ASIRI study should allow to evaluate the efficacy and the tolerance of Riluzole in children and young adults (6 to 20 years of age) with SMA. This is a multicentric, randomized, double-blind study versus placebo, with two parallel groups treated and followed during 2 years. It has been preceded by a 7 days pharmacokinetic phase, concerning 14 patients, aiming to provide information on the kinetic profile of Riluzole in children. The drug could stabilize patients condition, and especially interrupt paralysis progression; those are the desired effects.