neurologic disorders
Seizure Disorders
Classification of Seizure Disorders
In 2017, the International League Against Epilepsy (ILAE) developed a new classification system for seizures ( Classification reference ). Initial classification is by type of onset: Generalized onset Focal onset Unknown onset Focal-onset seizures are then classified by level of awareness (knowledge of self and environment). Level of awareness is not used to classify generalized-onset seizures because most of these seizures (but not all) impair awareness. All seizures are then classified, if possible, as Motor onset Nonmotor onset Responsiveness is not used to classify seizures, but it can be useful as a descriptor. Responsiveness can be intact or impaired whether awareness is impaired or not. In generalized-onset seizures, seizures originate in networks in both hemispheres. Awareness is usually impaired, and consciousness is usually lost. Generalized-onset seizures are classified as motor and nonmotor (absence) seizures. (However, nonmotor seizures may involve motor activity.) In generalized-onset motor seizures, motor activity is usually bilateral from the onset. When bilateral onset of motor activity is asymmetric, determining whether onset is focal or generalized may be difficult. Generalized-onset motor seizuresmay be further classified by type of seizure: Tonic-clonic seizures (formerly, grand mal seizures) Clonic seizures (sustained rhythmic jerking) Tonic seizures (generalized stiffening involving all limbs and without rhythmic jerking) Atonic seizures (loss of muscle tone) Myoclonic seizures (rhythmic jerking not preceded by stiffening) Myoclonic-tonic-clonic seizures (myoclonic jerking followed by tonic and clonic movements) Myoclonic-atonic seizures (myoclonic jerking followed by atonia) Infantile Spasms (formerly, infantile spasms) Generalized-onset nonmotor seizuresmay be further classified by type of seizure (defined by the earliest prominent feature): Typical absence seizures Atypical absence seizures (eg, with less abrupt onset or termination or with abnormal changes in tone) Myoclonic seizures (myoclonic jerking preceded by brief impairment of consciousness [absence]) Eyelid myoclonia All absence seizures are generalized-onset seizures. The following can help distinguish absence seizures from focal impaired-awareness seizures, although the distinctions are not absolute: Absence seizures tend to occur in younger people. They tend to start and end more suddenly. Usually, automatisms are less complex in absence seizures than in focal impaired-awareness seizures. Generalized-onset seizures result most often from metabolic disorders and sometimes from genetic disorders. Focal-onset seizures originate in networks in one hemisphere and may originate in subcortical structures. They may be discretely localized or more widely distributed. Focal-onset seizures may be classified by level of awareness: Focal aware seizures (formerly, simple partial seizures) Focal impaired-awareness seizures (formerly, complex partial seizures) If awareness is impaired during any part of the seizure, the seizure is classified as a focal impaired-awareness seizure. Focal-onset motor seizures may be further classified by type of seizure: Automatisms (coordinated, purposeless, repetitive motor activity) Atonic (focal loss of muscle tone) Clonic (focal rhythmic jerking) Epileptic spasms (focal flexion or extension of arms and flexion of trunk) Hyperkinetic (causing pedaling or thrashing) Myoclonic (irregular, brief focal jerking) Tonic (sustained focal stiffening of one limb or one side of the body) Awareness level is usually not specified for atonic seizures or epileptic spasms. In focal-onset tonic seizures, stiffening involves only one limb or one side of the body, usually with no loss of consciousness. electroencephalography (EEG) may show contralateral focal epileptiform abnormalities. In contrast, in generalized-onset tonic seizures, stiffening involves all limbs, with or without loss of consciousness, and EEG may show bilateral epileptiform abnormalities. Focal-onset nonmotor seizures may be further classified based on the earliest prominent feature: Autonomic dysfunction (autonomic effects such as gastrointestinal (GI) sensations, a sense of heat or cold, flushing, sexual arousal, piloerection, and palpitations) Behavior arrest (cessation of movement and unresponsiveness as the main feature of the entire seizure) Cognitive dysfunction (impairment of language or other cognitive domains or positive features such as déjà vu, hallucinations, illusions, or perceptual distortions) Emotional dysfunction (manifesting with emotional changes, such as anxiety, fear, joy, other emotions, or affective signs without subjective emotions) Sensory dysfunction (causing somatosensory, olfactory, visual, auditory, gustatory, or vestibular sensations or a sense of heat or cold) Focal-onset seizures may evolve into a generalized-onset tonic-clonic seizure (called a focal-to-bilateral tonic-clonic seizure; formerly, secondary generalization), which causes loss of consciousness. Focal-to-bilateral tonic-clonic seizures occur when a focal-onset seizure spreads and activates the entire cerebrum bilaterally. Activation may occur so rapidly that the initial focal-onset seizure is not clinically apparent or is very brief. Seizures are usually classified as unknown-onset seizures when information about onset is lacking. If clinicians acquire more information about the seizures, these seizures may be reclassified as focal-onset or generalized-onset. Seizures of unknown onset can be motor or nonmotor. Unknown-onset motor seizures may be further classified as Tonic-clonic Epileptic spasms Unknown-onset nonmotor seizures may be further classified as Behavior arrest Tonic-clonic seizures with an obscure onset are often classified as unknown-onset seizures. Seizures that are later identified as epileptic spasms or behavior-arrest seizures may initially be classified as unknown-onset seizures. Detailed video EEG monitoring can help clarify whether onset is focal or generalized; doing so is important because if onset is focal, the cause may be treatable. 1.Fisher RS, Cross JH, D'Souza C, et al: Instruction manual for the ILAE [International League Against Epilepsy] 2017 operational classification of seizure types.Epilepsia58 (4):531–542, 2017. doi: 10.1111/epi.13671 