Electroencephalography (EEG) records brain waves, but it doesn't read minds or measure IQs. Instead, it's used to detect the level of electrical activity in the brain. Your brain cells communicate by electrical impulses, and an EEG measures and records these electrical impulses to detect anything abnormal.
An EEG measures primarily grey matter or higher brain function. The largest part of the brain is comprised of the cerebrum, which is split into right and left hemispheres. The cerebrum controls voluntary actions, thought, speech, and memory. In humans, the cerebrum comprises most of the brain, while in other mammals it's relatively small. This allows us to perform much more complicated actions than other species can.
The outer layer of the cerebrum, called the cerebral cortex, is responsible for most higher brain functions such as thought, reasoning, memory, and voluntary muscle movement. The cerebral cortex is mostly made up of neurons, which are nerve cells that carry messages throughout the body. In turn, the activity of the cerebral cortex is regulated by two structures that are deeper in the brain: the thalamus, which is located in the center of the brain and carries signals from the sensory organs to the brain, and the reticular activating system, which sends signals to tell us to go to sleep and to wake us up.
The electrical activity of all these structures is the primary focus of the EEG.
How is an EEG Done?
The procedure is not painful in any way. A technician arranges the electrodes, usually a dozen, at specific sites on the child's head, fixing the electrodes in place with sticky paste. The patient must remain still and lie down while the EEG is taken. Motion can interfere with the EEG - sometimes this might be difficult for an awake child, but it's necessary to record an accurate EEG. However, the patient can't be sedated to achieve stillness because the sedative can cause an inaccurate reading. Natural sleep EEGs, however, can be very helpful.
In the past, the electrodes transmitted the electrical signal by wires to a receiver and amplifier, and the results were recorded on a continuous roll of paper. These days, it's possible to send these signals wirelessly to a computer, where they can be stored in the computer's memory or on a disk. Technological advances have allowed for more expanded EEG testing and the combination of EEG testing with video monitoring of seizure activity.
Through a surgical procedure, wireless transmission also allows the placing of electrodes within the skull, or even within the brain itself, to precisely identify the location of any areas of unstable electrical impulses. These areas are known as seizure foci.
Why are EEGs done?
EEGs help in the diagnosis and management of:
• Seizure disorders
The EEG is the key test in diagnosing, pinpointing, and managing certain areas of unstable electrical impulses in the brain that cause epilepsy (in other words, the EEG tries to find the seizure foci). EEGs can identify other very rare seizure syndromes, and are also used to judge the effectiveness of anti-convulsion therapy, which is used in the treatment of seizure disorders.
An encephalopathy is any disease that alters brain function, such as Parkinson's disease or cerebral palsy. Encephalopathies can be caused by kidney, liver, or respiratory failure, or chemical imbalance in the blood (called metabolic encephalopathy), low blood sugar (hypoglycemia), or drug overdoses. Others may be due to infection and other causes of brain inflammation (encephalitis), or due to the effects of injured neurons (this case is very rare).
• Stupor and Coma
Stupor is defined as reduced or slowed responsiveness, and coma is defined as unresponsiveness (not waking up, or if appearing awake, then not reacting to stimuli). Although EEGs may record the lack of electrical activity in some comatose patients, which can be used as evidence of brain death, in many other instances EEGs turn up treatable conditions like otherwise undetected seizures disorders or chemical imbalances. EEGs may also uncover signs of an otherwise unexpected good prognosis, like otherwise undetectable responses to stimuli, or evidence of normal sleep patterns.
In the past, EEGs played a bigger roll than they do now in the diagnosis of brain injury due to strokes and in the diagnosis and identification of brain tumors. Computer-assisted techniques such as CT-scans (an X-ray image of the body, also called a CAT scan); MRIs (magnetic resonance imaging, which uses radio waves and magnetic fields to produce and image); and PET scans (positron emission tomography, which uses a radioactive tracer that is injected into the body to help form a picture), have taken over most of the tasks of diagnosing these conditions, since they tend to be more sensitive and specific in the diagnosis of strokes and tumors and are simpler to do and easier to interpret than EEGs.
How Long Will It Take To Get Results?
EEGs are read (analyzed and interpreted) by specially trained electroencephalographers, almost always neurologists. How long the analysis and interpretation takes depends on how long the EEG record itself is, the frequency and complexity of the potential abnormality being sought, and the availability of a competent interpreter. Since records can be transmitted now by telephone or Internet, even complex EEGs are now usually read and interpreted within a day or two, sometimes at sites far distant from where they're performed. Emergency EEGs done on coma patients are often simple to read, and may be available within an hour of the record's completion.
Updated and reviewed by: Frederick Meier, MD
Date reviewed: July 2004
Originally reviewed by: Neil Izenberg, MD, and Frederick Meier, MD