How is epilepsy diagnosed?
There is no diagnostic test for a seizure or for epilepsy. The doctor’s diagnosis is based on a thorough investigation of a first seizure (including any witness observations), a physical examination, family history, and supportive tests such as the EEG, CT Scan, and MRI.
The description of what happens during during the seizure, along with any changes that happen before or after the seizure, are important features used by a healthcare providor to determine the seizure category and the seizure type. The duration of the seizure is another important feature.
What is an EEG?
The electroencephalograph or EEG directly measures electrical activity in the brain–brain waves–through the skin. In this harmless test, small sensors called electrodes are attached to the patient’s scalp. The electrical activity picked up by each sensor is graphed onto an EEG printout.
Tests are done on people with epilepsy commonly show uneven activity or large changes in the voltage of brain waves (spikes). Different patterns of activity from different spots on the scalp point to different kinds of epilepsy.
How accurate is the EEG test?
The EEG is not foolproof. It can only measure abnormal electrical activity that occurs during the test period. Sometimes, the brain of the person with epilepsy functions perfectly normally during the test. Or the electric patterns that the device is looking for happen too deep in the brain to be picked up by the scalp electrodes of the EEG.
When the EEG doesn’t find anything unusual, it is common for the patient to get a continuous, 24-hour EEG monitoring in hospital. About 20% of people with epilepsy have normal EEGs, and a small percentage of people who don’t have epilepsy have abnormal EEGs.
What other tests are sometimes used?
Neuro-imaging tests are often recommended, even in cases of long-standing epilepsy, when its cause is unknown.
- CAT or CT scan stands for computerized axial tomography, which uses computer processing and x-rays to make a computer image of the brain in three dimensions
- The Magnetic Resonance Imager (MRI) can better define the structures of the brain in three dimensions. All electric currents make magnetic fields, and the MRI measures the strength of these fields
- MRS (Magnetic Resonance Spectroscopy) provides information about the brain’s functioning and biochemistry which can be used in conjunction with structural MRI or CT images
- Functional MRI (fMRI) can look at discrete areas of brain activation
- PET (Positron emission tomography) scanning is a highly specialized, expensive and largely unavailable technique that detects cerebral blood flow and metabolism
- SPECT scanning is much cheaper and technically simpler than PET scanning for determining cerebral blood flow.