Abnormal results on an electroencephalogram or EEG may show brain waves that are less active than normal for the person's age and level of alertness, called slow waves, or waves that resemble spikes or sharp waves and indicate epilepsy, states the Epilepsy Foundation. Generalized epilepsy is characterized by spike-and-wave discharges. EEGs also show where epilepsy seizures originate, such as in the temporal lobe although brain activity is normal between seizures.
In an EEG, electrodes are placed in specific locations on the skull, according to the Epilepsy Foundation. The EEG amplitude machine records signals from the electrodes in different combinations, called montages.
Electrical patterns that are not the same on both sides of the brain are abnormal. Bursts of activity, which may be caused by a tumor, stroke, epilepsy, infection or injury, are also abnormal, explains WebMD. Abnormal patterns that affect the entire brain, such as slow waves, may be caused by a generalized condition, such as intoxication, encephalitis or a metabolic disorder that affects the chemical balance of the body. An EEG showing a high amount of delta or theta wave activity, which is associated with sleep, may indicate brain injury or illness. If the EEG shows no electrical activity, it is said to be flat-lining. This is a sign of coma or brain death caused by a lack of oxygen.