EEGs record electrical activity in the brain using electrodes on the scalp. Neurologists analyze wave patterns to identify abnormalities and locate abnormal brain activity. Alpha, beta, delta, mu, and theta waves have different frequencies and can indicate different conditions. Artifact from the environment, muscle activity, and eye movement must be considered during analysis.
An electroencephalogram (EEG) is a process by which researchers and neurologists record the electrical activity of nerve cells in the brain using several electrodes attached to the scalp. After 20 to 40 minutes of data collection, clinicians can evaluate the pattern of oscillations from the synchronized activity of large groups of nerve cells for abnormalities that reflect brain dysfunction or disease. The most commonly observed wave patterns, including alpha, beta, and delta waves, fluctuate in the frequency range of one to 20 megahertz (MHz), with each wave having its own sub-frequency range. Through EEG analysis, neurologists can identify abnormal wave patterns and locate abnormal brain activity.
EEGs show alpha waves, ranging from eight to 13 MHz, originating from the back of the brain, with waves of greater amplitude on the dominant side of the brain. Alpha waves occur when the patient is relaxed with their eyes closed. The waves decrease in height when the eyes are open. Abnormal alpha activity on EEG analysis may indicate a comatose condition. A difference in wave size between the two sides that exceeds 50 percent indicates hindbrain abnormalities.
Delta waves, with slow frequencies of one to four MHz, can be recorded in the frontal areas of most adults. These waves occur during normal sleep, but can be abnormally present in patients with hydrocephalus (too much fluid in the brain), metabolic imbalances, and deep brain tissue lesions. On the other hand, beta waves, typically at frequencies greater than 13 MHz, are also found frontally in alert, active, and busy patients. On EEG analysis, beta waves may be attenuated in patients who have taken sedatives.
Mu waves occur at the same frequency as alpha waves and come from areas of the brain that control senses and movement. Waves occur more prominently during states of relaxation than during periods of alert activity. In cases where excessive mu waves occur, autism may be a possible diagnosis. Theta waves occur with sleepiness or excitement and are measured in areas other than where the mind is actively engaged in a task. Theta waves are noticed in EEG analysis when the patient is trying to suppress a thought or avoid an action.
The spikes and sharp fluctuations of the waves may reflect seizure activity in epilepsy, drug-induced seizures, or trauma-related seizures. During EEG analysis, these rapid and transient wave changes must be interpreted with regard to the potential for artifacts from the environment, muscle activity, eye movement or flutter, and tongue movement. Artifact is a critical consideration when conducting an EEG in a patient with Parkinson’s disease or tremor. Heartbeat artifacts can also produce “noise” on an EEG reading.
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