An electroencephalogram (EEG) records the electrical activity of the brain using small, metal discs called electrodes. These electrodes are temporarily attached to the scalp to detect electrical impulses generated by brain cells. The EEG translates this activity into wavy lines that a specialist analyzes to assess brain function. Since brain cells communicate constantly via electrical signals, the EEG provides a continuous, real-time picture of this dynamic process. It is a painless, non-invasive test used to evaluate various neurological conditions.
Defining the Electroencephalogram
The fundamental science behind the EEG lies in measuring synchronized electrical impulses, commonly known as brain waves, produced by masses of neurons. When a large number of brain cells fire together, the resulting electrical signal is strong enough to be picked up by the electrodes placed on the scalp. The resulting pattern of waves is classified by its frequency, measured in Hertz (Hz), and its amplitude.
Different mental states correlate with distinct types of brain waves. Delta waves are the slowest (0.5–4 Hz) and are associated with deep, restorative sleep. Theta waves (4–8 Hz) often appear during light sleep, drowsiness, or deep relaxation.
Alpha waves (8–12 Hz) are prominent when a person is awake but relaxed, such as when resting with eyes closed. These waves tend to disappear when a person opens their eyes or focuses on a mental task. Beta waves (13–30 Hz) are the fastest and dominate during periods of active concentration, alertness, and problem-solving. The neurologist interprets the mix of these wave types to understand the brain’s baseline activity and note any deviations.
Conditions Diagnosed by an EEG
The primary reason a doctor orders an EEG is to detect changes in brain activity that may point to a neurological disorder. The most frequent application is diagnosing and managing epilepsy or other seizure disorders. An EEG can capture the characteristic abnormal electrical discharges, such as spikes and sharp waves, that occur during or between seizure events.
Beyond seizure disorders, the EEG investigates sleep disorders, including narcolepsy and sleep apnea. It also helps assess individuals with an altered state of consciousness, such as those in a coma, or confirms brain death. The test assists in localizing and evaluating brain damage caused by head injuries, strokes, or infections like encephalitis. Other conditions, including brain tumors, certain types of dementia, and metabolic conditions, may also show recognizable patterns on an EEG.
Preparing for and Undergoing the Test
Preparation for an EEG usually begins the night before the procedure. Patients are instructed to wash their hair but avoid using conditioners, gels, or styling products, as these can prevent the electrodes from sticking properly. The doctor may also ask the patient to avoid caffeine for several hours prior to the test, since stimulants can alter the brain’s electrical patterns.
The test is performed by a specialized technician, who first measures the patient’s head to accurately mark the placement of the electrodes. Between 16 and 25 small metal discs are then attached to the scalp using a conductive paste or a specialized cap. The electrodes are connected by wires to the EEG machine, which amplifies and records the electrical signals.
A routine EEG usually takes between 20 and 40 minutes, though the entire appointment may last up to an hour and a half to account for preparation and cleanup. During the recording, the patient is asked to relax quietly with their eyes closed. The technician may ask the patient to perform specific actions to activate different brain patterns, such as opening and closing their eyes, breathing deeply and rapidly (hyperventilation), or looking at a flashing light (photic stimulation). In some cases, a sleep-deprived EEG is requested, requiring the patient to get only a few hours of sleep the night before, which can increase the likelihood of recording abnormal activity.
Interpreting EEG Results
Once the recording is complete, a neurologist or clinical neurophysiologist analyzes the printout or digital display. The specialist systematically evaluates the background activity, which is the dominant rhythm of the brain waves when the patient is awake and resting. A normal adult background rhythm shows an 8–13 Hz alpha rhythm when the eyes are closed.
The specialist looks for abnormal patterns that deviate from this expected rhythm, particularly epileptiform discharges. These include sharp waves and spikes, which are rapid, high-amplitude voltage changes that indicate cortical hyperexcitability and are a hallmark finding in epilepsy. Generalized slowing, where the dominant brain frequency drops below 8 Hz, can suggest more widespread brain dysfunction, such as an encephalopathy.
However, the presence of abnormal activity alone does not automatically confirm a diagnosis. The neurologist must correlate the EEG findings with the patient’s symptoms, medical history, and other test results. For example, a single sharp wave may not be significant, but frequent discharges in a specific brain region can help localize a seizure focus. The final diagnosis relies on a comprehensive clinical assessment.