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Epilepsy is a chronic seizure disorder characterized by seizures that usually recur unpredictably in the absence of a consistent provoking factor. Most epilepsy syndromes are genetically heterogeneous; pathogenic variants in different genes cause the same syndrome in different individuals or families, and only a fraction of the potential genetic causes have so far been identified. Genetic assessment is enhanced when clinical information is available. Although a positive test result can confirm or suggest that an individual has a specific syndrome, a negative test result might be uninformative. For instance, fewer than one in five individuals with autosomal dominant nocturnal frontal lobe epilepsy have a pathogenic variant in any of the genes currently associated with that disorder. Conversely, in syndromes with incomplete penetrance and variable expressivity, a positive test result in an unaffected family member does not necessarily mean that the individual will develop epilepsy in the future, nor can it predict the specific phenotype if the individual does. An important example of this problem is genetic epilepsy with febrile seizures plus (GEFS+), in which some family members with a SCN1A variant remain unaffected, and phenotypes in affected family members vary from simple age limited febrile seizures to severe epileptic encephalopathies.1