MECP2-Associated Rett Syndrome Without Developmental Regression—A Case Series

Clinical phenotype expansion of neurodevelopmental disorders is increasingly important. With accessibility and diagnostic ability of genetic testing expanding, new clinical criteria will continue to be elucidated. Here we describe a case series of 4 female pediatric patients seen for global developmental delay in non-genetic, specialty clinics who received unexpected genetic diagnoses of <i>MECP2</i>-associated Rett syndrome (RS). These results highlight broadening clinical presentation for RS. None of the patients in this cohort met clinical RS diagnostic criteria at the time of genetic testing, as there was no period of typical development or developmental regression. These cases demonstrate both the importance of broad genomic sequencing for patients with global developmental delay and that increasing understanding of atypical and mild presentations of syndromes like RS will continue to evolve as genetic testing becomes standard of care for clinical features like developmental delay.