Can manual ability be measured with a generic ABILHAND scale? A cross-sectional study conducted on six diagnostic groups

Abstract

Objectives: Several ABILHAND Rasch-built manual ability scales were previously developed for chronic stroke (CS), cerebral palsy (CP), rheumatoid arthritis (RA), systemic sclerosis (SSc) and neuromuscular disorders (NMD). The present study aimed to explore the applicability of a generic manual ability scale unbiased by diagnosis and to study the nature of manual ability across diagnoses.

Design: Cross-sectional study.

Setting: Outpatient clinic homes (CS, CP, RA), specialised centres (CP), reference centres (CP, NMD) and university hospitals (SSc).

Participants: 762 patients from six diagnostic groups: 103 CS adults, 113 CP children, 112 RA adults, 156 SSc adults, 124 NMD children and 124 NMD adults.

Primary and secondary outcome measures: Manual ability as measured by the ABILHAND disease-specific questionnaires, diagnosis and nature (ie, uni-manual or bi-manual involvement and proximal or distal joints involvement) of the ABILHAND manual activities.

Results: The difficulties of most manual activities were diagnosis dependent. A principal component analysis highlighted that 57% of the variance in the item difficulty between diagnoses was explained by the symmetric or asymmetric nature of the disorders. A generic scale was constructed, from a metric point of view, with 11 items sharing a common difficulty among diagnoses and 41 items displaying a category-specific location (asymmetric: CS, CP; and symmetric: RA, SSc, NMD). This generic scale showed that CP and NMD children had significantly less manual ability than RA patients, who had significantly less manual ability than CS, SSc and NMD adults. However, the generic scale was less discriminative and responsive to small deficits than disease-specific instruments.

Conclusions: Our finding that most of the manual item difficulties were disease-dependent emphasises the danger of using generic scales without prior investigation of item invariance across diagnostic groups. Nevertheless, a generic manual ability scale could be developed by adjusting and accounting for activities perceived differently in various disorders.

Figure 1

Flow diagram illustrating the analysis process steps. CP, cerebral palsy; CS, chronic stroke; DIF, differential item functioning; NMDa, neuromuscular adults; NMDc, neuromuscular children; PCA, principal component analysis; RA, rheumatoid arthritis; SSc, systemic sclerosis.

Figure 2

Disease-specific patterns of item difficulty according to the bimanual or proximal nature of the activities showing a differential item functioning. Differences between item difficulty ratings specific to each diagnostic group (δ_specific) and the average item difficulty for all diagnoses (δ_mean) are shown for each disorder (CS, chronic stroke; CP, cerebral palsy; RA, rheumatoid arthritis; SSc, systemic sclerosis; NMDc, neuromuscular children; NMDa, neuromuscular adults). Boxes indicate the 25% and 75% limits (the IQR); the vertical line inside each box indicates the median; vertical bars outside each box indicate the 10% and 90% limits and dots indicate the 5% and 95% outliers.

Figure 3

PCA results based on differences between disease-specific difficulty of the split DIF items and the average item difficulty across all diagnoses. CP, cerebral palsy; CS, chronic stroke; NMDa, neuromuscular adults; NMDc, neuromuscular children; RA, rheumatoid arthritis; SSc, systemic sclerosis.

Figure 4

Box plots showing the distribution of manual ability measures for each diagnosis. Boxes indicate the 25% and 75% limits (the IQR); the vertical line inside each box indicates the median; vertical bars outside each box indicate the 10% and 90% limits and dots indicate the 5% and 95% outliers. CP, cerebral palsy; CS, chronic stroke; NMDa, neuromuscular adults; NMDc, neuromuscular children; RA, rheumatoid arthritis; SSc, systemic sclerosis.