The electroencephalogram (EEG) is a non-invasive diagnostic tool that records the electrical activity generated by the brain, which appears as wavy lines on a display. This technology is foundational for evaluating neurological conditions, with its primary application being the diagnosis and classification of epilepsy and seizure disorders. The EEG captures the collective electrical impulses of millions of neurons, translating the brain’s complex communication into measurable patterns. By analyzing these patterns, clinicians can determine if a patient’s symptoms are caused by abnormal electrical discharges.
Understanding Normal Brainwave Activity
The healthy brain produces electrical activity that is typically rhythmic and symmetrical across both hemispheres. This normal background activity is defined by two key measurements: frequency, measured in Hertz (Hz), and amplitude, which reflects the wave’s voltage. The frequency of brainwaves changes predictably based on a person’s state of consciousness, providing a baseline for comparison when looking for abnormalities.
Four main frequency bands characterize a normal EEG tracing:
- Beta waves (greater than 13 Hz) are observed when a person is awake, alert, and actively concentrating.
- Alpha waves (8 to 13 Hz) are prominent when an individual is awake but relaxed with their eyes closed.
- Theta waves (4 to 7 Hz) are associated with drowsiness and the early stages of sleep.
- Delta waves (3 Hz or less) are characteristic of deep, non-rapid eye movement sleep in adults.
The consistency and appropriate distribution of these rhythms for the patient’s age and state define a normal EEG recording.
Identifying the Ictal Event
An ictal event refers to the period when a seizure is actively occurring. Its signature on the EEG is a sustained change from the normal background rhythm, characterized by the sudden onset of highly synchronized, high-amplitude, and often rhythmic electrical discharges. Essentially, a seizure appears as a chaotic electrical “storm” where a large population of neurons fire abnormally and in unison.
The specific appearance depends on the seizure type, providing information about its origin and spread. Generalized seizures, such as absence seizures, are marked by the classic 3 Hz spike-and-wave complex. This repetitive discharge appears synchronously across both hemispheres of the brain, overriding the normal background activity for the duration of the clinical event.
Focal seizures begin in a specific area of the brain and show onset as localized, rhythmic activity, often in the delta, theta, or alpha frequency bands. This localized discharge may evolve in frequency and amplitude as the seizure progresses. In some cases, a focal seizure can spread, or secondarily generalize, which is observed as the localized rhythmic pattern rapidly transforming into a widespread, bilateral discharge.
Interictal Findings and Diagnostic Clues
The majority of routine EEG recordings are performed when the patient is not actively seizing, known as the interictal period. Even during this time, the EEG can capture subtle electrical signatures that indicate an underlying predisposition to seizures. These brief, abnormal electrical transients are referred to as interictal epileptiform discharges (IEDs).
The most common IEDs are sharp waves and spikes, which are transient, high-amplitude waveforms that stand out from the background activity. A spike lasts 20 to 70 milliseconds, while a sharp wave is slightly more prolonged, lasting between 70 and 200 milliseconds. These waveforms represent the synchronized electrical activity in a hyperexcitable population of neurons.
The location and frequency of these discharges are valuable diagnostic clues for classifying the type of epilepsy. Spikes confined to one area suggest a focal epilepsy, while generalized spike-and-wave patterns imply a generalized epilepsy syndrome. Slow wave activity, particularly when intermittent and rhythmic over a localized area, can also be an interictal finding that points to irritation or dysfunction in that specific region.
Limitations of the EEG Test
Despite its utility, the EEG test has inherent limitations; a single normal recording does not definitively exclude epilepsy. Scalp EEG is highly specific—meaning an captured epileptiform pattern strongly suggests a seizure disorder—but it is not highly sensitive. The electrical source of a seizure may be too small or originate too deep within the brain to be detected by scalp electrodes.
Epileptiform activity often fluctuates, and a brief, routine recording may miss the abnormal discharge. Some individuals with confirmed epilepsy may never exhibit an abnormal pattern, even with serial recordings. To increase the likelihood of capturing activity, clinicians use activating methods such as hyperventilation, photic stimulation, or sleep deprivation, or they order long-term video-EEG monitoring.
Long-Term Monitoring
Long-term monitoring, often done in a hospital setting, allows for the safe reduction of anti-seizure medication to provoke an event. This increases the chance of recording both the clinical symptoms and the corresponding electrical activity. Ultimately, EEG findings must always be integrated with the patient’s full clinical history, symptoms, and neuroimaging results to establish a definitive diagnosis.