A neurodevelopmental disorder caused by variation in the KCND2 gene. This disorder is characterized by early-onset global developmental delay with impaired motor, speech and cognitive development. Patients often present muscle hypotonia, and less frequently, developmental epileptic encephalopathy, visual impairment and physical dysmorphisms. The disease is MONDO_1040003 (KCND2-related neurodevelopmental disorder with or without seizures).