SCN1A and encephalopathy, progressive, early-onset, with brain edema and/or leukoencephalopathy: Dravet syndrome, also previously known as severe myoclonic epilepsy in infancy (SMEI), is a severe childhood epilepsy that is caused in around 80–90% of people by loss of function mutations in SCN1A (summarised in Scheffer et al. (2009); also see Rosander and Hallböök (2015)).