Investigations and procedures
An electroencephalogram (EEG) is a recording of the electrical activity of the brain. It is obtained by placing electrodes on the scalp, using a jelly to reduce electrical resistance. A recording of at least half an hour is usually needed, to maximise the chances of picking up transient abnormalities. The patient needs to lie still, as electrical changes due to movement can interfere with the recording.
Its main use is for the classification of epilepsy, but is it may also be useful in the diagnosis of other brain disorders such as encephalitis. In epilepsy, different waveforms may be seen. The EEG is often normal between seizures, and anti-convulsant medication can alter the EEG.
· Characteristic abnormalities are described under Epilepsy, but include spikes and spike and wave discharges.
· Photic stimulation and hyperventilation are routinely performed to increase the sensitivity if the resting EEG is normal.
· Sleep-deprived EEG, which allows recordings when the patient dozes and wakes.
· EEG recording in status epilepticus is useful at demonstrating whether seizure activity is suppressed by medication, particularly in a paralysed, ventilated patient.
· The EEG is abnormal postictally, which can be helpful in the diagnosis of epilepsy.
· Focal abnormalities, e.g. localising abnormal electrical activity to the frontal lobe can suggest an underlying epileptogenic focus, e.g. a tumour or area of infarction or encephalitis, as well as occurring in focal status epilepticus.
Encephalitis, cortical necrosis (e.g. in stroke or postanoxic damage), metabolic brain disorders including drug-induced delirium, and tumours can cause either focal or generalised EEG abnormalities. EEG changes are seen even before MRI changes are evident.