Development of an 11-tier Functional Scale for Spinal Muscular Atrophy


Clinical Management

Poster Number: 8


Meghan Moore Burk, PT, DPT, Children's Hospital Colorado, Thomas Crawford, MD, Johns Hopkins Hospital, Kristin Krosschell, PT, Ann & Robert H. Lurie Children's Hospital of Chicago, Susan Apkon, MD, Children's Hospital Colorado

Background: With the development of three effective SMN-specific treatments, the conventional “Type” classification of spinal muscular atrophy (SMA), based upon the untreated natural history of progressive degeneration, is now anachronistic. While outcome is best predicted by the combination of SMN2 copy number and age of treatment, there is a need for an easily administered ordinal scale that assesses post-treatment functional ability. This scale targets new needs: assessment of the impact of therapies and variables of care, stratification criteria for new clinical trials and biomarker development, and a basis for population-based assessment of the impact of therapy. Objectives: To address these needs, a new scale must be a standardized, easily administered, and broadly applicable metric of the functional abilities compromised by SMA. The value of this scale will relate to its performance on a number of important dimensions. Meaningfulness is key: the scale tiers need to assess clearly delineated functional tasks that are relevant to the population. Next is the intertwined dimensions of reliability and granularity: increasing the number of categories cannot compromise reliable assignment. The functional scale should mesh with existing therapist-guided outcome measures and be operational without special training. To maximize statistical power, the scale tiers should segregate groups of roughly equal size. Results: The scale was developed in three phases: scale creation, content validity through nominal group process and consensus methods, and interrater reliability between expert and novice doctors and therapists. An initial nine-tier classification was developed, then vetted by comparison of the categorical ratings of 74 patients (≥2 years old) by 6 raters at one site (3MDs, 3PTs); further definitional refinements were then assessed in 112 patients at two other sites. Inter-rater reliability between/within expert and novice PTs and MDs demonstrated strong correlations. Post-hoc refinements were made at an all-evaluators conference. Inter-rater reliability of the final 11-tier scale was established using factitious cases among all participating clinicians. Conclusions: We demonstrate progress toward development of a sensitive and reliable functional SMA scale, independent of evaluator training/experience, that is applicable across the range of ages and disease severity. Planned further studies will engage community through patient/family self-assessment.