In 30% of patients with epilepsy, seizures are not adequately controlled with drugs. For this reason, when a patient has tried at least two antiepileptic drugs and seizures persist, making daily life difficult, his epilepsy is considered to be drug-resistant and should be studied to see if his condition requires surgical treatment.
The aim of this study is to locate the place in the brain where the epileptic seizures originate to see if that area can be eliminated without serious sequelae, because in any case, epilepsy surgery is aimed at improving the patient’s quality of life. To this end, the expectations of improvement with the operation and the possible sequelae must be taken into account, assessing whether the intervention is worthwhile overall.
Tests to be performed to study whether a patient can undergo surgery
There are a number of tests that should be performed to check whether a patient can undergo epilepsy surgery. These tests are as follows:
- Prolonged video EEG
The purpose of the video EEG, which is performed by placing electrodes, is to record and analyze in detail the patient’s usual seizures, as well as the epileptic activity seen on the EEG between seizures (intercritical activity). If seizure frequency is low, it may be necessary to decrease or discontinue the patient’s usual antiepileptic drugs to provoke seizures.
In rare cases, in addition to discontinuation of drugs, other procedures, such as keeping the patient awake, should be used. Logically, reduction or discontinuation of antiepileptic drugs may lead to more prolonged or severe seizures, such as secondarily generalized seizures or convulsions. For this reason, a peripheral venous line is always placed during admission so that sedative medication can be administered if necessary.
The seizures are videotaped so that the epileptologist can analyze the patient’s sensations, movements and behavior during the seizures and extract information about their origin in the brain. The different types of epilepsy are associated with different electrical patterns that the epileptologist has to recognize.
- Neuropsychological study
Patients evaluated for epilepsy surgery should undergo a detailed neuropsychological study.
This study provides information on the patient’s general intelligence and also on his or her abilities, both verbal and manipulative. If deficits in the function of certain brain areas are identified, it can be assumed that these areas do not function properly, and that they are somehow related to the onset or propagation of seizures. For example, patients with left temporal epilepsy often have impaired verbal memory.
Neuropsychological evaluation also helps to predict possible cognitive sequelae of epilepsy surgery, such as memory decline in the case of temporal epilepsy.
- Neuroimaging tests
High-resolution MRI, performed with an epilepsy-specific protocol, is an essential test during the pre-surgical evaluation of a patient with drug-resistant epilepsy. The presence of a lesion on MRI, especially if it is located in the same area where seizure onset was seen on surface EEG, is the factor most correlated with a good outcome of surgery. This does not mean that epilepsy surgery cannot be done in cases where the MRI is “normal”, but the prognosis is not as good.
There are several types of lesions that can be seen on MRI in epileptic patients:
- Tumors: in general those causing chronic epilepsy are benign or very slow growing. Epileptic seizures can also be seen in patients with malignant brain tumors and brain metastases.
- Vascular malformations: for example, cavernous angiomas or arteriovenous malformations. Some of these lesions may require treatment because of their risk of bleeding, regardless of how many seizures they are producing.
- Encephalomalacia and gliosis: these are ill-defined lesions, including cavities and scars, which are usually seen in various conditions, for example after severe head trauma.
- Mesial temporal sclerosis or hippocampal sclerosis: is a scar located in the inner part of the temporal lobe, in a structure called the hippocampus, which is involved in memory processes. This scar is associated with many cases of resistant adult epilepsy, and correlates with a good outcome after surgery. Approximately 70% of patients remain seizure-free.
- Cortical dysplasias are developmental malformations of the cerebral cortex that have occurred during the fetal stage, sometimes not manifesting until years after birth. Cortical dysplasias are a very frequent finding in pharmacoresistant epilepsies in children.
In selected cases, it may be necessary to complete the preoperative study with functional neuroimaging tests, which provide information on brain function. These tests include PET, which measures glucose consumption by the brain, and SPECT, which measures cerebral irrigation during and outside seizures.
Surgical interventions for epilepsy surgery
The different neurosurgical techniques used to treat refractory epilepsy are aimed at eliminating or disconnecting the epileptogenic zone, which is the region where the patient’s seizures start and which has been localized thanks to the tests described above.
- Lesionectomy: consists in the resection of the epileptogenic lesion. This procedure can be performed when it has been demonstrated that the seizures are initiated in the lesion.
- Lobectomy: consists in the resection of the cerebral lobe where the seizures start. The lobectomy may be more or less extensive, depending on the site of seizure onset and whether it is the language dominant hemisphere. The most frequently performed lobectomy is the anteromesial temporal lobectomy for the treatment of mesial temporal epilepsies secondary to hippocampal sclerosis.
- Hemispherectomy: this is a complex procedure that consists of disconnecting a complete hemisphere that is malfunctioning and causing the patient’s seizures. Hemispherectomy is performed in patients who already have severe neurological deficits dependent on the hemisphere to be disconnected.
What options are available in patients who cannot be operated on?
If the onset of seizures cannot be precisely localized, or the seizures start in multiple brain regions, or in areas whose resection would result in severe sequelae for the patient (such as difficulty speaking or moving an arm or leg), there are options other than medical treatment:
- The vagal stimulator is a device that emits an intermittent electrical signal over the left vagus nerve, and can significantly decrease seizure frequency in some patients. The vagal stimulator has to be implanted by a neurosurgeon, and the adjustment of the intensity parameters has to be done by an epileptologist. It can have adverse effects such as coughing, throat discomfort or changes in the tone of voice.
- The trigeminal stimulator is an external device to electrically stimulate two branches of the trigeminal nerve in the forehead. The stimulation has to be performed for several hours each day. Compared to the vagal stimulator, it has the advantage that it does not require surgical intervention for its placement and its use can be stopped if it is not effective. In a recent study we conducted, approximately 50% of patients benefited from this therapy.
- Some diets (ketogenic diet, modified Atkins diet) decrease seizure frequency in patients with refractory epilepsy. The ketogenic diet has been used mainly in children. The modified Atkins diet is easier to perform and also has good results. Both should be performed under medical supervision.