To further characterise CDKL5-related disorder, previously classified as an early-onset seizure variant of Rett syndrome, which is currently considered a specific and independent early-infantile epileptic encephalopathy. We describe the epileptic phenotype and neurocognitive development in three girls with CDKL5 mutations showing severe neurodevelopmental impairment, with different epileptic phenotypes and severity. The patients differed regarding age at epilepsy onset, seizure frequency, duration of "honeymoon periods", as well as EEG features. The "honeymoon period", defined as a seizure-free period longer than two months, represented, in our case series, a good indicator of the epilepsy outcome, but not of the severity of developmental impairment. However, even during the "honeymoon period", the interictal EEG showed epileptiform abnormalities, slowing, or a disappearance of physiological pattern. The natural history of CDKL5 disorder was compared between the three girls, focusing on the relationship between electroclinical features and neurological development. Our findings suggest that CDKL5 mutations likely play a direct role in psychomotor development, whereas epilepsy is one of the clinical features associated with this complex disorder.
Keywords: CDKL5 gene; Rett syndrome; developmental delay; epileptic encephalopathy; genetic epilepsy; honeymoon period; neurodevelopmental encephalopathy.