When using EEG to study a patient that is suspected of having epilepsy, some sort of provocation, such as hyperventilation or flashing lights, is commonly used. Frequently, the provocation methods bring out epileptic abnormalities in the EEG recording. Certain patterns in EEG indicate a tendency toward seizures. It should be noted, however, that there is a sliding scale in what is considered normal or abnormal EEG. Some epilepsy patients show completely normal EEG, whereas some relatives of an epileptic patient show abnormalities in their EEG that are typical for epilepsy even though they have no epilepsy symptoms. Thus, EEG cannot be used to rule out epilepsy.
Nonspecific vs. Specific Patterns in Epilepsy
Abnormal EEG patterns seen in epileptic patients are either nonspecific or specific. Nonspecific patterns can be seen in many conditions besides epilepsy, such as stroke, head trauma and brain tumor. Slowing, in which the rhythm of the brain waves is slower than expected, is an example of a nonspecific pattern commonly seen in epileptic patients. The International Federation of Societies for Electroencephalography and Clinical Neurophysiology says that specific patterns indicating a tendency toward seizures or epilepsy waves include spikes, spike-and-wave discharges and sharp waves.
Spikes
Epileptic seizures are caused by a group of neurons firing simultaneously. These are visible on EEG as sudden burst of electricity or "spikes" that are easy to discriminate from the background activity. The duration of a spike ranges from 20 to less than 70 milliseconds.
Sharp Waves
Just like spikes, sharp waves can easily be distinguished from the background activity. They have a pointed peak and duration of about 70 to 200 milliseconds. Spikes and sharp waves seen in a local brain area is a sign of a partial seizure as compared to generalized seizure.
Spike-and-slow-wave Complex
Spike-and-slow wave pattern consists of a spike that is followed by a slow wave. Typically, the slow wave is of higher amplitude than the spike. Multiple spike-and-slow-wave complexes can also be seen. They are just like spike-and-slow-wave patters except that one or more slow waves are accompanied by at least two spikes. If spike-and-wave patterns can be seen in a widespread area in both cerebral hemispheres, especially if they begin in both hemispheres at the same time, a patient is likely to have generalized epilepsy. A 3 Hz spike-and-wave activity is specific to an epilepsy type called petit mal.
References
- Epilepsy: Methods of Examination
- “Electroencephalography and Clinical Neurophysiology”; International Federation of Societies for Clinical Neurophysiology; November 1974
- "Canadian Journal of Neurology Science"; EEG in Epilepsy: Current Perspectives; M. Sundaram, R.M. Sadler, G.B. Young and N. Pillay; November 1999
