Functional Systems Organization of Multiple Sclerosis Living Disability Scale Items

Background: Multiple scales are capable of measuring elements of living disability and disease severity in multiple sclerosis (MS). The Incapacity Status Scale (ISS) and Expanded Disability Status Scale (EDSS) are two such measures, with the former assessing disability and the latter assessing neurological function based on functional systems scores. While there is a face-valid association between these constructs, prior research has not established a method of interpreting them in a joint context.

Objectives: To organize living disability report items into domains of functional neurological systems and assess the interpretability of the resulting scores.

Methods: Retrospective chart review was conducted for MS patients with neuropsychological testing data (N = 68), including the ISS and measures across multiple physical and cognitive domains. The ISS was assessed for internal consistency, and a varimax-rotated principal component analysis was performed. Averages of the items comprising each extracted component were compared to physical and cognitive measures by means of Pearson product-moment correlations.

Results: Two ISS questions were excluded from analysis due to high non-response rates: medical problems (86.8%) and sexual function (64.5%). A third, vision, was excluded due to poor internal consistency (item-total correlation = .137). 13-item ISS showed strong internal consistency (Cronbach’s α = .890). Four components were extracted based on eigenvalues greater than 1.000, corresponding to pyramidal/cerebellar, bowel/bladder, brainstem, and cerebral functions. Averages of the items associated with each extracted component correlated significantly with multiple external measures of function.

Conclusions: Living disability items map onto associated functional neurological systems as used in the EDSS. Scores representing these functional systems, produced through ratings of subjective disability, relate strongly to external functional measures.