Electroencephalogram (EEG)

The electroencephalogram is a means to assess the degree of damage to the brain in cases of trauma, or to measure the potential for seizure activity. It is used also in sleep studies to determine whether an individual has a sleep disorder and to study brain wave patterns during dreaming or upon sudden awakening.

The EEG is also a useful second-level diagnostic tool to follow-up a computerized tomogram (CT) scan to assist in finding the exact location of a damaged area in the brain. The EEG is one of a battery of brain tests available and is seldom used alone to make a diagnosis. The EEG tracing can detect an abnormality but cannot distinguish between, for example, a tumor and a thrombosis (site of deposit of a blood clot in an artery).

Although persons with frequent seizures are more likely to have an abnormal EEG than are those who have infrequent seizures, EEGs cannot be solely used to diagnose epilepsy. Approximately 10% of epilepsy patients will have a normal EEG. A normal EEG, therefore, does not eliminate brain damage or seizure potential, nor does an abnormal tracing indicate that a person has epilepsy. Something as simple as visual stimulation or rapid breathing (hyperventilation) may initiate abnormal electrical patterns in some patients.

If the EEG is taken at the time the patient has a seizure, the pattern will change. A grand mal seizure will result in sharp spikes of higher voltage and greater frequency (25–30 per second). A petit mal seizure also is accompanied by sharp spikes, but at a rate of only three waves per second.

Also, the EEG is not diagnostic of mental illness. The individual who is diagnosed with schizophrenia or paranoia may have an EEG tracing interpreted as normal. Most mental illness is considered to be a chemical imbalance of some sort, which does not create abnormal electrical activity. However, an EEG may be taken of an individual who exhibits bizarre, abnormal behavior to rule out an organic source such as thrombosis as the cause.

Patients being diagnosed for a brain disorder can be monitored on a 24-hour basis by a portable EEG unit. A special cap with electrodes is fitted onto the head where it will remain during the time the test is being run. The electroencephalograph is worn on the belt. A special attachment on the machine enables the patient to telephone the physician and transmit the data the machine has accumulated.