Brain electical activity has certain frequencies (the number of waves per second) that are normal for different levels of consciousness. For example, brain waves are faster when you are awake, and slower when you're sleeping. There are also normal patterns to these waves. These frequencies and patterns are what the EEG reader looks for.

Abnormal results on an EEG test may be due to:

• An abnormal structure in the brain (such as a brain tumor)
• Attention problems
• Tissue death due to a blockage in blood flow (cerebral infarction)
• Drug or alcohol abuse
• Head injury
• Inflammation of the brain (encephalitis)
• Hemorrhage (abnormal bleeding caused by a ruptured blood vessel)
• Migraines (in some cases)
• Seizure disorder (such as epilepsy or convulsions)
• Sleep disorder (such as narcolepsy)

Note: A normal EEG does not mean that a seizure did not occur.