Responsiveness and clinical correlates of continuous turns-amplitude analysis measures in myositis: A prospective longitudinal study.

OBJECTIVE

Turns-amplitude analysis (TAA) is a rapid quantitative electromyographic technique used in neuromuscular disorders, but its categorical interpretation (normal, myopathic, neurogenic) limits the granularity needed for longitudinal monitoring. We investigated whether continuous TAA-derived metrics could reflect longitudinal clinical change in idiopathic inflammatory myopathies (IIM).

METHODS

In this prospective single-center study, 30 adults with IIM underwent TAA of biceps brachii, quadriceps, and iliopsoas at baseline, 6, and 12 months. From each cloud, we extracted descriptors of cloud position, shape, point distribution, and the turns-amplitude relationship. Clinical improvement was defined using EULAR/ACR response criteria. Correlations with motor unit action potential descriptors, clinical and biopsy variables were assessed. Longitudinal trajectories were compared between improving and non-improving patients.

RESULTS

Percentage of myopathic points (mean |ρ|=.41), centroid Y (0.35), and regression-based metrics (0.34) showed the strongest alignment with clinical severity. Group-by-time interactions were found for percentage of myopathic points, slope of the linear regression constrained through the origin, and centroid Y. In an exploratory ranking, regression-based metrics performed best overall.

CONCLUSIONS

Continuous TAA cloud metrics showed clinicopathological associations and captured longitudinal differences between improving and non-improving patients.

SIGNIFICANCE: These findings support cloud analysis as a rapid, reproducible adjunct for tracking IIM recovery in research settings.