The measurement of ambulatory impairment in multiple sclerosis

The objective of this study was to examine the relationships between continuous measures of ambulatory impairment in MS patients and their ordinal counterparts. Much of the disability caused by MS is due to ambulatory impairment. The Expanded Disability Severity Scale (EDSS) and the Ambulation Index (AI) are ordinal measures of MS severity based largely on the maximal distance subjects can walk (Dmax) and the time to walk 8 m (T8), respectively. At EDSS levels 6.0 to 7.0 and AI levels 3 to 6, scores are defined more by the use of ambulatory aids, rather than by Dmax or T8. We determined Dmax (up to 500 m), T8, the EDSS score, and the AI in 237 ambulatory MS patients. The maximal distance subjects could walk and T8 were strongly related to their ordinal counterparts (Spearman r = 0.65 and 0.91, respectively), but the continuous measures showed considerable variability within EDSS and AI levels that the ordinal scales did not reflect. Most of the variability occurred at EDSS levels 6.0 to 7.0 and AI levels 3 to 6. Because the use of an aid did not clearly predict Dmax or T8, many patients in these ranges had better ambulatory function based on the continuous measures than those with less disability according to the ordinal scales. We found that Dmax and T8 provide more precise information about ambulatory impairment in MS than do the EDSS and AI, allowing better discrimination of differences between patients and potentially greater sensitivity to detect therapeutic effects in clinical trials.