Some brain tumors can cause seizures over many years without causing any other symptoms. These tumors are generally very slow growing, benign tumors. The goals of surgery in such a situation are to cure both the epilepsy and the tumor. As it turns out, these goals have an extremely high likelihood of success.
Astrocytomas and oligodendrogliomas arise from cells in the brain called glia. These cells surround the neurons and support their metabolic and physiologic functions. Glial tumors come in two varieties, fast-growing and slow-growing.
Fast-growing glial tumors often come to the patient's attention because of a neurological symptom, such as weakness or loss of vision. This type of tumor may also cause seizures, but because of the onset of the neurological symptoms, the tumor is generally discovered early on, perhaps after only one or two seizures. Faster-growing tumors, called anaplastic astrocytomas or glioblastomas, often come back after surgery.
Patients who have had multiple seizures for 3 years or more generally have the slow-growing type of tumor. Slow-growing glial tumors represent as many as 70% of tumors that cause epilepsy in the general population. These tumors are more common in adults with epilepsy; they may not be seen on a CT scan if the tumor is very small. Experience has shown that a complete surgical removal of these tumors can lead to excellent seizure control with a good chance of a cure. As a rule, the longer the patient has had seizures, the slower the growth rate of the tumor, and the greater the chance for a cure. Depending on the type of surgery performed, as many as 80-90% of patients stop having seizures after surgery. A recurrence of seizures, even many years after the surgery, may indicate a re-growth of the tumor. For this reason, patients with glial tumors should have an MRI scan on a yearly basis to ensure that their tumor has not returned.
Gangliogliomas and dysembryoplastic neuroepithelial tumors (DNT) are tumors composed of a combination of glial cells and neuronal cells. They are more common in children and are often found in the temporal lobe of the brain. The most common symptom of these tumors is partial seizures, which are often present for several years before the diagnosis is made. These tumors may not appear on CT scan, which contributes to the delay in diagnosis. Gangliogliomas and DNTs are extremely benign tumors and are generally cured with surgery. As opposed to the glial tumors described above, the goal of surgery for glioneuronal tumors is predominantly seizure-control. These tumors are often located next to areas of cortical dysplasia (areas of the brain's cortex that contain tissue abnormalities). Cortical dysplasia occurs as a result of abnormal neuronal development that can cause hyperexcitability in a population of neurons. This area of hyperexcitable neurons is the site of origin of the adjacent seizures. Surgery that removes the tumor tissue but not the adjacent areas of abnormal cortical tissue may not cure the seizures. For this reason, the surgical treatment of epilepsy caused by presumed glioneuronal tumors often requires the use of a technique called electrocorticography.
Epileptic seizures in patients with brain tumors do not originate in the tumors themselves, but from the area of the brain next to the tumor. Electrocorticography, or ECoG, is currently the most sensitive method for localizing the area in the brain where the epilepsy starts. This technique must be performed by a neurosurgeon, preferably an epilepsy surgeon. A grid or strip of electrodes is placed on the surface of the brain. Occasionally, a depth electrode can also be placed deep into the brain to record from structures that are difficult to reach with grids and strips. Electrical recordings are then made directly from the brain in the operating room. The surgeon can also leave the electrodes in contact with the brain and thereby monitor the patient for several days in the hospital until a few seizures occur. The recordings are interpreted by a neurologist with the knowledge and experience in distinguishing between normal and abnormal cortical activity.
The use of ECoG in the treatment of low-grade tumors that cause epilepsy is controversial. Some surgeons do not feel that ECoG is necessary to treat epilepsy caused by tumors. It has been demonstrated that removal of the tumor without EcoG guidance can cure epilepsy in as many as 60% of patients. There is mounting evidence, however, that the use of ECoG may help the surgeon cure as many as 80-90% of patients. ECoG allows the surgeon to identify the site of onset of the epilepsy, usually adjacent to the tumor, and remove this area as well. These areas of brain that initiate epilepsy can be normal tissue or, as in glioneuronal tumors, may be areas of cortical dysplasia. Extensive 'mapping' of the brain may also be performed in order to ensure that these epilepsy-producing areas of the brain are not important for other functions such as movement, language, or memory.
Tumors Located in the Temporal Lobe
The temporal lobe is a unique part of the brain. Structures called the "hippocampus" and "amygdala" on one side of the temporal lobe, are important for memory and emotion while structures on the other side, called the "neocortex, are important for language. For reasons that are not well-understood, the hippocampus and amygdala have a very high likelihood of producing seizures, even if the tumor is not contained in these structures. In many circumstances, the greatest chance of curing the seizures will require removal of the hippocampus and amygdala in addition to the tumor. This is a controversial idea since the hippocampus and amygdala may appear to be normal. For this reason, epilepsy doctors have tried to identify which patients would be helped the most if the hippocampus and amygdala were removed.
Probably the most important factor in making the decision to remove these tissues is whether the hippocampus is working and how its removal would affect the patient. This decision requires the aid of a neuropsychologist who can perform a number of psychological tests, including a Wada test. The Wada test uses a drug to put the hippocampus 'to sleep' for a few minutes to simulate the effects of the proposed surgery and thereby helps to predict the effect of the surgery on the patient. It appears that if someone has had seizures for many years, has had seizures since a very young age, has a tumor located very close to the hippocampus, or has evidence that the hippocampus is not functioning, the chance of curing seizures with surgery is much greater if the hippocampus and amygdala are removed in addition to the tumor. Microscopic analysis of the tissue removed at surgery under the above-mentioned circumstances often shows that the hippocampus is, in fact, abnormal.
Tumors that cause epilepsy are some of the most benign tumors found in the brain. The majority can be cured with surgery. In addition, epilepsy caused by tumors has the highest rate of cure after surgery, assuming the proper procedure is performed with ECoG, neuropsychological testing, Wada testing and stimulation mapping. In some cases of temporal lobe epilepsy, removal of the hippocampus may be necessary. For this reason, patients with epilepsy who learn that they may have a brain tumor must realize that this is not necessarily bad news. The prognosis is generally excellent.