EEG and Epilepsy Diagnosis

Reviewed by: HU Medical Review Board | Last reviewed: November 2021

Epilepsy is a neurological (brain) disease that causes people to have seizures. Epilepsy is a type of seizure disorder in which clusters of nerve cells in the brain (neurons) behave abnormally. Neurons normally create tiny electrical signals in a regular rhythm. These signals tell other parts of the brain and body what to do.1,2

With epilepsy, neurons create too many electrical signals, too quickly. Some doctors describe this as an electrical storm in the brain. This storm causes a wide variety of physical and mental changes, such as:1,2

  • Loss of consciousness
  • Muscle spasms
  • Loss of muscle tone
  • Changes in senses and emotions
  • Sudden stillness
  • Staring
  • Repetitive movements like lip-smacking

An electroencephalogram (EEG) is a test that measures electrical activity in the brain. Your test results will be reviewed by a neurologist, a doctor who specializes in nervous system disorders. Usually, an epileptologist or neurophysiologist will read your EEG. These doctors are neurologists with extra training in reading and interpreting EEGs.3

Not all seizures are caused by epilepsy. Seizures may be caused by a host of other health conditions. An EEG can help confirm epilepsy or rule out other conditions.

Preparing for an EEG

An EEG is a painless test. It can be performed in a hospital, in a doctor’s office, or at home. Sometimes, video is used to record the EEG session. The test may last from 20 minutes to hours in a hospital or office. An EEG can also last 1 to 3 days if the person wears the device home or stays in the hospital for a longer period.4

A technician will stick many electrodes to your head. Electrodes are small, metal, cup-shaped disks connected to a box by wires. The box is connected to an EEG machine. These wires record the electrical activity in your brain.5

For EEGs performed in a hospital or office, the test may include flashing lights or fast breathing in an attempt to provoke abnormal brain activity. Video provides a visual record of a person’s seizures, and this can help diagnose the particular type of seizure.4,6

After the test, you may have some sticky goo in your hair or on your scalp. Hair conditioner will help remove the glue.3

When an EEG may be needed

Your doctor may order an EEG for several reasons:3

  • You have had a first seizure, and they need to confirm the seizure was caused by epilepsy.
  • You have been diagnosed with epilepsy, and they need to know more about it.
  • You need to prepare for epilepsy surgery.
  • You are about to change or have changed anti-seizure drugs.
  • They suspect you are in non-convulsive status epilepticus.

What do the test results mean?

An EEG measures electrical activity at the time of the test. Some people with certain types of epilepsy have unusual patterns in their brainwaves even when they are not having a seizure. This can help your doctor understand more about your epilepsy.3

About half of people have an EEG that shows normal brain activity. That is because a person’s brain waves may return to normal between seizures. An EEG done several hours or days later may not pick up on any changes. So unless someone has a seizure during their EEG, the test will appear normal.6

Many people with true epileptic seizures can have a normal EEG even during a seizure. This is because the EEG does not always capture seizures that are very deep in the brain or in small areas between the scalp electrodes. Even if an EEG is normal, a doctor can still diagnose you with epilepsy based on your health history.6

An EEG does not always tell a doctor if there is damage to the brain or some physical issue with the brain. It may sometimes suggest these things. An MRI will be most helpful to show whether there is structural damage in the brain.3

Other tests for epilepsy

To diagnose epilepsy, your doctor will also order other tests and conduct a physical exam. The other tests may include:1,2

  • Urinalysis
  • Brain imaging tests
  • Blood tests
  • Spinal tap (lumbar puncture)
  • Hospital monitoring
  • Genetic testing

Once you have been diagnosed with epilepsy and are taking anti-seizure drugs, you may need regular blood work.

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