Value of an EEG
An EEG (electroencephalogram) is a recording of the surface charges of the brain using small surface electrodes applied to the scalp with a special conductive paste. Normal brain waves consist of a number of defined rhythms at different frequencies, amplitude and distribution. With altered brain function such as epilepsy these rhythms may alter showing waveforms of a specific type.
The indications for an EEG are:
- To define seizure type – absence episodes, generalised epilepsy or partial seizures. This may affect drug therapy and management.
- To define a focal abnormality in the brain where epileptic events may arise. This allows a more focused review of CT or MRI scans.
- To help in the assessment of attacks of altered awareness eg possible epileptic aura such as déjà vu phenomenon.
- Unexplained fluctuating or altered consciousness/awareness. This can be seen in non convulsive seizures. Presentation includes slow mentation and slow speech. Precipitants include withdrawal of benzodiazepine medication. Such altered behaviour can also be seen as a manifestation of epilepsy after cerebral injury e.g. from stroke.
- To confirm non epileptic stress attacks with frequent events allowing capture of an episode on EEG. A normal EEG in a typical attack would not support epilepsy.
- To assess specific EEG changes in a rapid onset decline in cognitive function. This may show changes suggestive of a specific diagnosis.
A normal EEG does not exclude epilepsy.
If treatment is begun for epilepsy, ideally an EEG should be done before the patient takes medication if information about a focal abnormality or seizure type is required. Sleep deprivation is also used to increase the possibility of recording epileptiform activity.
Where an EEG is not indicated:
- EEG should not be used to diagnose epilepsy per se. This is primarily a clinical diagnosis.
- EEG is not necessarily of use in refractory epilepsy, unless this is to review seizure type or to define a seizure focus.
- EEG is not helpful in decisions as to whether to discontinue medication, having been seizure free – unless there are absence attacks. Such decisions are driven more by clinical concerns and the consequences of a further seizure.
- EEG is generally not helpful in addressing the question of loss of consciousness – query faint or epilepsy. Epilepsy is a clinical diagnosis but an EEG may be useful if uncertainty exists after a full clinical assessment.
Compiled by Stuart Mossman Neurologist FRACP MD