The characteristic features of typical absences allowed their identification as early as 1705, when Poupart wrote the first description. In the 20th century, Sauer introduced the term “pyknolepsy,” which Adie subsequently used in characterizing the syndrome of childhood absence epilepsy (16,17). Although typical absences generally manifest in childhood, persistence into adult life may occur (18,19), and occasionally they can present de novo in adulthood (20). Typical absences usually last about 10 seconds (5,7,21). In the study by Penry and associates (7) of 374 absence seizures in 48 patients, the mean duration was 9 seconds (range, 1 to 45 seconds). Postictal confusion is either absent or very brief, lasting 2 or 3 seconds. Video-EEG monitoring has allowed a moment-by-moment analysis of the attacks.

The ILAE classification recognizes subtypes of typical absence seizures (10), and mixed forms with various combinations can be seen.

The frequency of these various features depends on how carefully they are looked for and the referral base from which cases are drawn. On the basis of ordinary history taking, simple absences and absences with mild clonic phenomena are by far the most common, followed by absences with automatisms and decreased postural tone. Studies using videotapes increase the frequency of the less commonly identified types and mixed forms (7). The categorization of typical absences into the above subtypes, although useful descriptively, probably has no clinical or neurobiologic significance. Many authors now refer only to simple absences and complex absences, the latter describing attacks with any combination of clonic, atonic, tonic, automatic, or autonomic features (22,23).

In the electroencephalography laboratory, typical absences are precipitated by hyperventilation in virtually all untreated patients and by photic stimulation in approximately 15% of individuals. The level of hypocapnia that induces absence seizures appears to vary among individuals (24). In everyday life, absences often occur with tiredness or boredom and are generally suppressed by attention, although demanding tasks and, in some patients, specific mental activities, such as calculation, may precipitate absences (25). Overbreathing during physical exercise decreases the frequency of absences (26).

Atypical absences have not been studied as thoroughly as typical absences and have been distinguished from the latter mainly by EEG criteria (5,21). The clinical features of atypical absences have resisted easy recognition because the patients are often mentally retarded and exhibit multiple other seizure types, both features being typical of symptomatic or cryptogenic generalized epilepsies (see Chapter 22). In addition, onset and offset are not as crisply delineated as are those of typical absences, partly because polypharmacy in intellectually disabled patients makes detection of altered awareness difficult. Careful analysis of videotaped seizures, however, often shows a change in behavior when the ictal discharge terminates (21).

Atypical absences average about 5 to 10 seconds in length (5,21,27), although they can be prolonged to the extent of absence status. Minor myoclonic, tonic, atonic, and autonomic features, as well as automatisms, may accompany the altered awareness. Tonic seizures, characterized by rhythmic fast activity, frequently occur, and the clinical and EEG features merge into those of the atypical absence (21,23). Unlike typical absences, however, atypical absences are usually not precipitated by hyperventilation or photic stimulation (21,28).

The relationship of absence and myoclonus is complex, and the nomenclature in the literature is confusing. Rhythmic jerking of the eyelids and facial muscles is common in typical absences, but more widespread myoclonus as part of absence seizures is rare. In some patients, however, discrete myoclonic seizures involving the extremities (without impairment of consciousness) occur independently of absences. In principle, the distinction between absences with myoclonus and independent absence with myoclonic seizures is obvious, being based on impaired consciousness with myoclonic movements. In practice, however, this distinction can be ill defined in some patients and may require video-EEG monitoring to establish.

Tassinari and colleagues (29,30) described the myoclonic absence. Not specifically identified in the ILAE classification, this seizure involves 10 to 60 seconds of rhythmic jerking of the shoulders, arms, and legs, with tonic contraction around the shoulders. It has a poor prognosis and appears to be specific to the syndrome of epilepsy with myoclonic absences (see causes). The myoclonic absence cannot be confused clinically with true absence because of its obvious convulsive features; the name derives from an association with typical 3-Hz spike-and-wave discharges.

At the onset of an attack, some patients with absences and photosensitivity have jerking of the eyelids with upward eye deviation. Some experts regard these seizures as a special type known as eyelid myoclonia with absences (19,31, 32, 33). Others believe that the so-called eyelid myoclonia represents voluntary (or subconscious) eye blinking that precipitated photosensitive absences (34, 35, 36). Characteristically, voluntary eye closure is followed by slow upward eye movement and rapid fluttering of the eyelids. Some patients continue to show eye blinking, without impaired consciousness, when the epileptiform discharges have been suppressed by medication. Thus, continued blinking must not be uncritically accepted as evidence of uncontrolled absences. Whether the clinical features and natural history of these patients warrant designation as a separate subgroup remains unproved. Similarly, the proposal to designate perioral myoclonia with absences as a distinct group requires further study (37).

Delgado-Escueta and associates (38) used the terms “myoclonus absence,” “myoclonic absence with 4- to 6-Hz multispike-and-wave complexes,” and “juvenile absence with 8- to 12-Hz rhythms” to denote other allegedly distinct types of absence. Although attacks resembling absences with unusual electroclinical features undoubtedly occur, we find this nomenclature unhelpful and unsubstantiated by published data.

In absences with focal features (39), cerebral damage is often present, and at least some of the attacks probably represent frontal lobe seizure mimicking absences (22,40).

A number of other EEG patterns are associated with staring spells, but they are not clearly associated with absence seizures.

Although interesting descriptively, the distinctions among multiple subtypes of typical absences likely have no clinical or neurobiologic significance (59). The important and practical diagnosis is that of idiopathic generalized epilepsy with absence seizures (60), as explained in Chapter 25. Similarly, atypical absence and its variants indicate a diagnosis of symptomatic or cryptogenic generalized epilepsy of the Lennox-Gastaut type. It is the syndromic diagnosis, not the identification of the seizure type, which is most useful for management (61).

Distinguishing absence seizures from simple daydreaming and inattentiveness is a common challenge in pediatrics. Many children are occasionally inattentive; this is a very common complaint in the outpatient setting. The family or teacher reports brief episodes of staring and unresponsiveness with no significant motor manifestations. Is the child having absence seizures or innocent nonepileptic staring spells? Based on a questionnaire given to parents, several features were identified that can help distinguish the two scenarios in otherwise normal children (62). Three features suggest nonepileptic events: (a) the events do not interrupt play; (b) the events were first noticed by a professional,
such as a schoolteacher, speech therapist, occupational therapist, or physician (rather than by a parent); and (c) the staring child is responsive to touch, or “interruptible” by other external stimuli. These features each have approximately 80% specificity for suggesting nonepileptic staring episodes. Several factors are associated with an epileptic etiology, including twitches of the arms or legs, loss of urine, or upward eye movement. Thus, video-EEG monitoring may not be necessary in otherwise normal children with staring spells, a normal routine EEG, positive responses to the nonepileptic types of questions, and no positive responses to the epileptic types of questions. Other features that are suggestive of nonepileptic or behavioral, rather than epileptic, staring include lower age and lower frequency of episodes (63). Similarly, sustained inattention is more often associated with attention deficit hyperactivity disorder than with absence seizures (64).