Seizure types

In the field of neurology, seizure types are categories of seizures defined by seizure behavior, symptoms, and diagnostic tests. The International League Against Epilepsy (ILAE) 2017 classification of seizures is the internationally recognized standard for identifying seizure types. The ILAE 2017 classification of seizures is a revision of the prior ILAE 1981 classification of seizures. Distinguishing between seizure types is important since different types of seizures may have different causes, outcomes, and treatments.

History
In ~2500 B.C., the Sumerians provided the first writings about seizures. Later in ~1050 B.C., the Babylonian scholars developed the first seizure classification, inscribing their medical knowledge in the stone tablets called Sakikku or in English "All Diseases." This early classification identified febrile seizures, absence seizures, generalized tonic-clonic seizures, focal seizures, impaired awareness seizures, and status epilepticus. Samuel-Auguste Tissot (1728–1797) authored Traité de l’Epilepsie, a book describing grand état (generalized tonic-clonic seizures) and petit état (absence seizures). Jean-Étienne Dominique Esquirol (1772–1840) later introduced grand mal (generalized tonic-clonic seizures) and petit mal to describe these seizures. In 1937, Gibbs and Lennox introduced psychomotor seizures, seizures with "mental, emotional, motor, and autonomic phenomena." Henri Gastaut led the effort to develop the ILAE 1969 classification of seizures based on clinical seizure type, electroencephalogram (EEG), anatomical substrate, etiology, and age of onset. The ILAE 1981 classification of seizure included information from EEG-video seizure recordings, but excluded anatomical substrate, etiology, and age factors as these factors were "historical or speculative" rather than directly observed. The ILAE 2017 classification of seizures closely reflects clinical practice, using observed seizure behavior and additional data to identify seizure types.

Focal vs. generalized seizure onset
A seizure is a paroxysmal episode of symptoms or altered behavior arising from abnormal excessive or synchronous brain neuronal activity. A focal onset seizure arises from a biological neural network within one cerebral hemisphere, while a generalized onset seizure arises from within the cerebral hemispheres rapidly involving both hemispheres. Seizure symptoms, seizure behavior, neuroimaging, seizure etiology, EEG, and video recordings help distinguish focal from generalized onset seizures. Unknown onset seizures occur if the available information is insufficient to distinguish focal from generalized onset seizures with a $\ge 80%$ confidence. Focal to bilateral tonic-clonic seizure indicates that the seizure begins as focal seizure then later evolves to a bilateral tonic-clonic seizure.

Aware vs. impaired awareness
The classification distinguishes focal aware seizures from focal impaired awareness seizures. Aware means aware of self and surroundings during the seizure, verified when a person can recall events having occurred during the seizure. Impaired awareness occurs even if the recall of events is only partially impaired. Impaired awareness may occur at any time during the seizure. If the level of awareness cannot be determined, the level of awareness is unspecified; this usually occurs for atonic seizures and epileptic spasm seizures.

Motor seizures
A motor seizure has prominent movement, increased muscle contraction, or decreased muscle contraction as the initial predominant seizure feature. Atonic seizures are a brief 0.5-2 second lapses in muscle tone commonly leading to a fall. Epileptic spasm seizures are brief 1-2 second proximal limb and truncal flexion or extension movements, often repeated. Hyperkinetic seizures occur as high amplitude truncal and limb movements such as pedaling, thrashing, and rocking movements. Myoclonic seizures are brief jerks of limbs or body lasting milliseconds. Tonic seizures are abrupt increases in muscle tone greater than 2 seconds in duration. Clonic seizures occur as rhythmic body jerks. Myoclonic-atonic seizures begins with one or more jerks (myoclonic phase) followed by a loss of muscle tone (atonic phase). Myoclonic-tonic-clonic seizures begin with one or more jerks (myoclonic phase), then body stiffening (tonic-phase), then rhythmic jerks (clonic phase). Tonic-clonic seizures begin as symmetrical bilateral body stiffening (tonic phase) followed by rhythmic jerks (clonic phase). Myoclonic, atonic, tonic, and myoclonic-atonic seizures may cause abrupt falls, called drop attacks, similar to cataplexy. Automatism seizures occur with repetitive stereotyped behaviors.

Non-motor seizures
A non-motor seizure may begin with a sensory, cognitive, autonomic, or emotional symptom, behavioral arrest of activity, or impaired awareness with minor motor activity as the initial predominant seizure feature.

