CLASSIFICATION OF SEIZURES AND EPILEPSIES
The most recent International League Against Epilepsy (ILAE) classification of epileptic seizures and epilepsies (epilepsy syndromes), published in 2010, revises past classifications using terminology and concepts appropriate for the modern era (Berg et al. 2010; Berg and Millichap 2013; Muro and Connolly 2014). Seizures are divided into three categories: generalized, focal (formerly called partial), and epileptic spasms. Focal seizures originate in neuronal networks limited to part of one cerebral hemisphere. Generalized seizures begin in bilateral distributed neuronal networks. A seizure can begin focally and later generalize. Seizures can originate in the cortex or in subcortical structures. Using a detailed history, EEG findings, and ancillary information, a physician can often categorize the seizure/epilepsy type, after which an appropriate diagnostic evaluation and treatment plan is formulated.
The main subtypes of generalized seizures are absence, generalized tonic-clonic (GTC), myoclonic, and atonic (Table 1). Absence seizures (formerly called petit mal) involve staring with unresponsiveness to external verbal stimuli, sometimes with eye blinking or head nodding. GTC seizures (formerly called grand mal) consist of bilateral symmetric convulsive movements (stiffening followed by jerking) of all limbs with impairment of consciousness. Myoclonic seizures consist of sudden, brief (“lightning-fast”) movements that are not associated with any obvious disturbance of consciousness. These brief involuntary muscle contractions may affect one or several muscles; therefore, myoclonic seizures can be generalized or focal. Atonic seizures involve the loss of body tone, often resulting in a head drop or fall.
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The clinical manifestations of a focal seizure depend on the area of cortex involved. For example, a focal seizure arising from the occipital lobe may present with visual phenomena; from the precentral gyrus, with rhythmic clonic or tonic motor activity; and from the postcentral gyrus, with sensory symptoms, such as paresthesias. When consciousness is impaired during a focal seizure, that is, the patient is unable to respond normally to verbal or tactile stimuli, the seizure is classified as dyscognitive (formerly called complex partial); seizures arising from the temporal lobe are often dyscognitive. Some seizures are preceded by an aura, which is a focal seizure wherein a patient retains awareness and describes motor, sensory, autonomic, or psychic symptoms. An aura precedes a focal dyscognitive or generalized seizure by seconds or minutes and is most often experienced by patients with temporal lobe epilepsy.
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The origin of the third category of seizure type, epileptic spasms, is uncertain. Epileptic spasms are manifest by sudden extension or flexion of extremities, held for several seconds, and then recur in clusters. Epileptic spasms can occur at any age; when they begin in the first year of life, they comprise a syndrome called infantile spasms (IS) (West syndrome [WS]; see below).
Epilepsies (epilepsy syndromes) (Table 2) were previously classified according to their onset site (generalized or related to a specific cortical localization) and etiology, that is, whether the cause was known (symptomatic) or not known (idiopathic). Here, we use the 2010 revised guideline for classification of seizures and epilepsy (Berg et al. 2010). The updated system takes into account expanding knowledge of structural and genetic causes, and includes the ictal semiology (seizure type), syndrome diagnosis (if present), and degree of functional impairment. New classification schemes will continue to evolve as knowledge about epilepsy pathophysiology, and genetics emerges.
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