Placebo Controlled Trial of Dextromethorphan in Rett Syndrome

The study will last for 3 months and will be limited to MECP2 mutation-positive children, one year - 9.99 years of age. This clinical trial, which is a placebo-controlled study, will randomize patients to the drug or placebo to determine the benefits of DM vs placebo on cognition, behavior, or seizures if present. Your child will stay twice in the Pediatric Clinical Research Unit (PCRU) at Johns Hopkins ICTR, for 3 days during each admission. The first hospital stay will be for 3 days, before she starts the DM or placebo. The follow-up 3-day hospital stay will be 3 months after she starts taking DM or placebo. There will also be two interim follow up evaluations at 2 weeks and 1 month after she starts taking the DM or placebo consisting of a neurological evaluation, EKG, and blood work, which can take place at your local doctor's office or at Johns Hopkins, and will be paid for by this study. Our research nurse or research associate will contact you at least weekly during the first month, and at least monthly thereafter until the end of the 3-month study.