Absence Seizures

In the untreated patient, absence seizures can occur quite frequently during the day. They sometimes occur in clusters, particularly when the child is tired or drowsy. In a child not on antiepileptic drugs, typical absence seizures can almost always be precipitated by hyperventilation.

There are four major syndromes in which typical absence seizures are a major component: childhood absence epilepsy (pyknolepsy), juvenile absence epilepsy, juvenile myoclonic epilepsy, and epilepsy with myoclonic absences. The absence epilepsies appear to have a complex genetic basis. Atypical absence seizures, a form of absence seizures, usually occur in cognitively impaired children who have other seizure types. Unlike typical absence seizures, atypical absence seizures are often longer and have a less distinct onset.

The EEG reveals a bilateral, synchronous symmetric, three-cycles-per-second, spike-and-wave discharge with normal interictal background activity. The interictal EEG in this disorder is distinctive and easily distinguished from other forms of generalized epilepsies. In atypical absence, the spike-and-wave discharges are irregular in frequency and shape and occur at a frequency that is less than three cycles per second.

OTHER GENERALIZED SEIZURES

Myoclonic Seizures. Myoclonic seizures are characterized by sudden, brief (<350 msec), shocklike contractions that may be generalized or confined to the face and trunk or to one or more extremities, or even to individual muscles or groups of muscles. Myoclonic seizures result in short bursts of synchronized electromyographic activity. The contractions of muscles are quicker than the contractions with clonic seizures. Any group of muscles can be involved in the jerk. Myoclonic seizures may be dramatic, causing the patient to fall to the ground or be quite subtle, resembling tremors. Because of the brevity of the seizures, it is not possible to determine if consciousness is impaired. Myoclonus may occur as a component of an absence seizure or at the beginning of a GTC seizure. The interictal EEG pattern seen in patients with myoclonic seizures typically consists of generalized spike-wave discharges.

Tonic Seizures. Tonic seizures are brief seizures (usually less than 60 seconds) consisting of the sudden onset of increased tone in the extensor muscles. If standing, the child usually falls to the ground. The seizures are longer than myoclonic seizures. Electromyographic activity is dramatically increased in tonic seizures. There is impairment of consciousness during the seizure, although in short seizures this may be difficult to assess. The EEG ictal manifestations of tonic seizures usually consist of bilateral synchronous spikes of 10 to 25 Hz of medium to high voltage with a frontal accentuation.

Atonic Seizures. Atonic (astatic) seizures, or drop attacks, are characterized by a sudden loss of muscle tone. They begin suddenly and without warning and cause the patient, if standing, to fall quickly to the floor. Because there may be total lack of tone, the child has no means to protect himself or herself, and injuries occur. The attack may be fragmentary and lead to dropping of the head with slackening of the jaw or dropping of a limb. In atonic seizures, there should be a loss of electromyographic activity.

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