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What Is an EEG?

By Deneatra Harmon
Updated Mar 03, 2024
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An EEG, also called an electroencephalography or electroencephalogram, is a test that detects the brain’s electrical activity or brain waves. Doctors often use the test to evaluate head injuries or to diagnose neurological disorders. Depending on the reason for the EEG, the patient may be awake or asleep during the test. Patients may have to devote anywhere from one to four hours to taking the test in a doctor’s office or hospital.

The EEG test monitors and records brain wave activity through the use of electrodes, which consist of small metal disks that attach to the patient’s scalp with a special adhesive. The electrodes connect to wires that are attached to a recording device and an amplifier. The amplifier connects to a computer monitor so that a neurologist can interpret the test results. The electrodes help to detect any electrical activity that is generated from the patient’s brain cells.

Patterns of the brain’s electrical activity are often categorized into four different types of wave rhythms during the EEG — alpha, beta, delta, and theta. These are used by neurologists to examine how the brain functions or reacts while the person sleeps or remains awake in up to a 30 minute period. The patient is tested with activity in each stage, ranging from sedation to responses to light or other stimuli, and sudden bursts of energy.

Each wave type is recorded at a different rate and indicates something different in relation to the patient's brain patterns. Alpha waves, which record up to 12 cycles per second, indicate when the patient is awake, but with eyes closed. Recorded at a pattern of up to 30 cycles per second, beta waves record the brain’s response to sedative medications. Doctors usually examine delta waves in children or during a patient’s deep sleep, as these are the shortest bursts of activity from 0.5 to 3.5 cycles per second. Theta waves are most often examined in children and young adults and are recorded at up to 7 cycles per second.

Neurologists often administer electrical activity tests to children and adults to diagnose head trauma, brain diseases, or sleep disorders. Patients with a brain injury from an accident may undergo an EEG so that doctors can evaluate changes in the brain. The exam may also be used to detect a brain tumor or a brain disease such as encephalitis. Electrical activity from the test most commonly detects the presence of epilepsy or other seizure disorders in patients. Alzheimer’s disease or narcolepsy may also be diagnosed with help from an EEG.

Preparation for the electroencephalography test usually varies depending on the patient’s condition. Hair should be washed with just shampoo the night before the exam, as electrodes would be placed on the scalp. The patient should avoid using conditioners or other hair products because they may interfere with test results. The doctor may request that the patient stops taking medications or avoids any caffeinated foods and beverages for several hours. Fasting is prohibited before the test because low blood sugar can skew the results.

Instructions may be given regarding sleep schedule that are specific to each patient. For instance, a patient may be instructed to reduce sleep the night before if she is required to sleep during the EEG. At the doctor’s office or hospital room, the patient may lie in a bed or recline in a chair with up to 25 electrodes placed on the head. Proper procedures call for the patient to remain still and relaxed, with eyes closed even though he or she may be awake. Results of the EEG may be available several hours after the test.

The Health Board is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
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