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,
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 brain's anatomy, EEGs evaluate brain's physiology. So while imaging tests won't replace electroencephalograms, EEGs won't replace imaging tests, either. The testing methods look at brain from completely different points of view, each valuable in its own way.
Broken into its parts, term electroencephalogram means "electrical brain recording."
Hans Berger, a German scientist, got ball rolling in 1924 with first recordings of human brain-waves. Since then, basic principles of recording EEGs have remained same, but electronics have gotten better—and smaller. In an EEG recording, tiny voltage-fluctuations from brain are picked up by a standard array of metal disks attached to scalp and are then amplified electronically in order to create a permanent recording.
If you are old enough, you might remember 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 back of 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 same on both sides of brain.
The premier use of EEGs is to evaluate people with known or suspected epilepsy (seizure disorders). Seizures are highly electrical events in brain that cause temporary alterations in patient's consciousness, perceptions or behavior.
In healthy circumstances, brain's approximately 20 billion brain cells perpetually signal each other via electrical impulses. Collectively, these impulses traveling among networks of brain-cells are means by which brain performs its functions—like perceiving, pondering, remembering, calculating and deciding. A poetic scientist described 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 brain's circuits and disrupt their normal functions. Suddenly, 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 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 periods between attacks—while patient feels normal—that can reveal a tendency to epilepsy and even identify specific sub-types.
