Design and Validation of a Specific Clinical Outcome Measure for Adolescent Andadult Patients With Spinal Muscular Atrophy: An Interim Rasch Analysis

OBJECTIVES: The SMA Life study aims to develop and validate a new clinical tool to assess key motor and beyond motor function dimensions relevant to the functional status of adolescent and adult patients with spinal muscular atrophy (SMA).

METHODS: This prospective, non-interventional ongoing study is being conducted at five centers in Spain and includes patients aged 16 years or older with a confirmed diagnosis of SMA 5q. 107 patients were recruited and 106 eligible for evaluation. A panel of neurologists, physiatrists, and patient representatives designed a 53-item clinical tool assessing seven key dimensions (bulbar function, breathing, axial, lower and upper limb function, fatigability, and other symptoms). This instrument was administered at baseline, 12 months, and 24 months. An interim Rasch analysis was conducted after the initial visit to evaluate its psychometric properties, such as model fit, dependency, reliability and construct validity.

RESULTS: Rasch analysis was used to select the best items for constructing the instrument, resulting in a final selection of 41 items. A global score, with different weights for each item, and an unweighted global score for each dimension of the tool were defined. Reliability, assessed using Cronbach’s alpha, was 0.939. A MADaQ3 value close to 0.1 supported the assumption of local independence. To assess the construct validity, Spearman correlation coefficients between global and dimension scores and the Hammersmith Functional Motor Scale Expanded, Revised Upper Limb Module, and SMA Independence Scale were estimated, showing strong correlations across all scores.

CONCLUSIONS: This interim baseline validation showed a reliable 41-item clinical tool with strong psychometric properties, effectively assessing functional status in adolescents and adults with SMA. Further prospective assessments are necessary to confirm these finding supporting its use in clinical and research settings.