VOL: 98, ISSUE: 38, PAGE NO: 36
William Blows, PhD, BSc, RMN, RGN, RNT, OStJ, is lecturer in biological sciences, St Bartholomew School of Nursing and Midwifery, City University, London
The activity of the brain produces tiny electromagnetic energy waves, called brainwaves, which penetrate the skull and can be recorded via electrodes attached to the scalp. This recording is known as an electroencephalogram (EEG) and the tests are valuable in many aspects of neurology, particularly in the diagnosis of epilepsy.
Brain activity is classified into four types of electromagnetic wave, which differ from each other by the number of cycles per second (Hz). These are delta, theta, alpha and beta waves:
- Delta waves have a frequency of 1-3Hz and have a large amplitude (they are strong waves that show as tall peaks on the tracing). They are a feature of normal deep sleep for people of all ages and may also be seen normally in infants (coming from the immature cerebrum). They may be found in waking adults with brain tumours, brain damage or obstructions of the brain’s blood supply.
- Theta waves have a frequency of 4-7Hz and a smaller amplitude than delta waves. They are normally seen in children and originate from the temporal lobes. There may be some increase in theta activity in adults who are angry, or in those over the age of 60, but in general adults with increased theta activity are likely to have some brain disorder, such as a tumour.
- Alpha waves have a frequency range of 8-12Hz, with a smaller amplitude than theta waves. These are characteristic of a brain output from a resting adult who has his or her eyes closed. They arise mainly from the occipital lobe, which includes the visual cortex, and disappear when the eyes are opened. They also decrease as a result of anxiety or disturbance by sudden noise, and disappear during sleep.
- Beta waves have a frequency of 13-35Hz and have the smallest amplitude of the four types of wave. They are best recorded from the frontal and parietal lobes and are triggered by visual activity or mental stimuli, for example if the subject is paying close attention to something or is anxious. They are also seen more frequently in patients treated with benzodiazepine and barbiturate drugs.
A series of electrodes is attached to the patient’s scalp in a pattern that is carefully measured and marked out across the top of the head (Fig 1). The attachments are made using sticky jelly, which transmits the tiny electrical potential differences (voltages) from the skin to each electrode. These voltages are measured in milliVolts (mV), and have to be amplified many times by the EEG recorder to show a tracing. The older pen-and-paper recorders are being replaced by computerised on-screen recorders, which record and store the tracing. This can then be manipulated as required.
Patients are placed in a reclining position to make them feel relaxed and comfortable. They should try to keep still and quiet because any muscle movement can have an unwanted effect on the tracing. Children are usually more relaxed and cooperative if their parents stay with them throughout. The procedure takes the form of recording the electrical potential differences between two electrodes on the scalp.
Different patterns of connection between electrodes and the recorder are called montages. For example, a bipolar montage records the electrical potential difference between successive electrodes along an anteroposterior (front-to-back) or transverse (side-to-side) line.
A unipolar montage records the electrical potential differences between an active electrode and one neutral reference point. Sixteen lines of EEG tracings are recorded simultaneously, with an electrocardiogram (ECG) added along the base. At the start of each line two electrodes are identified, with the line of tracing representing the potential difference between these electrodes.
In Figs 2-4, the top line is a record of the potential difference between electrodes T4 and T6, the locations of which are identified in Fig 1.
A period of recording with the subject at rest is made in different montages selected by the technician to establish a baseline before some specific tests can be introduced. These tests start with patients being asked to open and close their eyes at particular points during the test. This causes normal, characteristic brainwave changes, which the neurologist will examine.
The patient may also be asked to hyperventilate for a few minutes. This removes carbon dioxide rapidly from the blood, which in turn changes the blood from pH7.4 to pH7.8 (allowing it to become more alkaline).
The increased nerve excitability created by this process is recorded on the EEG, and how long this takes to return to normal is also noted. A strobe light is then shone at the patient. This starts with a single flash, followed by short periods of increasingly faster flash rates.
The recording takes 40-60 minutes to complete. After the test the electrodes are easily removed and the jelly can be washed off the hair. An EEG of a normal adult with eyes closed has a predominance of alpha waves from the occipital lobe, and mainly beta waves from the frontal lobes. Theta and delta waves are more often associated with sleep and correspond to specific depths of sleep. Some patients may fall asleep during the test, which allows the sleep pattern of brainwaves to be recorded.
Epilepsy and cerebral tumours
Marked changes can be identified in an EEG of a patient who has an epileptic seizure during a recording. The tracing will show larger amplitudes than the normal background amplitude - that is, higher voltages (Fig 3).
The waveform pattern is referred to as spike and wave: tall spikes with waves between that have a cycle of about 3Hz. Focal-point epilepsy, such as temporal-lobe epilepsy, shows as an abnormal, sharp wave arising from one part of the brain. This is picked up only by a few electrodes.
In the case of temporal-lobe epilepsy, the lateral electrodes over the temporal lobe will register the abnormal wave pattern (Fig 4). Cerebral tumours and other space-occupying lesions do not themselves produce any EEG wave patterns but can disturb the normal patterns of brainwaves produced by the neurons surrounding a tumour. Some EEG abnormalities associated with cerebral tumours are noted above, and it is significant from this that disturbance of the normal tracing often takes the form of a slow wave (2-7Hz). Identification of the electrodes showing this disturbance will help to localise the lesion.
The patient does not require any specific nursing care before having an EEG, but nurses should provide information and support as some patients may be anxious about the procedure. Patients need to be relaxed for this investigation. Fear or anxiety can cause changes in brainwave patterns or generate unwanted signals from tense head and neck muscles.
Drugs which may interfere with the EEG tracing, such as tranquillisers, stimulants and anticonvulsants, may be withheld for about 24 hours before the test. Some dietary stimulants, such as tea and coffee, may also be withheld before an EEG. Some EEG departments employ a nurse to provide continued patient support throughout and to assist with the patient’s mobility if required. Given that a proportion of patients are likely to have epilepsy, nursing staff on site can assist if a patient has a fit.
Blows, W.T. (2000)Systems and diseases: nervous system 1. Nursing Times; 96: 35, 41-44.
Blows, W.T. (2001)The Biological Basis of Nursing: Clinical Observations. London: Routledge.
Hickey, J.V. (1997)The Clinical Practice of Neurological and Neurosurgical Nursing. Philadelphia, PA: Lippincott.