HISTORICAL NOTE AND NOMENCLATURE
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 (Commission on Classification and Terminology of the International League Against Epilepsy 1981; Blume et al 2001). In the recent 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 (Blume et al 2001). 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 (Tassinari et al 1997). 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 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 are usually classified among generalized seizure types (Commission on Classification and Terminology of the International League Against Epilepsy 1981; Engel 2001). However, ictal atonic events may be also recognized in focal seizures, such as falling seizures of frontal origin (Satow et al 2002), focal atonic seizures (Hanson and Chodos 1978), and negative myoclonus (Tassinari et al 1995).

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 (Hunt 1922). Hunt called the condition “static epilepsy,” ie, a “form of epilepsy characterized by sudden losses of postural control” (Hunt 1922). In 1945, Lennox proposed the term “akinetic seizures” for attacks of this type, which he rebaptized “astatic” in 1951 (Lennox 1945; 1951). 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 (Lennox 1960). 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 (Gastaut et al 1966a; 1966b). Lastly, in 1981, the Commission on Terminology and Classification of the ILAE established the term “atonic seizure” for falling attacks with loss of tone (Commission on Classification and Terminology of the International League Against Epilepsy 1981).

CLINICAL MANIFESTATIONS

Atonic seizures may recognize a generalized or focal origin.

Generalized atonic seizures. The introduction of video polygraphic systems in the analysis of falling seizures has demonstrated that seizures characterized by pure diminution or loss of tone (atonic seizures) are distinctly rare, at variance with what was previously believed on the basis of only direct clinical observation. Only a few polygraphically documented atonic seizures are available in the literature: atonic seizures were present in 1 of 15 children studied by Ikeno and coworkers and in 9 of 45 patients with epileptic falls observed by Egli and coworkers (Egli et al 1985; Ikeno et al 1985). Gastaut found atonic seizures in 3 of 2000 epileptic children followed at the center St. Paul of Marseille (Gastaut et al 1966b).

From clinical observation, 2 forms of atonic seizures have been recognized:

(1) Brief atonic seizures (effondrements epileptique or drop seizures) in which loss of tone may be restricted to the head (head drop) or involve all postural muscles, leading to a slumping to the ground. If consciousness is lost, this loss is extremely brief, and the patients are able stand up immediately after the fall.
(2) Prolonged atonic seizures (akinetic seizures) in which the loss of consciousness and the generalized atonia last from 1 to several minutes. The patient falls to the ground and remains mute and motionless.

Polygraphic recording may reveal additional subtle clinical signs, such as bradycardia or brief arrest of respiration.

Partial atonic seizures. At least 3 types of ictal atonic phenomena may be seen in partial seizures:

(1) Drop attacks
(2) Focal atonic seizures
(3) Negative myoclonus

Epileptic drop attacks (or falling seizures) may occur in the context of partial epilepsies, being usually due to symmetric or asymmetric tonic stiffening caused by discharges involving the frontal lobe or supplementary motor areas (Tassinari et al 1997; Tinuper et al 1998). More rarely, brief drop attacks are considered to result from atonic mechanisms, although video-polygraphic demonstration is available only in a few cases in the literature (Satow et al 2002; Tassinari 2003).

Focal atonic seizures are partial seizures with ictal paresis or paralysis of one or more parts of the body, which must be distinguished from the more common postictal Todd paralysis. Several reports in the literature with video-EEG documentation of these seizures are now available (Hanson and Chodos 1978). The ictal paralysis (usually hemiparesis) may be preceded or accompanied by a somatosensory aura.

Negative myoclonus is defined as an interruption of tonic muscular activity (50 to 400 msec in duration), time-locked to a spike or a sharp wave in the contralateral centro-parietal cortex on the EEG, without evidence of an antecedent myoclonia (Tassinari et al 1995). Clinically, the negative myoclonus may be unilateral or bilateral and is manifest by brief lapses in tone that interfere with motor coordination, causing “instability” or, more often, dropping of objects from the hands, head nodding, or falls. Negative myoclonus may occur in a variety of extremely different epileptic conditions, including idiopathic and symptomatic entities. It is now classified as an autonomous epileptic seizure type (Engel 2001)

LOCALIZATION
Because of the variety of seizures featuring atonia as the main clinical and polygraphic event, it is likely that different mechanisms are implicated, involving cortical and subcortical structures.

