All study subjects will be randomly assigned to receive treatment using methylphenidate at a dose of 20-30 mg/day depending on weight (20 mg/day for <30 Kg and 30 mg/day for >30 Kg)+ Carnitine 500-1500 mg/day (depending on weight ) or methylphenidate at a dose of 20-30 mg/day depending on weight (20 mg/day for <30 Kg and 30 mg/day for >30 Kg)+ Placebo for a 6 week double blind, randomized clinical.