Neurophysiological investigations

Published on 09/04/2015 by admin

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Last modified 09/04/2015

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Neurophysiological investigations

The electroencephalogram

The electroencephalogram (EEG) records the electrical activity of the brain. Its major use is in the diagnosis and characterization of epilepsy. An EEG is usually recorded between seizures (interictal) but sometimes during seizures (ictal). The diagnosis of epilepsy is clinical and a normal interictal EEG does not exclude a diagnosis of epilepsy. The EEG is also useful in the diagnosis of encephalitis, coma and dementia, especially if rapidly progressive. It is complementary to imaging techniques such as CT and MRI and should not be used to look for structural pathology.

Method

The patient rests back and 20 electrodes are attached over the scalp with glue (Fig. 1). These are connected to a multichannel recorder, which generates a paper tracing or a computer record. This is often synchronized with a video recording of the patient. A skilled technician monitors the recording throughout, to detect and eliminate artefacts. The EEG is recorded with the patient’s eyes open and closed and several methods may be used to enhance sensitivity of the technique, routinely including forced hyperventilation for 3 min and stroboscopic photic stimulation at 1–50 Hz. Another technique to increase sensitivity is to deprive the patient of sleep before the EEG, then allow the patient to fall asleep during the recording.

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Fig. 1 EEG leads on a patient.

EEG interpretation

There are two features in the interpretation of the interictal EEG: background rhythm and paroxysmal EEG changes. Normal background rhythm in waking adults is 8–13 Hz ‘alpha’ activity, best seen over the occipital cortex when the individual’s eyes are shut. It varies with the age of the patient, changing especially in children as the brain matures. In sleep, the EEG is slower and varies according to the stage of sleep.

Abnormalities of background rhythm

Focal slow waves may represent focal structural lesions (tumour, infarct, etc.) and a brain scan is usually indicated. Widespread slow waves are seen as part of diffuse encephalopathic processes, often due to metabolic disturbance such as renal or hepatic failure, drug intoxication, encephalitis, advanced degenerative processes or sometimes thalamic or brain stem lesions that affect arousal. Faster background rhythms are usually due to drugs, especially benzodiazepines or barbiturates.

Paroxysmal interictal EEG changes

During the EEG, much briefer discharges may be seen. Spikes (<70 ms) and sharp waves (70–200 ms) are brief, near-simultaneous discharges of large numbers of neurones.

Spikes and sharp waves may be focal (affecting only part of the brain) or generalized (simultaneously affecting all parts of the brain) (Figs 2 and 3). Focal spikes suggest epilepsy due to a focal disturbance and imply a focal structural cause; neuroimaging should be considered. Generalized spikes are seen as part of the generalized epilepsies, which usually start in childhood or adolescence (pp. 74–75

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