TESTS IN EPILEPSY: BRAIN IMAGING INVESTIGATIONS AND THE CT SCAN

Posted on December 9, 2009, under Epilepsy.

The EEG is a ‘functional’ investigation, recording the brain’s function through normal and abnormal electrical activity. Imaging procedures or brain scans provide information about the brain’s structure, and revealing normal and abnormal anatomy. Most, if not all patients who have epilepsy need to have at least one EEG, fewer than perhaps 1 in 5 or 1 in 6 patients need to have an imaging investigation. Research is underway to determine who should be scanned.

Two types of imaging techniques are currently available in the developed world; these are the computerized tomographic (CT) brain scan and magnetic resonance imaging (MRI).

The CT scan-This is an abbreviation the computerized axial tomography (CAT) scan. The technique was developed in the 1970s and is a type of X-ray investigation. Tomography is a word dating from earlier X-ray techniques. The patient lies still on a table whilst a rotating X-ray machine takes two-dimensional pictures of the head from many different angles or positions. The information is then processed by a computer to produce pictures (or images) at different levels of the brain. The test is safe, and other than keeping the head still, there are no particular precautions to be taken. Children may have to be given a sedative drug or short anaesthetic so that they can keep still for the scan. The test takes approximately 15-20 minutes. If an area of interest is seen on the initial images, some contrast (special dye) is injected into a vein in the hand or arm and then the scan repeated. The dye may enhance contrast in areas of interest and give more detailed information. CT scanning has proved to be very useful in detecting structural abnormalities within the brain, such as strokes, infections, tumours, and congenital malformations which may cause epilepsy. However, only 20-25 per cent of patients with epilepsy referred to special centres will have an abnormal CT scan. Abnormalities on the CT scan in patients who have epilepsy are more likely to be found in the following situations:

• patients whose seizures affect only one side of the body;

• patients whose EEG shows a persistent slow wave abnormality on one side of the brain;

• when epilepsy starts in newborn babies and continues;

• when epilepsy starts in later life; and

• if the patient has abnormal findings on neurological examination, for example, mild weakness down one side of the body, or changes in the reflexes.

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