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Management of epilepsy in childhood and adolescents requires accurate characterization of seizures and epilepsy syndrome. The inability to do so may result in the selection of inappropriate treatment options. As an example, diagnosing absence epilepsy for staring in the context of complex partial epilepsy may result in the choice of carbamazepine instead of an antiabsence seizure agent. Carbamazepine is well known to exacerbate absence seizures and is rarely associated with spike-wave stupor. Failure to accurately classify epilepsy syndromes may also limit the ability to prognosticate and limit decisions regarding medication withdrawal.
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The types of generalized seizures include: typical absence, atypical absence, myoclonic, generalized tonic–clonic, tonic and atonic seizures. Generalized seizures often have an age-dependent presentation. The spectrum of generalized seizures as a function of age of onset is shown in Figure 20–1.
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The generalized epilepsy syndromes encompass generalized seizure types associated with specific clinical accompaniments, age, EEG, and the presence or absence of neurological deficits and imaging findings.1 Generalized seizures occurring in the context of a normal neurological and cognitive examination and in the context of normal cerebral imaging typically indicate one of the idiopathic generalized epilepsies. Examples include childhood absence, juvenile absence, and juvenile myoclonic epilepsy. In contrast generalized seizures occurring in the context of cognitive impairment, neurological deficits and/or imaging lesions suggest one of the symptomatic generalized epilepsies. An intermediate group referred to as the cryptogenic generalized epilepsies may have normal imaging and laboratory investigations but are associated with developmental, cognitive, and neurological deficits. Examples of symptomatic or cryptogenic epilepsy include West and Lennox–Gastaut syndromes.
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Generalized seizures can occur in the context of any of these three broad categories. Differentiating between them carries important therapeutic and prognostic information.
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The putative mechanism of the generalized epilepsies involves activation of the thalamic relay system, reticular neurons, and cortical pyramidal neurons. Thalamic relay neurons activate cortical pyramidal neurons in either a tonic or burst mode. T-type calcium channels underlie the burst mode. Drugs, such as ethosuximide, that are effective in controlling absence seizures affect the T-type calcium currents.
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DIFFERENTIATING GENERALIZED FROM PARTIAL SEIZURES
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A generalized seizure implies that the entire cerebrum is electrically involved at seizure onset (Fig. 20–1). Generalized seizures typically manifest clinically at the onset of the electrographic discharge. In contrast, partial seizures evolve to start in one cerebral hemisphere and propagate to involve expanding areas of brain until clinical manifestations become obvious. Epilepsia partialis continua seizures present with continuous recurrent clonic motor manifestations that are focal and associated with repetitive electrographic discharges.
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