Gastaut and Broughton considered “epileptic drop attacks” to be related to very brief but intense inhibitory mechanisms, suggesting the importance of participation of the motor cortex (Gastaut and Broughton 1972). Data from electrical stimulation of human cerebral cortex reveal the existence of frontal regions whose activation produces inhibition of voluntary movement (Luders et al 1995). These regions, defined as “negative motor areas” have been localized in the superior bank of the sylvian fissure near the rolandic representation of the face (“primary negative motor area”) (Luders et al 1995), and in the rostral portion of the supplementary motor area, in a region immediately anterior to the primary motor cortex (“supplementary negative motor area”) (Lim et al 1994). The importance of these cortical inhibitory areas is well established in the genesis of focal atonic seizures and negative myoclonus but is only postulated in the origin of atonic falling seizures (Satow et al 2002). An alternative explanation is that epileptic discharges in the primary sensorimotor cortex can produce negative motor phenomena via direct inhibition of the spinal motorneuron pool as suggested in humans by means of direct cortical stimulation through subdural electrodes (Ikeda et al 2000) or ictal electrocorticogram (Matsumoto et al 2000).

The good results of callosotomy in the treatment of epileptic drop attacks in the context of partial epilepsies suggest that bilateral synchronization of the epileptic discharge via the corpus callosum is a crucial mechanism. Whether this may lead to a bilateral activation of inhibitory cortical areas or of cortical regions projecting downward to brainstem structures is not known. In the brainstem, the direct or indirect (via the cortico-reticular efferents) activation of the pontine reticular formation, which is responsible for atonia or inability to move in REM sleep and in cataplexy, could produce a motor inhibition and underlie atonic seizures.

PATHOPHYSIOLOGY
Generalized atonic seizures. Gastaut and colleagues studied the clinical features of the brief atonic seizures with video recording and found that the patients fall onto the ground in less than 1 second, with a typical sequence of events: (a) head drop (lasting for 250 msec), (b) trunk and legs drop (from 250 to 800 msec) (Gastaut et al 1966b). After an intermediate period of about 1 second, the patients resume a standing position in about 2 seconds. Oguni and colleagues, by examining the video-polygraphic features of 69 “atonic epileptic drop attacks” recorded in 2 patients with myoclonic-astatic epilepsy, observed that the clinical manifestations ranged from collapsing and landing on the buttocks to head nodding only (Oguni et al 1992). Detailed video analysis of the drop attacks with the patient in standing position demonstrated the first manifestations to be flexion at the waist and knees, followed by further knee flexion, leading to falling straight down and landing on the buttocks.

The ictal EEG of atonic seizures discloses a generalized poly-spike-wave discharge, with loss of tone usually being associated with the wave component of the spike-and-wave complexes. Other ictal patterns include low- or high-voltage fast activity, flattening, or a burst of poly-spikes followed by generalized spike-wave activity. Interictal EEGs are not specific to the atonic seizures and consist of slow spike-wave activity or poly-spike-wave complexes.

Partial atonic seizures. Epileptic drop attacks of atonic origin in the context of partial epilepsies have been studied in detail by Satow and colleagues in 2 patients, 1 with frontal lobe epilepsy and the other with parietal lobe epilepsy (Satow et al 2002). Paroxysmal diminution of muscle tone mainly involved the axial muscles in both patients. The falls were slow, taking 2 to 5 seconds to fall down. Ictal EEG records showed low-voltage fast activity in the fronto-central area followed by repetitive spikes at the midline fronto-central area in the patient with frontal lobe epilepsy and rhythmic spikes in the left central area in the patient with parietal lobe epilepsy. Interictal FDG-PET disclosed hypometabolic regions consistent with the clinical and EEG findings.

Focal atonic seizures may have a long duration (longer than 30 minutes) and, therefore, may present as status epilepticus. Ictal EEG was recorded in 30 patients and showed spike-wave or slow wave discharges (53%) or rhythmic ictal activity (47%) in frontal or centroparietal areas (63%), in the temporal lobe (20%), or distributed over a wide scalp region (17%) contralateral to the paralyzed limb (Guerrini et al 2002). In the only 2 patients with ictal electrocorticogram, the seizure discharges involved the mesial frontal or the primary sensorimotor cortices (Noachtar and Luders 1999; Matsumoto et al 2000).

