An EEG (electroencephalogram) is a recording of your electrical brain activity.
EEG is an abbreviation for “electroencephalogram” which is a recording of electrical activity that is generated by neurons in the brain. This procedure is very safe and painless and can be done as both an outpatient or as an inpatient status depending on your condition.
There are several reasons and conditions on why your doctor might order an EEG. It’s one of the most useful tools used in diagnosing epilepsy. The EEG will be performed by a trained EEG technologist and reviewed by a neurologist. An outpatient EEG procedure usually takes up to two hours while an inpatient’s test duration could last up to several days.
Learn more about what to expect and how to prepare for your EEG before, during and after the procedure below.
It’s important that your hair be clean and free of sprays, oils, creams and lotions. Shampoo your hair the night before or the morning of your test, and don’t use conditioner or any other hair products. Try not to consume any foods or drinks that contain caffeine (such as coffee, tea, cola and chocolate) during the eight hours before your test.
Before the EEG
Before the day of your EEG, tell your doctor if you’re taking any medication and continue to take your medication unless your doctor tells you differently. If you’re taking sedatives, tranquilizers, muscle relaxers, sleeping aids or anti-seizure medications, it’s likely your doctor will tell you to stop taking these before your test because they affect your brain’s usual electrical activity and cause abnormal test results.
Don’t consume foods or drinks that contain caffeine (such as coffee, tea, cola and chocolate) during the eight hours before your test.
We’ll get the most information from your EEG if we are able to take measurements while you sleep. Adult patients are asked to stay awake for 24 hours prior to their test. Younger patients should be kept up later than normal and woken up a few hours early on the day of their test.
During the EEG, we’ll attach electrodes to your scalp. It’s important that your hair be clean and free of sprays, oils, creams and lotions. Shampoo your hair the night before or the morning of your test and don’t use conditioner or any other hair products
During the EEG
The EEG will be performed by an EEG technologist, and the record will be read by a neurologist. An outpatient EEG usually takes up to two hours.
During the test, you’ll be asked to lie on your back on a bed or table or relax in a chair with your eyes closed. The technologist will attach 16 to 25 flat metal discs (electrodes) to different areas of your head, using a sticky paste to hold them in place. The electrodes are attached by wires to a computer that records the electrical activity inside your brain. The activity shows as a series of wavy lines on a computer screen.
Lie still with your eyes closed during the recording, and don’t talk to the technologist unless you need to. The tech may ask you to do different things during the test to record your brain activity during those times. You may be asked to breathe deeply and rapidly (hyperventilate). You may be asked to look at a bright, flashing light called a strobe; this is called photic or stroboscopic stimulation. You may be asked to go to sleep.
If you’re asked to breathe rapidly, you may feel lightheaded or have some numbness in your fingers. This is normal and will go away a few minutes after you start breathing normally again. If you have a seizure disorder such as epilepsy, a seizure may be triggered by the flashing lights or by hyperventilation. If this occurs, the technologist is trained to take care of you.
There is no pain with an EEG. If paste is used to hold the electrodes, some may stay in your hair after the test and you’ll have to wash your hair to remove it.
An EEG is a very safe test. The electrical activity of your brain is recorded, but at no time is any electrical current put into your body. An EEG should not be confused with electroshock (electroconvulsive) therapy.
The EEG will measure the electrical activity of your brain, otherwise known as your brain waves. There are several types:
Alpha waves have a frequency of 8 to 12 cycles per second. Alpha waves are present only when you are awake and mentally alert – but your eyes are closed.
Beta waves have a frequency of 13 to 30 cycles per second. These waves are normally found when you’re alert or have taken high doses of certain medications, such as benzodiazepines.
Delta waves have a frequency of less than 4 cycles per second. These waves are normally found only when you are asleep, or in young children.
Theta waves have a frequency of 4 to 7 cycles per second. These waves are normally found only when you are asleep or in young children.
After the EEG
Your test will be reviewed by a neurologist, and you’ll receive your results in two to three business days.
In adults who are awake, it’s normal for the EEG to show mostly alpha waves and beta waves, for the two sides of your brain show similar patterns of electrical activity, and for there to be no abnormal bursts of electrical activity and no slow brain waves on the EEG tracing.
If flashing lights (photic stimulation) are used during the test, one area of your brain (the occipital region) may have a brief response after each flash of light, but the brain waves are normal.
If the two sides of your brain show different patterns of electrical activity, this may mean there’s a problem in one area or side of your brain.
If the EEG shows sudden bursts of electrical activity (spikes) or sudden slowing of brain waves in your brain, this may be caused by a brain tumor, infection, injury, stroke, or epilepsy. If you have epilepsy, the location and exact pattern of the abnormal brain waves may indicate what type of epilepsy or seizures you have. Keep in mind that in many people with epilepsy, an EEG may appear completely normal between seizures; an EEG by itself may not diagnose or rule out epilepsy or a seizure problem.
If the doctor thinks you have epilepsy but your EEG is normal, the technologist running the test may have you look at a flashing light (photic stimulation), breathe quickly and deeply (hyperventilation), or sleep during the test. These techniques sometimes show epileptic EEG patterns that didn’t show up at first. Or, your doctor may repeat the EEG more than once.
An EEG done during a seizure will almost always show abnormal electrical patterns. The test can help your doctor diagnose non-epileptic seizures that may be due to stress, emotional trauma or mental illness.
An EEG records changes in your brain waves that may not be in just one area of your brain. A problem affecting your entire brain – such as drug intoxication, infections (encephalitis) or metabolic disorders (such as diabetic ketoacidosis) that change the chemical balance in your body may cause these kinds of changes.
If you’re an adult and your EEG shows delta waves or too many theta waves, this may mean you have a brain injury or illness, although some medications can also cause this.
If the EEG shows no electrical activity in the brain (a "flat" or "straight-line" EEG), this means brain function has stopped. This is usually caused by lack of oxygen or blood flow inside the brain and may happen when someone has been in a coma. In some cases, severe drug-induced sedation can cause a flat EEG.
Video EEG Testing
CoxHealth’s Neurodiagnostic Lab can also perform video EEG testing. Video EEG records your seizures on a computer so your doctor can see what happens just before, during, and right after a seizure.
This test can help your doctor determine the specific area of your brain where your seizures are originating and can help diagnose psychogenic seizures. These may look like seizures, but do not affect the electrical activity in your brain. Short-term, outpatient monitoring may last up to six hours, while long-term monitoring is done in the hospital and typically lasts three to seven days.
During a video EEG, you’ll be given a push button to use when you experience your symptoms/spells. If you’re unaware of your symptoms/spells, it’s very important to bring a friend or family member to stay with you, who can push the button for you and record details on a daily log.
The day of your test, bring comfortable clothing, including a shirt that buttons up the front. Or you can wear a hospital gown.
Brain mapping is similar to an EEG. During this test, electrodes are placed on your scalp to transmit your brain’s electrical activity to a computer that makes a color-coded map of these signals. The “map” includes mathematical measures such as 'coherence', 'co-modulation', 'phase', etc. All the data from your recording can be compared to several databases of normal brainwaves.