An Electroencephalogram (EEG) is a non-invasive medical test that measures the electrical activity of the brain, capturing the tiny signals produced by brain cells, or neurons, as they communicate. An EEG provides a graphical representation of brain function, allowing healthcare professionals to observe patterns related to various brain conditions.
Understanding the Electroencephalogram (EEG)
The EEG records brain waves, electrical impulses generated by the synchronized firing of millions of neurons in the brain’s cerebral cortex. Small metal discs, known as electrodes, are carefully placed on the scalp. These electrodes detect and amplify the subtle electrical signals, transmitting them to a machine that records the brain’s activity as wavy lines or traces on a computer screen.
Brain waves are categorized by their frequency, measured in Hertz (Hz), and reflect different states of consciousness. Delta waves (0.5-4 Hz) are associated with deep sleep, while theta waves (4-8 Hz) are prominent during light sleep or drowsiness. Alpha waves (8-13 Hz) occur when a person is awake but relaxed, and beta waves (13-30 Hz) signify an active, alert, and focused mental state.
The Role of EEG in Seizure Diagnosis
An EEG is a primary tool for diagnosing and managing seizure disorders, including epilepsy. Seizures result from sudden, uncontrolled bursts of abnormal electrical activity in the brain. The EEG helps to identify these irregular electrical discharges, which are distinct from normal brain wave patterns.
The test can confirm a seizure diagnosis, differentiate seizures from other conditions that might mimic them, and classify the type of seizure, such as generalized or focal. The EEG can pinpoint the specific area of the brain where seizures originate. While a routine EEG may not always capture an actual seizure, it can detect atypical brain wave patterns between seizures, known as interictal epileptiform discharges (IEDs), often seen in epilepsy.
Visualizing Seizure Activity on EEG
Normal brain waves on an EEG tracing appear regular and symmetrical. In contrast, seizure activity manifests as sudden, high-amplitude, and often rhythmic electrical discharges that significantly disrupt the background activity. These abnormal patterns indicate hyperexcitability within the brain’s cortical regions.
Common EEG patterns associated with seizures include spikes, sharp waves, and spike-and-wave discharges. A spike is a brief, pointed discharge lasting less than 70 milliseconds, while a sharp wave is similar but lasts longer, between 70 and 200 milliseconds. Both spikes and sharp waves are followed by a slow wave, representing the recovery period of the affected neurons.
Generalized seizures, such as absence seizures, are characterized by a regular, symmetrical 3 Hz spike-and-wave complex that appears abruptly on a normal EEG background. Focal seizures, on the other hand, may be associated with spikes or sharp waves localized to a specific brain region, such as temporal spikes for temporal lobe epilepsy.
The EEG Procedure and Interpretation Basics
Before an EEG, patients are advised to wash their hair the night before and avoid using conditioners, gels, or sprays, as these can interfere with electrode adhesion. Patients should inform their healthcare provider about any medications they are taking, as some can affect brain activity. Avoiding caffeine for at least eight hours prior to the test is also recommended.
During the procedure, a technician places about 20 small disc electrodes on specific scalp locations using a conductive adhesive gel. Patients are asked to remain still to minimize recording artifacts. A routine EEG lasts between 20 minutes to an hour, but other types, like sleep-deprived or ambulatory EEGs, may be used to increase the likelihood of capturing seizure activity. The recorded EEG data is then analyzed and interpreted by a neurologist or neurophysiologist, who reviews the brain wave patterns for diagnosis.