The Electroencephalogram, or EEG, is a non-invasive tool used to measure the electrical activity produced by the brain’s neurons. This activity results from the synchronized communication of millions of brain cells, which creates detectable electrical signals. These tiny voltage fluctuations are captured and amplified, producing a continuous stream of wavy lines that represent the brain’s ongoing function. The resulting record is a representation of its electrical rhythm, offering a functional snapshot of brain health.
Reading an EEG involves interpreting these patterns in the context of a patient’s state, such as whether they are awake, asleep, or stimulated. This requires understanding how the data is collected, the types of normal waves, and the visual characteristics of abnormal patterns. This technology helps clinicians diagnose various neurological conditions.
The Physical Setup and Recording Process
The process begins with the careful placement of small metal discs, called electrodes, onto the scalp using a conductive paste or a specialized cap. Technicians follow the standardized International 10-20 system to ensure activity from specific brain regions is consistently recorded. This system identifies electrode positions by measuring distances between specific bony landmarks on the skull.
The electrical signals generated by the brain are typically in the microvolt range, requiring an amplifier. The EEG machine amplifies these minute signals to produce a visible, readable tracing. This conditioning process also filters out unwanted noise, such as electrical interference or muscle movement, ensuring a clean recording of the brain’s activity.
The collection method varies depending on the diagnostic goal. A routine EEG is a short recording, often 20 to 40 minutes, performed in a clinic setting to capture a baseline or immediately obvious abnormalities. For conditions with infrequent events, a portable ambulatory EEG allows the patient to wear a small recorder for 24 to 72 hours, capturing brain activity during normal daily life. Video EEG monitoring synchronizes the electrical recording with a video camera, allowing clinicians to correlate a patient’s observable behavior, like a seizure, with the corresponding brainwave pattern.
Understanding the Basic Waveforms
The wavy lines on an EEG are defined by frequency and amplitude. Frequency, measured in Hertz (Hz), indicates how quickly a wave repeats itself, reflecting the speed of the underlying neural activity. Amplitude describes the height of the wave, which is a measure of the voltage or strength of the electrical signal. Clinicians categorize brain activity into four main frequency bands, each corresponding to a different state of consciousness.
The fastest activity is the Beta rhythm (13 to 30 Hz), associated with active mental concentration and alert wakefulness. This low-amplitude, high-frequency activity is dominant during focused thought. When the mind relaxes, such as during quiet rest with the eyes closed, the activity slows down to the Alpha rhythm (8 to 12 Hz). This is a higher-amplitude, rhythmic wave that typically appears most prominently over the back of the head.
Slower rhythms appear as consciousness diminishes. The Theta rhythm (4 to 7 Hz) is characteristic of drowsiness, light sleep, and early childhood. Excess Theta activity in an awake adult suggests diffuse brain dysfunction or reduced alertness. The slowest rhythm is the Delta wave (below 4 Hz) has the highest amplitude, dominating the EEG during deep, dreamless sleep. Outside of deep sleep or infancy, Delta activity in an adult is considered an abnormal sign of significant brain pathology.
Interpreting Contextual Patterns
Interpreting a recording involves assessing the overall pattern based on symmetry, reactivity, and morphology. A healthy brain exhibits symmetry, meaning the electrical activity should look nearly identical across both hemispheres in terms of frequency and amplitude. A significant difference between the left and right sides, where one side is slower or lower in amplitude, points toward a focal issue in the compromised hemisphere.
Reactivity refers to the expected change in wave patterns in response to external stimuli. For instance, the Alpha rhythm should promptly disappear when a person opens their eyes or performs a mental calculation. A lack of this expected change, known as a non-reactive pattern, suggests a widespread problem with the brain’s ability to respond to its environment.
Clinicians look for two types of slowing when assessing abnormalities. Focal slowing is characterized by an increase in Delta or Theta waves restricted to a specific area of the head, suggesting a localized structural lesion like a stroke, tumor, or bleed. In contrast, generalized slowing involves a widespread increase in slow-wave activity across the entire recording, associated with metabolic issues, drug effects, or diffuse conditions like encephalopathy.
The presence of spikes and sharp waves is the electrical signature of epilepsy. These are transient, high-amplitude discharges that look like abrupt peaks against the background rhythm. A spike lasts less than 70 milliseconds, while a sharp wave lasts between 70 and 200 milliseconds. These patterns indicate a predisposition for seizures. Another element is the periodic discharge, a pattern of sharp, repetitive waveforms occurring at regular intervals across the entire brain, often seen in severe conditions such as Creutzfeldt-Jakob disease or severe anoxic brain injury.
Common Diagnostic Applications
The EEG’s ability to record real-time brain function provides information that structural imaging cannot offer. The most common application is in the diagnosis and classification of epilepsy and seizure disorders. The EEG captures interictal (between seizures) spikes and sharp waves that confirm an electrical propensity for seizures. It also captures the specific pattern of an actual seizure, allowing clinicians to classify the seizure type, which is crucial for selecting the correct anti-seizure medication.
In the realm of sleep disorders, the EEG is a component of polysomnography. By analyzing the frequency bands, the EEG accurately identifies the different sleep stages, from light sleep (Theta) to deep sleep (Delta) and REM sleep. This allows specialists to diagnose disorders like narcolepsy or confirm that abnormal movements occurring during sleep are seizures rather than typical sleep phenomena.
The EEG is also used in critical care settings to monitor brain activity in unconscious patients. It helps determine the depth of anesthesia during surgery, where patterns like burst suppression indicate a very deep level of sedation. In patients in a coma, the EEG tracks the severity of brain dysfunction and monitors for non-convulsive seizures. Finally, the EEG is used as an ancillary test to confirm brain death, requiring a recording to demonstrate complete electrocerebral inactivity—no electrical activity greater than two microvolts for a sustained period under strict technical criteria.