Change in muscle strength over time in spinal muscular atrophy types II and III. A long-term follow-up study

Whether muscle strength deteriorates with time in spinal muscular atrophy (SMA) types II and III is still debated. We present a long-term follow-up study on muscle strength in 30 patients with SMA types II and III. Median follow-up time was 17 years. Median number of assessments was four. All patients were assessed by Manual Muscle Testing (MMT), Brooke upper limb scale and EK scale. There was a difference in muscle strength of the upper limbs from first to last assessment in SMA II (p < 0.0001) and SMA III patients (p < 0.02). In SMA II patients, the rate of yearly decline in strength (% MRC score) was 0.22 units (p < 0.03). The decline was independent of the grade of muscle strength at entry. In SMA II patients the decline in muscle strength was reflected in a loss upper limb function as measured by Brooke upper limb scale (p < 0.0001) and motor function as measured by EK scale (p < 0.0001), a loss of great importance to the patients’ need for practical assistance. This study demonstrates loss of muscle strength over time in SMA II and III patients. Because of the very slow deterioration, it takes years to detect this change, which has to be taken into account in future treatment trials.