Epilepsy is a brain disease defined by recurring, unprovoked seizures. The formal diagnostic threshold, set by the International League Against Epilepsy, requires at least two unprovoked seizures more than 24 hours apart. Around 50 million people worldwide live with the condition, and roughly 5 million new cases are diagnosed each year.
The Three Diagnostic Criteria
A person meets the clinical definition of epilepsy if any one of the following is true:
- Two or more unprovoked seizures occurring at least 24 hours apart
- One unprovoked seizure combined with a high probability (at least 60%) of another seizure within the next 10 years, based on brain imaging, EEG findings, or the circumstances of the first seizure
- Diagnosis of a recognized epilepsy syndrome, a specific pattern of seizure type, age of onset, and EEG features that clinicians can identify as a known condition
That second criterion is important because it means some people can receive an epilepsy diagnosis after just one seizure, if there’s strong evidence that more are likely. For example, someone whose brain MRI shows an old stroke lesion or a structural abnormality in an area known to generate seizures may already cross the 60% recurrence threshold.
How a Seizure Differs From Epilepsy
Having a seizure does not automatically mean you have epilepsy. Many people experience a single seizure at some point in their lives and never have another. Seizures can be provoked by specific triggers: high fever in children, extremely low blood sugar, alcohol withdrawal, or a severe head injury. These provoked seizures don’t count toward an epilepsy diagnosis because the brain was reacting to a temporary insult, not generating seizures on its own.
Epilepsy specifically refers to the brain’s ongoing tendency to produce unprovoked seizures, meaning seizures that happen without an obvious immediate trigger. Unless there’s brain damage, a family history of epilepsy, or other neurological abnormalities, most people who have a single seizure won’t go on to have more.
What Happens in the Brain During a Seizure
A seizure is a temporary disruption of brain function caused by nerve cells firing excessively and in unison. Normally, neurons communicate through carefully timed electrical signals. During a seizure, two things go wrong simultaneously. First, individual neurons become hyperexcitable, meaning they fire rapid bursts of electrical signals in response to much weaker stimulation than usual. Second, large groups of neighboring neurons get recruited into firing together in lockstep, a phenomenon called hypersynchrony.
The result is a wave of abnormal electrical activity that can stay in one area of the brain or spread widely. Where the activity occurs and how far it spreads determines what the seizure looks like from the outside, whether that’s a brief staring spell, involuntary jerking of one arm, or a full-body convulsion with loss of consciousness.
Types of Seizures
Seizures fall into three broad categories based on where the abnormal electrical activity starts:
- Focal onset seizures begin in one specific area of one hemisphere of the brain. You might remain fully aware during a focal seizure (experiencing unusual sensations, a strange taste, or involuntary movement in one part of the body) or your awareness may be impaired.
- Generalized onset seizures appear to start in both hemispheres simultaneously. These include absence seizures (brief staring episodes common in children) and tonic-clonic seizures (the classic convulsion with stiffening and rhythmic jerking).
- Unknown onset seizures are classified this way when a doctor can’t determine where the activity originated, often because nobody witnessed the beginning of the episode or testing was inconclusive.
A person with epilepsy may experience only one type of seizure or several different types, depending on the underlying cause and the areas of the brain involved.
What Causes Epilepsy
Epilepsy has no single cause. In many cases, the origin is never identified. When a cause is found, it typically falls into one of several categories: structural brain damage from stroke, traumatic injury, or tumors; infections that affect the brain such as meningitis or encephalitis; and genetic factors.
Genetics play a larger role than many people realize. Research presented at the American Epilepsy Society Annual Meeting found that more than 40% of adults with epilepsy carry a genetic variant linked to the condition. These aren’t always inherited in a straightforward way. Some variants increase susceptibility rather than directly causing seizures, and many people carry variants whose significance isn’t yet understood. Still, the finding suggests that genetic testing can be informative even for adults who developed epilepsy later in life without an obvious structural cause.
How Epilepsy Is Diagnosed
Diagnosis typically starts with a detailed history of the seizure events, including what happened before, during, and after each episode. An EEG (a test that records the brain’s electrical activity through sensors on the scalp) is the primary tool. It can detect the abnormal electrical patterns characteristic of epilepsy even between seizures.
Brain imaging, usually an MRI, helps identify structural causes like scar tissue, tumors, or developmental abnormalities. In ambiguous cases, video EEG monitoring in a hospital setting is considered the gold standard. This involves recording the patient on camera while simultaneously tracking brain electrical activity, allowing doctors to match visible symptoms with what’s happening electrically in real time.
This kind of monitoring is especially important for distinguishing epileptic seizures from psychogenic non-epileptic seizures (PNES), which look similar from the outside but are not caused by abnormal electrical discharges. PNES episodes show no epileptic brain activity on EEG. Certain physical features also help with differentiation: gradual onset, side-to-side head shaking, and asynchronous limb movements are more common in PNES than in epileptic seizures.
When Epilepsy Is Considered Resolved
Epilepsy is not necessarily a lifelong diagnosis. Some epilepsy syndromes are age-dependent, meaning children outgrow them as their brains mature. For others, epilepsy can be considered resolved if a person has been seizure-free for at least 10 years, with at least the last 5 of those years off medication. At that point, the risk of future seizures drops low enough that the active disease label no longer applies, though it doesn’t guarantee seizures will never return.