EEG Seizure Patterns: What Do They Look Like?

An electroencephalogram (EEG) is a non-invasive test that measures and records the brain’s electrical activity. Brain cells generate tiny electrical impulses to communicate, and an EEG captures this activity using small metal discs called electrodes attached to the scalp. The recording displays these signals as wavy lines. Seizures are the result of sudden, abnormal bursts of these electrical signals, and an EEG is a primary tool used to observe these disturbances and diagnose conditions like epilepsy.

The EEG Procedure for Seizure Evaluation

The EEG process begins with a technician painlessly attaching 16 to 25 electrodes to the scalp using a special paste or cap. These electrodes only record electrical signals and do not transmit any electricity. A routine EEG, the most common initial test, lasts between 20 and 40 minutes while the patient rests quietly.

To increase the chance of capturing abnormal brain activity, the technician may use activation procedures. These include hyperventilation, which involves deep and rapid breathing, or photic stimulation, which uses bright, flashing lights. Both methods can trigger brainwave patterns associated with certain types of epilepsy.

If a standard EEG is inconclusive, a doctor might recommend longer or more specialized monitoring. A sleep-deprived EEG is performed after the patient has had less sleep, as fatigue can make abnormal brainwaves more likely to appear. An ambulatory EEG allows a person to go about their daily activities while a portable device records brain activity for one to three days.

In more complex cases, a video EEG (VEEG) is performed in a hospital. This procedure combines continuous EEG with video and audio recording over several days. Synchronizing the EEG data with the video allows doctors to precisely match a person’s physical actions with the electrical activity in their brain.

Identifying Seizure Activity on an EEG

When examining an EEG, a neurologist looks for deviations from the brain’s normal, rhythmic electrical patterns. The electrical activity associated with a tendency for seizures appears as distinct, disruptive events known as epileptiform discharges. These represent the brain’s potential for generating seizures.

The two primary types of abnormal findings are interictal and ictal patterns. Interictal epileptiform discharges are brief bursts of abnormal electrical activity that occur between seizures, appearing as sharp, pointed waveforms called “spikes” or “sharp waves.” These patterns stand out from the smoother waves of normal background activity.

An ictal pattern is the electrical signature of a seizure itself, recorded during an actual clinical event. Unlike brief interictal spikes, an ictal pattern is sustained and shows a clear evolution, often beginning with a burst of high-frequency activity that changes over seconds to minutes.

Observing an ictal pattern provides definitive proof of a seizure. Because capturing one during a short EEG is not always possible, the presence of interictal spikes is significant, as they serve as important markers for an underlying seizure disorder.

Types of EEG Seizure Patterns

The patterns on an EEG also help classify the type of epilepsy, which is a major factor in determining the most appropriate treatment. The primary distinction is between generalized and focal seizure patterns, based on where the abnormal electrical activity begins.

Generalized seizure patterns appear to start in all parts of the brain simultaneously, with abnormal discharges occurring across both hemispheres from the onset. A classic example is the “generalized 3-Hz spike-and-wave” pattern, which is the hallmark of absence seizures. This pattern consists of a repeating sequence of a spike followed by a slow wave occurring about three times per second.

Focal seizure patterns originate in a specific, localized area of the brain, such as the temporal or frontal lobe. The EEG shows the abnormal spikes or seizure activity starting in one region. From this point of origin, the seizure activity may remain contained or spread to other areas of the brain.

Interpreting EEG Results

An EEG provides valuable information, but its results must be interpreted carefully within the context of a person’s overall medical situation. The recording is just one tool in the diagnostic process, and a neurologist integrates the findings with a patient history, descriptions of the events, and a physical examination.

A person with epilepsy can have a completely normal EEG result. Seizure activity and the interictal discharges that suggest a risk for seizures can be infrequent and may not occur during the short window of the test. This is why longer monitoring may be recommended if the clinical suspicion for epilepsy is high.

Conversely, an abnormal EEG does not automatically mean a person has epilepsy. Some individuals may have epileptiform discharges on their EEG but never experience a seizure. In these cases, the findings may not require treatment unless clinical symptoms develop.

Ultimately, the neurologist must determine if the patterns on the recording align with the patient’s symptoms. For example, a disruptive electrical pattern in the temporal lobe is highly significant if the patient describes episodes characteristic of temporal lobe seizures. Other tests, such as an MRI of the brain, are often used to look for structural causes that might explain the EEG findings.