Seizure and epilepsy classification

An epileptic seizure is currently defined as a transient occurrence of symptoms and/or signs due to abnormal excessive or synchronous neuronal activity in the brain.^[1] Seizures can be classified based on outward signs/symptoms and EEG features. Having a seizure by itself does not confer a diagnosis of epilepsy. Epilepsy is defined as having any one of the following conditions:^[2]

• At least two unprovoked seizures occurring greater than 24 hours apart
• One unprovoked (or reflex) seizure and a probability of further seizures similar to the general recurrence risk (at least 60%) after two unprovoked seizures, occurring over the next 10 years.
• Diagnosis of an epilepsy syndrome

Non-epileptic seizures^[3] can mimic seizures but do not share the same pathophysiological mechanisms. Non-epileptic seizure can be divided in two groups:

• Organic non-epileptic seizure (ONES), generally correlated with a physiological cause. They include fainting (syncope) and metabolic causes (e.g. diabetes, hyperammonemia, etc)
• Psychogenic non-epileptic seizure (PNES), associated with mental or emotional processes. PNES include different types of paroxysmal events, in particular dissociative seizures (the most common type), panic attacks and factitious seizures.

It is important to recognize these disorders in the evaluation of paroxysmal events as misdiagnosis rates in epilepsy and ineffective treatments are prevalent throughout the world. The gold standard to a correct diagnosis is video-EEG monitoring. Unfortunately, there are some conditions in which epileptic and non-epileptic events can co-exist making the diagnosis and management particularly challenging.

Classification

Earlier classification

In the earliest preserved historical documents, epilepsy was not seen as a homogeneous disease. The first description and classification of seizure types date back to the 7th century BC with Babylonian’s stone tablets (Sakikku)^[4] in which different types of seizures were mentioned and correlated with variable prognostic significance. At that time, seizures were attributed to demonic possession or super-natural events. The first historical monograph on epilepsy was written by Hippocrates around 400 BC, titled "On the Sacred Disease", around 400 BC. In this text, epilepsy was attributed to an imbalance of body fluids with additional influence from external forces such as changes in the weather. In the Middle Ages, the terms "grand mal" arose, referring to larger episodes of whole body convulsions. Non-convulsive seizure types then started to be described; in 1705, Poupart first used the term "absence seizures", and Tissot described absence seizures in more detail in 1770.^[5] In 1838, the French psychiatrist Hean-Etienne Dominique Esquirol was the first to use the term "petit mal" to directly contrast these non-convulsive seizures to convulsive "grand mal" seizures.

Modern epileptology began with John Hughlings Jackson who was the first author to carefully dissect seizure spread, recognizing that seizures can be "focal" in onset and then spread through the brain, now known as the "Jacksonian March" although he never endeavored to design a systematic classification. The advent of electroencephalography (EEG) in the early 1940s also had a profound effect on seizure definition and classification, and the "electroclinical" approach was born.  However, by the mid-twentieth century there was still a need for standardization.

History of ILAE seizures classification

In the 1960s, the World Health Organization (WHO) set up expert panels in order to delineate definitions and terminologies for seizures and epilepsy. The team was led by Henri Gastaut who coordinated the production of a Dictionary of Epilepsy.^[6] A new update of seizure classification was introduced in 1981 subsequently to the introduction of video-EEG, a revolutionary approach in seizure diagnosis because, for the first time, it introduced objective methods of analyzing seizures. The 1981 classification was therefore a fundamentally semiological classification (i.e. based on symptoms and signs). However, the urge to ameliorate the definition and classification of seizure and epilepsy led the ILAE to publish a series of revised ‘official’ definitions. First in 2001 than in 2006, a revised glossary of terms was published by the ILAE Taskforce on Classification and Terminology, with the stated intention of being "descriptive and phenomenologic, and providing a standard terminology for health workers to communicate what is observed and what a patient reports during a seizure".^{[citation needed]}

ILAE classification of seizures and epilepsy 2017

In 2017 ILAE Taskforce on Classification and Terminology introduced a new update of seizure classification pointing out some criticisms and several classification issues. In particular:

1. The problem regarding some seizure types, for example, tonic seizures or epileptic spasms, that can have either a focal or generalized onset.
2. The difficulty in classifying and discussing seizures without a clear onset
3. The fact that a retrospective seizure description often does not specify a level of consciousness, and altered consciousness, although central to many seizures, is a complicated concept.
4. Some terms in current use do not have high levels of community acceptance or public understanding, such as "psychic," "partial," "simple partial," "complex partial," and "dyscognitive."
5. Some important seizure types are not included.

