Development of the SMA EFFORT: A new approach to characterize perceived physical fatigability in spinal muscular atrophy

Background: Fatigue and fatigability are commonly reported in spinal muscular atrophy (SMA). Physical fatigability, proposed to be the most relevant to SMA pathophysiology, encompasses performance-based and perceived physical fatigability (PPF) assessments. While performance-based measures have highlighted physical fatigability as an SMA hallmark, PPF is not well characterized due to the lack of disease- and construct-specific scales. Objective: Our aim was to create a patient-reported outcome measure tailored for SMA, named the SMA EFFORT, to improve PPF assessment. Here, we describe the scale development process, assess scale properties, and discuss future research and application. Methods: SMA and scale development experts organized a comprehensive physical activity item bank, relevant across the SMA phenotypic spectrum. Activities were systematically categorized by varying intensities and durations. The SMA EFFORT was completed by an international cohort of individuals with all types of SMA. To compare PPF across demographic and clinical variables, SMA EFFORT PPF percent (PPF%) composite scoring was established. Results: One hundred eighteen participants completed the SMA EFFORT. Total PPF% scores were broadly distributed within functional groups, with differences between non-sitters (35.1 ± 21.0) and sitters (24.9 ± 15.1) ( p = 0.006), and those with (34.4 ± 18.1) and without respiratory support (26.4 ± 17.8) ( p = 0.02). Participants treated with disease modifying therapy (DMT) showed similar scores to those without treatment ( p = 0.70). Further, no differences in scores were observed in participants with scoliosis surgery and those without ( p = 0.71). Subscale analyses revealed differences in mean PPF% subscale scores by functional group. Conclusions: The novel SMA EFFORT standardizes PPF ratings by anchoring activity to intensity and duration. Item and scale data insights will inform the next iteration, which will undergo additional investigation. The SMA EFFORT aims to improve upon current measures to better assess treatment impact on physical well-being across the SMA spectrum.