Natural history studies in SMA have primarily focused on infants and children. Natural history studies encompassing all age groups and SMA types are important for the interpretation of treatment effects of recently introduced SMN augmenting therapies. This cross-sectional study investigated muscle strength, Hammersmith Functional Motor Scale (Expanded) scores and the patterns of muscle weakness in relation to age and SMA type. A total of 180 patients with SMA types 1-4 in the age range 1-77.5 years and median disease duration of 18 years (range 0-65.8 years) was included. With the exception of the early phases of disease in which children with SMA types 2 and 3 may achieve new motor skills and show a temporary increase in muscle strength, cross sectional data suggest that declining muscle strength and loss of motor skills over time are characteristic for all SMA types. Mean loss of strength is at least 1 point on the MRC and 0.5 point on the HFMS(E) scores per year. Trend lines compatible with deterioration of motor function and muscle strength start in childhood and continue into adulthood. The age at loss of specific motor skills is associated with disease severity. Triceps, deltoid, iliopsoas and quadriceps are the weakest muscles in all patients. Hierarchical cluster analysis did not show a segmental distribution of muscle weakness as was suggested previously.