The ILAE 2017 Seizure Classification has a columnar, non-hierarchical (meaning that levels can be skipped) organization. This new classification tends to stress the concept that epilepsy is a network disease rather than a symptom of brain abnormalities. From a network perspective, seizures could arise in neocortical, thalamocortical, limbic, and brainstem networks. The first step is recognizing whether the initial manifestation of the seizure is "focal" of "generalized". The semiology (symptoms/signs) that occurs during a seizure can give clues to the precise region, lobe, or hemisphere that is involved in seizure onset and propagation. In addition, EEG and video-EEG data may help clinicians to identify the specific seizure type. However, in the event seizure onset may remain undetermined, the term "unknown" can be applicable.^{[citation needed]}

Changes in seizure type classification from 1981 to 2017

The main changes in the ILAE classification system over time have been:^{[citation needed]}

1. The term "partial" has changed to the term "focal".
2. Seizures are classified by onset as focal, generalized, or unknown.
3. Seizures of unknown onset may have features that can still be classified using other terms.
4. "Aware" or "Impaired Awareness" is used as a classifier of focal seizures.
5. The terms dyscognitive, simple partial, complex partial, psychic, and secondarily generalized have been eliminated
6. Terminology includes automatisms, autonomic, behavior arrest, cognitive, emotional, hyperkinetic, sensory, and focal to bilateral tonic–clonic seizures. Atonic, clonic, epileptic spasms, myoclonic, and tonic seizures can be either focal or generalized.
7. Generalized seizure types now include absence with eyelid myoclonia, myoclonic absence, myoclonic–tonic–clonic, myoclonic–atonic, and epileptic spasms

Focal seizure

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Focal seizures originate within networks limited to one hemisphere. They may be discretely localized or more widely distributed with preferential propagation patterns that can involve the same and/or opposite hemisphere. Focal seizure classification is undertaken at two levels.

• The first level is the presence of "awareness". Retained awareness means that the person is aware of self and environment during the seizure, even if immobile. In this context, awareness refers to perception or knowledge of events occurring during a seizure, not to knowledge of whether a seizure occurred. If awareness is impaired at any point in the seizure, the seizure is a "focal impaired awareness seizure". A focal aware seizure corresponds to the prior term "simple partial seizure", whereas a focal impaired awareness seizure corresponds to the prior term "complex partial seizure."
• The second level is represented by the symptoms/signs of a seizure, which is the most useful feature for identification of the regional brain network from which the seizure arises. Hence, focal seizures are sub-grouped as having motor and nonmotor signs and having symptoms at the onset. Focal seizures are classified by their initial onset feature, even if this is not the most prominent feature overall in the seizure. For example, a seizure beginning with a sudden inability to understand language followed by impaired awareness and clonic left arm jerks would be classified as a "focal impaired awareness (nonmotor onset) cognitive seizure" (progressing to clonic left arm jerks). If both motor and nonmotor signs are present at the seizure start, the classification would fit with the more prominent symptoms.

Seizures are classified by 2 dimensions (level of awareness and involvement of movement):

