilepsy in older adults. About 10% of stroke survivors develop what’s known as poststroke epilepsy. The onset is variable, but 40% to 80% of these cases emerge within the first year after the stroke. When a stroke kills brain tissue, the surrounding area undergoes changes that can make it electrically unstable over time.
Alzheimer’s disease and other forms of neurodegeneration also increase epilepsy risk in older adults. As brain cells progressively deteriorate, the disruption to normal signaling pathways can trigger seizures, sometimes before dementia symptoms become obvious.
Infections That Cause Epilepsy
Brain infections are a major cause of epilepsy worldwide, particularly in lower-income regions. Meningitis (infection of the membranes surrounding the brain), encephalitis (infection of the brain itself), and brain abscesses can all damage tissue in ways that lead to chronic seizures.
The single most common preventable cause of epilepsy in the developing world is neurocysticercosis, a parasitic infection caused by the pork tapeworm. People become infected by consuming undercooked pork or water contaminated with tapeworm eggs, or through poor hygiene. The larvae travel to the brain and form cysts, which cause inflammation, chronic headaches, and seizures. The World Health Organization identifies it as a common cause of seizures globally.
Immune and Metabolic Causes
In autoimmune epilepsy, the body’s immune system mistakenly attacks brain cells or their receptors. This is a relatively recently recognized category. The immune attack disrupts normal signaling and can trigger recurrent seizures. These cases sometimes respond to treatments that suppress the immune system, which distinguishes them from other forms of epilepsy.
Metabolic disorders, many of them inherited, can also cause epilepsy. These conditions disrupt the chemical processes brain cells depend on for normal function. In newborns, even temporary metabolic problems like low blood sugar, low calcium, or electrolyte imbalances can provoke seizures.
How Causes Differ by Age
The most likely cause of epilepsy depends heavily on when seizures first appear. In newborns, the typical triggers are brain malformations, oxygen deprivation during birth, bleeding in the brain, metabolic problems, and maternal drug use during pregnancy. In infants and young children, fevers are the most common seizure trigger (these are called febrile seizures, and most children who have them do not go on to develop epilepsy), along with infections and, less commonly, brain tumors.
In older children and young adults, genetic factors, congenital conditions like Down syndrome or tuberous sclerosis, head trauma, and progressive brain diseases become more prominent. In adults over 65, stroke and Alzheimer’s disease dominate as causes.
How Epilepsy Is Diagnosed
A single seizure doesn’t necessarily mean you have epilepsy. The clinical definition requires at least two unprovoked seizures occurring more than 24 hours apart. However, epilepsy can also be diagnosed after just one seizure if the risk of having another is estimated at 60% or higher over the next 10 years. This might be the case if brain imaging reveals a structural abnormality or if an EEG shows a pattern strongly associated with recurrent seizures. A diagnosis can also be made if the pattern of seizures fits a recognized epilepsy syndrome.
The 60% threshold was chosen because it roughly matches the recurrence risk that already exists after two unprovoked seizures, which falls between 60% and 90%. After a single seizure with no identifiable cause, the recurrence risk at five years is closer to 50%, which is why many doctors take a watch-and-wait approach after a first event.
When No Cause Is Found
Despite advances in genetic testing and brain imaging, about half of all epilepsy cases have no identifiable cause. This doesn’t mean nothing is wrong. It means current technology can’t yet pinpoint the specific change responsible. In many of these cases, a combination of subtle genetic predispositions and environmental factors likely work together to lower the brain’s seizure threshold. The condition is still real, still treatable, and still follows the same patterns of diagnosis and management as epilepsy with a known cause.