Developmental and epileptic encephalopathies after negative or inconclusive genetic testing: what is next?

The redefinition of classical electroclinical syndromes and the emergence of neurogenetics has led to a revolution in the field of developmental and epileptic encephalopathies (DEEs). In this context, advances in genetic techniques are leading to the final diagnosis of a large proportion of patients with DEE. However, up to 50% of patients with DEE remain undiagnosed. For patients with uncertain genetic etiology, there is a pressing need for the implementation of new targeted treatments and precision medicine. In some undiagnosed patients, genetic reanalysis with further in-depth or reverse phenotyping are valuable diagnostic tools to clarify new variants of uncertain significance. In other cases, the implementation of new bioinformatic algorithms is required for the update and reassessment of previously generated genetic data. Moreover, many other clinical tools have been developed for the management of patients of DEEs after a negative or inconclusive genetic testing. In this review, we highlight advances and limitations of new diagnostic strategies used in DEE patients without a known genetic etiology. Finally, we provide a wide perspective on aspects that will need further research, especially in non-Mendelian inheritance DEEs, such as those related to somatic mosaicism of the central nervous system or epigenetic and oligogenic mechanisms