An electroencephalogram (EEG) is a test in which electrodes are placed on a patient’s head to record the brain's electrical activity as a series of brain waves. The information is used to diagnose or monitor a number of conditions including epilepsy, infections, metabolic disorders, head injuries, structural problems in the brain and sleep disorders.

Patients should follow any preparatory recommendations suggested by their physician prior to the test. These may include changes to diet or medication regimen. Patients are usually instructed to wash their hair the night before the test, but to avoid using conditioners or any products that leave residue (e.g., gels, oils).

An EEG typically takes place at a health care provider’s office or a hospital. On the day of the test, up to 25 flat metal discs called electrodes are placed on the patient’s head. As electrical activity occurs in the brain, the electrodes measure what is taking place and send the information to the EEG machine.

Patients may be asked to perform several actions during the test such as breathing deeply or watching a flashing light. These are designed to stimulate brain activity that can be viewed on the EEG.

Following the test, the electrodes are removed, and the patient is allowed to return home. No recovery time is required. A specially trained neurologist known as an electroencephalographer analyzes the results and reports a diagnosis to the patient’s physician. Appropriate treatment follows based on this diagnosis.

A standard EEG is a very safe procedure that poses few risks to patients.

An electroencephalogram is a test that measures the brain's electrical activity. Also known as an EEG, it may be used to diagnose epilepsy, infections, metabolic disorders, head injuries, structural problems in the brain, sleep disorders and other problems. An EEG also is sometimes performed to confirm brain death in a comatose patient.

The brain is split into two major hemispheres that together make up the cerebrum. The outer later of the cerebrum (cerebral cortex) is responsible for most higher brain functions. These include planning, organization skills, visual spatial orientation, judgment, thought, reasoning, memory and voluntary muscle movement. Neurons make up most of the cerebral cortex. These neurons are nerve cells that carry messages throughout the body. The cerebral cortex in turn is regulated by two deeper brain structures: the thalamus (carries signals from sensory organs to the brain) and the reticular activating system (sends signals that tell the body when to sleep and wake up).

An EEG records the electrical activity in these parts of the brain and displays it as a series of wave patterns. This is either drawn onto a moving piece of graph paper or is recorded into a computer. Computer recording has become the method of choice in recent years. Normal electrical activity is represented on the recording as a series of lines that make recognizable patterns. People with unusual brain activity may display abnormal patterns such as sharp spikes or waves.  

In addition to the standard EEG technique, there are several modifications that build upon this approach. They include:

• Ambulatory EEG. Electrodes are attached to special headgear and transmitted to a portable recorder, which takes down information about the electrical activity in a patient’s brain over a 24-hour period. This type of EEG may be used to observe unpredictable brain activity in patients with certain conditions (e.g., epilepsy).
  
  
• Sleep EEG (polysomnograms). Measures brain activity while the patient is sleeping. This type of EEG may be used to diagnose a variety of sleep disorders. It may also be used to observe seizure activity in people whose seizures occur mainly while they are sleeping. Polysomnograms may last from a few hours to an entire night.
  

  

• Other types of EEG. In some cases, information about activity from deeper inside the brain may be required for a diagnosis. For this type of information, a physician may recommend a surgical procedure whereby electrodes are placed within the skull – or even on the brain – to help gather these readings. Depending on where the electrodes are inserted, the patient may experience minor discomfort during this procedure.

A standard EEG is generally a very safe procedure. They have been used commonly in the United States since the 1950s. In the past, physicians used EEGs to diagnose damage to the brain after strokes and to find brain tumors. However, imaging tests such as computed axial tomography (CAT) scans and magnetic resonance imaging (MRI) have taken over this role because they are easier to use and more sensitive for diagnosing these conditions than EEGs. Currently, EEG are most commonly used to diagnose and monitor epilepsy.

EEGs may help diagnose the source of symptoms that might indicate a brain disorder. These symptoms include confusion, coma, and problems related to thought and memory. For example, EEGs may be used to help diagnose any disease that alters brain function (encephalopathies), such as:

• Infections and other sources of brain inflammation (encephalitis)
  
  
• Illnesses such as Parkinson’s disease or cerebral palsy
  
  
• Kidney, liver or respiratory failure
  
  
• Chemical imbalance in the blood (metabolic encephalopathy)
  
  
• Low blood sugar (hypoglycemia)
  
  
• Drug overdoses
  
  
• Injured neurons

EEGs are often used to diagnose or monitor seizure disorders such as epilepsy, a brain disorder in which the patient suffers recurrent seizures. Patients who are admitted to a hospital following a seizure will probably have a continuous EEG with video monitoring. Physicians can use the EEG to identify a seizure as it occurs and try to look for body movements that might indicate which part of the brain is responsible for the seizure.

EEGs can also help physicians identify the source of other rare seizure syndromes. In some cases, EEGs are used during treatment known as electroconvulsive therapy (ECT). This treatment is sometimes used to treat severe cases of depression that have not responded to medication. A sedated patient is given electrical shocks to induce a small seizure. The EEG is used to verify that the seizure has occurred, because it is usually not visible in a sedated patient.

Other conditions that can be diagnosed or monitored using and EEG include:

• Cognitive disorders. EEG can help reveal electrical disturbances in the brain associated with conditions such as delirium or dementia.
  
