A seizure is a transient disturbance of brain function caused by abnormal, excessive electrical activity among groups of neurons in the brain. This disorganized electrical surge can manifest in a variety of ways, ranging from brief staring spells to full-body convulsions. An Electroencephalogram (EEG) is a non-invasive diagnostic test that uses small metal discs (electrodes) placed on the scalp to measure these electrical impulses. The primary purpose of the EEG is to record brain wave patterns, but the question remains whether this test captures every seizure event.
The Relationship Between Seizures and EEG Activity
The direct answer to whether all seizures show up on an EEG is no. An EEG is a powerful diagnostic tool, but it only records the brain’s electrical activity during the short time the test is being performed. The likelihood of a routine, short-duration EEG recording an actual seizure event, known as ictal activity, is low.
When a seizure is captured, it appears on the EEG tracing as a distinct pattern of disorganized, high-amplitude waves. These abnormal signals are called epileptiform discharges, often appearing as sharp waves or spikes. However, the most common finding in a person with epilepsy is a pattern of abnormal waves that occur between seizures, referred to as interictal activity. The presence of this interictal activity is highly suggestive of epilepsy, but its absence does not automatically rule out the condition.
Factors That Limit EEG Detection
Several factors can prevent a seizure’s electrical signature from being detected on a standard scalp EEG. One major limitation is the location of the seizure focus within the brain. Seizures originating deep in the brain, such as in the mesial temporal lobe, may not generate an electrical field strong enough to travel through the skull and scalp to reach the surface electrodes.
The brief nature of many seizures also contributes to missed detection. A routine EEG is a short test, typically lasting only 20 to 40 minutes, which is a small window in which to capture a sporadic event. Furthermore, patient movement, such as muscle tensing or eye blinking, can create artifacts—electrical interference that can obscure a subtle seizure signal. Standard electrode placement only covers the cortical surface, meaning activity localized to small, deep structures can be missed.
Advanced EEG Monitoring Techniques
To overcome the limitations of a routine EEG, specialized monitoring procedures increase the probability of capturing a seizure or abnormal interictal activity. Video-EEG monitoring (VEEG) continuously records the patient in an inpatient setting, simultaneously capturing electrical brain activity and corresponding physical behavior. This extended recording helps doctors correlate clinical symptoms with electrical changes over a period of days.
For patients whose seizures occur infrequently or primarily at home, an Ambulatory EEG offers a portable solution. This device allows the patient to wear the equipment for several days while going about their daily routine outside of the hospital. Additionally, physicians employ activation procedures during an EEG to intentionally provoke electrical activity. These techniques include sleep deprivation before the test, deep breathing (hyperventilation), or exposure to flashing lights (photic stimulation).
Comprehensive Seizure Diagnosis Beyond the EEG
The diagnosis of a seizure disorder is fundamentally a clinical decision, relying heavily on the patient’s medical history and eyewitness accounts of the event. Even a negative EEG result does not negate a clear history of seizures witnessed by a reliable observer. In such cases, the EEG functions as one piece of evidence, not the final determinant.
Neuroimaging techniques identify structural abnormalities in the brain that may be the underlying cause of seizures. Magnetic Resonance Imaging (MRI) provides detailed images of soft tissues, allowing doctors to spot lesions, tumors, stroke damage, or developmental malformations. Similarly, Computed Tomography (CT) scans identify acute issues like bleeding or structural changes.
Other functional scans help pinpoint the seizure focus by assessing brain metabolism and blood flow. Positron Emission Tomography (PET) scans and Single-Photon Emission Computed Tomography (SPECT) scans reveal areas of the brain that are either underactive or overactive, which often correlates with where the seizure originates. These non-electrical tests are invaluable for confirming the diagnosis and planning treatment when the EEG remains inconclusive.