EEG
Meaning of EEG: One of the few tests that watches the brain working moment by moment instead of photographing its structure.
Definition of EEG
EEG stands for electroencephalogram, a recording of the electrical activity of the brain made through small electrodes attached to the scalp. What it captures is the summed activity of large populations of nerve cells, displayed as continuous waves whose frequency and pattern shift with alertness, sleep stage and any abnormal electrical discharge.
What do the wave patterns mean?
| Rhythm | When it appears |
|---|---|
| Beta | Fast and small; alert, thinking, concentrating |
| Alpha | Relaxed with eyes closed, and it disappears the moment the eyes open |
| Theta | Drowsiness and light sleep, and normal in young children while awake |
| Delta | Slow and large; deep sleep, and abnormal in a waking adult |
| Spikes and sharp waves | Brief tall discharges associated with a tendency toward seizures |
A recording therefore describes a state as much as a structure, which is why the same person can produce quite different tracings an hour apart, and why sleep stage is scored from these patterns during a sleep study, including the distinctive pattern of REM sleep.
What is it used for?
Its main use is around seizure activity: confirming it, classifying the type, distinguishing an epileptic event from something that resembles one, and locating where seizures begin when surgery is being considered. Beyond that it helps assess unexplained altered consciousness, where a diffusely slowed background points toward a metabolic or toxic disturbance of brain function; it forms part of sleep studies; it is used to gauge depth of anesthesia; and it contributes to assessment of brain function after cardiac arrest. Continuous monitoring in intensive care detects seizures that produce no visible movement at all.
What are its limits?
Skull and scalp blur and weaken the signal, so the recording mostly reflects the outer surface of the brain and activity arising deep inside may never appear. A tracing between seizures is frequently normal, so a normal result does not rule out epilepsy, while minor irregularities turn up in people who never have a seizure. Muscle tension, eye movements, sweating and nearby electrical equipment all generate artifacts that can imitate real findings. Recording during sleep, after a night of sleep deprivation, or over hours and days with video alongside raises the chance of capturing what is being looked for.
Also known as
Used in a sentence
An EEG was ordered to evaluate the patient's recent episodes of altered awareness.
Good to know
An EEG recorded between seizures can appear normal even in someone with a seizure disorder, since abnormal electrical activity is not continuously present, so a normal result does not exclude the condition.
Word origin
Where “EEG” comes from
Formed from electro-, referring to electrical activity, encephalo-, from Greek enkephalos, meaning brain, and -gram, from Greek gramma, meaning something written.