Electroencephalogram (EEG)

EEG (Electroencephalography)

Results

An electroencephalogram (EEG) is a test that measures and records the electrical activity of your brain. Special sensors (electrodes ) are attached to your head and hooked by wires to a computer. EEG test results are ready on the same or the next day.

There are several types of brain waves:

• Alpha waves have a frequency of 8 to 12 cycles per second. Alpha waves are present only in the waking state when your eyes are closed but you are mentally alert. Alpha waves go away when your eyes are open or you are concentrating.
• Beta waves have a frequency of 13 to 30 cycles per second. These waves are normally found when you are alert or have taken high doses of certain medicines, such as benzodiazepines.
• Delta waves have a frequency of less than 3 cycles per second. These waves are normally found only when you are asleep or in young children.
• Theta waves have a frequency of 4 to 7 cycles per second. These waves are normally found only when you are asleep or in young children.

Electroencephalogram (EEG)

Normal:	In adults who are awake, the EEG shows mostly alpha waves and beta waves.
	The two sides of the brain show similar patterns of electrical activity.
	There are no abnormal bursts of electrical activity and no slow brain waves on the EEG tracing.
	If flashing lights (photic stimulation) are used during the test, one area of the brain (the occipital region) may have a brief response after each flash of light, but the brain waves are normal.
Abnormal:	The two sides of the brain show different patterns of electrical activity. This may mean a problem in one area or side of the brain is present.
	The EEG shows sudden bursts of electrical activity (spikes) or sudden slowing of brain waves in the brain. These changes may be caused by a brain tumor, infection, injury, stroke, or epilepsy. When a person has epilepsy, the location and exact pattern of the abnormal brain waves may help show what type of epilepsy or seizures the person has. Keep in mind that in many people with epilepsy, the EEG may appear completely normal between seizures. An EEG by itself may not diagnose or rule out epilepsy or a seizure problem.
	The EEG records changes in the brain waves that may not be in just one area of the brain. A problem affecting the entire brain—such as drug intoxication, infections (encephalitis), or metabolic disorders (such as diabetic ketoacidosis) that change the chemical balance in the body, including the brain—may cause these kinds of changes.
	The EEG shows delta waves or too many theta waves in adults who are awake. These results may mean brain injury or a brain illness is present. Some medicines can also cause this.
	The EEG shows no electrical activity in the brain (a "flat" or "straight-line" EEG). This means that brain function has stopped, which is usually caused by lack of oxygen or blood flow inside the brain. This may happen when a person has been in a coma. In some cases, severe drug-induced sedation can cause a flat EEG.

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Author:	Monica Rhodes	Last Updated: July 29, 2008
Medical Review:	Anne C. Poinier, MD - Internal Medicine Colin Chalk, MD, CM, FRCPC - Neurology
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