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An electroencephalogram (EEG) is the most important diagnostic tool in the evaluation of patients with seizures. An EEG measures the electrical activity of the brain and is shown as brainwaves (squiggly lines) that can be printed on paper or stored in a computer for later review by a physician.
A normal electroencephalogram (EEG) is shown to the right. EEG recording is a safe and pain free testing process. EEG equipment has come a long way over the past 20 years from large cumbersome polygraph equipment to smaller more advanced digital technology. The biggest impact in the field of EEG technology over the past 30 year was the application of the computer to the process. The digital EEG device is a computer connected to an amplifier that amplifies the brain signal so that it can be easily seen. The EEG signal is small, in the microvolt range; it has to undergo drastic amplification to be seen. The heart by contrast puts out millivolts (milli is a thousandth of a volt), while the brain puts out microvolts (micro is a millionth of a volt). The amplifier is connected to the electrodes (small metal discs) that are attached to a patient's head with a gel or cream that makes the electrical connection. The electrodes connect from the head to the recording system, which are attached to the scalp according to a standardized system of electrode placement called the Ten Twenty placement.
The ten-twenty placement system (right) is a standardized method for measuring the head for electrode placement. A technologist measures a patient's head according to this standardized system and attaches small discs to the head. The electrodes may also be attached to the scalp by means of collodion (the base for fingernail polish). Collodion is often used for electrode placement for long-term epilepsy monitoring. It adheres well to the scalp and electrodes do not come off easily with movement or sweat.
There are a few disadvantages of collodion, such as the smell of ether or fingernail polish, as well as the time it takes to place the electrodes (45 minutes to an hour). Patients have to remain very still during electrode placement.
A routine screening test of about 30 to 45 minutes is performed when seizures are suspected. It is important for the patient to fall into a light sleep during the procedure once an awake portion has been recorded. Sleep often brings out abnormalities that may not be seen in the awake state. Only 50 percent of abnormalities may be seen in the awake state and may increase to 85 percent in the asleep state, particularly for patients with epilepsy or seizures. Patients are frequently asked to open and close their eyes. A flashing light is used to see whether the patient is photosensitive, that is, if he or she has an unusual sensitivity to light. Some patients suffer from photosensitive epilepsy and may experience small seizures as a response to the flashing light. This procedure is called photic stimulation and is considered to be an activation procedure meant to bring out abnormalities seen with seizure activity.
Sleep is also considered an important activating procedure that may show seizure acitivity. Sleep deprivation is often used to ensure that patients fall asleep during the procedure. Sleep deprivation may be defined as a few hours less than the usual amount of sleep a patient usually obtains each night. Sleep deprivation for this purpose is also often used to activate seizures or abnormal activity otherwise not seen.
Another routine activating procedure is called hyperventilation, where the patient is asked to breathe in and out rapidly for three to five minutes. This often brings out seizures known as staring spells or absence. Staring spells are a common type of seizure that have a typical brainwave pattern called the "3 per second spike and wave discharge" (Right). This pattern is usually seen in patients with absence or what used to be referred as petit-mal seizures. These seizures may be seen as a result of hyperventilation. This is most often seen in young children under the age of ten years.
If routine recordings do not produce evidence of seizures, 24-hour continuous long-term video and EEG monitoring may be recommended. This type of monitoring may be done in the hospital, or as an outpatient. Monitoring that is performed as an outpatient is called ambulatory monitoring. During both procedures, inpatient or outpatient, combined or synchronized digital video recording is obtained. These studies help the epileptologist diagnose the epilepsy and identify where the seizures are coming from as well as to classify the epilepsy type. Identifying the seizure type or classifying the epilepsy is important for proper treatment and management. There are many anti-epileptic drugs (AEDs) that are given for specific epilepsy types. This also helps to distinguish between true epileptic seizures caused by the brain and non-epileptic seizures caused by other physiologic or psychological factors.
In certain situations people who have never experienced seizures may show epileptic abnormalities on their EEGs. This means that a genetically they may have a propensity for a certain type of epilepsy. Generally speaking this type of epilepsy is easy to treat with anti-epileptic drugs (AEDs) and in many cases patients outgrow this type of seizure disorder.
Physicians who interpret the EEG results rely on the EEG as the most reliable indicator of a seizure disorder along with other complimentary information such as the medical history and other tests.
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