EEG: The Brain's Electrical Map

The EEG is the most commonly used and most asked-about test in diagnosing epilepsy. In this chapter, we explain what the EEG is, how it is done, how to prepare your child for it, and what the results mean.

What Is an EEG?

An EEG (electroencephalography) is a test that records the electrical activity produced by the brain. Small metal electrodes are placed on the scalp; these electrodes pick up the brain's waves and turn them into a wave graph. The EEG is painless; the electrodes do not send any current into the brain — they only listen to the brain's own signals. You can think of it like a very sensitive microphone that “listens to the brain's voice.”

What Is an EEG For?

An EEG can capture abnormal electrical patterns specific to epilepsy (such as spikes, sharp waves, and spike-and-wave complexes). These findings help both to support the diagnosis and to identify the seizure type and syndrome. For example, absence epilepsy, SeLECTS (self-limited epilepsy with centrotemporal spikes), and West syndrome each show distinct, recognizable patterns on the EEG.

Two Very Important Facts

A normal EEG does not prove that there is no epilepsy. The EEG shows only the activity at the moment it is recorded; if seizures are infrequent, everything may look normal at that time. That's why the EEG is sometimes repeated.

An abnormal EEG, on its own, does not establish a diagnosis of epilepsy. Some healthy children can also show minor EEG changes. The EEG is always interpreted together with the history and examination.

Types of EEG

• Standard (routine) EEG: Takes about 20 to 40 minutes. Trigger methods such as deep breathing (hyperventilation) and flashing light (photic stimulation) are often applied.
• Sleep (sleep-deprived) EEG: Some abnormalities appear only during sleep. For this reason, the child may be kept partly sleep-deprived the night before the test so that they can fall asleep during it.
• Long-term / video-EEG monitoring: Done over hours or days, with simultaneous video recording. Used to “catch” seizures and to distinguish true epileptic seizures from non-epileptic events (we'll cover this in Chapter 18).

How to Prepare Your Child for an EEG

Explain the EEG to your child in language that isn't frightening: “They'll put little buttons on your head, and a machine will draw a picture of your thoughts; it won't hurt at all.” Having the hair clean before the test and free of gel, cream, or spray helps the electrodes make good contact. If a sleep EEG is requested, follow the sleep-deprivation instructions your doctor gives. For young children, bringing a favorite toy or blanket makes things easier.

Common Phrases in an EEG Report

When you read an EEG report, you may come across some technical phrases. Interpreting them fully is your doctor's job; but recognizing a few common terms makes the report feel less foreign. “Normal EEG” means no clear abnormality was found during the recording (but this does not rule out epilepsy). Phrases like “epileptiform discharge,” “spike,” “sharp wave,” or “spike-and-wave” refer to patterns that may be associated with epilepsy. “Focal slowing” may point to a change in function in a specific area of the brain. Rather than trying to interpret these terms on your own, ask your doctor what the report means; a good doctor will be glad to translate it into language you can understand.

Families' Common Worries About the EEG

• “Does the EEG send current into the brain?” No. The EEG only listens to the brain's own signals; it does not send any current into the brain and does not cause pain.
• “Is the gel put on the hair harmful?” No. It washes off easily afterward. It's enough for the hair to be clean and product-free before the test.
• “Can the flashing-light test start a seizure?” During photic stimulation, a brief response can rarely be seen in light-sensitive children; this is done in a controlled setting, safely, to help with the diagnosis.
• “Does keeping the child sleep-deprived harm them?” The partial sleep deprivation requested for the test is temporary and harmless; its purpose is to capture findings that appear during sleep.