What is SMEI or Dravet syndrome?

Dr. Charlotte Dravet first described Dravet syndrome in 1978 as Severe Myoclonic Epilepsy of Infancy (SMEI). Dravet syndrome is at the most severe end of the Dravet Spectrum Disorders. The key features of Dravet syndrome are febrile seizures and status epilepticus.

The first seizures usually happen before one year of age, with no known cause other than fever or illness. Seizures that are brought on by fever are called febrile seizures. The first seizure may be a tonic clonic seizure or hemiclonic seizure. Tonic clonic seizures involve a stiffening of the arms and legs (tonic phase) and jerking of the arms, legs, and head (clonic phase). Tonic clonic seizures are also called grand mal. Hemiclonic seizures are similar to tonic clonic seizures, though only one side of the body is convulsive.

Seizures progress to be frequent and intractable in SMEI, meaning that seizures do not respond well to treatment. They also tend to be prolonged, lasting more than 5 minutes. Prolonged seizures may lead to status epilepticus, a medical emergency. Status epilepticus is defined as a seizure that lasts more than 30 minutes, or seizures that occur in clusters, one after another.

Seizures usually become afebrile, which means they occur spontaneously without fever. Other types of seizures appear in early childhood, including myoclonic seizures, atypical absence and complex partial seizures. Nocturnal seizures, or seizures that occur at night, are common in Dravet syndrome.

What is SMEB?
Severe myoclonic epilepsy borderline (SMEB) is a term that is used to describe a subset of Dravet syndrome patients having less severe symptoms and moderate, rather than severe, developmental delays. Myoclonic seizures are not typical in SMEB.

Children with Dravet Syndrome are likely to develop other complications. Developmental delays appear between two and four years of age. This may or may not include regression or loss of developmentally attained skills. Children may be delayed or impaired in speech, exhibit autistic- like behaviors, or lose their ability to control movement (ataxia). Children may have difficulty sleeping, be prone to infection, or become sensitive to temperatures, visual patterns, and environmental lighting.

The absence of disease complications should not delay genetic testing when prolonged febrile seizures occur in infancy. The degree to which complications are seen in Dravet syndrome is much more variable than when the disease was first described.

Most, but not all, patients with Dravet syndrome test positive for SCN1A gene mutations. Patients who do not have a positive result for a SCN1A mutation may have a mutation in part of the SCN1A gene that is not examined for mutations in standard tests. All types of SCN1A mutations are seen in Dravet syndrome. These include missense, nonsense, frameshift, insertion, deletion, and duplication. Mutations of SCN9A, SCN2B, PCDH19 and GABRG2 genes are also seen rarely in those diagnosed with Dravet syndrome.

Approximately three out of ten Dravet patients have “no detectable” genetic mutation. It is expected that more precise genetic testing in the future will find genetic mutations in these patients.

For general information on gene mutations:

Summary of Dravet syndrome (SMEI)

  • Febrile seizures
  • Seizures begin in infancy
  • Intractable seizures
  • Status epilepticus
  • Tonic clonic seizures
  • Myoclonic seizures
  • Seizures become afebrile
  • Poor response to treatment
  • Moderate to severe developmental delay
  • SCN1a mutation

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