A Proposed Diagnostic Scheme for People with Epileptic Seizures and with Epilepsy: Report of the ILAE Task Force on Classification and Terminology

Abstract

The International League Against Epilepsy (ILAE) made a major contribution when it established standardized classifications and terminology for epileptic seizures and syndromes. This provided a universal vocabulary that not only facilitated communication among clinicians, but also established a taxonomic foundation for performing quantitative clinical and basic research on epilepsy. Much, however, has changed since the adoption of the currently used Classification of Epileptic Seizures in 1981 (1) and the Classification of Epilepsies and Epileptic Syndromes in 1989 (2). Consequently, the Executive Committee of the ILAE, which took office in July 1997, agreed that review and revision of the current classification system would be a priority for this Executive term. A Task Force on Classification and Terminology was appointed, which divided itself into four working groups concerned with Descriptive Terminology for Ictal Events; Seizures; Syndromes and Diseases; and Impairment. During the course of several meetings and vigorous e-mail discussions, the Task Force agreed that it would not be possible to replace the current international classifications with similar revised and updated classifications that would be universally accepted and meet all the clinical and research needs such a formal organizational system would be expected to provide. Rather, the Task Force is proposing a diagnostic scheme that makes use of standardized terminology and concepts to describe individual patients (Table 1). Within this diagnostic scheme, a variety of approaches to classification are possible, and some are presented here by way of example only. The Task Force views the development of specific classifications as a continuing work in progress. Flexible and dynamic classifications will be revised periodically based not only on rapidly emerging new information, but also on the resolution of problems that will inevitably be identified through use. At this point, the proposal does include several definitive changes in concepts and terminology (Table 2), and classifications are presented as examples of what could be devised in the future. Although each new ILAE classification has represented considerable effort on the part of acknowledged experts from many different countries, they have always met with a certain degree of resistance from the international epileptology community. This is because, in part, a rigid classification shapes the manner in which future generations of clinical and basic neuroscientists think about epilepsy and epileptic phenomena, thereby influencing (perhaps unduly) clinical practice and research. For instance, in the current Classification of Epileptic Seizures, the division of partial seizures into "simple" and "complex" inappropriately created the impression that impairment of consciousness had certain mechanistic implications related to limbic system involvement. Confusion, and at times vociferous objections, resulted in part from the fact that the 1970 International Classification of Epileptic Seizures had used the term "complex partial seizures" synonymously with "temporal lobe seizures"(3). Over the past two decades, detailed investigations of the anatomic substrates of ictal semiology, based largely on work carried out in epilepsy surgery centers, have strongly suggested that fundamental mechanisms of certain limbic seizures are different from those of neocortical seizures, and that both can be associated with impairment of consciousness or not. Consequently, the designation of partial seizures as "simple," or "complex," has in the process lost meaningful precision. Indeed, the 1981 Classification of Epileptic Seizures was purposely based purely on ictal phenomenology and associated EEG findings rather than anatomic substrates and pathophysiologic mechanisms, because insufficient information was available at the time to permit the authors to do otherwise. It is the belief of the Task Force that adequate evidence now exists to permit creation of a list of seizure types that represent diagnostic entities, as opposed to phenomenologic descriptions, based on known or presumed common anatomy and pathophysiology. Such diagnostic entities would, like syndromes, have etiologic, therapeutic, and prognostic implications, and could be used to supplement syndromic diagnoses, or stand alone when syndromic diagnoses cannot be made. The 1981 Classification of Epileptic Seizures also has been criticized because it is not purely semiologic; post hoc etiologic information and EEG data are often required to use it properly, and the dichotomy of "partial" versus "generalized" belies a need to avoid anatomic implications. The Task Force believes that a purely descriptive phenomenologic approach to defining ictal semiology has definite clinical value, and the new diagnostic scheme proposed here includes a modification of a previously proposed classification of ictal phenomenology (4), as an option that can be used in