Increased brain glutamate and its N-methyl-D-aspartate (NMDA) receptors found in the brain of younger Rett syndrome (RTT) patients cause toxic damage to neurons (the brain's nerve cells), and contributing to EEG spikes. Dextromethorphan (DM) acts by blocking NMDA/glutamate receptors. This study is being done to determine if DM will prevent the harmful over-stimulation of the neurons thereby reducing EEG spike activity. Treatment with DM consists of one of 3 different doses (0.25 mg/kg per day; or 2.5 mg/kg/day; or 5mg/kg/day), and aims to find out which dose if any will help improve EEG abnormalities, behavior, cognition, and reduce seizures, as well as improve breathing abnormalities, motor capabilities, bone density, and GI dysfunction.