Dravet Spectrum Disorders describe a group of related epilepsies having a similar genetic cause, most commonly mutations in the SCN1A gene which encodes a sodium ion channel, NaV1.1, found in the brain. The Dravet Spectrum Disorders include the following syndromes, listed from least severe to most severe:
In these disorders, the first seizures usually occur before one year of age, with no known cause other than fever or illness. Seizures that occur with fever are called febrile seizures. In FS, the least severe form of this spectrum of disorders, the seizures remain associated with fever and subside with age. In GEFS+, febrile seizures are followed by afebrile seizures, which occur without fever. Seizures are typically generalized tonic clonic (GTCS) in nature; these involve stiffening followed by jerking/twithing of all extremities.
The more severe seizure syndromes--ICE-GTC, SIMFE, SMEB, and SMEI--also begin with febrile seizures. These may be GTCS, but may be clonic or hemiclonic (jerking/twitching of all extremities or just one side). Later, myoclonus (brief muscle jerks) as well as other seizure types emerge in a majority of affected patients.
Seizure severity and control varies along the spectrum of severity of these disorders, from mild impairment in FS to very severe in SMEI. Developmental delay and cognitive impairment also increase with severity of the seizure disorder along this spectrum, from mild or insignificant in FS to severe in SMEI.
Mutations in genes SCN1A and SCN1B that encode the protein components of the brain sodium ion channel Nav1.1 were first discovered in GEFS+ patients in 1999 and 2000. In 2001, scientists discovered that mutations in the SCN1A gene that encodes Nav1.1 sodium ion channels also cause SMEI, the most severe form of epilepsy in the Dravet Spectrum. In FS and GEFS+, there is usually a family history of febrile seizures or epilepsy, and these forms of epilepsy are inherited within the affected families in a genetically dominant pattern. In contrast, patients with SMEI usually have an SCN1A gene mutation that is "de novo", meaning that it is not inherited from their parents. Early diagnosis of these disorders in infancy or early childhood is critical for optimal treatment and best outcome. Genetic testing can help to diagnose the disorder. It gives clues about what the parents might expect.
Other names for Dravet Spectrum Disorders include the following, or some variation.
These diseases occur equally in both genders, and have no geographic or ethnic boundaries. Febrile seizures are very common, occurring in up to 4 out of 100 children at some point in childhood. Most children who have febrile seizures do not have a Dravet Spectrum Disorder or any other inherited form of epilepsy. As researchers and doctors learn more about these disorders, and awareness about the clinical spectrum broadens, the number of people diagnosed with Dravet Spectrum Disorders is increasing.
Understanding the causes of these diseases is an ongoing challenge to researchers. Genetic mutations are present in many patients with Dravet Spectrum Disorders. Most, but not all, patients test positive for a mutation in the SCN1A gene. The presence of the SCN1A mutation usually means a diagnosis of a Dravet Spectrum Disorder. However, three out of ten Dravet syndrome patients may have “no detectable” genetic mutation, yet they have all the clinical features of the disorder. These patients may have mutations in the parts of the SCN1A gene that are not currently examined in genetic testing for the disease. As genetic testing technology becomes more complete and precise, researchers suspect that similar genetic mutations will be found in most people with a clinical diagnosis of a Dravet Spectrum Disorder.
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