The EEG records the ever-present, ongoing electrical activity generated by the neurons in the brain, hence it is also referred to as a "brain wave" test. Abnormal EEG signals include little electrical "explosions" such as the spikes, spike and wave, and sharp waves that are common in epilepsy even when children are not in the midst of a clinical seizure. Indeed, the EEG is usually done in the interictal state-the time in between clinical seizures. (The actual seizure is referred to as the ictus). Epileptiform activity refers to the spikes, spike and wave and sharp waves. The finding of epileptiform activity may help determine whether or not a child is epileptic.
During EEGs, hyperventilation (breathing in and out fast) and photic stimulation (flashing strobe lights) may induce epileptiform activity or even seizures. In addition, either sleep, or the transition states between waking and sleep, may also activate the EEG. For children, we prefer to always record both waking and sleep states. For a routine EEG, recording electrodes are placed on the scalp with some form of paste or easily removable glue. Children are studied for approximately one to two hours as outpatients. Occasionally, it may be necessary to sedate a child with a mild medication to obtain sleep. Sometimes activation of EEG abnormalities by other medications is necessary. In certain difficult to diagnose patients or hard to control epileptics, it may be necessary to make use of long term combined EEG and video monitoring on our inpatient LTM (or long-term monitoring) service. These video EEGs help to correlate the clinical, or observable, seizures of the child with the findings on the EEG.
How to prepare for an EEG
Most tests in the Clinical Neurophysiology Laboratory are scheduled for two hours. These include EEG, brain stem auditory response, and visual evoked potentials. A prolonged EEG is scheduled for approximately four hours, as are brain electrical mapping (BEAM) studies. Electromyograms (EMGs) take approximately one hour.
Patient preparation for EEGs is very important. In order for the technician to obtain readings during awake, drowsy, and asleep periods, we ask that the patient be sleep deprived the night prior to the test. Patients older than eight years should receive four hours of sleep only, between midnight and 4:00 a.m. Younger children should sleep one-half their normal sleep hours and be awakened at 4:00 a.m. Infants (under one year of age) do not need to be sleep deprived the night before unless the test is scheduled at 8:00 a.m. In this case, the child should be awakened early so that they will be tired by 9:00 a.m. Infants and children who take naps should be deprived of naps on the day of the test.
The family should plan to arrive at Children's approximately 20-30 minutes prior to test time in order to park and to register at the Fegan Registration area. Please make sure that you have received pre-authorization for the test from your insurance company, if this is necessary.
What to Expect During an EEG?
Small, non-invasive electrodes (usually 16 to 32 in number) are placed upon a patient's scalp, after careful measurement by a trained technologist, with paste or a glue-like substance to hold them in place. Low voltage signals (5-500 microvolts) are amplified by the EEG machine and stored digitally. The resulting polygraphic display, looking much like a multiple channel seismograph, is typically read by unaided visual inspection. The physician interpreting such a tracing is usually a neurologist with special training in EEG. Such an individual is often referred to as a neurophysiologist or electroencephalographer. Psychiatrists, neurosurgeons, and psychologists may also interpret EEGs but to do so, like neurologists, they require special EEG training. Board certification is available in EEG and other aspects of neurophysiology from several organizations. Similarly EEG technologists should have special training in EEG and may become "registered".
Interpreting the EEG Report
Techniques for interpretation of EEG by visual inspection have changed little since the discovery of EEG in the 1920s by Berger and its extension to clinical issues in the 30s and 40s by Gibbs and Lennox. Typically the EEG is screened for features that stand out ( transient responses) like the spike or spike and wave associated with epilepsy. Next, the frequency or spectral content of the remaining EEG background is visually evaluated. There are four broad spectral bands of clinical interest: delta (0-4 Hz), theta (4-8 Hz), alpha (8-12 Hz), and beta (above 12 Hz). Not everyone agrees on the exact boundaries of these rhythms and many subdivide these bands, especially beta. Pathology typically increases slow activity (delta, theta) and diminishes fast activity (alpha, beta). Thus overlying a localized brain tumor one would expect increased slowing and decreased fast activity. Similarly following a global brain insult one might expect globally increased slowing and decreased fast activity. However, there are many exceptions to this oversimplified explanation. EEG interpretation requires considerable skill and often years of clinical experience. The mere determination of whether an EEG spectral band is normal, increased, or decreased may require years of experience. Some have likened EEG reading to the grading of equine or canine conformation by judges who have spent their careers learning what to look for. EEG interpretation is as much an art as a science.