The most recent classification of seizure types by the International League Against Epilepsy (ILAE) defines atonic seizure as “sudden loss or diminution of muscle tone without apparent preceding myoclonic or tonic event lasting one to two seconds or more, involving head, trunk, jaw or limb musculature.”1 As it affects mostly the axial muscles, it can lead to a fall or, if not affecting the whole trunk, to a head “nod” or “drop.” The term astatic seizure encompasses a variety of seizure types leading to “loss of erect posture that results from an atonic, myoclonic or tonic mechanism.”1 Thus, atonic seizure implies abnormal muscle tone, whereas astatic seizure refers simply to a fall and does not require any knowledge about tonicity.
There is inconsistency in the literature and clinical practice regarding the term atonic seizure. Historically, the following terms were used synonymously: epileptic drop attack, akinetic seizure, astatic seizure, and atonic seizure—all denoting seizures that cause a fall. The term atonic seizure should be used exclusively for seizures with loss of tone, according to the consensus statement by the ILAE in 1981.2
Although some authors may consider the label epileptic drop attack as referring to atonic seizures, drop attacks or falls may be seen with myoclonic, tonic, myoclonic-tonic, and myoclonic-atonic seizures, as well as frontal and temporal lobe seizures.2,3 Confusion is likely unless the episode is properly named according to the clinical sequence of movements and the degree and type of alteration of muscle tone, both of which require video-encephalography (EEG) with electromyogram (EMG) for appropriate classification.2
The incidence rate of generalized atonic seizures alone is difficult to determine because of the inconsistent use of the term and the lack of proper assessment. Adding to the difficulty is the fact that an individual patient typically does not have solely atonic seizures, but rather they are one type in the setting of brain abnormalities causing multiple seizure types, as in Lennox-Gastaut syndrome (LGS). Furthermore, a pure form of a generalized atonic seizure is seldom encountered; that is, the seizure may be accompanied by an initial myoclonic or tonic component. The best estimate regarding the incidence of atonic seizures is deduced from the literature of atonic seizures in patients with LGS and myoclonic-astatic epilepsy (Doose syndrome).
It is estimated that LGS accounts for ∼3 to 10% of all childhood epilepsy.4 In a Finnish study, the annual incidence was 2 in 100,000 children, corresponding to other studies.5 In patients with LGS, studies investigating the percentage of patients with pure atonic seizures found that they occur in ∼50% of cases.6 However, they comprise only 15 to 20% of seizures that lead to a fall. In a study of 2000 patients with epilepsy published by Gastaut et al, only 3 had atonic seizures ...