Severe myoclonic epilepsy in infancy (SMEI) is a rare disease, characterized by febrile and afebrile, generalized and unilateral, clonic or tonic–clonic seizures that occur in the first year of life in an otherwise apparently normal infant. They are later associated with myoclonus, atypical absences, and partial seizures. Developmental delay becomes apparent within the second year of life and is followed by definite cognitive impairment and personality disorders of variable intensity. In the borderline form, children do not present with myoclonic symptoms but have the same general picture. SMEI is a channelopathy and the genetic studies have shown a mutation in the SCN1A gene in 70 to 80% of the patients, including the borderline forms. At present, there are no well-established correlations between genotype and phenotype. The electroencephalograms, often normal at the onset, display both generalized and focal anomalies, without a specific electroencephalographic pattern. As a rule, neuroimaging is normal. All seizure types are resistant to antiepileptic drugs and status epilepticus is frequent. Some drugs have been shown to aggravate the seizures and must be avoided. Two recent drugs have been proved to partially control the convulsive seizures and the status epilepticus. Therefore, it is crucial to diagnose this epilepsy soon after its onset in order to prescribe the most appropriate treatment.
Among nonsymptomatic epilepsies exhibiting several types of generalized seizures in children two syndromes were progressively identified: epilepsy with myoclonic–astatic seizures (MAE) and nonsymptomatic Lennox–Gastaut syndrome (LGS). Various approaches based on etiology, electroclinical semiology, and mathematical analysis have progressively helped to distinguish these two conditions. Both conditions preferentially affect boys. The course is stereotyped in MAE, characterized by progressive worsening of epilepsy, usual pharmacoresistance at onset and tonic–clonic seizures, myoclonus and frequent episodes of myoclonic status epilepticus. EEG shows 3Hz spike wave bursts characteristic of idiopathic generalized epilepsy together with slowing of the tracing. In LGS, major seizures are mainly atypical absences and tonic seizures with 0.5–2Hz slow spike-waves and eventually focal anomalies. Prognosis in both syndromes ranges from recovery without sequelae to pharmacoresistant epilepsy that has improved over the past 2 decades with the new generation antiepileptic compounds. Iatrogenic factors may contribute to the poor prognosis, mainly in MAE. Pathophysiology remains speculative for both syndromes: although both share factors of brain maturation, MAE is probably mainly related to genetic predisposition whereas LGS results from some unidentified cortical brain malformation. In unfavorable cases, there may therefore be a continuum between both syndromes. They need to be distinguished from other epilepsy syndromes and inborn errors of metabolism that begin in the same age range: atypical idiopathic benign epilepsy, frontal lobe epilepsy with secondary bisynchrony, ring chromosome 20, ceroid lipofuscinosis, and nonsymptomatic late-onset spasms.
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