Rasch analysis of the modified Fatigue Severity Scale in neuromuscular disorders and comparison between sex, age and diagnoses

Joshua P. J. Fulton MBChB, James K. L. Holt PhD, Roger J. Mills PhD, Carolyn A. Young MD

Introduction/Aims Fatigue is a common and debilitating symptom encountered in the neuromuscular clinic. The 7-item Fatigue Severity Scale (FSS-7) is a Rasch-modified assessment validated in inflammatory neuropathies but not across a typical neuromuscular patient population. The aim of this study was to validate this measure in neuromuscular disorders and to compare between patient sex, age and diagnoses.

Methods The modified FSS-7 was mailed to patients recruited from a specialist neuromuscular clinic at the Walton Centre. Responses were subjected to Rasch analysis and descriptive statistics were performed on the Rasch converted data.

Results The mFSS-7 met the Rasch model expectations with an overall Chi-square probability of 0.4918, a strict unidimensional scale free from differential item functioning (DIF) that satisfied the model with substantial test–retest reliability using Lin's concordance correlation coefficient 0.71 (95% CI 0.63–0.77). A 15.7% ceiling effect was observed in this patient cohort. Post hoc analysis did not show any significant difference in fatigue between sex, age or neuromuscular diagnoses.

Discussion The self-completed Rasch mFSS-7 showed acceptable test–retest reliability across patients with varied disorders under follow-up in a specialist neuromuscular clinic. The ceiling effect constrains its use for those with the most severe fatigue. Future considerations could include assessment of the benefits of clinical interventions, particularly multidisciplinary team input or dedicated fatigue clinics.

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