This article includes discussion of atypical absence seizures, Lennox-Gastaut syndrome, Dravet syndrome, epileptic encephalopathy with spike-and-wave during sleep, and epilepsy with myoclonic-atonic seizures. The foregoing terms may include synonyms, similar disorders, variations in usage, and abbreviations.
Atypical absences are epileptic seizures that primarily occur in children with severe learning and neurologic disabilities of epileptic encephalopathies, mainly, Lennox-Gastaut syndrome. They are distinct from typical absences in that onset and termination is slow, impairment of consciousness is mild, and they are often associated with significant tone disturbances. The ictal EEG shows diffuse spike waves that are slower than the typical absence, usually between 1.5 and 2.5 Hz. Patients with atypical absences also suffer from concurrent tonic, atonic, and other types of epileptic seizures according to the primary epileptic syndrome. Management is usually difficult, and prognosis is that of the underlying disorder.
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• Atypical absences are generalized epileptic seizures of mainly severe epilepsies in children with learning difficulties who also suffer from frequent seizures of another type.
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• Atypical absence seizures are characterized by a slow, insidious start and end with usually mild impairment of consciousness and significant atonic symptoms.
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• Ictal EEG shows diffuse spike and slow wave discharges with a varying range of frequencies at less than 2.5 Hz. Interictal EEG is often abnormal.
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• Atypical absences are common in Lennox-Gastaut syndrome and less prevalent in Dravet syndrome, epileptic encephalopathy with continuous spike-and-wave during sleep, and epilepsy with myoclonic-atonic seizures.
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• Prognosis and management is that of the underlying disease and syndrome.
Historical note and terminology
Poupart, in 1705, was the first to describe absences (77). Tissot described a girl with absences “avec un tres leger movement dans les yeux” and frequent generalized tonic-clonic seizures (78). The term “epileptic absence” was first used by Calmeil (14), and shortly thereafter Esquirol coined the term “petit mal” (26), which probably corresponds to what is now known as “typical absence seizures.” Gowers gave a most accurate description of the absence seizures “without conspicuous convulsions” (38). Friedman reported a long-term favorable prognosis but believed that these absences were not epileptic (32). Sauer coined the name “pyknolepsy” (from the Greek word pyknos, meaning closely packed, dense, or aggregated) (66). Adie defined pyknolepsy as a distinct epileptic syndrome of childhood, probably of what we now recognize as childhood absence epilepsy (01).
The recognition of atypical absence seizures and their differentiation from typical absence seizures became possible with the advent of EEG recordings. In 1939, Gibbs and associate described the EEG discharges of diffuse slow spike-waves (characteristic of atypical absences), which they differentiated from the faster and more rhythmical spike-waves seen in “petit mal” absences (characteristic of typical absences) and which, consequently, they designated “petit mal variant” (37). The distinction and clinical correlates of fast and slow spike-waves was identified by Lennox, who observed that patients showing discharges of the petit mal variant seemed to differ from those showing the discharges of typical petit mal absences in that their ictal symptoms were atypical; their mental functions were nearly always affected, and they did not respond to the then classical treatment of petit mal absences with diones (50; 51). Lennox’s petit mal triad consisted of (1) the slow spike-and-wave interictal EEG pattern, (2) mental retardation, and (3) three types of seizures (myoclonic jerks, atypical absence, and drop attacks). However, the petit mal triad of Lennox (50), which was misused and misunderstood, was clarified by the Gastaut school of Marseilles who defined their main clinical correlates of what they called “childhood epileptic encephalopathy with diffuse slow spike-waves” (23; 35). Most of the cases of “childhood epileptic encephalopathy with diffuse slow spike-waves” belong to what now is recognized as Lennox-Gastaut syndrome. Epileptic encephalopathy with continuous spike-and-wave during sleep, Dravet syndrome, epilepsy with myoclonic-atonic seizures, and syndrome other than the Lennox-Gastaut syndromes that produce atypical absences, were distinguished from Lennox-Gastaut syndrome soon thereafter.
It was also Gastaut who introduced the term “atypical absences” to differentiate them from “typical absence seizures.” This nomenclature of typical and atypical absences has been adopted by the ILAE Commissions (33), and it is still valid in all newest ILAE reports on classification.
Terminology and classification. According to the 1981 ILAE Classification of epileptic seizures, atypical absences are generalized epileptic seizures that are different from typical absences in that:
(A) The ictal EEG discharge is more heterogeneous and may include irregular, less than 2.5 Hz, spike-and-slow complexes fast or other paroxysmal activity; these are bilateral but often irregular and asymmetrical.
(B) The interictal EEG usually has abnormal background and paroxysmal activity (such as spikes or spike-and-slow-wave complexes) that is frequently irregular and asymmetrical.
(C) Clinically, the seizures may have: (1) changes in tone that are more pronounced than in typical absences and (2) onset or cessation that is not abrupt (17).
The ILAE classification core group recognized typical, phantom, myoclonic, and atypical absences (25), commenting that in atypical absences, “there are variable manifestations of this ictal event, some involving hypotonia and atonia. Better criteria for characterizing atypical absences will also be discussed by the working group on atonic seizures” and “absence seizures can rarely represent propagation from localized cortical areas, usually in the frontal lobe. There may be a continuum between these events and generalized atypical absences” (25).
The 2010 ILAE Commission report proposed that absence seizures should be simplified in (A) typical absences, (B) atypical absences, and (C) absence with special features (eg, myoclonic absence, eyelid myoclonia) (06).
The latest ILAE position paper of the operational classification of seizure types recognizes typical absences, atypical absences, absences with eyelid myoclonia, and myoclonic absences (29; 28). "An absence is atypical because of slow onset or termination or significant changes in tone supported by atypical, slow, generalized spike and wave on the EEG." However, in this paper all absences are classified as "generalized nonmotor (absence) seizures" though this (nonmotor) does not convey the complex semiology of absence seizures that often manifest with significant motor manifestations as analyzed in a narrative review (80).
The ILAE Commission diagnostic manual of epilepsies describes atypical absence seizures as follows (Commission on Classification and Terminology of the International League Against Epilepsy 2014):
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An atypical absence seizure has less abrupt onset and offset of loss of awareness than typical absence seizures. They are often associated with other features such as loss of muscle tone of the head, trunk or limbs (often a gradual slump) and subtle myoclonic jerks. Atypical absence seizures often occur in individuals with intellectual impairment. The loss of awareness may be minimal with the patient continuing an activity, but more slowly or with mistakes.
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CAUTION: These seizures can be difficult to recognize in a patient with ongoing slow (less than 2.5 Hz) generalized spike-and-wave on EEG; careful correlation between EEG and clinical state is recommended.
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EEG background/interictal/activation: Please refer to specific syndromes and etiologies in which this seizure type occurs.
EEG ictal: Slow (less than 2.5 Hz) generalized spike-and-wave discharges accompany atypical absences. These may be facilitated by hyperventilation; however, hyperventilation may be difficult to achieve in children with intellectual impairment.
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Differential diagnosis: Daydreaming or inattention; focal seizures with dyscognitive features.
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Related syndromes: Lennox-Gastaut syndrome, Dravet syndrome, Epilepsy with myoclonic-atonic seizures, Epilepsy with myoclonic absences (Commission on Classification and Terminology of the International League Against Epilepsy 2014).