  
• Creutzfeldt-Jakob disease. Commonly known as mad cow disease, a form of brain damage that causes a rapid decrease of mental function and movement. An EEG reveals characteristic symptoms of the disease as long as it has been present for at least three months.
  
  
• Narcolepsy. A central nervous system (CNS) disorder in which a patient repeatedly falls asleep, often at unexpected and inopportune moments. An EEG is often used to measure the electrical activity of the brain when a patient is undergoing polysomnography, which involves overnight observation in a sleep center.
  

  

• Sleep apnea. Temporary cessation of breathing during sleep. EEGs taken during polysomnography can reveal brain wave patterns that indicate patterns of repeated awakenings that are associated with this condition.
  
  
• Stupor (reduced or slowed responsiveness) or coma (lack of responsiveness). EEGs may be used to detect seizure disorders or chemical imbalances that cause these conditions. In other cases, EEGs are used to detect a lack of electrical activity in comatose patients that may indicate brain death.

Patients are urged to follow any preparatory recommendations suggested by their physician. This usually includes washing their hair prior to the procedure and not using any hair spray, gel, conditioner or other substances until the procedure is finished.

Certain medications can interfere with test results, so patients may be asked to temporarily alter their medication regimen. Patients should not stop taking any medications without first consulting a physician. Patients also may be asked to avoid all caffeine and food intake for about eight hours prior to the test. 

A physician may recommend that the patient avoids sleeping for a period of time prior to the procedure. For people who have a seizure disorder, this may be advised because sleep deprivation is known to increase the brain’s electrical activity. It may also be useful for patients who will be required to sleep during the procedure (an EEG known as a polysomnography) as reducing sleep prior to the test can make it more likely that the patient will be able to fall asleep when required.

Patients may be advised to have someone drive them to and from the site of the EEG test, especially if they have altered important medication regimens or are sleep deprived.

Electroencephalograms (EEG) are administered by an expert known as an EEG technician. They take place in a specially designed room in either a hospital or a health provider’s office.

Patients lie on a table or in a reclining chair. Small, round, metal adhesive electrodes are placed at various points on the patient’s scalp and held in place with temporary glue. Typically, between 16 and 25 of these flat sensors are used. In some cases, these electrodes are connected to wires that run to an amplifier and a recording machine. In other cases, the electrodes are connected wirelessly to a computer. As the electrodes sense small changes in voltage, the information is collected and recorded. This record is known as a tracing.

Patients are required to close their eyes and to remain still during the procedure. After a baseline reading has been established, patients may be asked to perform actions such as breathing deeply and rapidly (hyperventilate). They also may be exposed to a flashing light or may be asked to stare at a patterned board. Such actions and activities are known to trigger electrical activity in the brain. The EEG monitors and records the electrical responses to these stimuli.

Testing may be stopped at times to allow patients to rest or reposition themselves. Fatigue can alter brain wave patterns, so keeping the patients well-rested is an important part of an effective EEG.

The length of an EEG procedure varies. The procedure usually takes about an hour. However, some forms of EEG can take up to 24 hours or longer. This is especially the case for people with seizure disorders who may require a longer period of time to record patterns of abnormal activity in the brain. Additionally, polysomnogram EEGs (used to diagnose some sleep disorders) are usually part of an overnight sleep study that takes up to 12 hours.

There are no significant risks associated with electroencephalogram, and most patients do not experience any significant discomfort. In some cases, needle electrodes are used, in which case patients may feel pricking sensations when they are inserted.

Following the test, the electrodes are removed, and the patient is allowed to return home. No recovery time is required. Patients will likely be instructed to return to any previous medication regimen. Patients who have temporarily stopped taking seizure medications should have someone else drive them home.

The information gathered during an electroencephalogram (EEG) is sent to a specially trained neurologist known as an electroencephalographer. This physician analyzes the reading and results are typically ready within a day or two. This may vary depending upon the complexity of the information and the diagnosis. In some cases, it can take a couple of weeks before patients learn their test results.

A negative test occurs when brain waves are found to have a normal frequency and amplitude. It is important to note that a negative test does not rule out disorders.  For example, patients with seizures may not experience them during the EEG. Some abnormalities may also occur deep in the brain and may not be detected by an EEG.

Abnormal brain waves represent a positive response and may indicate many conditions, including:

• Brain structure defect such as a tumor or abscess
  
• Cerebral infarct (tissue death caused by blockage of the blood supply)
  
• Encephalitis (inflammation of the brain)
  
• Head injury
  
• Hemorrhage (bleeding resulting from a broken blood vessel)
  
• Seizure disorders
  
• Sleep disorder

Preparing questions in advance can help patients to have more meaningful discussions with healthcare professionals regarding their conditions. Patients may wish to ask their doctor the following questions related to electroencephalograms:

1. Why do you recommend that I have an electroencephalogram?
   
   
2. What type of electroencephalogram will I undergo?
   
   
3. Can you explain in detail how the procedure will unfold?
   
   
4. What information will my electroencephalogram provide?
   
   
5. How should I prepare for my electroencephalogram?
   
   
6. Are there any medications I should stop taking before the electroencephalogram?
   
   
7. What will happen if I have a seizure during the test?
   
   
8. Should I plan to have someone drive me home after the test?
   
   
9. When will I know my results?
   
   
10. What happens if I fall asleep during the test?