Electroclinical Phenotype-Genotype Homogeneity in Drug-Resistant “Generalized” Tonic-Clonic Seizures of Early Childhood

Abstract

Purpose Children with refractory focal to bilateral tonic-clonic seizures, despite normal high-resolution imaging, are often not subjected to genetic tests due to the costs involved and instead undergo multimodality presurgical evaluation targeted at delineating a focal onset. The objective of this study was to ascertain genotype-phenotype correlations in this group of patients. Method An online hospital database search was conducted for children who presented in 2019 with drug-resistant epilepsy dominated by nonlateralizing focal-onset/rapid generalized (bilateral) tonic-clonic seizures (GTCS), subjected to presurgical evaluation and subsequent genetic testing due to absence of a clear focus hypothesis. Results Phenotypic homogeneity was apparent in 3 children who had onset in infancy with drug-resistant GTCS (predominantly unprovoked and occasionally fever provoked) and subsequent delayed development. 3-Tesla magnetic resonance imaging (MRI) scans were negative and video EEG documented a homogeneous pattern of multifocal and/or generalized epileptiform discharges with phenomenology favoring probable focal-onset/generalized-onset bilateral tonic-clonic seizures. All 3 tested positive for SCN1A gene variants (heterozygous missense substitution variants in 2 children, one of which was novel and a novel duplication in one that led to frameshift and premature truncation of the protein), suggestive of SCN1A-mediated epilepsy. This electroclinical profile constituted 3 out of 25 patients with SCN1A-epilepsy phenotypes at our center. Conclusions These cases suggest that children with early-onset drug-resistant “generalized” epilepsy are likely to have a genetic basis although the presentation may not be typical of Dravet syndrome. Hence, genetic testing for SCN1A variants is recommended in children with drug-resistant MRI negative focal-onset/generalized-onset bilateral tonic-clonic seizures before subjecting them to exhaustive presurgical workup and to guide appropriate treatment and prognostication.