Value of an EEG
An EEG (electroencephalogram) is a recording of the electrical charges of the brain. This is obtained by 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 with epilepsy these rhythms may change. The EEG then assists with clarifying the clinical diagnosis.
The indications for an EEG are:
- To define the seizure type – absence episodes, generalised epilepsy or focal seizures. This may affect anticonvulsant 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 brain scans.
- To help in the assessment of attacks of altered awareness, including possible epileptic aura, such as the déjà vu phenomenon.
- Unexplained fluctuating or altered consciousness/awareness. This includes slow thinking processes 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 a stroke.
- To confirm frequent non-epileptic stress attacks that allow capture of an episode on an EEG. A normal EEG result in a typical attack would not support a diagnosis of epilepsy.
- To assess specific EEG changes in a rapid onset decline in cognitive function (e.g. dementia). This may show changes suggestive of a specific dementia diagnosis.
Note: An EEG is not 100% sensitive for detecting epilepsy and only shows characteristic abnormalities in 25% of adult patients with seizures. Sleep deprivation is also used to increase the possibility of recording epileptiform activity.
Ideally an EEG should be done before the patient takes medication, so that information about a focal abnormality or seizure type can be obtained.
Where an EEG is not indicated:
Contact Us to see if an EEG scan is right for you.
- EEG should not be used to diagnose epilepsy per se. This is primarily a clinical diagnosis.
- EEG is not necessarily of use in treatment resistant epilepsy, unless this is to review seizure type or as part of a workup 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 not of good predictive value after the first seizure in determining whether further epileptic events are going to occur, although this is debatable.
- EEG is generally not helpful in addressing the cause of undefined loss of consciousness without any other clinical pointers to an epileptic attack.
Compiled by Stuart Mossman Neurologist FRACP MD