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

Incidence/Prevalence Rate

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 (0.15%).⁷ Doose syndrome, or myoclonic-astatic epilepsy, is an uncommon epileptic syndrome in which atonic seizures are seen. The incidence of Doose syndrome is estimated to be 0.32 per 100,000 children.⁵ A detailed analysis by Oguni et al. suggested that pure atonic seizures may be more common in this disorder than in LGS.⁸

Age of Onset

The age of onset of atonic seizures depends on the epilepsy syndrome with which they are associated. For example, the typical age of onset for patients with LGS ranges from 1 to 8 years.^4,9 Myoclonic-astatic epilepsy tends to present in early childhood from 1 to 5 years of age.^2,10 Rarely, atonic seizures may present in older age groups.¹¹

Gender Distribution

The literature does not provide gender distribution selectively for atonic seizures. However, specific epilepsy syndromes may have unique gender differences; for example, LGS affects boys more commonly than girls.⁴

Associated Epilepsy Syndromes and the Syndrome’s Natural History

The two most common epilepsy syndromes associated with atonic seizures are Lennox-Gastaut syndrome and myoclonic-astatic epilepsy, or Doose syndrome.

LGS is defined as the combination of (1) several seizures types, among them tonic, atonic, myoclonic, atypical absence, and generalized tonic-clonic seizures; (2) mental retardation; and (3) typical EEG findings consisting of multifocal spikes, generalized slow spike-wave activity (<2.5 Hz), and bursts of generalized polyspikes or paroxysmal fast activity (PFA) during sleep.⁴ Unfortunately, the prognosis of patients with LGS is poor. About 90% of patients have intractable seizures, require lifelong medication, and oftentimes need to be institutionalized. Virtually none of the patients are cognitively normal, even if the seizures remit.^4,12–15

Myoclonic-astatic epilepsy (Doose syndrome) is considered an idiopathic or cryptogenic generalized epilepsy characterized by (1) a combination of myoclonic and myoclonic-astatic seizures, absence seizures, and generalized tonic-clonic seizures starting in childhood; and (2) occurrence in a previously normal child without any identifiable brain abnormality on imaging.^10,16 The etiology is unclear in most patients, but a genetic cause or susceptibility is suspected. Long-term follow-up reveals a favorable outcome for most patients. More than two thirds have a complete remission of their seizures, and the majority of patients have normal intelligence. Only about one fifth of patients have continuing, intractable seizures and develop cognitive impairment.¹⁷

Pathologic Substrates

There is no uniform cause for atonic seizures. As in the typical child with LGS, the history and imaging can yield heterogeneous results. For example, the patient may have a history of seizures since early infancy, such as infantile spasms, or the seizures may start suddenly at a later age. Similarly, magnetic resonance imaging may show various malformations, such as cortical dysplasia, perinatal infarcts, or tuberous sclerosis; alternatively, it may be normal. Myoclonic-astatic epilepsy is felt to be due to an unidentified genetic etiology.

The pathophysiology of atonic seizures is not known. The three prevailing hypotheses discussed in the literature are activation of negative motor areas, diffuse cortical inhibition, and activation of brainstem centers leading to efferent inhibition of spinal neurons, all leading to abrupt loss of muscle tone.⁹

Cortical stimulation studies have demonstrated regions in the frontal lobe that lead to atonia. The premotor area anterior to the rolandic region associated with facial movements and areas anterior to the supplementary motor areas can both cause motor inhibition when activated.¹⁸ EEG studies comparing atonic seizures associated with generalized spike-wave discharges and absence seizures have noted similarities that suggest that atonia may be due to diffuse cortical inhibition in the motor cortex.¹⁹ The final proposed theory suggests that the generalized atonia is mediated through the reticular activating formation of the brainstem that modulates atonia during rapid eye movement (REM) sleep. Animal studies examining the effects of stimulation of the nucleus reticularis pontis oralis demonstrated that it produced inhibition of extensor muscle tone. Similarly, perfusion of pontine nuclei with pentylenetetrazol resulted in hypotonia.⁹ Both generalized and partial seizures could stimulate this region, leading to a transitory loss of muscle tone.

Ictal Behavior

Atonic seizures are clinically defined by apparent loss of muscle tone of the whole or part of the body. Generalized atonic seizures typically affect the axial musculature to a varying degree. For example, they may range from a subtle isolated drop of the head to collapse of the patient with limp extremities (See Video 15-1). This potentially leads to severe injuries because of the complete lack of countergravity muscle activity and the inability of the patient to perform any protective maneuver (See Video 15-2).

The fall of a patient, practically folding onto himself or herself, happens within 1 second. However, muscle atonia has to last >250 msec to produce a fall.²⁰ Video-EEG studies provide a detailed analysis of the sequence of movements accompanying an atonic fall. There is an initial head drop. If the patient is standing, the hip and knee joints flex, and the body falls straight downward with the arms hanging limply; if the patient is sitting, he or she may fall in any direction, depending on the center of gravity.^7,21 The patient does not experience an aura, and there is often no postictal confusion with atonic seizures.²² Muscle tone returns quickly, allowing most patients to arise immediately. However, there are some that appear to be paralyzed for several seconds. Clusters of atonic seizures, lasting minutes to hours, are possible, and they occur preferentially in the morning or after awakening from sleep. Preservation of consciousness is difficult to assess because, during the seizure, the patient is not able to respond with any type of action that requires muscle activity, including talking, and immediately afterward irritation and pain guide the most prominent behavior. Another problem is that patients are usually young children who are often cognitively impaired and unable to effectively communicate.⁷

Most authors will lump atonic seizures with associated myoclonic components among pure generalized atonic seizures, as long as the prominent feature of the seizure is loss of tone. A myoclonic component is often seen as initiating part of the seizure, such as a brief extension of the trunk or extremities, and may make it hard to decide how to label the event: myoclonic, myoclonic-atonic, or atonic seizure.²³