Electroencephalograms (EEGs):
Catching a Brain Wave


Gary E Cordingley, MD, PhD
Would you believe that a brain-test invented in 1924 can detect abnormalities
invisible to the latest-generation MRI scanner? The test in question is the
electroencephalogram or brain-wave test, and for certain medical conditions it is
the key to accurate diagnosis.
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.


(C) 2005 by Gary Cordingley