electroencephalogram

electroencephalogram (EEG) Recording of electrical activity from the brains of animals was first reported by the British physiologist Caton in 1875. Berger, a German psychiatrist, described the human EEG in 1929, but it was only after a further description of ‘the Berger rhythm’, by Adrian and Matthews in Cambridge five years later, that it began to be used in research and diagnosis.

Electroencephalography records, from electrodes placed on the scalp, the weak electrical activity generated by the brain, its voltage ten times smaller than that from the heart displayed by an electrocardiogram (ECG). This makes the EEG very sensitive to interference from muscle activity in the scalp or from electronic equipment in the vicinity. Moreover, whereas in the heart the origin of the activity is very well understood, the EEG records the collective activity of large populations of neurons in the cerebral cortex. For all the sophistication of modern recording equipment and computerized analysis of the records, it has to be accepted that the EEG remains a relatively crude measuring device, although it enables recognition of the different phases of normal sleep, and is of diagnostic value in some abnormal conditions.

A number of electrodes (typically about 20) are positioned on the scalp and connected in pairs, yielding 8–16 channels, each recording the potential between two electrodes. Each electrode receives signals from an area of cortex of 2–3 cm diameter. But a third of the cortex is inaccessible, in the depths of the indentations (the sulci), on the basal surface, or hidden within the larger folds of the brain. Some 10–15 min of recording results in 20–30 pages of paper that can be bound and read like a book — and often also analyzed by computer.

Information lies in the frequency and amplitude (voltage) of the waves recorded in different channels. At rest, relaxed and with the eyes closed, the frequency of these waves is 8–12 Hz (cycles/sec). This ‘alpha’ activity is believed to reflect the brain in ‘idling’ mode, because if the person then either opens the eyes, or does mental arithmetic with the eyes closed, these waves disappear, to be replaced by irregular patterns (so-called desynchronized activity). In normal sleep there is characteristic higher voltage activity, in patterns which vary according to the level of sleep.

When there is severe diffuse brain abnormality, such as encephalitis or conditions causing coma, there will be usually be no alpha activity, whilst in the vegetative state there may be alpha activity that fails to desynchronize on eye opening. Faster frequencies (beta, at >13 Hz) or slower (theta, at 4–8 Hz) can be normal in infancy and childhood. Even slower ‘delta’ waves (<4 Hz) can be normal in sleep and in infancy, but in awake adults indicate severe abnormality. When localized they may indicate pathology such as a tumour or abscess in the brain, causing the adjacent cortex to produce abnormal rhythms; however, modern imaging techniques have replaced EEG as a means of detecting and locating such lesions.

It is in the investigation of epilepsy that EEG has proved most useful — both in diagnosing epilepsy as the cause of abnormal behaviours and in localizing the site in the brain from which abnormalities are originating. During an epileptic seizure there are bursts of high voltage activity in the region of the brain affected. The problem is the low probability of a patient having an attack during routine recording. However, the record in a person with epilepsy is often abnormal between attacks, and these abnormalities are more likely to be found if the patient hyperventilates (producing temporary alkalosis in the brain), or is fasting (with temporary mild hypoglycaemia). But some patients who never have a clinical seizure may show such abnormalities on EEG, whilst a third of patients who do have seizures have a normal record at times between attacks. To increase the chance of securing a recording during an attack patients may be fitted with electrodes that transmit by radio to a receiver. Such telemetry allows 24-hour recording during normal activities — which may also be monitored on video in order to visualize any seizure that occurs.

When patients with epilepsy are being investigated with a view to possible surgery the location of the seizure-producing lesion may be identified by using electrodes placed to cover areas not available to those placed on the scalp. Electrodes introduced through the cheek to the base of the skull can detect activity from the under surface of the brain, whilst those inserted through a burr hole (a small opening in the skull) on to the surface of deeper parts of the brain, or even into the substance of the brain, bring other areas under surveillance.

A visual, auditory, or somatic stimulus will normally evoke an altered wave form from the appropriate area of the cortex (for example the occipital region for a visual input). It is therefore possible with the EEG to explore the integrity of sensory pathways by means of these evoked potentials. This method is also used during operative procedures to help surgeons to identify such sensory pathways by observing the effects of electrical stimulation on the EEG, so as to avoid damaging them.

An occasional use of the EEG is in confirming the diagnosis of brain death, when an isoelectric (flat) record may be obtained, but technical difficulties can lead to equivocal results. In some places, but not in the UK, EEG is mandatory after clinical tests have been completed.

Bryan Jennett

See also convulsions; epilepsy.