Juvenile myoclonic epilepsy

Juvenile myoclonic epilepsy (JME), also known as Janz syndrome, is a form of genetic generalized epilepsy (also known as idiopathic generalized epilepsy), representing 5–10% of all epilepsy cases. Typically it first presents between the ages of 12 and 18 with myoclonic seizures (brief, involuntary, single or multiple episodes of muscle contractions caused by abnormal excessive or synchronous neuronal activity in the brain). These events typically occur after awakening from sleep, during the evening or when sleep deprived. JME is also characterized by generalized tonic–clonic seizures, and a minority of patients have absence seizures. The genetics of JME are complex and rapidly evolving as over 20 chromosomal loci and multiple genes have been identified. Given the genetic and clinical heterogeneity of JME some authors have suggested that it should be thought of as a spectrum disorder. The prevalence of JME is approximately 0.1–0.2 per 100,000, constituting approximately 5–10% of all epilepsies. Some studies suggest that JME is slightly more common in females than males. The onset of symptoms ranges between the ages of 8 and 36 years, peaking between 12 and 18 years. Approximately 15% of children with childhood absence epilepsy and juvenile absence epilepsy subsequently develop JME. In most cases, myoclonic jerks precede the first generalized tonic–clonic seizure by an average (mean) of 3.3 years. A long-term population-based study suggested that 25 years after seizure onset, 17% of people with JME had all seizure types resolved, and 13% only experienced myoclonus despite having discontinued medication, meaning that approximately a third no longer had troublesome seizures. There are three principal seizure types which may occur in JME: myoclonus, generalized tonic–clonic seizures and absence seizures. Approximately one-third of patients have all three seizure types. The majority of patients (58.2%) have frequent myoclonic jerks and uncommon generalized tonic–clonic seizures.