[1] Childrens Hosp Philadelphia, Philadelphia, PA 19104 USA
[2] Univ Sydney, Sydney, NSW, Australia
[3] Childrens Hosp Westmead, Sydney, NSW, Australia
[4] Univ Iowa, Dept Pediat, Carver Coll Med, Iowa City, IA 52242 USA
[5] IRCCS Fdn, Carlo Besta Neurol Inst, Milan, Italy
[6] UCL Inst Child Hlth, London, England
[7] Great Ormond St Hosp Sick Children, London, England
[8] UCL Inst Neurol, MRC Ctr Neuromuscular Dis, Queen Sq, London, England
[9] Nemours Childrens Hosp, Neuromuscular Program, Div Neurol, Orlando, FL USA
[10] Univ Rochester, Dept Neurol, Rochester, NY USA
[11] Univ Penn, Dept Biostat & Clin Epidemiol, Perelman Sch Med, Philadelphia, PA 19104 USA
[12] Univ Iowa, Dept Neurol, Carver Coll Med, Iowa City, IA 52242 USA
[13] Univ Penn, Childrens Hosp Philadelphia, Dept Neurol, Perelman Sch Med, Philadelphia, PA 19104 USA