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THE TREATMENT OF EPILEPSY: SURGERY

The surgical treatment of epilepsy is becoming increasingly useful, particularly when the seizures are not controlled by anti-epileptic drugs. However, surgery must only be undertaken after a careful detailed assessment of the patient. This, and the operation, should only be carried out in recognized specialist centres. This is because both the assessment of the patient, and the operation itself involve expert and sophisticated procedures—and clearly surgery is an irreversible treatment.

Surgical treatment depends on two main principles or ideas. The first is that a local abnormal area of brain can be entirely removed, leaving behind only healthy, normal brain. The second is that the spread of the seizure discharge, can be prevented by cutting the nerve fibres which cause the discharge. Penfield and Rasmussen, two Canadian neurosurgeons, were the pioneers of surgery for epilepsy and much of the surgical assessment and treatment of patients today is based on their early work. One of the most important questions that must be answered before surgery can be considered is from where

precisely within the brain do the seizures originate. When the cause is a tumour or cyst, then this is relatively easy, but frequently the cause is an area of brain that developed abnormally in fetal life. The identification of the abnormal part of the brain relies upon magnetic resonance imaging, and the use of special electrodes to try and record or ‘capture’ the epileptic discharge. The scalp electrodes (used in a routine EEG) are not usually sensitive enough for this task, and so other electrodes, called depth electrodes, are frequently used. They are also called ‘sphenoidal’ or ‘foramen ovale, electrodes because this describes how they are placed close to the brain. Electrodes may even have to be placed directly on the surface of the brain, or, as fine silver needles, within its substance. Because these special electrodes are in very close contact with the brain, there is a much greater chance that they will pick up the epileptic discharge.

As well as these assessments, people being considered for surgery may also need detailed psychological evaluation, specifically to try and identify which side of the brain is responsible for language and memory, so that these areas are not damaged during the operation. Consideration must also be given to avoid operating in those parts of the brain responsible for movement—it would be unacceptable to stop the seizures at the expense of causing a paralysis on one side of the body (hemiplegia), which might result in losing the ability to walk or write.

Before a patient is considered for surgical treatment of their epilepsy, it must have been shown that the patient’s seizures could not be adequately controlled using anti-epileptic drugs. For how long a patient should not have been controlled is dependent on individual circumstances. It is unwise to operate too early, as the epilepsy might remit (stop) spontaneously, although this is unlikely in the difficult epilepsies. However, if surgery is delayed for too long, then this may limit the potential success of the operation, either because the patient has suffered irreversible educational and social consequences of repeated seizures, or because other parts of the brain which were previously normal may have developed abnormal foci of electrical activity as a result of the continuing activity of the primary focus. Generally speaking most patients with difficult, drug-resistant epilepsy are being considered for surgery too late; surgery can safely be undertaken in children—even young infants. Most specialists would now consider that if acceptable seizure control has not been achieved using optimal doses of anti-epileptic drugs after one to two years, then surgery should be considered as the next step in a patient’s treatment. It has been estimated that many patients in the UK might currently benefit from surgery, but only about 200 operations per year are at present being performed.

There are four types of surgical procedure that are currently undertaken:

• removing a large, identifiable lesion such as a tumour or cyst.

• removing an entire cerebral hemisphere. This is done when the whole of one side of the brain is abnormal, this being associated with a hemiplegia (weakness down one side of the body). The operation sounds dramatic, but is often successful leading to a complete resolution of seizures and, frequently, an improvement in the hemiplegia. Hemispherectomy is particularly useful in children with the Sturge-Weber syndrome.

• removing a small or large lesion which has been identified on the basis of detailed specialized EEG recording and imaging. This procedure is the one frequently used in temporal lobe epilepsy, where different parts and amounts of the temporal lobe are removed. Advances in imaging have led to the identification of subtle structural abnormalities in the temporal lobes, which are responsible for seizures.

• carrying out a disconnection procedure; this is to try and separate the focus (site of abnormal electrical activity) of origin of the seizure from other parts of the brain, by cutting the nerve fibres which allow the epileptic discharge to spread. Operations attempted have included division (cutting) of the corpus callosum. This is a large band of fibres which transmits electrical information from one hemisphere to another. A more sophisticated, technically difficult procedure (called subpial transection) appears to be more successful.

Overall, the results of epilepsy surgery are encouraging, as many as 60-70 per cent of people who have operations for epilepsy have no further seizures, whilst another 10-20 per cent are much improved. Patients undergoing a hemispherectomy or temporal lobectomy do better than patients who have a corpus callosotomy. For some patients who have had to live with uncontrolled seizures for many years, a cure of their epilepsy following surgery may come as something of a ‘shock’, requiring a major adjustment in their lives. These patients need careful and expert support and counselling.

It must be emphasized again that patients must be assessed carefully in specialist centres before undergoing surgical treatment of their epilepsy. No one person can have a guarantee that their seizures will stop.

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