An electroencephalogram, commonly known as an EEG, serves as a primary diagnostic tool in neurology, offering a window into the brain’s electrical activity. This non-invasive test records the tiny electrical signals generated by brain cells through electrodes carefully placed on the scalp. Its fundamental purpose in the context of seizures is to detect and characterize the abnormal electrical discharges associated with these neurological events.
The EEG Procedure
Undergoing an EEG test involves a straightforward and painless process for the patient. Before the appointment, individuals are advised to wash their hair to ensure it is clean and free of conditioners or styling products, which can interfere with electrode adhesion. Patients are instructed to avoid caffeine and other stimulants on the day of the test, as these substances can alter brain activity. During the procedure, a trained technologist applies electrodes to specific points on the scalp using a conductive paste or by fitting a cap containing the electrodes.
Once the electrodes are securely in place, they connect to a recording device that amplifies and displays the brain’s electrical signals as wavy lines on a monitor or paper. The patient lies still with their eyes closed, though they might be asked to open and close their eyes or perform simple tasks. Technologists may employ activation procedures to provoke seizure activity, such as hyperventilation (deep, rapid breaths) or photic stimulation (looking at a flashing light). EEG recordings range from brief outpatient tests (20-40 minutes) to prolonged inpatient monitoring lasting several days.
Identifying Seizure Activity on an EEG
A neurologist interprets the complex patterns of brainwaves recorded by the EEG, looking for deviations from normal activity. A healthy, awake adult exhibits rhythmic alpha waves, appearing as smooth, consistent oscillations, particularly over the back of the head when eyes are closed. Faster, lower-amplitude beta waves are present over the front of the head when the person is alert and engaged. Slower theta and delta waves are more prominent during sleep or in certain brain regions.
When a seizure occurs, the EEG recording displays distinct changes. The hallmark of seizure activity, known as “ictal” activity, involves sudden, abnormal, and repetitive electrical discharges. These can manifest as “spikes” (very fast, pointed deflections lasting less than 70 milliseconds) or “sharp waves” (slower, more rounded peaks lasting 70-200 milliseconds). These discharges disrupt the normal background rhythms, appearing as disorganized, high-amplitude, or abnormally rhythmic patterns that propagate across the electrodes. The specific appearance of these abnormal waves provides important information about the nature and origin of the seizure.
Types of EEG Seizure Patterns
The EEG is important in classifying different seizure types based on their distinct electrical signatures. Generalized seizures, for instance, are characterized by abnormal electrical activity that involves both hemispheres of the brain simultaneously from the onset. A classic example is the “3-Hz spike-and-wave” pattern, characteristic of absence seizures. This pattern consists of a spike immediately followed by a slow wave, repeating approximately three times per second, observed broadly across the scalp.
In contrast, focal seizures, also known as partial seizures, originate in a specific, localized area of one brain hemisphere. On an EEG, this appears as abnormal discharges confined to a particular group of electrodes, indicating the seizure’s point of origin. The pattern might begin as localized rhythmic theta or delta activity, or repetitive spikes or sharp waves, before spreading to adjacent areas. Complex, high-frequency discharges, appearing as a rapid, desynchronized pattern, are associated with tonic-clonic seizures, reflecting widespread, intense neuronal firing. The precise location and propagation of these patterns help neurologists determine if a seizure is focal or generalized, which guides treatment decisions.
Distinguishing Seizures from Other Brain Activity
Interpreting an EEG requires careful distinction between seizure activity and other electrical signals. One important concept is “interictal epileptiform discharges,” which are abnormal spike-like or sharp wave patterns occurring between seizures. These discharges suggest an underlying tendency for seizures but do not represent an active seizure event. Their presence indicates an irritable brain state, providing supportive evidence for a seizure disorder diagnosis, even without a clinical seizure during recording.
Identifying “artifacts” is another challenge in EEG interpretation; these are non-brain signals that can contaminate the recording and mimic seizure activity. Common artifacts include electrical signals from eye movements (e.g., blinks or lateral eye shifts), which produce large, slow deflections in the frontal electrodes. Muscle tension, particularly from jaw clenching or scalp muscles, can create fast, irregular, high-frequency activity that might obscure brainwaves. External electrical interference from nearby medical equipment or power lines can introduce unwanted noise. Skilled EEG technologists and neurologists are trained to recognize these artifacts, ensuring they are not mistaken for genuine brain activity and allowing for an accurate diagnosis.