DIFFERENTIAL DIAGNOSIS
Because many seizure types and different mechanisms may be responsible for epileptic drops and falls, video-polygraphic monitoring is mandatory to achieve a correct diagnosis.

Aside from pure atonic seizures, epileptic drops and falls may be caused by myoclonic-astatic seizures, myoclonic seizures, and (more frequently) tonic seizures (including axial spasms).

Myoclonic-atonic seizures are attacks in which the more or less pronounced loss of tone (usually leading to a sudden fall) is preceded by symmetrical myoclonic jerks of the arms or irregular twitching of the face. The ictal EEG shows generalized spike-wave discharges (at 2 to 3 Hz), with the spike being usually associated with myoclonia and the slow wave with the ensuing atonia.

Myoclonic seizures causing sudden falls have been exceptionally recorded; in these cases the jerks were associated with generalized spike wave discharges at 3 to 3.5 Hz.

Tonic seizures represent the most common cause of sudden fall to the ground in children with Lennox-Gastaut syndrome. Tonic seizures may be axial, axial and proximal appendicular, or global, and are characterized by sudden flexion of the neck and body, rising of arms in a semiflexed position, extension of the legs, contraction of facial muscles, rolling of the eyes, apnea, tachy- or bradycardia, dilated pupils, enuresis, and facial flushing. Two types of “generalized” tonic seizures associated with falls have been described, namely short tonic seizures (or spasms) and prolonged tonic seizures.

Short tonic seizures, or axial spasms, resemble those of infantile spasms and are characterized by an abrupt and massive increase of tone, maximal in the neck and shoulder girdle. The ictal EEG shows either generalized slow waves or no changes, or occasionally a slight attenuation.

Prolonged tonic seizures are characterized by an overt and sustained increase of tone over the axial and segmental muscles, over both agonist and antagonist muscles. The ictal EEG expression is either a run of low-voltage fast activity, or a brief burst of generalized spike-wave, or poly-spike-wave activity.

Tonic posturing and stiffening is also responsible for the vast majority of epileptic falls occurring in partial epilepsies.

Video polygraphic monitoring also may be needed to exclude drop attacks of non epileptic origin. Meissner and colleagues analyzed the diagnosis in 108 consecutive patients with a history of drop attacks and found the following etiologies: unknown (64%), cardiac (12%), cerebrovascular insufficiency (8%), combined cardiac and cerebrovascular disease (7%), seizures (5%), vestibular (3%), and psychogenic (1%) (Meissner et al 1986). Other conditions to be included in the differential diagnosis also include cataplexy, syncopes, and breath-holding spells.

DIAGNOSTIC WORKUP

As stated before, video-polygraphic monitoring represents the only investigation capable to provide the proper diagnosis of atonic seizures. In fact, definition of atonic seizures rests on the identification of the following neurophysiological features:

• Interruption of ongoing EMG discharges corresponding to the clinical event (head nodding, fall, etc.). The loss of tone detectable by EMG may involve only a limited number of muscles, accounting for different clinical expressions.
• Presence of an EEG correlate, detectable by routine surface EEG.

Therefore, video-polygraphy allows the recognition of epileptic or nonepileptic falls caused by different mechanisms and pathophysiology.

Ancillary diagnostic tools may be considered for specific purposes. MRI is needed in partial atonic seizures to disclose specific etiologies, such as cortical malformations (Guerrini et al 1998). Stereo-EEG recording and ictal SPECT may be required in partial atonic seizures of symptomatic origin when a surgical therapy is postulated.

Syndromes and diseases in which the seizure type occurs

Atonic seizures may occur in the context of generalized and partial epilepsies.

Epileptic falls and head nods of true atonic origin have been documented in the Lennox-Gastaut syndrome and myoclonic-astatic epilepsy of early childhood (Oguni et al 1992), usually in combination with other seizure types.

In epilepsy with electrical status epilepticus during slow sleep, atypical absences with atonic components, epileptic falls, and negative myoclonus have been described in subgroups of patients (Tassinari et al 1985), formerly reported under the heading of atypical benign epilepsy of childhood (Aicardi and Chevrie 1982).

“Epileptic drop-attacks” are also described in partial epilepsies, particularly of frontal origin and with marked bilateral synchrony. In these cases, however, the falls are usually associated with tonic stiffening, or polygraphic documentation is scanty. Satow and colleagues reported pure atonic seizures in 2 patients with partial epilepsy (of frontal and parietal origin) studied by means of prolonged video-EEG monitoring (Satow et al 2002).

