Dravet syndrome

Dravet syndrome
Classification and external resources
Specialty Neurology
OMIM 607208
DiseasesDB 33728
GeneReviews

Dravet syndrome, also known as severe myoclonic epilepsy of infancy (SMEI), is a type of epilepsy with seizures that are often triggered by hot temperatures or fever.[1] It often begins around six months of age.[1]

Signs and symptoms

Dravet syndrome has been characterized by prolonged febrile and non-febrile seizures within the first year of a child’s life. This disease progress to other seizure types like myoclonic and partial seizures, psychomotor delay, and ataxia.[2] it is characterized by cognitive impairment, behavioral disorders, and motor deficits.[3] Behavioral deficits often include hyperactivity and impulsiveness, and in more rare cases, autistic-like behaviors.[3] Dravet syndrome is also associated with sleep disorders including somnolence and insomnia.[3] The seizures experienced by people with Dravet syndrome become worse as the patient ages since the disease is not very predictable when first diagnosed.[3] This coupled with the range of severity differing between each individual diagnosed and the resistance of these seizures to drugs has made it challenging to develop treatments.[3]

Dravet syndrome appears during the first year of life, often beginning around six months of age with frequent febrile seizures (fever-related seizures). Children with Dravet syndrome typically experience a lagged development of language and motor skills, hyperactivity and sleep difficulties, chronic infection, growth and balance issues, and difficulty relating to others. The effects of this disorder do not diminish over time, and children diagnosed with Dravet syndrome require fully committed caretakers with tremendous patience and the ability to closely monitor them.[4]

Febrile seizures are divided into two categories known as simple and complex. A febrile seizure would be categorized as complex if it has occurred within 24 hours of another seizure or if it lasts longer than 15 minutes. A febrile seizure lasting less than 15 minutes would be considered simple. Sometimes modest hypothermic stressors like physical exertion or a hot bath can provoke seizures in affected individuals.[4] However, any seizure uninterrupted after 5 minutes, without a resumption of postictal (more normal; recovery-type; after-seizure) consciousness can lead to potentially fatal status epilepticus.

Causes

In most cases the mutations in Dravet syndrome are not hereditary and the mutated gene is found for the first time in a single family member.[2] In 70-90% of patients, Dravet syndrome is caused by nonsense mutations in the SCN1A gene resulting in a premature stop codon and thus a non-functional protein.[2] This gene normally codes for neuronal voltage-gated sodium channel Na(V)1.1.[5] In mouse models, these loss-of-function mutations have been observed to result in a decrease in sodium currents and impaired excitability of GABAergic interneurons of the hippocampus.[5] The researchers found that loss of NA(V)1.1 channels was sufficient to cause the epilepsy and premature death seen in Dravet syndrome.[5][6]

Genetics

The genotypic explanation of the disorder has been located on the specific voltage-gated sodium channel genes known as SCN1A and SCN2A. These genes are located on the long (q) arm of chromosome 2 at position 24.3 and code for the alpha subunit of the transmembrane sodium channel protein. A mutation in either of these two genes will cause an individual to develop dysfunctional sodium channels, which are crucial in the pathway for sending chemical signals in the brain, causing the phenotypic display of myoclonic epilepsy from the individual. A properly functioning channel would respond to a voltage difference across the membrane and form a pore through which only sodium ions can pass. The influx of sodium induces the generation of action potential by temporarily changing the charge of the cell. When the gene is mutated, the eventually translated protein improperly folds its pore segment within the cell membrane because it has different amino acid chemistry, which renders the channel inactive. It is also possible for a mutation to reduce the number of channels produced by an individual, which leads to the development of Dravet syndrome.[7]

Currently, the SCN1A gene is the most clinically relevant; the largest number of epilepsy related mutations characterized thus far occur in this gene.[4][8] Typically, a missense mutation in either the S5 or S6 segment of the sodium channel pore results in a loss of channel function and the development of Dravet syndrome. A heterozygous inheritance of an SCN1A mutation is all that is necessary to develop a defective sodium channel; patients with Dravet syndrome will still have one normal copy of the gene.[7]

Diagnosis

According to the Dravet Syndrome Foundation, the diagnostic criteria for DS requires patient to present with multiple of the following symptoms:[9]

Treatment

Seizures in Dravet syndrome are difficult to manage, but they can be somewhat reduced by anticonvulsant drugs such as clobazam, stiripentol, topiramate and valproic acid.[10] Because the course of the disorder varies from individual to individual, a standard treatment protocol is difficult to establish. A diet high in fats and low in carbohydrates also may be beneficial, known as a ketogenic diet. Although diet adjustment can help, it does not eliminate the symptoms. Until a better form of treatment or cure is discovered, those with this disease will have myoclonic epilepsy for the rest of their lives.[4]

Certain anticonvulsant drugs that are classed as Sodium Channel Blockers are now known to make seizures worse in most Dravet patients. These drugs include carbamazepine, gabapentin, lamotrigine, and phenytoin.[11][12]

