Surgical Treatment of Seizures
(c) G. Rees Cosgrove, MD, F.R.C.S.(C) and Andrew J. Cole MD, FRCP(C) Departments of Neurology and Neurosurgery, Massachusetts General Hospital Epilepsy Center Harvard Medical School, Boston, Massachusetts.
In the majority of patients with epilepsy, seizures can be well controlled with appropriate medication. However, current estimates indicate that 20 – 30% of patients with epilepsy are refractory to all forms of medical therapy. (1) These medically intractable patients are candidates for surgical treatment in an attempt to achieve better seizure control. Another group of patients who might benefit are those whose seizures may be relatively well controlled but who have certain characteristic presentations or lesions that strongly suggest surgical intervention might be curative. Overall, the single most important determinant of a successful surgical outcome is patient selection. This requires detailed presurgical evaluation to characterize seizure type, frequency, site of onset, psychosocial functioning and degree of disability in order to select the most appropriate treatment from a variety of surgical options. This type of evaluation is best carried out at a multi-disciplinary center experienced in the investigation and treatment of epilepsy.
In this article, we will outline the essential elements of the presurgical evaluation as well as the diagnostic surgical procedures required for invasive EEG monitoring and then describe the variety of therapeutic surgical options including indications, techniques, results and complications of each procedure.
There are many types of seizures and different forms of epilepsy. A seizure is defined as a paroxysmal, self-limited change in behavior associated with excessive electrical discharge from the central nervous system. Epilepsy is defined as a condition of recurrent seizures and medical intractability as recurrent seizures despite optimal treatment under the direction of an experienced neurologist over a two to three year period.
In the past, seizures have been classified based upon their clinical manifestations, which had some relevance for patients and physicians but was of limited diagnostic or prognostic value. This classification scheme is based entirely on the distinct behavioral and electrophysiologic features of the seizures themselves and purposely avoids implications as to the specific underlying pathophysiological mechanisms or substrates. (2) According to this classification, an epileptic disorder is defined as either being generalized, partial (focal) or undetermined. Primary generalized seizures start as a disturbance in both hemispheres synchronously without evidence of a localized onset. The manifestations of these seizures tend to be major motor seizures of a tonic, clonic, tonic-clonic, myoclonic or atonic type. They also include minor events of the petit mal (absence) type. Partial forms of epilepsy start in a focal area of the brain and may remain localized without alteration of consciousness. These events are referred to as simple partial seizures and the symptoms vary with the area of the brain affected. If the event spreads and alters consciousness it is referred to as a complex partial seizure. If the event spreads further and leads to a major motor seizure it is referred to as a secondarily generalized seizure and may be quite difficult to distinguish from the primary generalized forms. Partial seizures often arise from the limbic structures of the temporal and frontal lobes but can occur from any cortical region and are often quite refractory to medical therapy alone. In general, patients with partial seizure disorders are the most amenable to surgical intervention.
A second classification divides the clinical epilepsies into idiopathic, symptomatic and syndromic forms based upon their presumed etiologies. (3) Idiopathic forms include some of the generalized seizure types that may have familial patterns but without a prominent genetic component such as simple febrile convulsions of childhood. Symptomatic forms are those caused by a recognized central nervous system (CNS) lesion. Included in this type are cases of known structural pathology, metabolic abnormalities or neurodegenerative processes. Syndromic forms include disorders that may be idiopathic or symptomatic but seem to follow a clear and predictable course. These include childhood and juvenile absence epilepsies, juvenile myoclonic epilepsy and the Lennox-Gastaut Syndrome. The ability to place a patient in one or another of the syndromic categories has the advantage of providing a basis for predicting long-term prognosis.
Both classifications have certain advantages and are not mutually exclusive but can be combined to provide helpful information. From a surgical point of view however, dividing the seizure types into either generalized or focal appears to be the most useful. This is because most surgical decisions are based upon defining those seizures that originate in one focal area of the brain and localizing that area as a prelude to resection.
The goal of epilepsy surgery is to identify an abnormal area of cortex from which the seizures originate and remove it without causing any significant functional impairment. The primary components of the presurgical evaluation include a detailed clinical history and physical examination, advanced neuro-imaging, video-EEG monitoring, neuropsychological testing and assessment of psychosocial functioning. (Table 1) The major surgical questions one hopes to answer with this evaluation are: 1) are the seizures focal or generalized? 2) if focal, are they temporal or extratemporal in origin ? 3) is there a lesion associated with the seizures ?; and 4) if surgery is undertaken what functional deficits, if any, might be anticipated?
Psychosocial evaluation is also extremely important to assess current level of functioning and to ensure realistic goals and attitudes are engendered in both the patient and their family prior to surgery.
Therapeutic Surgical Options
Epilepsy surgery began as removal of gross structural lesions of the brain. With the addition of EEG data from preoperative and intraoperative recordings, areas of removal expanded to include tissue that was grossly normal in appearance but known to give rise to epileptiform activity. Small areas of resection were soon replaced by partial lobectomies and more extensive cortical resection. While resection techniques (lesionectomy, lobectomy, hemispherectomy, corticectomy) generally yield the best surgical results, disconnection (callosotomy, subpial transection) and augmentation (cerebellar and vagal stimulation) techniques remain worthwhile considerations (Table 2).
The success or failure of the surgical treatment of epilepsy depends in large part on the proper selection and investigation of patients. Recent advances in imaging and long term EEG monitoring have allowed for a greater accuracy in the localization of the seizure focus with overall surgical results better than those of the prior decades. Continued investigation into the basic mechanisms of the epilepsies as well new forms of medical and surgical therapy is necessary in order to help the many patients with severe and disabling intractable seizures.
