Epilepsy is diagnosed through a combination of clinical history, brain wave recordings, and imaging, not from a single test. The formal criteria require either two unprovoked seizures more than 24 hours apart, or a single unprovoked seizure with at least a 60% chance of another occurring within 10 years. A diagnosis can also be made when someone fits a recognized epilepsy syndrome.
Why Your Account of the Seizure Matters Most
The diagnostic process almost always starts with a detailed description of what happened before, during, and after the event. Since you likely won’t remember the seizure itself, the account of someone who watched it is extremely valuable. Doctors will want to know several specific things: what you were doing right before the event, whether your eyes moved to one side, whether your limbs jerked or stiffened, whether you lost bladder control, and how long it took you to return to normal afterward.
These details help distinguish a true seizure from other events that can look similar. Fainting (syncope), for example, can cause jerking movements, but the pattern is different. In fainting episodes, people typically have fewer than 10 jerks that are irregular and lose muscle tone completely. In convulsive seizures, there are usually more than 20 jerks, they’re rhythmic, and full loss of tone doesn’t occur. Researchers have described this as the “10/20 rule,” a practical way clinicians separate the two.
Blood Tests to Rule Out Other Causes
Before diagnosing epilepsy, doctors need to confirm the seizure wasn’t provoked by something temporary. Blood tests check for low blood sugar, electrolyte imbalances (the salts that regulate fluid balance in your body), signs of infection, and sometimes drug or alcohol levels. If a seizure was triggered by one of these, it’s considered provoked and doesn’t count toward an epilepsy diagnosis. Only unprovoked seizures, those without an obvious external trigger, meet the criteria.
EEG: Recording Your Brain’s Electrical Activity
An electroencephalogram, or EEG, is the most important test for confirming epilepsy. Electrodes are placed on your scalp to detect abnormal electrical patterns between seizures. The catch is that a single routine EEG misses a lot. Only about 13% of people with epilepsy show characteristic abnormalities on a standard recording.
To improve those odds, your doctor may order a sleep-deprived EEG, where you stay up most of the night before the test. Sleep deprivation makes abnormal brain activity more likely to surface, and studies show roughly 41% of epilepsy patients show abnormalities on a sleep-deprived recording compared to 13% on a standard one. This is especially useful for generalized epilepsy, the type that involves both sides of the brain from the start. For focal epilepsy, which begins in one area, the advantage of sleep deprivation over simply repeating a standard EEG is less clear.
A normal EEG does not rule out epilepsy. It simply means no abnormal activity was captured during that particular recording window.
Video-EEG Monitoring
When a routine EEG isn’t enough, you may be referred to an epilepsy monitoring unit for continuous video-EEG recording. This involves staying in a hospital room with electrodes on your head and a camera running around the clock, so doctors can watch what happens to your body and your brain waves simultaneously during an actual event.
This level of monitoring is used in several situations: when the diagnosis is uncertain and doctors need to see whether your episodes are epileptic seizures or something else entirely, when seizures need to be classified as focal or generalized to guide medication choices, when there are concerns about seizures happening without visible symptoms, or when surgery is being considered for drug-resistant epilepsy. The stay typically lasts several days, sometimes longer, depending on how frequently events occur.
Brain Imaging With MRI
Most people with a new epilepsy diagnosis will get an MRI of the brain. The goal is to find a structural cause for the seizures. Doctors look for specific abnormalities: areas where the brain’s outer layer didn’t develop normally (called cortical dysplasia), scarring in the memory-related structures of the inner temporal lobe, small tangles of blood vessels, tumors, or other developmental irregularities.
Higher-powered MRI scanners (3 Tesla versus the standard 1.5 Tesla) are better at picking up subtle findings, particularly small areas of abnormal brain development that can be invisible on lower-resolution scans. The improved contrast at the boundary between gray and white matter is what makes the difference. If your initial MRI looks normal but seizures continue, your neurologist may recommend rescanning on a more powerful machine with a specialized epilepsy protocol.
Classifying the Seizure Type
Diagnosis doesn’t stop at confirming epilepsy exists. The type of seizure matters because it determines which treatments work best. Under the current international classification system, seizures fall into three categories based on where they start in the brain.
- Focal onset seizures begin in networks on one side of the brain. They may stay localized or spread. Symptoms depend on the area involved and can range from a strange sensation or déjà vu to jerking in one arm to a full convulsion.
- Generalized onset seizures engage both sides of the brain from the beginning. These include the classic tonic-clonic (grand mal) convulsions as well as absence seizures, where someone briefly stares and becomes unresponsive.
- Unknown onset seizures are those where nobody witnessed the beginning, so doctors can’t yet determine the starting point.
The EEG pattern, the clinical description of the event, and MRI findings all feed into this classification. Getting it right is important because some medications that help focal seizures can actually worsen certain generalized types.
When Genetic Testing Is Recommended
Genetic testing isn’t part of every epilepsy workup, but it’s increasingly used when the cause remains unexplained after standard evaluation. The American Epilepsy Society identifies several situations where genetic testing adds value: epilepsy that begins in childhood (especially severe forms), drug-resistant epilepsy with no clear cause, epilepsy accompanied by developmental delays or other neurological problems, brain malformations visible on MRI, or a family history of epilepsy in two or more close relatives.
When a genetic cause is found, it can sometimes change treatment. Certain genetic variants affect how the brain’s ion channels work, and specific medications target those pathways more effectively than standard options. In some metabolic forms of epilepsy, dietary changes or supplements can make a meaningful difference. For many families, identifying a genetic cause also ends what can be a long and frustrating diagnostic journey.
How Long Diagnosis Takes
For someone who arrives at the emergency department after two clear convulsive seizures witnessed by others, with a characteristic EEG pattern, diagnosis can happen within days. But for many people, the process is slower. Seizures may be subtle or infrequent. The first EEG may be normal. Events might be hard to distinguish from panic attacks, fainting, or sleep disorders. In these cases, diagnosis can take weeks to months and may require repeated testing or an inpatient video-EEG stay before the picture becomes clear.
If your initial evaluation is inconclusive, that doesn’t mean epilepsy has been ruled out. It means the evidence isn’t sufficient yet, and further monitoring or repeat testing may be the next step.