• Aware
• Impaired Awareness

and

• Motor onset
  • Focal clonic seizure
    • Focal hemiclonic seizure
  • Focal tonic seizure
  • Focal motor seizure with dystonia
  • Focal myoclonic seizure
  • Focal atonic seizure
  • Focal motor seizure with paresis/paralysis
  • Focal epileptic spasms
  • Focal hyperkinetic seizure
  • Focal automatism seizure
    • Orofacial (lip smacking, lip pursing, chewing, swallowing, clicking, eye-blinking)
    • Manual (unilateral or bilateral, fumbling, tapping, manipulating or exploratory movements with the hands)
    • Pedal (bilateral or unilateral movements of the feet/legs, these may include pacing, walking or running. The movement is more reminiscent of normal movements in amplitude, and is less frenetic or rapid in comparison to the movements seen in focal hyperkinetic seizures involving the legs)
    • Perseverative (the movement consists of an inappropriate continuation of pre-seizure movement)
    • Vocal (single or repetitive sounds such as shrieks or grunts)
    • Verbal (single or repetitive words, phrases or brief sentences)
    • Sexual
    • Other (automatisms can include head nodding, undressing and a range of other automatic movements)
  • Focal motor seizure with dysarthria/anarthria
  • Focal motor seizure with negative myoclonus
  • Focal seizure with version
• Non motor onset
  • Focal sensory seizure
    • Somatosensory seizure
    • Visual seizure
    • Olfactory seizure
    • Auditory seizure
    • Gustatory seizure
    • Vestibular seizure
    • Focal sensory hot-cold sensation seizure
    • Focal sensory with cephalic sensation seizure
  • Focal cognitive seizure
    • Aphasic seizure
    • Anomic seizure
    • Agnosic seizure
    • Seizure with memory impairment
    • Dyslexic seizure
    • Seizure with dejà-vu or jamais-vu
    • Seizure with hallucination
    • Seizure with dissociation
    • Seizure with forced thinking
    • Seizure with dyscalculia
    • Seizure with dysgraphia
    • Seizure with left-right confusion
  • Focal emotional seizure
    • Seizure with fear
    • Gelastic seizure
    • Dacrystic seizure
    • Pleasure seizure
    • Seizure with anger
  • Focal autonomic seizure
    • Seizure with tachycardia/bradycardia
    • Seizure with epigastric sensation
    • Seizure with pallor
    • Seizure with hyper/hypoventilation
    • Seizure with piloerection, erection
    • Seizure with urge to urinate/defecate
    • Seizure with lacrimation
    • Seizure with pupillary dilatation/constriction
  • Focal behavioural arrest seizure

"Focal to bilateral tonic-clonic" seizure

Focal seizures can spread widely in the brain to engage bilateral networks, including cortical and subcortical structures, resulting in a tonic-clonic seizure with loss of consciousness. This seizure type is known as a "focal to bilateral tonic-clonic seizure", a term that corresponds to the 1981 phrase "partial onset with secondary generalization." The term "to bilateral" rather than "secondary generalized" was used to further distinguish this focal-onset seizure from a generalized-onset seizure which engages bilateral networks rapidly at onset. In order to make an accurate diagnosis, clinicians collect relevant supporting information (seizure videos, results of video-EEG monitoring, MRI and other tests) to achieve a high level of confidence.^{[citation needed]}

Generalized seizure

A generalized seizure is conceptualized as originating at some point within, and rapidly engaging, bilaterally distributed networks which can include cortical and subcortical structures, but do not necessarily include the entire cortex.^[7] In contrast to focal seizures, awareness is not a classifier for generalized-onset seizures, because the vast majority of generalized seizures present with impaired awareness or full loss of consciousness. However, awareness and responsiveness can be at least partially retained during some generalized seizures (e.g. absence epilepsy with eyelid myoclonia or myoclonic seizures). From the semiological point of view, it is not always easy to recognize a generalized seizure from focal or focal to bilateral ones. For example, generalized seizures can be asymmetric, rendering it difficult to distinguish them from focal-onset seizures.

Generalized-onset seizures are classified into two types: motor and non-motor seizures. This classification is similar to the 1981 classification, with the addition of myoclonic–atonic seizures, myoclonic–tonic–clonic seizures, myoclonic absence, and absence seizures with eyelid myoclonia.^{[citation needed]}

