The Spinal Muscular Atrophy Functional Classification System

To develop a functional mobility classification for spinal muscular atrophy (SMA), reflecting changing phenotypes in the era of disease-modifying agents, with associations to hip displacement, scoliosis, and pulmonary function determined.

Patients with SMA and Hammersmith Functional Motor Scale Expanded (HFMSE) assessments were included. The Spinal Muscular Atrophy Functional Classification System (SMAFCS), comprising levels I to IV, was constructed, referencing Functional Mobility Scale groupings. Receiver-operating characteristic analysis determined HFMSE thresholds by SMAFCS. Associations of hip displacement, scoliosis, and pulmonary function were determined.

Sixty patients (62% female) were included, with 312 HFMSE assessments (mean 5.3 [SD 2.7] per patient) over 7 years 6 months (SD 3 years 11 months). HFMSE thresholds by SMAFCS level were I, ≥27; II, ≥22, <27; III, ≥10, <22; IV, <10 (area under the curve 0.62-0.89; p < 0.001). HFMSE versus SMAFCS age trajectories were modeled, significant for levels I (p = 0.002), III (p < 0.001), and IV (p = 0.002) but not level II (p = 0.48). The prevalences of hip displacement and scoliosis by SMAFCS level were respectively I, 61%; II, 50%; III, 92%; IV, 96% (p = 0.008) and I, 42%; II, 50%; III, 85%; IV, 96% (p < 0.001). Pulmonary function (forced vital capacity percent of predicted) worsened by SMAFCS level: I, 96 (SD 15); II, 83 (SD 12); III, 75 (SD 16); IV, 48 (SD 27) (p < 0.001).

These preliminary data suggest the SMAFCS could be a valuable tool to stratify gross motor function and predict risk of clinical outcomes in SMA. Further work is required to study reliability and responsiveness to change.