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Diagnosing Epilepsy: EEG's Limits - Topic Overview

Electroencephalography (EEG) is thought to be the most useful test in confirming a diagnosis of epilepsy, but it is not foolproof.

  • Some people with abnormal EEG results do not have epilepsy. This is not common.
  • About 50% of people with epilepsy will have normal results on their first EEG.1 If epilepsy is still suspected, a follow-up EEG may be done. This second test may be a sleep-deprived EEG, in which the test is done after you have been forced to stay awake for a longer period of time than usual. A sleep-deprived EEG can sometimes reveal abnormalities that did not show up on the regular EEG.
  • From 10% to 40% of people with epilepsy will have normal EEG results even after having several EEG tests done.1

Video and EEG monitoring records seizures on videotape and computer so that the doctor can see what happens just before, during, and right after a seizure occurs. The video records what you are doing while the EEG records the electrical activity occurring in your brain. This type of monitoring may be used:

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Understanding Epilepsy -- Symptoms

 Seizures are the basic indicator of epilepsy. They vary widely: Staring straight ahead, repetitive swallowing, and lapsing into complete immobility for a few seconds characterize absence (petit mal) seizures, which can recur many times in a day. Tonic/clonic (grand mal) seizures, which usually last several minutes, typically begin with a loss of consciousness and a fall, followed by rigidity, then jerking motions and incontinence of urine. After the seizure ends, there is usually a period...

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  • When your medical history and repeated EEGs are not enough to figure out what kinds of seizures you are having. Simultaneous video and EEG recording can provide important clues about what type of seizure you have had.
  • To evaluate your condition before you have epilepsy surgery.
  • To diagnose seizures that are not from epilepsy, such as psychogenic seizures.

    This information is produced and provided by the National Cancer Institute (NCI). The information in this topic may have changed since it was written. For the most current information, contact the National Cancer Institute via the Internet web site at http:// cancer .gov or call 1-800-4-CANCER.

    WebMD Medical Reference from Healthwise

    Last Updated: March 12, 2014
    This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.
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