I have seizures, but my EEG was normal. Why?
The neurologist has just informed you that your EEG is normal even though you recently experienced your first tonic-clonic seizure. This test result puzzles you. Does a normal EEG mean that you didn’t have a seizure and the headache and muscle aches you felt were related to some other illness?
- A normal EEG does not mean that you did not have a seizure. Approximately one-half of all EEGs done for patients with seizures are interpreted as normal. Even someone who has seizures every week can have a normal EEG test. This is because the EEG only shows brain activity during the time of the test. If you aren't having a seizure at that time, there may not be any unusual brain waves for the test to record.
- During a seizure, the electrical activity is abnormal. Once the seizure is over, the brain rapidly returns to normal in most individuals. When an EEG is done several hours or even days later, it misses the changes in electrical activity that occurred during the actual seizure.
- The likelihood of recording a seizure during a routine EEG is small. The EEG generally records brain waves between seizures, called interictal brain waves. These waves may or may not show evidence of seizure activity. The neurologist looks for spikes or sharp waves ("epilepsy waves") to confirm the diagnosis, but the absence of these abnormal brain waves does not mean you didn't have a seizure in the past.
- Specific techniques, like flashing lights or 2 to 5 minutes of deep breathing (hyperventilation), often are used to provoke abnormal brain waves so they can be recorded. Recording the "epilepsy waves" is helpful because it confirms the diagnosis and may identify the type of seizure disorder, but it is not necessary for diagnosis and treatment.
What other information is used to diagnose seizures?
Making a diagnosis of seizures does not depend only on the results of the EEG. The neurologist also considers several other types of information.
- A description of the event from you and anyone who saw it is probably the most important information. Does it sound like a complex partial seizure or a fainting spell?
- The results of a physical examination and perhaps an MRI scan of your brain will be evaluated for relevant abnormalities.
- Your past medical history to see if it includes injuries or illnesses that would make you more likely to have seizures. For example, if you've had meningitis, your risk of seizures is three times greater.
Based on all of this information, your neurologist may diagnose seizures with confidence even though the result of your EEG was normal. The normal EEG does not mean that the neurologist was wrong in saying that you had a seizure.
Keep taking the seizure medicine that was prescribed. If you have a normal EEG and stay free of seizures for two years or more, the neurologist may suggest that you can stop taking it!