1. Robb, P: Focal epilepsy: the problem, prevalence, and contributing factors. In Advances in Neurology, eds. D. Purpura, J. Penry, R.D. Walter, pp. 11-22. New York: Raven Press, 1975.
2. Commission on Classification and Terminology of the International League Against Epilepsy: Proposal for Revised Clinical and Electro-graphic Classification of Epileptic Seizures. Epilepsia 26:258-268, 1985.
3. Commission on Classification and Terminology of the International League Against Epilepsy: Proposal for Classification of Epilepsies and Epileptic Syndromes. Epilepsia 26:268-278, 1985.
4. Lessor R, Modic M, Weinstein M, et al. MRI in patients with intractable epilepsy, Arch Neurol 43:367,371,1986.
5. Engel, J., Kuhl, N., Phelps, M., Crandall, P. Comparative localization of epileptic foci in partial epilepsy by PET and EEG. Ann Neurol 12:529-37, 1982.
6. Lee, B., Marklan, O., Siddiqui, A., Park, H., Mack, B., et al. Single photon emission computed tomography (SPECT) brain imaging, intractable complex partial seizures. Neurology 36:1471-77, 1986.
7. Marks DA, Katz A Hoffer P, et al. Localization of extratemporal epileptic foci during ictal single photon emission computed tomography. Ann Neurol 31:250-255,1992.
8. Gotman, J., Ives, J., Gloor, P., eds. Long-Term Monitoring in Epilepsy (Suppl. 37 to Electroencephalography and Clinical Neurophysiology). Amsterdam: Elsevier, 1985.
9. Wada J, Rasmussen T: Intracarotid injection of sodium amobarbital for the lateralization of speech dominance; experimental and clinical observations. J Neurosurg 17:226-282.1960.
10. Barnett GH, Burgess RC, Awad IA, et al: Epidural peg electrodes for the presurgical evaluation of intractable epilepsy. Neurosurgery 27:113-115,1990.
11. Wyler, A.R., Ojemann, G.A., Lettich, E., Ward, A.A: Subdural strip electrodes for localizing epileptogenic foci. J. Neurosurg. 60:1195-1200,1984.
12. Van Buren JM: Complications of surgical procedures in the diagnosis and treatment of epilepsy. In: Engel J Jr, (ed), Surgical Treatment of the Epilepsies. New York: Raven Press, 1987:465-475.
13. Spencer, S. Depth electroencephalography in selection of refractory epilepsy for surgery. Ann. Neurol. 9:207-14, 1981.
14. Ojemann, G.A. Surgical therapy for medically intractable epilepsy. J. Neurosurg. 66-489-909, 1987.
15. Lessor RP, Luders H, Klem G, et al: Extraoperative cortical functional localization in patients with epilepsy. J Clin Neurophysiol 4:27-53,1987.
16. Dinner DS, Luders H, Lessor RP, et al: Invasive methods of somatosensory evoked potential monitoring. J Clin Neurophysiol 3:113-130,1986.
17. Cosgrove GR, Buchbinder BR, Jiang H: Functional magnetic resonance imaging for intracranial navigation in Neurosurgical Clinics of North America. Maciunas R (ed), WB Saunders and Co:Philadelphia, 1995.
18. Spencer, D., Spencer, S., Mattson, R., Williamson, P. Intracerebral masses in patients with intractable partial epilepsy. Neurology 34(4):432-36, 1984.
19. Rasmussen, T. Surgical treatment of complex partial seizures: results, lessons and problems. Epilepsia Suppl. 1 24:65S-76S, 1983.
20. Spencer DD, Spencer SS, Mattson RH, et al: Access to the posterior medial temporal lobe structure in surgical treatment of temporal lobe epilepsy. Neurosurgery 15:667-671,1984.
21. Weiser, H.G., Yasargil, M.G. Selective amygdalohippocampectomy as a surgical treatment of mesiobasal limbic epilepsy. Surg. Neurol. 17:455-57, 1982.
22. Rasmussen, T. Cortical resection in the treatment of focal epilepsy. In Neurosurgical Management of the Epilepsies. Adv. in Neurol. 8:139-54, 1975.
23. Rasmussen, T. Surgery of frontal lobe epilepsy. In Neurosurgical Management of the Epilepsies. Adv. in Neurol. 8:197-205, 1975.
24. Lieb, J.P., Rausch, R., Engel, J., Brown, W.J., Crandall, P.H. Changes in intelligence following temporal lobectomy: relationship to EEG activity, seizure relief and pathology. Epilepsia 23:1-13, 1982.
25. Rasmussen, T. Hemispherectomy for seizures revisited. Can. J. Neurol. Sci. 10:71-78, 1983.
26. Gates, J.R., Rosenfeld, W.E., Maxwell, R.E., Lyons, R.E. Response of multiple seizure types to corpus callosum section. Epilepsia 28:28-34, 1975.
27. Morrel F, Whisler WW, Bleck TP: Multiple subpial transections: a new approach to the surgical treatment of epilepsy. J Neurosurg 70:231-239.1989.
28. Ojemann, G.A., Ward, A.A. Stereotaxic and other procedures for epilepsy. In Neurosurgical Management of Epilepsy, Adv. in Neurol. 8:241-65, 1975.
29. Cooper, I.S., Amin, I., Riklan, M., Waltz, J.M., Poor, T.P. Chronic cerebellar stimulation in epilepsy. Arch. Neurol. 33:559-70, 1976.
30. Ramsey RE, Uthman B, Ben-Menachem E, et al: Efficacy of vagal nerve stimulation in partial seizures: double blind comparison of two stimulus paradigms. Epilepsia (suppl) 32:90-91,1991.