DEFINITIONS AND EPIDEMIOLOGY

Any epidemiological approach requires solid definitions of the events and conditions that are under study. Consensual agreement on even the definition of the terms seizure and epilepsy has proved difficult.

An epileptic seizure can be defined as a transient occurrence of signs and/or symptoms due to abnormal or excessive synchronous brain activity.

Epilepsy is a family of disorders of the brain characterized by an enduring predisposition to epileptic seizures and by the neurobiological, cognitive, psychological, and social consequences of this condition.

Epileptic seizures are pleomorphic, although usually stereotyped for a given individual. Unlike most neurological disorders, the majority of patients with epileptic seizures do not have permanent physical signs and can be diagnosed only by taking a history or by the chance observation of a seizure. Diagnosis is a discretionary judgment that may vary depending on the skill and experience of clinicians and the quality of available information from witnesses. Electroencephalographic and other complementary investigations are useful in classifying epilepsy but are of limited help in making the diagnosis. Some patients with seizures may never seek medical attention because they ignore or misinterpret their symptoms, or indeed they may be unaware of them. In practice, both false-positive and false-negative diagnoses are common.

The definitions of epilepsy have often included the notion of unprovoked seizures and of recurrence. Epileptic seizures may occur as a result of a variety of acute brain insults or metabolic disorders. Those seizures triggered by clear precipitants are termed “acute symptomatic seizures” but, despite exhibiting clear epileptiform phenomenology, they are not classified as epilepsy. The distinction between provoked and unprovoked seizures, however, is not always clear-cut or reliable for epilepsy diagnosis. Indeed, precipitating factors such as lack of sleep or alcohol may facilitate seizures in well-established epileptic diseases.

The notion of recurrence, which theoretically implies at least two seizures, is not a necessary characteristic for epilepsy diagnosis. The occurrence of one seizure may be sufficient when clinical or paraclinical elements suggest that there is an enduring alteration in the brain that increases the likelihood of future seizures. More than recurrence, it is the potential for recurrence of seizures that defines epilepsy. All of these distinctions and evolving concepts increase the difficulty of diagnostic accuracy and case ascertainment necessary for the accurate study of epilepsy epidemiology.

The overall annual incidence of epilepsy is generally believed to be around 50 per 100 000 (range, 40 to 70 per 100 000/year) in industrialized countries, but socioeconomically deprived people are at greater risk. First seizures are estimated to be around 70 per 100 000 annually. There is a mild predominance in males. The specific incidence as a function of age shows a bimodal curve, with two peaks, one in childhood, with figures of more than 100 per 100,000 during the first year of life, although there is evidence of a decrease in childhood incidence in recent decades. A second peak is observed in elderly people with an estimate of 150 per 100,000 over the age of 80 years, which is attributed to the many prevalent causes of brain damage at that age, with cardiovascular diseases being the most frequent (Fig. 50-1).

Figure 50-1 Epidemiology of epilepsy in terms of incidence and lifetime cumulative risk.

In terms of prevalence, several studies conducted in different regions of the world have suggested that epilepsy affects 5 to 8 per 1000 population. These estimates are based on individuals with active epilepsy, defined as patients with a seizure in the previous 5 years or still receiving antiepileptic drug treatment.

When isolated seizures are included, the cumulative risk or incidence, which measures the lifetime risk of having a nonfebrile seizure, is very high, ranging from 2% to 8% between 40 and 80 years of age.

From the difference between incidence and prevalence of active epilepsy, it is apparent that most patients with epilepsy cease to have seizures or die. It is also likely that the condition remits in many patients. However, epilepsy is associated with increased mortality, particularly but not exclusively in patients with brain lesions.

CLINICAL APPROACH TO EPILEPTIC DISORDERS

The clinical approach should consist of a description of the ictal semiology, using as far as possible a standardized terminology. A description of the ictal event(s) should be carefully obtained from the patient and relatives without reference to etiology, anatomy, or mechanisms. Detailed descriptions of the onset and evolution of focal ictal phenomena are not always necessary to make a diagnosis of partial seizure, but they are useful in patients who are candidates for surgical treatment or for research designed to elucidate the anatomical substrates or pathophysiological mechanisms underlying specific clinical behaviors. Electroencephalographic video monitoring, including direct interaction with the patient during an event, is the investigation that permits the most accurate and detailed analysis of symptoms.

The next step is to characterize the epileptic seizure type relative to the list of different types defined by the International Classification of Seizures (Table 50-1). It is essential to identify or rule out seizure types by detailed questioning of patients and relatives. Localization within the brain should be specified when this is appropriate; in the case of reflex or provoked seizures, the specific stimulus should also be specified. Electroencephalographic data are often incorporated at this stage of the diagnostic process, as many seizures are also associated with specific electroclinical characteristics. Age at onset is also a critical piece of information. Certain specific seizure types may by themselves be more or less indicative of diagnostic entities with etiological, therapeutic, and/or prognostic implications.

TABLE 50-1 International Classification of Seizures (1981)

A syndromic diagnosis should then be made whenever possible, again using the common terms proscribed by the International Classification of Epilepsies and Epileptic Syndromes. This syndromic diagnosis relies on the grouping of information: on seizure type or types, pattern of recurrence or frequency, age at onset, personal and familial antecedents, natural history, and other neurological and extraneurological manifestations. The electroencephalographic data, both interictal and sometimes ictal, are essential, as is a brain imaging examination. Magnetic resonance imaging is indicated in the majority of situations but can be omitted when certitude is reached about the diagnosis of an idiopathic generalized syndrome.

Clinical and paraclinical investigations can also help to identify specific etiologies. The etiology can consist of a specific disease frequently associated with epileptic seizures or syndromes, a genetic defect, or a specific pathological substrate, for instance, for the symptomatic focal epilepsies.

Finally, it is essential to evaluate the degree of impairment caused by the epileptic condition and to try to establish a prognosis in parallel to making decisions about the therapeutic measures to be exhibited. These steps are strongly influenced by analysis of seizure type and the syndromic diagnosis.

EPILEPTIC SEIZURES

Three great categories have been identified—generalized, partial or focal seizures, and unclassified seizures. Here they are described as individual events, with a well-defined temporal course from onset to termination. Almost every type of seizure can occur continuously or repetitively, thus constituting status epilepticus. In this case, some of the seizure characteristics may become less readily identifiable.