Developmental and epileptic encephalopathies (DEE) refers to a group of rare and severe neurodevelopmental disorders where genetic etiologies can play a major role. This study aimed to elucidate the genetic etiologies of a cohort of 53 Vietnamese patients with DEE. All patients were classified into known electroclinical syndromes where possible. Exome sequencing (ES) followed by a targeted analysis on 294 DEE‐related genes was then performed. Patients with identified causative variants were followed for 6 months to determine the impact of genetic testing on their treatment. The diagnostic yield was 38.0% (20/53), which was significantly higher in the earlier onset group (<12 months) than in the later onset group (≥12 months). The 19 identified variants belonged to 11 genes with various cellular functions. Genes encoding ion channels especially sodium voltage‐gated channel were the most frequently involved. Most variants were missense variants and located in key protein functional domains. Four variants were novel and four had been reported previously but in different phenotypes. Within 6 months of further follow‐up, treatment changes were applied for six patients based on the identified disease‐causing variants, with five patients showing a positive impact. This is the first study in Vietnam to analyze the genetics of DEE. This study confirms the strong involvement of genetic etiologies in DEE, especially early onset DEE. The study also contributes to clarify the genotype–phenotype correlations of DEE and highlights the efficacy of targeted ES in the diagnosis and treatment of DEE.

Ohtahara syndrome is one of the earliest and most severe forms of developmental and epileptic encephalopathy. Over the last decade, the rapid advances in molecular techniques, especially in high-throughput sequencing (HTS), have revealed that a majority of Ohtahara patients have genetic etiology. About 20 genes have been found to be related to this syndrome so far, and Next Generation Sequencing (NGS) technique is now an important genetic test for this syndrome. This study was conducted on 4 patients with Ohtahara syndrome referred to Children’s Hospital 2, Ho Chi Minh City. Whole-exome sequencing (WES) following targeted analysis on 283 epileptic encephalopathy–related genes was performed to identify disease-causing variants of the patients. Following multi-step bioinformatics analysis, trio-based Sanger sequencing confirmation, and variant classification according to standards of The American College of Medical Genetics and Genomics – 2015, we have identified 2 pathogenic mutations in 2 patients: OH3 (KCNQ2, c.868G>A, p.G290S) and OH4 (SCN2A, c.788C>T, p.A263V). The results of this study contribute to verifying the role of genetic factors in Ohtahara syndrome in Vietnamese patients. This study also confirms that NGS in general and WES, in particular, are reliable and useful in detecting genetic causes of Ohtahara syndrome, thereby, assisting in diagnosis and treatment of this syndrome.