Like many sophisticated machines, the human body uses electricity to send messages from one part of the body to another in order to control and co-ordinate its many activities. The ‘wires’ along which these messages are passed are the nerve cells and their long connecting axones. The voltage of these stimuli is minute – as small as a fraction of a millivolt, in fact -but it is detectable by metal electrodes attached to the body surface. These can be used to pick up the signals and feed them into an amplifier, after which they can be displayed on a video screen or recorded by a moving pen making a trace on a paper strip. Electrical nerve signals can be detected from many parts of the body, but the two organs most often monitored in this way are the brain and the heart. Because the currents emitted are so small, it is necessary to exclude any possible sources that might disturb the measurements of brain and heart activity. Movement of muscles should be avoided, so the patient is encouraged to sit or lie in as quiet and relaxed a manner as possible.
Usually, these tests are used to confirm (or exclude) a doctor’s provisional diagnosis or to monitor a patient during recovery from illness and during convalescence. The procedures are painless and harmless.
Electroencephalogram (EEG) Normally, the brain, which consists of innumerable neurones, is active constantly. It produces a characteristic pattern of electrical impulses that varies in different circumstances. The electrical waves are slower in frequency during sleep, for example. Closing the eyes produces a characteristic pattern, as does activity or concentration. Brain injury or abnormality may alter the normal pattern of electrical activity, so the EEG can be used with some success to seek an explanation for episodes of unconsciousness, for example, or to diagnose some types of brain disorder. The EEG is interpreted by a doctor. It does not reveal every abnormality present, however. For example, about 75 per cent of people who have epilepsy have abnormalities in their EEG recording, but an abnormal EEG does not necessarily mean that a person has epilepsy, nor does a normal EEG prove that he or she does not have epilepsy. However, a patient’s progress on drug therapy can be fairly effectively monitored by making a series of EEGs during the course of treatment. This is particularly useful in the case of some anti-epileptic drugs because their effectiveness varies from patient to patient. The EEG may also be used to establish ‘brain death’. An EEG recording takes about one hour to carry out. During the procedure the patient lies on a couch or sits in a chair. Twenty-four small silver discs, the electrodes, are attached to the scalp at specific points with a special conductive glue. (The procedure does not involve any shaving of the hair.) Wires lead from the electrodes to the EEG machine, called an encephalograph, which records the electrical activity in the different areas of the brain when the electrodes are attached. On most machines there are 12 channels, each recording a particular aspect of brain function. Epileptic activity in one part of the brain, for example, may show up on the EEG as characteristic ‘spikes’ in that area. The extent of damage caused by a stroke can be examined by making an EEG recording, as can paralysis of part of the brain from a haemorrhage. Interspaced at regular intervals, EEGs may show improvement or worsening of these conditions.