Atonic seizures are currently defined as epileptic attacks characterized by a sudden loss or diminution of muscle tone, which may be fragmentary, confined to a segment, or massive, leading to a slumping to the ground. They are currently diagnosed only by means of polygraphic recording, which should demonstrate the interruption of EMG discharges associated with an EEG correlate. Due to these strict criteria, atonic seizures cannot account for the vast majority of falling seizures, which are usually caused by a variety of other motor events (tonic-clonic, tonic, myoclonic, myoclonic-atonic, etc.). In this update, the clinical and neurophysiological findings are reviewed, with special emphasis on the classification of atonic seizures (which may recognize a generalized or focal origin) and the description of mechanisms generating atonic events in humans.
Historical note and terminology
Atonic seizures are currently defined as epileptic attacks characterized by a sudden loss or diminution of muscle tone, which may be fragmentary, confined to a segment (limb, jaw, head), or massive, leading to a slumping to the ground (05; 04; 02; 09). In the glossary of descriptive terminology for ictal semiology, it has been emphasized that in atonic seizures, the loss or diminution of muscle tone is “pure,” without apparent preceding myoclonic or tonic events (04). Admittedly, the term atonic seizure refers to a specific pathophysiological mechanism (ie, loss or diminution of tone) and cannot account for the vast majority of falling seizures. Falling seizures, otherwise termed epileptic drop attacks, are a heterogeneous group of epileptic seizures in which the fall represents the main or only feature in the absence of any major motor phenomena (37). Falling seizures may be caused by a variety of seizure types (either generalized or partial) and mechanisms, including massive myoclonus, tonic contraction, pure atonic events, or the combinations of motor phenomena, as in the case of myoclonic-astatic atonic seizures. These observations may explain the controversies in the terminology of epileptic falls and the high number of terms used in the past (akinetic, atonic, astatic, static, drop, apoplectic, inhibitory, etc.).
Atonic seizures were usually classified among generalized seizure types (05; 08; 02). The new 2017 International League Against Epilepsy (ILAE) classification emphasizes that atonic seizures can be generalized as well as focal in origin. There have been prior descriptions of focal atonic seizures (16; 20), such as falling seizures of frontal origin (34), and negative myoclonus (38). The 2017 ILAE classification also notes that although generalized seizures usually have impairment in consciousness, it is difficult to assess consciousness in an atonic seizure.
Seizures characterized by a sudden fall have been known for a long time. Apart from early clinical notes by Tissot and Newman dating back to the 18th and 19th centuries, respectively, the first detailed description of atonic seizures was given by Hunt in 1922 (17). Hunt called the condition “static epilepsy,” ie, a “form of epilepsy characterized by sudden losses of postural control” (17). In 1945, Lennox proposed the term “akinetic seizures” for attacks of this type, which he renamed “astatic” in 1951 (22; 23). These seizures were considered to belong to the electroclinical group of petit mal absences; Lennox suggested the term “petit mal triad” to include absences, myoclonic jerks, and akinetic attacks (24). In 1966, Gastaut and coworkers described the Lennox-Gastaut syndrome, in which falling seizures were a characteristic seizure type, and also reported 4 cases of polygraphically studied atonic seizures (11; 12). Lastly, in 1981, the Commission on Terminology and Classification of the ILAE established the term “atonic seizure” for falling attacks with loss of tone (05).