Electroencephalography, commonly known as EEG, is a medical test that measures the electrical activity produced by the brain. It is a non-invasive procedure, meaning it does not involve any incisions or insertion of instruments into the body. This diagnostic tool helps medical professionals understand how the brain functions and provides insights into its electrical patterns. The information gathered from an EEG can assist in diagnosing various neurological conditions.
Understanding Electroencephalography
The brain’s intricate network consists of billions of neurons, specialized cells that communicate through tiny electrical impulses. These impulses generate electrical activity detectable from the scalp. An electroencephalograph uses electrodes placed on the scalp to pick up these faint signals, amplifying them for recording and analysis.
The recorded electrical activity manifests as rhythmic patterns known as brain waves, which reflect different states of consciousness and brain function. For instance, alpha waves typically appear when a person is awake but relaxed, often with eyes closed. Beta waves are associated with active thinking, alertness, and problem-solving, indicating a more engaged mental state.
Theta waves are observed during states of drowsiness, light sleep, or deep meditation. Delta waves, the slowest and highest amplitude waves, are characteristic of deep, dreamless sleep. By analyzing the presence, absence, and characteristics of these wave patterns, neurologists can gain insight into the underlying electrical health and activity of the brain.
Why an EEG is Performed
An EEG serves multiple diagnostic purposes. It is frequently employed to identify and monitor seizure disorders, such as epilepsy, by detecting characteristic abnormal electrical discharges that occur during or between seizures. The test can reveal specific spikes or sharp waves that indicate a predisposition to seizures, even when a patient is not actively experiencing one.
The procedure also plays a role in evaluating sleep disorders, including narcolepsy and insomnia, by recording brain activity during different sleep stages. This allows clinicians to identify disrupted sleep architecture or specific patterns associated with sleep-related breathing disorders. Following head injuries or strokes, an EEG can assess the extent of brain damage and monitor recovery by observing changes in brain wave patterns.
For patients in a coma, an EEG helps determine the level of brain activity and responsiveness, aiding in prognosis. It is a standard procedure for confirming brain death, as a flat or absent EEG tracing indicates a complete lack of electrical activity in the brain.
The EEG Procedure
Patients are advised to wash their hair before the procedure to ensure electrodes adhere properly. Avoiding caffeine and certain medications, such as sedatives, is also recommended, as these can affect brain activity and alter the results.
During the procedure, a technician will measure the patient’s head to determine the precise placement of electrodes onto the scalp. A conductive paste or gel is used to ensure good electrical contact between the electrodes and the skin.
Once the electrodes are in place, the patient will lie quietly on a bed or recline in a chair while the brain’s electrical signals are recorded. The patient may be asked to perform simple actions, such as opening and closing their eyes, or to engage in specific activities like hyperventilation (deep, rapid breathing) or looking at a flashing light. These activities can sometimes provoke abnormal brain activity that might not be apparent during resting states, helping to uncover underlying conditions.
Interpreting EEG Results
Neurologists analyze the recorded brain wave patterns. They examine several key characteristics of the waves, including their frequency, which refers to how many waves occur per second. The amplitude, or the height of the waves, indicates the strength of the electrical activity.
The morphology, or shape of the waves, provides additional diagnostic clues, as certain shapes can be indicative of specific neurological conditions. Neurologists assess the symmetry of brain activity, observing whether the electrical patterns are similar on both sides of the brain. Normal brain activity on an EEG typically shows a symmetrical pattern with expected frequencies and amplitudes corresponding to the patient’s age and state of consciousness.
Abnormal patterns might include unusually slow or fast waves, excessive amplitude, or the presence of sharp spikes. For example, focal slowing in a specific area could suggest a localized brain lesion or injury.
Different Types of EEG and Safety
Beyond the standard routine EEG, several specialized types of electroencephalography tests are employed to gather more specific information or to monitor brain activity over extended periods.
Sleep-Deprived EEG
A sleep-deprived EEG, for instance, is performed after a patient has had limited sleep, which can increase the likelihood of provoking abnormal brain activity or seizures that might not appear during a routine, rested state.
Ambulatory EEG
Ambulatory EEG involves the patient wearing a portable EEG device for one or more days, allowing continuous recording of brain activity while they go about their daily routines at home. This extended monitoring can capture infrequent events or patterns that might be missed in a shorter, in-clinic test.
Video EEG
Video EEG combines continuous EEG recording with simultaneous video monitoring, enabling clinicians to correlate specific brain wave changes with observable physical behaviors or clinical events, which is particularly useful in diagnosing and classifying seizure types.
EEG is a very safe procedure. It is non-invasive and does not involve the administration of any electrical shocks to the patient. The electrodes only detect the brain’s natural electrical activity. The most common potential side effect is mild skin irritation from the electrode paste, which is rare and usually resolves quickly after the test.