Seizures have been grouped in several ways: according to their presumed etiology, that is, idiopathic (primary) or symptomatic (secondary); their site of origin; their clinical form (generalized or focal); their frequency (isolated, cyclic, or repetitive, or the closely spaced sequence of status epilepticus); or by special electroencephalographic (EEG) patterns. A distinction must be made between the classification of seizures: generalized tonic clonic (grand mal), absence (petit mal), myoclonic, partial, and others, and the classification of the epilepsies, or epileptic syndromes, which are specific diseases, many of which may manifest several seizure types. These are discussed later in the chapter. A further distinction is made by clinical and EEG features. This approach allows for the reasonable predictability of response to specific medications and to some extent, in prognosis.
Basically, this classification divides seizures into two types—focal (formerly termed partial), in which a focal or localized onset can be discerned clinically or by EEG, or generalized, in which the seizures appear to begin bilaterally. Generalized seizures are of two types—convulsive and nonconvulsive. The common convulsive type is the tonic-clonic (grand mal) seizure. Less common is a purely tonic, purely clonic, atonic, and myoclonic. The typical nonconvulsive generalized seizure is the brief lapse of consciousness or absence (petit mal); included also under this heading are minor motor phenomena. Focal seizures can also be convulsive or uncommonly, nonconvulsive. Furthermore, focal seizures can secondarily generalize into any of the types listed above.
The classification followed here was first proposed by Gastaut et al in 1970 and has been refined repeatedly by the Commission on Classification and Terminology of the International League against Epilepsy. This nomenclature, based mainly on the clinical form of the seizure, and its EEG features, has been adopted worldwide and is generally referred to as the “International Classification.” A modified version of it is reproduced in Table 15-1.
Focal seizures are further classified according to their additional features such as a specific subjective experience (aura), motor, autonomic, and most importantly, whether awareness or consciousness is disturbed; the latter was formerly called partial complex seizure, now termed focal seizures with dyscognitive features. In reality, an aura represents the initial phase of a focal seizure; in some instances, it may constitute the entire epileptic attack.
A complementary classification is provided by considering the epilepsy syndromes, a group of somewhat diverse, age-dependent and usually genetically determined entities that arise without underlying structural abnormalities. The syndromes are characterized the age of onset, type of seizure, and often, by a particular EEG pattern. By contrast, epilepsies manifesting as seizures that begin locally and may evolve into generalized tonic-clonic seizures, termed secondarily generalized tonic clonic seizures (termed bilateral convulsive in Fig. 15-1), generally have no such genetic component and are usually the result of underlying brain disease, either acquired or a result of congenital malformations or metabolic defects. Quite often, the initial focal phase is not appreciated, leading to misdiagnosis. An increasing frequency and severity of this group of disorders with age reflects the accumulation of focal cerebral damage from trauma, strokes, and other damage.
We begin our discussion with a practical approximation of the classification shown in Fig. 15-1, followed by consideration of a number of well-defined epilepsies and epileptic syndromes.
A proposed current classification of epileptic syndromes based on the age of onset of the seizure disorder is shown in Fig. 15-2, and the distribution of the seizure types for each age epoch, obtained and aggregated from several sources is shown in Fig. 15-3. There has been substantial progress in defining the molecular basis of familial and hereditary epilepsies over the last decade; it is likely that these insights will lead to modification of both the clinical classifications and management of the epilepsies.