PROGNOSIS AND COMPLICATIONS

Whatever the origin and mechanisms of falling seizures in either generalized or partial epilepsies, their occurrence frequently represents an ominous sign because they are frequent, resistant to therapy, and physically dangerous, and they portend personality changes. In a series of 31 patients with epileptic drop attacks (of tonic or atonic origin) in partial epilepsy (mostly of frontal type), 74% of patients had a bad prognosis, and 45% were mentally retarded (Tinuper et al 1998).


MANAGEMENT

Medical treatment. The medical treatment of atonic seizures is usually disappointing and unsatisfactory because of the resistance to any conventional antiepileptic drug. Valproic acid associated with benzodiazepines is usually considered the treatment of choice. ACTH and hydrocortisone may have immediate and sometimes spectacular effects, but these are usually followed by relapse. Some of the newer antiepileptics such as lamotrigine, felbamate and topiramate have shown some promise for the treatment of the Lennox-Gastaut syndrome, with atypical absences and falling seizures being particularly improved. Combination therapy of valproic acid and lamotrigine has been proposed as the treatment of choice for falling seizures in partial epilepsy.

Surgical treatment. Callosotomy is widely considered to be the surgical treatment of choice in patients with falling seizures who are not candidates for classical resective surgery. From a review of the literature, it appears that patients with tonic or atonic falls had a reduction of about 80% in seizure frequency after callosotomy in most instances. The aim of callosotomy is to limit the spread of convulsive discharges from one hemisphere to the other, and, therefore, patients with mainly lateralized foci with secondary generalization benefit the most (Oguni et al 1991).

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ILAE

ILAE Copyright Notice

ABBREVIATIONS

EEG: electroencephalogram
REM: rapid eye movement
PSW: poly-spike-wave
SW: spike-wave
FDG-PET: F18-labeled fluorodeoxyglucose positron emission tomography
EMG: electromyogram
MRI: magnetic resonance imaging
SPECT: single photon emission tomography

SYNONYMS

astatic seizures
drop seizures

MAJOR KEYWORD DESCRIPTORS

diminution of tone
drop attacks
falling seizures
inhibitory motor areas
loss of tone
negative motor areas
negative myoclonus

MINOR KEYWORD DESCRIPTORS

akinetic
apoplectic
astatic
generalized and focal seizures
inhibitory
negative myoclonus
static

AGE OF PRESENTATION

01-23 months
02-05 years
6-12 years

AGE OF TYPICAL PRESENTATION

02-05 years

GLOSSARY 

Atonic seizures:Atonic seizures are epileptic attacks characterized by a sudden loss or diminution of muscle tone, which may be fragmentary or massive. Atonic seizures may be generalized or focal, but are usually seen in children with the Lennox-Gastaut syndrome. They must be differentiated from other seizure types that may cause sudden falls, such as myoclonic-astatic seizures, myoclonic seizures and tonic seizures.

ILLUSTRATION CAPTIONS

Figure 1: Atonic seizure
Figure 2: Focal atonic seizure

LEGENDS of FIGURES

Figure 1: A rather irregular multiple spike-and-wave discharge is associated with abrupt and complete loss of postural tone, causing the patient to fall down. After a few seconds the child returns to the erect position.

Figure 2: This young boy with a complex epileptic encephalopathy is holding the arms outstretched. The tracing shows the appearance of a fast rhythm involving C4; one second after the discharge onset one can observe the fall of the left arm (Delt.L.) and of the head (Cervic).

PERMUTED TOPIC, SYNONYMS, VARIANTS
Atonic seizures
atonic seizure

RELATED SUMMARIES
Juvenile myoclonic epilepsy
Lennox-Gastaut syndrome
Myoclonic-astatic epilepsy of childhood
Myoclonic-atonic seizures
Subcortical laminar heterotopia

DIFFERENTIAL DIAGNOSIS
myoclonic-astatic seizures
myoclonic seizures
tonic seizures
axial spasms
partial epilepsy
cardiac disease
cerebrovascular insufficiency
combined cardiac and cerebrovascular disease
seizures
vestibular disorders
psychogenic disorders
cataplexy
syncopes
breath-holding spells