detail where appropriate. Similarly, the previous dichotomous classifications based on concepts of "partial" or "localization related" versus "generalized" abnormalities created the false impression that epileptic seizures, or epilepsy syndromes, were due to either localized disturbances in one hemisphere or disturbances involving the entire brain. A variety of conditions between focal and generalized epileptogenic dysfunctions include diffuse hemispheric abnormalities, multifocal abnormalities, and bilaterally symmetrical localized abnormalities. Although concepts of partial and generalized epileptogenicity have value, perhaps more with respect to ictal events than to syndromes, it is neither appropriate nor useful to attempt to contain all seizures and syndromes within one or the other of these categorizations. The term partial itself has come under criticism because it implies part of a seizure, or part of a syndrome, rather than a seizure or syndrome that begins in part of one hemisphere. For this reason, the 1989 Classification of Epilepsies and Epileptic Syndromes replaced the term partial with "localization-related." This latter terminology has been cumbersome and is not consistently used. The Task Force is now proposing that the terms partial and localization-related be replaced with the older term "focal," which remains in common use. It must be strongly emphasized, however, that the term focal does not mean that the epileptogenic region is a small, well-delineated focus of neuronal pathology; focal seizures, as well as focal syndromes, are almost always due to diffuse, and at times widespread, areas of cerebral dysfunction. Another change in terminology evident in this document is the omission of the words "convulsion" and "convulsive" in the list of epileptic seizure types and epilepsy syndromes. The Task Force thought that these are nonspecific lay terms, and at times improperly used. Consequently it was agreed to be consistent, not only in descriptive ictal terminology, but also in naming epileptic seizure types and syndromes, to avoid these terms. For instance, the Task Force is proposing that the term "febrile convulsions" be replaced by "febrile seizures." There also has been dissatisfaction with the terms "idiopathic" and "cryptogenic." Problems with the former have resulted from misunderstanding of the correct definition of idiopathic, which means a disorder unto itself, sui generis, and not etiology unknown. Problems with the latter have been due to an imprecision in definition; cryptogenic is usually used to designate conditions that are not idiopathic, or are presumed to be symptomatic, when the etiology has not been determined, but it also is used by some for conditions in which it is not known whether they are idiopathic or symptomatic. The Task Force has been unable to find an acceptable alternative to the term idiopathic, which, when used correctly, confers a useful taxonomic concept. The terms "benign" and "genetic" were discarded because not all idiopathic epilepsies are necessarily benign, and not all genetic epileptic conditions (e.g., the progressive myoclonus epilepsies) are idiopathic. Although the term "essential" also is used in medicine to convey the same meaning, the Task Force believes that most epileptologists have now learned to use the term idiopathic correctly, and that there is value in maintaining continuity. Consequently, it is recommended that the terms idiopathic and symptomatic be retained, but that the term cryptogenic, although still acceptable, be replaced by the more precise term "probably symptomatic." Therefore, some epilepsy syndromes are referred to as either idiopathic or symptomatic, but a dichotomous classification system that attempts to categorize all syndromes in this manner has been avoided. Another important criticism of previous rigid syndromic classifications has been a failure to recognize the fact that some syndromes are well accepted, whereas others are controversial, or lack sufficient data. Formally recognizing a syndrome by including it in an official international classification may give it inappropriate legitimacy, whereas failing to recognize a syndrome in the official classification can discourage studies that are necessary to lead to its acceptance. Any official ILAE-sanctioned list of epilepsy syndromes must differentiate between universally accepted syndromes and those in development, and must also be sufficiently flexible to permit additions and deletions of syndromes as new information becomes available. The rapidly moving field of genetics has contributed greatly in recent years to our understanding of many diseases, including some epileptic disorders, but the relationship between genetic disturbances and phenotypic expression remains complicated and poorly understood. Because a single, relatively well defined, idiopathic epilepsy syndrome can be due to more than one genetic abnormality, and different members of a family sharing a

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• University of California, Los Angeles US

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