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

Abstract

AbstractIntroduction/AimsFatigue 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.MethodsThe 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.ResultsThe 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.DiscussionThe 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.