The facioscapulohumeral muscular dystrophy Rasch-built overall disability scale (FSHD-RODS)

Abstract

Background and objectives: Facioscapulohumeral muscular dystrophy (FHSD) is a debilitating inherited muscle disease for which various therapeutic strategies are being investigated. Thus far, little attention has been given in FSHD to the development of scientifically sound outcome measures fulfilling regulatory authority requirements. The aim of this study was to design a patient-reported Rasch-built interval scale on activity and participation for FSHD.

Methods: A pre-phase FSHD-Rasch-built overall disability scale (pre-FSHD-RODS; consisting of 159 activity/participation items), based on the World Health Organization international classification of disease-related functional consequences was completed by 762 FSHD patients (Netherlands: n = 171; UK: n = 287; United States: n = 221; France: n = 52; Australia: n = 32). A proportion of the patient cohort completed it twice (n = 230; interval 2-4 weeks; reliability studies). The pre-FSHD-RODS was subjected to Rasch analyses to create a model fulfilling its requirements. Validity studies were performed through correlation with the motor function measure.

Results: The pre-FSHD-RODS did not meet the Rasch model expectations. Based on determinants such as misfit statistics and misfit residuals, differential item functioning, and local dependency, we systematically removed items until a final 38-inquiry (originating from 32 items; six items split) FSHD-RODS was constructed achieving Rasch model expectations. Adequate test-retest reliability and (cross-cultural and external) validity scores were obtained.

Conclusions: The FSHD-RODS is a disease-specific interval measure suitable for detecting activity and participation restrictions in patients with FSHD with good item/person reliability and validity scores. The use of this scale is recommended in the near future, to determine the functional deterioration slope in FSHD per year as a preparation for the upcoming clinical intervention trials in FSHD.

Keywords: FSHD; Rasch-built disability scale; activity and participation; facioscapulohumeral dystrophy; outcome research; reliability; validity.

FIGURE 1

Item characteristic curve (ICC; grey line) for item 19 (ability to jump) as an example of item bias on factor age. ICC for item 19 (jump) showing uniform differential item functioning on age. The graph shows how patients aged <50 years (blue line) find this item easier to execute compared to the other two age categories (red [50 to 65 years] and green [>65 years] lines). Based on these findings, this item was split into item₁₉<50 years versus item₁₉≥50 years [Colour figure can be viewed at wileyonlinelibrary.com]

FIGURE 2

Facioscapulohumeral muscular dystrophy specific Rasch‐built overall disability scale (FSHD‐RODS) with 32 items. (a) Distribution of activity and participation assessment (ability location) of 714 patients with FSHD, assessed with the final FSHD‐RODS. A total of 0.3% of the patients demonstrated a floor effect and 6.6% demonstrated a ceiling effect (maximum scores). (b) Threshold map of the final 32 items (38 inquiries) as part of the FSHD‐RODS. The map shows the expected response for each item related to the ability of the patients using FSHD‐RODS. The easiest item was "able to fill in a form", the most difficult item was "able to run" for males 50 years and older. Zero logit is set as the average of item difficulty and patient ability. This means that a patient with a mean score would be able to clean the refrigerator (this item requires ‒0.457 logits) easily and would have a higher probability of being able to perform the easier tasks (these having a lower logit location score); conversely, this patient will have a higher chance of experiencing extra difficulty with the more difficult tasks and will most probably fail on these. (c) Graph demonstrating the relationship between the overall raw sum scores (vertical axis: ranging from 0 to 64: 32 items, maximum score per item is 2: 32 × 2 = 64) with the Rasch‐obtained corresponding interval scores (in logits; horizontal axis) showing the typical S‐shape pattern. In essence, this graph shows the transformation of raw ordinal‐based scores to interval Rasch‐based values. Three S‐shape figures are being presented with minor differences. (d) Graph showing the location of the 64 thresholds in the final FSHD‐RODS (32 items, three response options, meaning two thresholds per item) [Colour figure can be viewed at wileyonlinelibrary.com]

FIGURE 3

Association between the final facioscapulohumeral muscular dystrophy Rasch‐built overall disability scale (FSHD‐RODS) and the motor function measure (MFM) score. Significant associations were obtained between the two outcome measures (Spearman correlation coefficient 0.86)

FIGURE 4

Item difficulty hierarchy of the final facioscapulohumeral muscular dystrophy Rasch‐built overall disability scale (FSHD‐RODS) in the first versus the second assessment. Almost all items and patients were located within the 95% confidence interval (solid lines), reflecting ideal reliability