Current therapeutic treatments include cognitive rehabilitation through psychomotor and speech therapy.[3] In addition, valproate is often administered to prevent recurrence of febrile seizures and benzodiazapine is used for long lasting seizures, but these treatments are usually insufficient.[13] Stiripentol was the only drug for which a double-blind placebo trial was performed and this drug showed efficacy in trials for treatment of Dravet syndrome.[13] This compound acts as a GABAergic agent and as a positive allosteric modulator of GABAA receptor.[13] Stiripentol, which can improve, quote: "focal refractory epilepsy", as detailed in a 2014 Cochrane Library article (by Francesco Brigo and Monica Storti), as well as Dravet's syndrome (which is, by its very definition, usually, though not always, very refractory- resistant to treatment and with a tendency to recur), supplemented with clobazam and valproate was approved in Europe in 2007 as a therapy for Dravet syndrome and has been found to reduce overall seizure rate by 70%.[13] In cases with more drug resistant seizures, topiramate and the ketogenic diet are used as alternative treatments.[13][14]

Cannabidiol (CBD) has received orphan drug status in the United States, for treatment of Dravet syndrome which will allow it to be studied.[15]

Epidemiology

Dravet syndrome is a severe form of epilepsy. It is a rare genetic disorder that affects an estimated 1 in every 20,000-40,000 births.

History

Charlotte Dravet first described severe myoclonic epilepsy of infancy in France in 1978 and the name was later changed to Dravet syndrome in 1989.[16]

References

  1. 1 2 Lhatoo, edited by Simon Shorvon, Renzo Guerrini, Mark Cook, Samden D. (2013). Oxford textbook of epilepsy and epileptic seizures. Oxford [u.a.]: Oxford Univ. Press. p. 13. ISBN 9780199659043.
  2. 1 2 3 Selmer, K.K.; Eriksson, A-S; Brandal, K.; Egeland, T.; Tallaksen, C.; Undlien, D.E. (1 October 2009). "Parental SCN1A mutation mosaicism in familial Dravet syndrome". Clinical Genetics 76 (4): 398–403. doi:10.1111/j.1399-0004.2009.01208.x.
  3. 1 2 3 4 5 6 Granata, Tiziana (1 April 2011). "Comprehensive care of children with Dravet syndrome". Epilepsia 52: 90–94. doi:10.1111/j.1528-1167.2011.03011.x.
  4. 1 2 3 4 Miller, I.O. and Sotero de Menezes. M.A. SCN1A-Related Seizure Disorders. 2007 Nov 29 [Updated 2014 May 15]. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2015. Available from: http://www.ncbi.nlm.nih.gov/books/NBK1318/
  5. 1 2 3 Cheah, C; Catterall, W.A. (2012). "Characterizing the role of sodium channels in mouse models of Dravet Syndrome".
  6. ↑ Sudden death in epilepsy: Researchers finger possible cause. By Jennifer Couzin-Frankel. ScienceNow, 8 April 2015
  7. 1 2 Wallace, Robyn. A Plethora of SCN1A Mutations: What Can They Tell Us? Epilepsy Curr. 2005; 5(1): 17–20.
  8. ↑ Sugawara T, Tsurubuchi Y, Agarwala KL, Ito M, Fukuma G, Mazaki-Miyazaki E, Nagafuji H, Noda M, Imoto K, Wada K, Mitsudome A, Kaneko S, Montal M, Nagata K, Hirose S, Yamakawa K. A missense mutation of the Na+ channel alpha II subunit gene Na(v)1.2 in a patient with febrile and afebrile seizures causes channel dysfunction. Proc Natl Acad Sci U S A. 2001; 98: 6384–9. [PubMed]
  9. ↑ "Diagnostic Criteria". Dravet Syndrome Foundation. Retrieved 17 March 2015.
  10. ↑ "Dravet Syndome Foundation: Treatment". Retrieved 1 January 2016.
  11. ↑ "NICE: Epilepsies: diagnosis and management". Retrieved 1 January 2016.
  12. ↑ "SCN1A Patients Advised to Avoid Sodium Channel Blockers". Retrieved 1 January 2016.
  13. 1 2 3 4 5 Chiron, C; Dulac, O (1 April 2011). "The pharmacologic treatment of Dravet syndrome". Epilepsia 52: 72–75. doi:10.1111/j.1528-1167.2011.03007.x.
  14. ↑ http://onlinelibrary.wiley.com/doi/10.1002/14651858.CD009887.pub2/abstract
  15. ↑ "Cannabis-Derived Dravet Syndrome Drug Gets US Orphan Drug Approval". Nov 18, 2013. Retrieved 21 July 2015.
  16. ↑ Dravet, Charlotte (1 April 2011). "The core Dravet syndrome phenotype". Epilepsia 52: 3–9. doi:10.1111/j.1528-1167.2011.02994.x. Retrieved 17 March 2015.
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