An Electroencephalogram (EEG) is a medical test that measures the electrical activity of the brain. Electrodes placed on the scalp detect electrical impulses from brain cells (neurons). These impulses are recorded as wavy lines, visually representing brain activity. EEGs diagnose various brain conditions, such as epilepsy, sleep disorders, and encephalopathy, by showing how different brain areas function.
Understanding Normal Brainwave Activity
Normal brain activity on an EEG shows rhythmic electrical impulses, categorized by frequency. Measured in Hertz (Hz), these include delta, theta, alpha, and beta waves, each associated with different states of consciousness.
Delta waves, the slowest (0.5-4 Hz), are usually seen during deep sleep and in young children. Theta waves (4-8 Hz) are common during light sleep, deep relaxation, or meditative states. Alpha waves (8-12 Hz) are prominent when an individual is awake but relaxed, especially with eyes closed, and tend to disappear with mental effort or eye opening. Beta waves (12-30 Hz) are faster and dominate when a person is awake, alert, actively thinking, or problem-solving. The normal appearance of these brainwaves varies by age, state of consciousness, and location on the scalp.
Key Indicators of Abnormal EEG Patterns
Abnormal EEG patterns deviate from normal brainwave characteristics in frequency, amplitude, shape, or location. One common indicator is asymmetry, where brain activity differs significantly between the two hemispheres or specific regions. For example, a consistent amplitude difference or a slower frequency on one side compared to the other can suggest a localized problem.
Excessive slowing, where brainwaves operate at frequencies lower than expected for the patient’s age and alertness, is another sign of abnormality. This slowing can be diffuse, affecting the entire brain, or focal, concentrated in a specific area. Conversely, unusually fast activity in inappropriate states also indicates abnormality. Sudden, unexpected bursts of activity, known as paroxysmal activity, often suggest a seizure tendency.
Specific Abnormal Waveforms and Their Significance
Specific abnormal EEG waveforms provide clues about neurological conditions. Spikes and sharp waves are brief, high-amplitude discharges associated with epilepsy. Spikes typically last less than 70 milliseconds, while sharp waves last between 70 and 200 milliseconds, both indicating an epileptic focus. These can occur sporadically between seizures, known as interictal epileptiform discharges.
Spike-and-wave complexes consist of a rapid spike followed by a slower wave, forming a distinct pattern. A classic example is the 3-Hz spike-and-wave complex, which is a hallmark of absence epilepsy, a type of generalized seizure.
Generalized slowing indicates widespread brain dysfunction, often seen in conditions like encephalopathy due to metabolic imbalances, infections, or certain medications. This involves brainwaves predominantly in the theta or delta ranges when the patient should be awake.
Focal slowing, characterized by slow waves concentrated in a specific brain region, suggests a localized structural problem. This can be continuous or intermittent and is seen in conditions such as brain tumors, stroke, or localized brain injury.
Periodic discharges are repetitive, often sharp or spike-like, waveforms occurring at regular intervals. These can be generalized or localized and may be associated with severe brain dysfunction or a predisposition to seizures.
Factors Influencing EEG Appearance
Factors beyond brain pathology can influence EEG recordings, requiring careful interpretation to distinguish them from true abnormalities. Artifacts are unwanted signals originating from non-brain sources.
Common physiological artifacts include electrical activity from muscle movements (e.g., eye blinks, chewing, facial grimacing), appearing as high-frequency noise. Eye movements, for instance, create large electrical potentials that can obscure frontal EEG readings. External sources, like electrical interference from nearby equipment (e.g., 50/60 Hz powerline noise) or poor electrode contact, can also generate artifacts. The patient’s state (awake, drowsy, or asleep) significantly alters normal brainwave patterns. Medications, particularly sedatives or anticonvulsants, can also affect the EEG, potentially causing generalized slowing or altering normal rhythms. Interpreters must consider these variables to accurately assess the brain’s electrical activity.