Prospective and longitudinal natural history study of patients with Type 2 and 3 spinal muscular atrophy: Baseline data NatHis-SMA study by Chabanon, A, Seferian, AM, Daron, A, Péréon, Y, Cances, C, Vuillerot, C, De Waele, L, Cuisset, J-M, Laugel, V, Schara, U, Gidaro, T, Gilabert, S, Hogrel, J-Y, Baudin, P-Y, Carlier, P, Fournier, E, Lowes, LP, Hellbach, N, Seabrook, T, Toledano, E, Annoussamy, M, Servais, L