Asymmetric tonic seizures are usually a manifestation of frontal lobe epilepsy, also known as supplementary motor seizures. They are mainly characterized by asymmetric tonic posturing of the extremities with or without head turning. Seizures commonly originate in the supplementary motor area. They are usually brief and nocturnal and may not be associated with loss of consciousness. They can be disabling due to falls. Other regions, such as mesial parietal (precuneus) and posterior cingulate, can also manifest as asymmetric tonic seizures. They can be easily mistaken for nonepileptic events due to a paucity of EEG abnormalities during seizures. Etiologies are multiple and include mass lesions and malformations of cortical development. Seizures unresponsive to antiseizure medications may respond to epilepsy surgery.
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• Asymmetric tonic seizures are defined by tonic arm or leg posturing.
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• The seizures often originate in the supplementary motor area on the medial interhemispheric surface of the brain.
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• The seizures are characterized by short duration and nocturnal prominence and can have preserved consciousness, which makes them difficult to distinguish from psychogenic nonepileptic seizures.
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• Asymmetric tonic seizures are clinically very distinct but may show minimal EEG abnormalities.
Historical note and terminology
The supplementary motor area was initially defined in animal studies on monkeys in 1881 by Munk (30). C. and O. Vogt published many years of research in 1919 summarizing their findings of both the anatomy and physiology of this region in the monkey model, but they did not distinguish the mesial area as having any special functional significance (30). Grumbaum and Sherrington described movements in the upper extremity and the chest wall with stimulation for the cortex on the mesial surface anterior to the subdivision of the leg in the motor area (30). In 1951, Penfield and Welch showed that the supplementary motor area is a well circumscribed region within the longitudinal intrahemisphere fissure by stimulating the area in a series of both monkeys and human patients and eliciting motor movements (30). Penfield was also the first one who described supplementary motor area seizures with arm elevation and head turning (31). Following early descriptions of the supplementary motor area, asymmetric seizures were described and equated with onset in the supplementary motor area (27).
The historical classification of seizures was largely based on clinical observations. In 1960, the ILAE published the first classification scheme, which was updated in 1981 and 1989. Asymmetric tonic seizures in the 1989 classification were classified as simple partial seizures or complex partial seizures depending on if there was any loss of consciousness during the patient’s seizure event.
The latest ILAE classification in 2010 intended to bring the classification out of the realm of expert opinion to an objective and clinical neuroscience-based classification that could be easily incorporated into a physician’s clinical practice. They made changes to terminology and concepts but not to the list of epileptic syndromes (04; 05). The operational classification of seizure types by the ILAE lists asymmetric tonic seizures under focal onset seizures with or without impairment in awareness (motor onset tonic) of frontal lobe type (11). Alternative classifications listed asymmetric tonic seizures as simple motor tonic seizures (26).