An electroencephalogram (EEG) is a non-invasive test that records electrical activity in the brain. It works by picking up abnormal brain waves via electrodes that are attached to the scalp. EEGs are usually done to detect seizures and to diagnose epilepsy, but they can be used to evaluate or diagnose other conditions, such as sleep disorders or brain injuries. EEGs are also often used to monitor brain activity in someone who is in an induced coma or undergoing certain types of surgery. An EEG may be ordered by a general practitioner or by a neurologist—a doctor who specializes in disorders that affect the brain, spinal cord, and nerves.
An EEG is a measurement of the continuous electrical activity of the brain. This is detected via small metal discs called electrodes that are positioned in standardized patterns on the scalp. Each electrode has wires that attach to a computer, although according to the Epilepsy Foundation of America(EFA), wireless systems are being used more and more. The electrodes detect electrical activity produced by the brain and transmit this information to a computer, where it is processed and saved electronically or printed out. Brain waves are recorded as squiggly lines called traces, and each trace represents a different area in the brain.
EEGs most often are used to evaluate the presence or risk of seizures—abnormal electrical discharges in the brain that can cause confusion, agitation, uncontrolled movements, hallucinations, and even collapse. If you’re being evaluated for epilepsy, your neurologist will look for patterns on your EEG called epileptiform that can manifest as spikes, sharp waves, or spike-and-wave discharges. If abnormal activity shows up on your EEG, the trace can show where in your brain the seizure originated.
For instance, if you’re having generalized seizures, which means they involve both sides of your brain, there likely will be spike-and-wave discharges spread throughout your brain. If you’re having focal seizures, meaning that they involve just one area of your brain, there will be spikes and or sharp waves can be seen in that specific location.
Although the primary reason an EEG is done is to diagnose epilepsy, the test has many other uses. These include looking for abnormal brain activity that may be caused by:
- A head injury
- A brain tumor
- An infection such as encephalitis (inflammation of the brain that’s usually caused by a virus)
- Sleep disorders caused by seizures. For this purpose, an EEG may be done in conjunction with a standard sleep study called a polysomnogram, which monitors sleep stages and cycles to identify disruptions in sleep patterns, and why they may be occurring. In people with abnormal movements or behaviors during sleep, it can be important to rule out seizures as a potential cause.
An EEG might also be used to determine why someone is in a coma or state of delirium, if a person in a persistent coma is brain-dead, or to evaluate drug intoxication. Someone in a medically induced coma may have continual brain wave monitoring using an EEG to make sure they’re getting the correct level of anesthesia. A patient undergoing brain or vascular surgery may be monitored with an EEG to make sure the surgery is not causing permanent damage.
There are several types of electroencephalograms, as well as various versions of each. Your experience will be based on your specific situation. Very generally speaking, however, there are two basic types of EEG:
- Routine EEG: This basic test is typically done after someone has a seizure for the first time. This is ideally performed within 24 hours, which is why it’s important to call for emergency help or go to a hospital ER right away if you or someone else has a seizure. A routine EEG can be done with or without video monitoring, in which you will be videotaped during the test to see if you have abnormal brain waves during specific movements or activities.
- Ambulatory EEG: This test uses equipment that a person wears so that brain activity can be recorded continuously as they go about their normal activities. It can be done with video as long as there is another person to do the taping.
Risks and Contraindications
For most people, an EEG is perfectly safe and poses no significant risks. Note that the electrodes used for an EEG only pick up electrical charges; they do not emit electricity and are harmless.
In rare instances, an EEG can cause seizures in a person with a seizure disorder, which are brought on by deep breathing or flashing lights or if the person took less or none of their medication for the test. Rest assured that you will be monitored very carefully for this and treated with a fast-acting anti-seizure medication right away if it happens to you. There will also be oxygen and other safety equipment nearby in the event of a prolonged seizure.
If you’re being tested in a hospital for a prolonged period of time and are at risk of having severe seizures, other precautions may be taken during the test. For instance, a belt may be placed around your waist to keep you from falling or you may not be allowed to walk around. People who are truly in danger of getting hurt during a seizure may even be fitted with mitts so they don’t scratch themselves, or a restraint to prevent them from climbing out of bed. The sides of the bed may be padded.