Electroencephalography

Electroencephalography examines by means of scalp electrodes the spontaneous electrical activity of the brain. Tiny electrical potentials, which measure millionths of volts, are recorded, amplified and displayed on either 8 or 165 channels of a pen recorder. Low and high frequency filters remove unwanted signals such as muscle artefact and mains interference.

The system of electrode placement is referred to as the 10/20 system because the distance between bony points, i.e. inion to nasion, is divided into lengths of either 10% or 20% of the total, and the electrodes placed at each distance.

A switch changes recording from A 3„—

(parasagittal) to B (transverse). Other electrode arrangements are also 'preset'. The numbering 4«-r" indicates the write out from top to bottom of an 8-channel record.

Alpha rhythm (8-13 Hz - cycles/second). Symmetrical and present posteriorly with the eyes closed - will disappear or 'block' with eye opening

Beta rhythm (>13 Hz). Symmetrical and present frontally. Not affected by eye opening

Theta rhythm (4-8 Hz) 1 Seen in children and young

I adults with frontal and Delta rhythm < 4 Hz) J temporal predominance

These 'immature' features should disappear in adult life as the EEG shows 'maturation'

Normal rhythms

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1 sec

As well as recording a resting EEG using various 'preset' electrode arrangements, stressing the patient by hyperventilation and photic stimulation (a flashing strobe light) may result in an electrical discharge supporting a diagnosis of epilepsy.

More advanced methods of telemetry and foramen ovale recording may be necessary

- to establish the diagnosis of 'epilepsy' if doubt remains

- to determine the exact frequency and site of origin of the attacks

- to aid classification of seizure type.

Telemetry: utilises a continuous 24-48 hour recording of EEG, often combined with a videotape recording of the patient. Foramen ovale recording: a needle electrode is passed percutaneously through the foramen ovale to record activity from the adjacent temporal lobe.

Magnetoencephalography

A new technique which measures changes in the magnetic field generated by the brain's electrical activity. It allows detection of the depth and location of current changes with better temporal and spatial resolution than the EEG. 49

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