The Muscle Cramp Impact Index: A Patient-Centered Scale for the Assessment of Muscle Cramps.

Abstract

Muscle cramps are common, variably disabling, and lack a comprehensive validated tool to assess their multidimensional effect. Previous cramp studies have relied primarily on cramp frequency and intensity, despite qualitative work demonstrating that sleep disturbance, daytime functioning, and mental health are also important to patients. Guided by these findings, the aim of this study was to develop and validate a patient-reported outcome measure, the Muscle Cramp Impact Index (MCII), to assess the effect of muscle cramps across diverse etiologies and clinical settings. This was a multisite, prospective scale-development and validation study conducted in neuromuscular, amyotrophic lateral sclerosis, hepatology, and kidney clinics at the University Health Network/Mount Sinai Hospital and Sunnybrook Health Sciences Centre (2018-2022). MCII item generation was based on a previous qualitative study, literature review, and cognitive interviews with affected patients. Content validity was assessed through review by 27 international neuromuscular and muscle cramp experts. Adult patients experiencing muscle cramps within the preceding 2 weeks were enrolled. Draft items were evaluated for missing data, floor/ceiling effects, and exploratory factor analysis (EFA). Test-retest reliability was assessed in clinically stable patients over a 2-week interval using weighted κ and intraclass correlation coefficients (ICCs). Construct validity was examined through predefined hypotheses correlating MCII scores with established measures of sleep disturbance, mood, pain, stiffness, and quality of life. A total of 105 patients (40% female individuals; mean age 60.0 ± 14.2 years) completed the field testing. Eighteen draft items were refined to 16 after patient and expert review. All items demonstrated low missingness (<6%). EFA identified 3 domains-cramp frequency/distribution, sleep interference, and interference with daily life-and 2 items were removed for floor effects or low factor loadings, yielding a final 14-item measure (score range 0-42). The MCII demonstrated moderate correlations with EuroQol 5 Dimension, Epworth Sleepiness Scale, Beck Depression Inventory, and Sleep Disturbance scores. Test-retest reliability was excellent (n = 57; ICC 0.83, 95% CI 0.72-0.89), confirming strong measurement stability. The MCII is a patient-centered, psychometrically robust measure capturing the multidimensional effect of muscle cramps. Limitations include reliance on self-reported clinical stability and inability to verify respondent identity for mailed questionnaires. Further evaluation of responsiveness and validation in additional cramp populations is warranted to support broader implementation.