Classifying generalized seizures:^{[citation needed]}

• Motor onset
  • Tonic-clonic seizure and variants: bilateral and symmetric consists of a tonic (bilateral increased tone, lasting seconds to minutes) and then a clonic (bilateral sustained rhythmic jerking) phase, typically in this order, however variations such as clonic-tonic-clonic and myoclonic-tonic-clonic can also occur.
  • Clonic seizure: bilateral, sustained rhythmic jerking, it is distinguished from repetitive serial myoclonic seizures by the presence of more rhythmicjerking and loss of consciousness.
  • Tonic seizure: bilaterally increased tone of the limbs typically lasting seconds to a minute. They often occur out of sleep and with varying intensity of tonic stiffening. The individual is unaware during these events. At the beginning of tonic seizures with more intense stiffening, individuals may make an expiratory sound. More severe and prolonged tonic seizures may have a vibratory component which may be confused with clonic jerking
  • Atonic seizure: sudden loss or diminution of muscle tone without apparent preceding myoclonic or tonic features. Atonic seizures are very brief (<2 seconds) and may involve the head, trunk or limbs
  • Myoclonic seizure: a single or series of jerks (brief muscle contractions). Each jerk is typically milliseconds in duration
  • Myoclonic-atonic seizure: a myoclonic seizure followed by an atonic seizure, can result in a "drop attack"
  • Myoclonic-tonic-atonic seizure
  • Epileptic spasms: sudden flexion, extension or mixed flexion-extension of proximal and truncal muscles, lasting 1–2 seconds. Spasms typically occur in a series, usually on wakening. Subtle forms may occur with only chin movement, grimacing, or head nodding. Spasms may be bilaterally symmetric, asymmetric, or unilateral, depending on whether they are generalized or focal onset.
• Non motor onset
  • Typical Absence: abrupt onset and offset of altered awareness which can vary in severity. Memory for events during the seizures is usually impaired although there may be some retained awareness particularly for adolescents. Clonic movements of eyelids, head, eyebrows, chin, perioral or other facial parts may occur, most typically at 3 Hz. Myoclonus of limbs can rarely occur. Oral and manual automatisms are common and there may be perseveration of behaviors occurring prior to seizure onset. If an absence last longer than 45 seconds or it is associated with a consistent post-ictal phase (such as confusion or drowsiness), consider focal behavioral arrest seizure.
  • Atypical Absence: less abrupt onset and offset of loss of awareness than typical absence seizures. They are often associated with other features such as loss of muscle tone of the head, trunk or limbs (often a gradual slump) and subtle myoclonic jerks. The loss of awareness may be minimal.
  • Myoclonic absence: rhythmic myoclonic jerks of the shoulders and arms with tonic abduction that results in progressive lifting of the arms during the seizure. The myoclonic jerks are typically bilateral but may be unilateral or asymmetric. Perioral myoclonia and rhythmic jerks of the head and legs may occur. Seizures last 10–60 seconds and typically occur daily. Level of awareness varies from complete loss of awareness to retained awareness.
  • Absence with eyelid myoclonia: absence seizures accompanied by brief, repetitive, often rhythmic, fast (4–6 Hz) myoclonic jerks of the eyelids with simultaneous upward deviation of the eyeballs and extension of the head. Seizures are typically very brief (<6s in duration) and multiple seizures occur on a daily basis. Awareness is typically retained.

Unknown onset

Seizures of unknown onset may be referred to by the single word "unclassified" or with additional features, including motor, nonmotor, tonic-clonic, epileptic spasms, and behavior arrest. A seizure type of unknown onset may later become classified as either of focal or generalized onset, but any associated behaviors (e.g., tonic–clonic) of the previously unclassified seizure will still apply.^{[citation needed]}

Continuous seizures

Status epilepticus (SE) refers to continuous seizure activity with no recovery between successive seizures. According to the 2015 ILAE definition^[8] status epilepticus is a condition resulting either from the failure of the mechanisms responsible for seizure termination or from the initiation of mechanisms which lead to abnormally prolonged seizures (after a time point t₁). It is a condition that can have long-term consequences (after a time point t₂), including neuronal death, neuronal injury, and alteration of neuronal networks, depending on the type and duration of seizures. This definition is conceptual, with two operational dimensions: the first is the length of the seizure and the time point (t₁) at which point the seizure should be regarded as an "abnormally prolonged seizure." The second time point (t₂) is the time of ongoing seizure activity beyond which there is a risk of long-term consequences. In the case of convulsive (tonic-clonic) SE, both time points are based on animal experiments and clinical research. The time point of operational dimension 1 determines the time at which treatment should be considered or started, whereas the time point of operational dimension 2 determines how aggressively treatment should be implemented to prevent long-term consequences. The time domain may vary considerably between different forms of SE. Given the experimental evidence indicating irreversible brain damage after prolonged seizures^[9] and the potential threat of brain damage in humans, it has been suggested:^[10]

• The time of t₁ at
  • 5 minutes in convulsive SE,
  • 10 minutes in focal SE with impaired awareness
  • 10–15 minutes in absence status epilepticus
• The time of t₂ at
  • 30 minutes in convulsive SE,
  • >60 minutes in focal SE with impaired awareness

No data are available for t₂ time in absence status epilepticus

Awareness is not always impaired in status epilepticus. For example, myoclonic status epilepticus is characterized by ongoing (> 30 minutes) irregular jerking, often with partially retained awareness.

Epilepsia partialis continua refers to recurrent focal motor seizures (typically affecting hand and face, although other body parts may be affected), that occur every few seconds or minutes for extended periods (days or years).^[11] The focal motor features may exhibit a Jacksonian march. A Todd's paresis may be seen, which is a transient weakness of the affected body part.

References

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