The Rasch-transformed gastrointestinal symptoms rating scale in myotonic dystrophy type 1 (RT-GSRS-DM1)

BACKGROUND: Myotonic Dystrophy type 1 (DM1) is a multisystemic neuromuscular disorder. Gastrointestinal (GI) symptoms significantly impact quality of life, but remain under-assessed. Currently, no DM1 specific GI questionnaire is available. The Gastrointestinal Symptoms Rating Scale (GSRS) is widely used but lacks validation with modern clinimetric methods. OBJECTIVES: To evaluate the GSRS using Rasch analysis in DM1 patients and develop a disease-specific, interval-level GI symptom measure. METHODS: Rasch analysis evaluated item fit, threshold ordering, differential item functioning (DIF), local dependency, and unidimensionality. Model fit was evaluated using chi-square statistics, item and person fit residuals, and the Person Separation Index (PSI). RESULTS: Four hundred and three DM1 patients (206 women, mean age 48.3 years) completed the GSRS questionnaire. The GSRS initial data did not meet Rasch model expectations. Three items (hard stools, heartburn, diarrhea) were removed and the item nausea was split by age category. The item constipation had misfit exceeding the Bonferroni threshold, but was retained due to its clinical relevance. The final model did not fulfill Rasch requirements (item fit residuals: -0.23, SD 1.29; person fit residuals: mean -0.27, SD 1.15; item-trait Chi-square: p-value < 0.001; degrees of freedom: 60). Acceptable person separation index (0.76) was obtained. CONCLUSION: This study highlights the challenges of measuring GI symptoms in DM1. Although this research is an important first step, more research is needed for developing a questionnaire that reflects the patient experience whilst simultaneously considering measurement accuracy.