Typical absence seizures constitute the most characteristic epilepsy of childhood (“childhood absence”); rarely do the seizures begin before 4 years of age or after puberty. Another attribute is their great frequency (hence, the old term pykno, meaning “compact” or “dense”). As many as several hundred may occur in a single day, sometimes in bursts at certain consistent times of the day. They produce periods of inattention and may appear in the classroom when the child is sitting quietly rather than participating actively in his lessons. If frequent, they disturb attention and thinking to the point that the child’s performance in school is impaired. Less frequently, such attacks may last for hours with no interval of normal mental activity between them—so-called absence or petit mal status. Absence epilepsy of adolescent onset (“juvenile absence”) does not have the very high seizure frequency of the childhood type. Cases of absence status have also been described in adults with frontal lobe epilepsy (see in the following text). In contrast to childhood absence seizures, the disorder may last well into adulthood and be punctuated by generalized tonic-clonic seizures or a burst of seizures. Akinesia (motionlessness) is not unique to any seizure type.
The typical absence, with or without myoclonic jerks, rarely causes the patient to fall. Absence should be considered a separate entity because of its relative benignity. It may be the only type of seizure during childhood. The attacks tend to diminish in frequency in adolescence and then often disappear, only to be replaced in many instances by major generalized seizures. About one-third of children with absence attacks will, in addition, display symmetrical or asymmetrical myoclonic jerks without loss of consciousness, and about half will at some time have major generalized (tonic-clonic) convulsions.
Distinguished from typical absence seizures are variants in which the loss of consciousness is less complete or in which myoclonus is prominent, and others in which the EEG abnormalities are less regularly of a 3-per-second spike-and-wave type (they may occur at the rate of 2 to 2.5 per second or take the form of irregular 4to 6-Hz polyspike-and-wave complexes). Atypical absence is a term that was introduced to describe long runs of slow spike-and-wave activity, usually with no apparent loss of consciousness. External stimuli, such as asking the patient to answer a question or to count, interrupt the run of abnormal EEG activity. The current classification (Fig. 15-1) separates the disorder into groups that are identified as typical, atypical, and having special features, namely, myoclonic jerks or eyelid myoclonus.
In sharp contrast to the typical absence epilepsies, is a form that has its onset between 2 and 6 years of age and is characterized by atonic, or astatic, seizures (i.e., falling attacks), often succeeded by various combinations of minor motor, tonic-clonic, and partial seizures and by progressive intellectual impairment in association with a distinctive, slow (1to 2-Hz) spike-and-wave EEG pattern. This is the Lennox-Gastaut syndrome. Often it is preceded in earlier life by infantile spasms, a characteristic high-amplitude chaotic EEG picture (hypsarrhythmia), and an arrest in mental development, a triad sometimes referred to as the West syndrome (see further on). The early onset of atonic seizures with abrupt falls, injuries, and associated abnormalities nearly always has a grave implication—namely, the presence of serious neurologic disease. Prematurity, perinatal injury and metabolic diseases of infancy are the most common underlying conditions. This is essentially a form of symptomatic generalized epilepsy, in contrast to the foregoing idiopathic types such as typical absence epilepsy (petit mal). The Lennox-Gastaut syndrome may persist into adult life and is one of the most difficult forms of epilepsy to treat.