Electroencephalograms (EEGs): Catching a Brain Wave

by Gary Cordingley

In this era of rapidly developing technology for medical imaging, one test that might seem downright old-fashioned is an electroencephalogram (EEG) or brain-wave test. But by its very nature, the EEG test will continue to occupy a valuable niche in medical diagnosis that brain-imaging tests—like CT scans and MRI scans—will never fill. Rather than making pictures of the brain's anatomy, EEGs evaluate the brain's physiology. So while imaging tests won't replace electroencephalograms, EEGs won't replace imaging tests, either.

The testing methods look at the brain from completely different points of view, each valuable in its own way. Broken into its parts, the term electroencephalogram means "electrical brain recording."

Hans Berger, a German scientist, got the ball rolling in 1924 with the first recordings of human brain-waves. Since then, the basic principles of recording EEGs have remained the same, but the electronics have gotten better—and smaller. In an EEG recording, tiny voltage-fluctuations from the brain are picked up by a standard array of metal disks attached to the scalp and are then amplified electronically in order to create a permanent recording.

If you are old enough, you might remember the 1960s fascination with alpha waves which people tried to enhance via biofeedback devices. Alpha waves are rhythmic brain-signals oscillating between 8 and 13 times per second that can be measured from the back of the head during quiet wakefulness.

Additional voltage-rhythms encountered during clinical recordings include theta and delta waves that oscillate more slowly than alpha waves, and beta waves that oscillate more rapidly. All four rhythms can be seen in normal states of alertness, drowsiness or sleep, and should be the same on both sides of the brain.

The premier use of EEGs is to evaluate people with known or suspected epilepsy (seizure disorders). Seizures are highly electrical events in the brain that cause temporary alterations in the patient's consciousness, perceptions or behavior.

In healthy circumstances, the brain's approximately 20 billion brain cells perpetually signal each other via electrical impulses. Collectively, these impulses traveling among networks of brain-cells are the means by which the brain performs its functions—like perceiving, pondering, remembering, calculating and deciding. A poetic scientist described the brain and its normal functioning as "an enchanted loom where millions of flashing shuttles weave a dissolving pattern, always a meaningful pattern though never an abiding one; a shifting harmony of subpatterns."

But in epileptic attacks, salvo upon salvo of excessive discharges overwhelm the brain's circuits and disrupt their normal functions. Suddenly, the enchanted loom's patterns are no longer meaningful or harmonious.

A seizure-in-progress is readily detected by an EEG recording. However, most patients under evaluation for seizures don't oblige the doctor by having an attack during a typical 30-90 minute recording session. Fortunately, for purposes of diagnosis this is not usually necessary. Tell-tale changes in brain-waves are often present during the periods between attacks—while the patient feels normal—that can reveal a tendency to epilepsy and even identify specific sub-types.

Two other common uses for EEG testing are for assessment of confusional states and sleeping disorders.

In people with confusion or memory loss, an EEG can show different patterns depending on the cause. When confusion is caused by a depressed mood, the EEG remains normal. When confusion is caused by a degenerative dementia like Alzheimer's disease or by a metabolic condition like a drug-overdose, the brain's rhythms become slower.

For evaluation of sleeping problems, an EEG is just one component of a battery of recording systems. Through concurrent monitoring of other biological processes—like breathing, blood-oxygenation, heartbeat, muscle activity and eye movements—conditions such as narcolepsy and obstructive sleep apnea can be diagnosed.

So what can you expect if your doctor refers you for an EEG study? In many cases, you won't need to make any special preparations for the test, but in cases where epilepsy is under consideration, you might be asked to stay awake for much of the preceding night. Sleep-deprivation increases the likelihood of recording a seizure-related abnormality in patients who have them. You are usually allowed to eat before the test.

The testing-session lasts about two hours, though can be longer, especially if a sleeping problem is being evaluated. The EEG technologist uses much of the testing-session to attach about two dozen metal-disk electrodes to the scalp in standardized locations, and then to test the electrical characteristics of each electrode to ensure that good connections have been made.

Once the electrodes are in place the recording session can begin. For most of the recording you lie quietly with your eyes closed. The technologist measures the brain-waves during quiet wakefulness, and then if you fall asleep, that is recorded, too. In additional parts of the test, you might be asked to breathe rapidly and deeply for about three minutes (useful in detecting "absence" or "petit mal" epilepsy) or to watch flashing lights (useful in detecting certain other kinds of epilepsy). Then the electrodes are disconnected and you go home.

A physician subsequently reads the recording and makes a report. The report includes a description of the observed rhythms of brain-waves, details of any detected abnormalities and comments about their possible significance. Your own doctor uses this report along with what else is known about your condition to make a diagnosis.

If you need an EEG, the good news is that the test is not painful. No needles are involved. There is no need to shave or otherwise remove hair. The bad news is that the adhesive used to attach the electrodes to your scalp can take a day or two to scrub out, and might require mineral oil to remove. Beauty-shop treatments should be postponed until after the EEG appointment; the hair-dresser's efforts will be ruined if the EEG comes second.

About the Author

(C) 2005 by Gary Cordingley

Gary Cordingley, MD, PhD, is a clinical neurologist, teacher and researcher who works in Athens, Ohio. For more health-related articles see his website at: http://www.cordingleyneurology.com