Sensory seizures occur with somatosensory, olfactory, visual, gustatory, vestibular, or thermal sensations. Cognitive seizures occur with language impairment (e.g. aphasia, dysphasia, anomia), memory impairments (deja vu, jamais vu), hallucinations, persistent thought (forced thinking), and neglect. Autonomic seizures occur with palpitations, heart rate changes, nausea, vomiting, piloerection, lacrimation, pupil size changes or urge to urinate or defecate. Emotional seizures occur with fear, anxiety, laughing, crying, pleasure, or anger sensations. These initial symptoms are seizure auras. Behavioral arrest seizures occur as an abrupt cessation of movement.

Absence seizures occur with a sudden brief impairment in awareness, commonly less than 45 seconds. Typical absence seizures may be accompanied by rhythmic facial 3 per second facial movements. Atypical absence seizures occur with a less sudden impairment in awareness, often accompanied by a gradual head, limb, or truncal slumping. Myoclonic absence seizures occur with myoclonic jerks of arms and shoulders. Absence with eyelid myoclonia seizures occur with 4-6 per second eyelid myoclonic jerks and upward eye movement.

Descriptors
Descriptors are additional seizure behaviors or symptoms that are appended to the seizure diagnosis. Descriptors may be a non-predominant or non-initial seizure feature. Descriptors provide a more complete description of the seizure.

ILAE 2017 classification of seizure types
1 - Classify focal seizures as focal aware, focal impaired awareness, or focal unspecified awareness.

Focal onset seizure
During the typical 1 minute seizure, a person experiences a familiar (déjà vu) sensation, followed by picking and fumbling hand movements. After this seizure, the person cannot recall what was said during the seizure. Brain magnetic resonance imaging (MRI) shows left hippocampal sclerosis, a brain abnormality associated with focal seizures. This is a focal impaired awareness cognitive seizure with déjà vu. Appending a descriptor, this is a focal impaired awareness cognitive seizure with déjà vu followed by hand automatisms.

Generalized onset seizure
During the typical 10 second seizure, a child abruptly stops and stares with 3 per second rhythmic eye fluttering movements. After the seizure, the child cannot recall what occurred during the seizure. An EEG test shows 3 per second spike-wave pattern, an EEG pattern indicating a generalized onset seizure. This generalized onset non-motor (absence) seizure is a typical absence seizure. Appending a descriptor, this is a typical absence seizure with 3 per second eye fluttering movements.

Comparison of ILAE 2017 and ILAE 1981 classifications
Dyscognitive, simple partial, complex partial, psychic, and secondarily generalized are terms that apply only to the ILAE 1981 classification of seizures. The ILAE 2017 classification relies on intact awareness of self and surroundings, but the ILAE 1981 classification relies on intact consciousness, defined as a normal response to an external stimulus due to intact awareness and intact ability to respond. Unlike the ILAE 2017 classification, the ILAE 1981 classification specifies specific EEG patterns for each seizure type.

ILAE 1981 classification of seizure types
The associated EEG patterns are not included.

Partial seizures.
 * Simple partial seizures: consciousness is not impaired.
 * With motor signs
 * With somatosensory or special-sensory symptoms
 * With autonomic symptoms or signs
 * With psychic symptoms
 * Complex partial seizures: consciousness is impaired.
 * Simple partial onset, followed by impairment of consciousness
 * With impairment of consciousness at onset
 * Partial seizures evolving to secondarily generalized seizures
 * Simple partial seizures evolving to generalized seizures
 * Complex partial seizures evolving to generalized seizures
 * Simple partial seizures evolving to complex partial seizures evolving to generalized seizures

Generalized seizures.
 * Absence seizures
 * Absence seizures
 * Atypical absence seizures
 * Myoclonic seizures
 * Clonic seizures
 * Tonic seizures
 * Tonic-clonic seizures
 * Atonic seizures

Unclassified epileptic seizures

Continuous and subclinical seizures
Status epilepticus is a seizure "lasting longer than 30 minutes or a series of seizures without return to the baseline level of alertness between seizures."

Epilepsia partialis continua is a rare type of focal motor seizure, commonly involving the hands or face, which recurs with intervals of seconds or minutes, lasting for extended periods of days or years. Common causes are strokes in adults, and focal cortical inflammation in children: Rasmussen's encephalitis, chronic viral infections, or autoimmune encephalitis.

Subclinical seizures cause no symptoms and either no altered behavior or very minimal behavioral changes; the clinician recognizes these seizures as an evolving seizure pattern on an EEG recording.