The Electroencephalogram (EEG) is a medical test that records the brain’s electrical activity. The procedure involves placing small, metal disc electrodes onto the scalp, often using a conductive paste. These electrodes detect subtle electrical signals produced by communication between brain cells, which are then amplified and displayed as wavy lines. This tracing provides a functional view of the brain, unlike structural scans such as MRI or CT. Since epilepsy and seizures are disorders of abnormal electrical discharge, the EEG is the primary tool for studying these conditions. It allows physicians to observe ongoing electrical rhythms and identify disruptions suggesting a neurological irregularity.
The Role of EEG in Seizure Diagnosis
The EEG is central to diagnosing epilepsy because it captures the condition’s electrical signature. Epileptic seizures result from a sudden, abnormal burst of highly synchronized electrical activity among groups of neurons. Non-epileptic events, such as fainting or psychogenic seizures, do not originate from this disorganized electrical discharge. When a patient has a seizure-like episode while connected to the EEG, the test confirms if the event was truly epileptic by revealing characteristic abnormal brainwave patterns.
The EEG’s utility extends beyond capturing an active seizure, which is often difficult to time. It is also valued for detecting interictal epileptiform discharges (IEDs), which are abnormal electrical spikes or sharp waves occurring between seizures. These IEDs reveal underlying hyperexcitability in the brain’s electrical network, even when the patient feels normal. While IEDs alone do not confirm epilepsy, they strongly support the diagnosis when combined with a history of clinical seizures. Analyzing these patterns helps classify the seizure type for optimal treatment selection.
Types of EEG Monitoring
The duration and setting of the EEG test are adjusted based on seizure frequency.
Routine EEG
The routine EEG is the simplest and most common initial test, typically lasting 20 to 40 minutes. During this time, technicians may use “activation procedures” like deep, rapid breathing (hyperventilation) or flashing lights (photic stimulation) to attempt to provoke subtle electrical abnormalities.
Sleep-Deprived and Ambulatory EEG
If a routine test is inconclusive, a sleep-deprived EEG may be ordered, requiring the patient to have less sleep than usual. Sleep deprivation increases the likelihood of recording epileptiform activity, especially abnormalities that manifest during drowsiness or sleep. For infrequent events, an ambulatory EEG uses a portable recorder worn for one to three days. This allows continuous recording while the person engages in their normal daily routine, increasing the chance of capturing a rare event.
Video-EEG Monitoring (VEM)
The most comprehensive method is Video-EEG Monitoring (VEM), where the patient is admitted to a specialized Epilepsy Monitoring Unit (EMU). VEM simultaneously records the brain’s electrical activity and captures the patient’s physical behavior with a synchronized video camera. This dual recording is the gold standard because it allows neurologists to precisely correlate the physical manifestations of an event with the corresponding electrical changes. This correlation helps differentiate true epileptic seizures from non-epileptic attacks.
Interpreting Seizure Patterns
When a seizure occurs, the resulting pattern on the EEG tracing, known as the ictal recording, changes dramatically from the normal background rhythm. The hallmark of an active seizure is a pattern of rhythmic discharges—repetitive, high-amplitude waves that evolve in frequency and location. An electrographic seizure is defined as a rhythmic pattern of epileptic activity lasting ten seconds or longer, often with a frequency greater than 2.5 Hertz.
Interictal Discharges
The electrical signatures seen between seizures, IEDs, are informative for diagnosis and classification. IEDs primarily appear as Spikes or Sharp Waves, which are brief, transient electrical deviations standing out from the background activity. A spike is a fast discharge lasting 20 to 70 milliseconds, while a sharp wave is broader, lasting 70 to 200 milliseconds.
These abnormal waveforms often combine with a slower wave to form a Spike-and-Wave Complex, a pattern strongly associated with generalized epilepsy syndromes. For example, a generalized, symmetrical 3 Hertz spike-and-wave pattern is characteristic of typical absence seizures.
Focal vs. Generalized Discharges
The EEG differentiates between focal and generalized discharges, which is essential for seizure classification. Focal seizures begin with discharges localized to one specific brain region, such as the temporal or frontal lobe, which may or may not spread. Generalized discharges, in contrast, involve both hemispheres of the brain simultaneously from the onset, reflecting instantaneous engagement of the entire cortical network. Identifying the origin of the abnormal rhythm is a determining factor in selecting the appropriate anti-seizure medication.
Limitations and Ambiguities
Despite its utility, the EEG has several limitations that can lead to inconclusive results. A single routine EEG has a low diagnostic yield; a completely normal tracing does not rule out epilepsy. Since seizures are episodic, the brief recording window may miss the abnormal electrical activity, necessitating longer monitoring.
A common challenge is the presence of artifacts—electrical signals originating from sources other than the brain that contaminate the recording. Interpreters must carefully distinguish these artifacts from genuine cerebral activity to avoid misdiagnosis. Artifacts can be caused by:
- Muscle movement
- Blinking
- Sweating
- Electrical interference from nearby equipment
Furthermore, some electrical patterns resembling epileptiform discharges are normal variants that occur in healthy individuals, particularly during sleep. The interpretation requires a specialist to place the findings within the full clinical context, as an abnormal reading alone is rarely sufficient to confirm epilepsy. Conversely, some focal seizures originating deep within the brain may not register on the scalp electrodes, resulting in a normal EEG even during a clinically observed seizure.