An Electroencephalogram (EEG) is a medical test that measures the electrical activity produced by the brain. It is a non-invasive procedure where sensors are placed on the scalp to detect the tiny electrical signals generated by brain cells. This article offers a foundational understanding of EEG interpretation, demystifying the patterns seen on a recording and revealing what they can indicate about brain function.
What is an EEG and How is it Recorded?
An EEG detects the electrical impulses that brain cells use to communicate. Small, flat metal discs called electrodes are placed on specific points across the scalp to pick up these signals. The electrodes connect to an EEG machine, which amplifies these minute electrical charges. Amplification is necessary because the brain’s electrical signals measured from the scalp are very small, typically ranging from 10 to 100 microvolts.
Once amplified, these electrical signals are recorded and displayed as wavy lines on a computer screen, known as brainwaves. The characteristics of these waves, such as their frequency and amplitude, provide insight into brain activity. Frequency refers to the number of wave cycles per second, measured in Hertz (Hz), while amplitude represents the strength or height of the electrical signal. Different brain states, like being awake or asleep, produce distinct patterns of these waves.
Understanding Normal Brainwave Patterns
The brain produces continuous electrical signals, categorized into four main types based on their frequency. Each type associates with different states of alertness and mental activity. These patterns are considered normal when they occur in their expected contexts.
Beta waves are the fastest, ranging from 13 to 30 Hz. They are prominent when a person is actively thinking, concentrating, or engaged in problem-solving, reflecting alert wakefulness. Alpha waves, ranging from 8 to 12 Hz, are associated with a relaxed, wakeful state, often observed with eyes closed or during meditation, signifying quiet readiness.
Theta waves, ranging from 4 to 7 Hz, appear during drowsiness, light sleep, or deep meditation, indicating a transition to a less conscious state. Delta waves are the slowest, ranging from 0.5 to 4 Hz, and are characteristic of deep, dreamless sleep, reflecting profound rest and regeneration.
Recognizing Common Abnormalities
While normal brainwave patterns are well-defined, an EEG can reveal various abnormal findings, suggesting underlying neurological issues.
One abnormality category includes epileptiform discharges, which are sudden, brief electrical changes. These appear as spikes, sharp waves, or spike-and-wave complexes. Spikes are very brief, while sharp waves are slightly longer. These patterns often associate with seizure activity and indicate increased cortical excitability, though their presence alone does not always mean a seizure is occurring.
Generalized slowing is another common abnormality, characterized by abnormally slow waves (theta or delta frequencies) across wide brain areas during an awake state. This pattern suggests diffuse brain dysfunction. Focal slowing, conversely, involves slow waves confined to a specific region, indicating a structural problem. Asymmetry, significant differences in brainwave activity between hemispheres, can also be abnormal, suggesting a problem affecting one side more than the other.
Factors Influencing EEG Readings
EEG readings are not solely determined by underlying brain pathology; many non-pathological factors also influence recorded brainwave patterns. These influences require consideration for accurate interpretation.
Medications can alter brainwave frequencies and amplitudes. A person’s sleep state also impacts EEG patterns; a sleep EEG will look different from an awake recording, with slower waves predominating. Brainwave patterns also change with age, meaning what is normal at one age may be abnormal at another.
External influences and physiological processes can also create artifacts in the recording. Muscle movements, eye blinks, or electrical interference can mimic brain activity. These artifacts must be identified and filtered out during interpretation to avoid misdiagnosis. Technical issues, such as poor electrode contact, can also compromise recording quality, leading to unreliable results.
The Role of Medical Professionals in Interpretation
Accurate EEG interpretation requires specialized knowledge and extensive training. Neurologists and neurophysiologists are qualified professionals who interpret EEG results. They distinguish between normal variants, artifacts, and true abnormalities.
These professionals integrate EEG findings with a patient’s medical history, clinical symptoms, and other diagnostic tests. This holistic approach ensures a comprehensive understanding. An abnormal EEG pattern, for example, might be expected in a patient with a known seizure disorder, but its significance varies based on clinical context. This article provides a basic understanding of EEG principles, not a replacement for professional medical advice.