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Severe myoclonic epilepsy in infancy (SMEI)

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

Authoring team

This is a malignant syndrome of childhood epilepsy.

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.

Subsequent seizures may be of many types:

  • complex absences
  • partial clonic seizures
  • myoclonic jerks
  • complex partial seizures
  • apnoeic attacks

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

  • genetic studies have shown a mutation in the SCN1A gene in 70 to 80% of the patients, including the borderline forms

There are no well-established correlations between genotype and phenotype


  • electroencephalograms, often normal at the onset, display both generalized and focal anomalies, without a specific electroencephalographic pattern
  • neuroimaging is usually normal

Management (2):

  • requires specialist advice - refer to, a tertiary paediatric epilepsy specialist when a child presents with suspected Dravet syndrome
  • consider sodium valproate or topiramate as first-line treatment in children with Dravet syndrome. Follow the MHRA safety advice on sodium valproate
  • if first-line treatments are ineffective or not tolerated, and consider clobazam or stiripentol as adjunctive treatment


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