Overview of Electroencephalograpy

Electroencephalography (EEG) is the most specific test for diagnosing epilepsy because it records the electrical activity of the brain. It is a safe and painless procedure in which electrodes are applied to the patient’s scalp with a special paste or glue. These electrodes are then connected by wires to an electrical box, which in turn is connected to an EEG machine. Several types of EEG are used to diagnose epilepsy, including Routine EEG, Ambulatory EEG, and Video-EEG Monitoring.

How are EEG Recordings Interpreted?

Normal EEG Traces

EEG Traces of Generalized Epilepsy

The EEG machine records the brain’s electrical activity as a series of waveforms called traces. Each trace corresponds to a different region of the brain.

The EEG shows well-defined patterns of normal or abnormal brain electrical activity. Abnormal patterns may be either nonspecific or specific. "Nonspecific" refers to patterns that may be seen in a number of different conditions. For example, certain waves may be seen after head trauma, stroke, brain tumor, or seizures. "Slowing," in which the rhythm of the brain waves is slower than the rhythm that would be expected for the patient’s age, is a nonspecific pattern. "Specific" refers to patterns that indicate a tendency toward seizures, as "epilepsy waves" (spikes, sharp waves, and spike-and-wave discharges). Spikes and sharp waves occurring in a local area of the brain, such as the left temporal lobe, are markers of partial seizures.

Spike-and-wave discharges occurring in a widespread area over both cerebral hemispheres, and beginning simultaneously over both hemispheres, are markers of primary generalized epilepsy. In some cases, actual seizures may be recorded during the EEG, particularly in children with absence seizures who are asked to breathe deeply (hyperventilate) during the test.

Can an EEG be Normal Even if the Person Has Epilepsy ?

Because an EEG usually records the brain activity occurring between seizures, or interictal activity ("ictal" activity refers to a seizure), a person with epilepsy may have a normal EEG. Just as people with epilepsy behave normally most of the time (when the duration and frequency of seizures is compared to all the time in a given month, for instance), so is the EEG normal much of the time. Also, areas of abnormality may go undetected by the EEG if the abnormality arises deep in the brain (e.g., deep in the temporal lobe), "outside the reach" of the electrodes on the scalp, or if the volume of brain affected is too small to generate abnormal waves of sufficient size to be detected by the electrodes. To increase the chances of finding an abnormality on the EEG, it can be recorded in various circumstances:

  • During both wakefulness and sleep (sometimes a sleeping pill can be used)
  • After sleep deprivation (lack of sleep can cause epilepsy waves on the EEG)
  • With 3 to 5 minutes of deep breathing (hyperventilation)
  • With flashing lights (photic stimulation)
  • With special electrodes
  • For prolonged periods (ambulatory and video EEG)

How Safe is an EEG ?

The EEG itself is an extremely safe test. The wires used to record electrical activity only record: they do not deliver electrical current to the scalp. The technologist will, however, often perform "provocative" methods described below (hyperventilation and photic stimulation) that could provoke seizures in special populations.