Design: This is a Phase I/II safety/efficacy trial with an adaptive trial design: one year of pretreatment baseline period serves the dual purpose of collecting patient-specific biomarkers of disease progression and collecting longitudinal neuroimaging and clinical data for selection of primary outcome measures. This baseline period is then followed by a double-blind, idebenone versus placebo treatment phase for a total of 2 years. Based on preliminary sample size estimates, current enrollment calls for a total of 66 patients (33 per arm).