Prospective Study of the Natural History of Patients With Type 2 and 3 Spinal Muscular Atrophy

INCLUSION CRITERIA * Type 2 or 3 spinal muscular atrophy genetically confirmed * Age superior or equal to 2 years old up to 30 years of age included * For patients older than 6 years old, willing and able to comply with all protocol requirements and procedures. * For non-ambulant patients, able to sit upright in a wheelchair for at least three hours * Patients over 18 years of age and parent(s)/legal guardian(s) of patients \< 18 years of age must provide written informed consent prior to participating in the study and informed assent will be obtained from minors at least 7 years of age when required by regulation. * In France only: Affiliated to or a beneficiary of a social security category EXCLUSION CRITERIA * Previously treated with an investigational drug within 6 months prior the recruitment in this study. * Other condition which may significantly interfere with the assessment of the SMA and is clearly not related to the disease * Current or anticipated participation in any therapeutic investigational clinical studies. * Patients with specific contraindication to MRI (i.e. metallic foreign body, claustrophobia, and others deemed to be prohibitive by the investigators) will be allowed to participate, but MRI will not be performed. * For women : pregnancy or current breastfeeding