4  Understanding the EEG

4.1 What is an EEG?

An electroencephalogram (EEG) is a test that records the electrical activity of your brain. It’s the single most important test for diagnosing and managing epilepsy.

The good news: an EEG is completely painless and non-invasive. Nothing goes into your body, and the test doesn’t send any electricity into your brain - it only records the brain’s natural electrical signals.

4.2 Why Do We Need an EEG?

Your doctor orders an EEG to:

  • Look for abnormal electrical patterns that suggest epilepsy
  • Identify what type of seizures you’re having
  • Determine which part of the brain seizures are coming from
  • Distinguish epileptic seizures from other conditions
  • Monitor how well treatment is working
  • Help decide when it might be safe to stop medications

The EEG helps answer the question: “Does this look like epilepsy, and if so, what kind?”

4.3 Types of EEG Tests

4.3.1 Routine EEG

This is the most common type and typically lasts 20-40 minutes. You’ll sit in a comfortable chair or lie on a bed while a technician places electrodes on your scalp. The recording captures your brain activity during this brief window.

4.3.2 Sleep-Deprived EEG

Sometimes you’ll be asked to stay up late or wake up very early before your EEG. Why? Because:

  • Sleep deprivation can make epilepsy patterns more visible
  • Recording your brain during sleep is valuable for diagnosis
  • Some types of epilepsy are specifically related to sleep

If your EEG requires sleep deprivation, you’ll receive specific instructions about how many hours of sleep to skip.

4.3.3 Ambulatory EEG

This is a portable EEG that you wear at home for 24-72 hours (or longer). The electrodes stay on your head, and a small recording device hangs on your belt or goes in a pouch. You can go about most of your normal activities while it records continuously.

Ambulatory EEG is useful when:

  • Routine EEG is normal but seizures are still suspected
  • We need to capture your typical events
  • You have frequent seizures that might occur during the recording

4.3.4 Video-EEG Monitoring

This is the most comprehensive test, done in a hospital epilepsy monitoring unit. You stay in the hospital for several days with continuous EEG recording and video cameras. The goal is usually to capture your typical seizures on both video and EEG simultaneously.

Video-EEG monitoring is used when:

  • The diagnosis is unclear
  • We need to identify exactly where seizures start
  • You’re being evaluated for epilepsy surgery
  • We need to distinguish epileptic seizures from other events
  • Your neurologist wants to assess for seizure burden

4.4 What to Expect During a Routine EEG

4.4.1 Before the Test

Prepare your hair:

  • Wash your hair the night before or morning of the test
  • Don’t use conditioner, hair gel, spray, or other products
  • Hair should be completely dry
  • Remove braids, extensions, or weaves if possible

Other preparation:

  • Take your regular medications unless told otherwise
  • Bring a list of all current medications

For children:

  • Explain the test in simple, age-appropriate terms
  • Bring comfort items (favorite toy, blanket)
  • Let them know it won’t hurt
  • Consider showing them pictures or videos of EEG placement beforehand

4.4.2 During the Test

Electrode placement (15-20 minutes):

The technician will:

  1. Measure your head to find the right positions for electrodes
  2. Mark spots on your scalp with a colored pencil or marker
  3. Clean each spot
  4. Apply small metal disc electrodes with paste or gel
  5. Secure everything with gauze wrap

The electrodes are placed in standardized positions that cover different areas of your brain. A routine EEG typically uses 20-25 electrodes.

The paste or gel can feel cold and slightly uncomfortable, but the process usually doesn’t hurt. The prep work often takes longer than the actual recording!

The recording (20-40 minutes):

Once the electrodes are in place, you’ll:

  • Sit in a comfortable chair or lie on a bed
  • Need to stay still and relaxed
  • Keep your eyes closed for parts of the test
  • Be asked to open and close your eyes on cue
  • Try to avoid excessive muscle movement

The technician can see your brain activity on a computer screen in real-time. They’re looking for normal patterns and any unusual activity.

4.4.3 Activation Procedures

During the EEG, the technician will try several “activation procedures” designed to bring out epilepsy patterns:

Hyperventilation (deep breathing):

  • You’ll breathe deeply and rapidly for 3-5 minutes
  • This lowers carbon dioxide in your blood
  • Can activate certain types of epilepsy patterns
  • May make you feel lightheaded or tingly (this is normal and temporary)
  • Not done if you have certain heart or lung conditions

Photic stimulation (flashing lights):

  • A strobe light flashes at different frequencies
  • Tests for photosensitivity (light-triggered seizures)
  • Affects only about 3-5% of people with epilepsy
  • You can close your eyes if uncomfortable

Sleep:

  • If possible, they’ll record while you fall asleep
  • Sleep brings out many epilepsy patterns
  • This is why sleep deprivation before the test helps
What if I don’t have a seizure during EEG?

Many people worry about not having a seizure during the EEG. While it would be diagnostically useful, most EEGs don’t capture an actual seizure, and that’s okay! The test is still valuable because it can show patterns that indicate epilepsy even between seizures.

4.4.4 After the Test

The technician will:

  • Remove the electrodes (takes just a few minutes)
  • Give you a towel to wipe off excess paste
  • You can leave immediately

You can:

  • Resume all normal activities right away
  • Drive home (unless you did a sleep-deprived EEG)
  • Shampoo your hair (it may take 2-3 washes to remove all the paste)

4.5 Understanding Your Results

4.5.1 Normal vs. Abnormal

A normal EEG shows:

  • Organized background rhythms appropriate for age
  • Expected changes with eye opening/closing
  • Normal response to sleep
  • No epileptiform discharges

An abnormal EEG might show:

  • Spikes, sharp waves, or spike-and-wave patterns
  • Slowing of background rhythms
  • Asymmetry between brain sides
  • Patterns specific to certain epilepsy syndromes

4.5.2 Important Points About EEG Results

A normal EEG doesn’t always rule out epilepsy:

  • About 50% of people with epilepsy have a normal first EEG
  • Epilepsy discharges are intermittent - they may not occur during the brief recording
  • Repeat EEG may sometimes be needed
  • Sleep-deprived or prolonged EEGs are more sensitive

An abnormal EEG doesn’t always mean epilepsy:

  • Some abnormal patterns are not epileptic
  • About 1-2% of people without epilepsy have epilepsy-like patterns
  • Results must be interpreted along with clinical history
  • Context matters enormously