Estimates of the walking distance in multiple sclerosis patients and their effect on the EDSS

Abstract

Sirs: In multiple sclerosis (MS) patients the Expanded Disability Status Scale (EDSS) [5] is a frequently used disability score for the evaluation of clinical disease burden and progression. It helps to monitor the course of MS [6, 8] and is part of the 3-scale-model [3], which is recommended for the observation of the effectiveness of immunomodulatory therapies. Between the scores 4.0 and 7.5 the EDSS overrates the functions of the legs [7] because it depends only on the maximum walking distance. Outside clinical studies the walking ability is at most times not tested, but the patient estimates his maximum walking distance with the risk of misjudging his ability to walk, because patients and even doctors are inaccurate when estimating distances [10]. A misinterpreting of the MS-related disability could be the consequence. In our MS outpatient clinic we investigated 104 relapse-free patients with symptoms at the lower extremities, who were still able to walk, with regard to their estimated maximum walking distance. After informed consent we included all consecutive patients until the sample size of about 100 patients was obtained. The mean age was 42.4 years. 82 of the patients were women (79 %) and 22 were men (21 %). The EDSS ranged from 1.0 to 7.5 (median EDSS 5.0). The interview including the question asking the current maximum walking distance was performed by the doctor. About half to one hour later the real walking distance was measured by an assistant, who walked once with the patients a pre-measured route. We compared statistically the estimated with the actual walking distance and calculated the absolute value of the misjudgement (Fig. 1). A logarithmic presentation was necessary because of the large range of false estimates. A Spearman correlation analysis was used to prove the dependence of the extent of the misjudgement on the real walking distance and the real EDSS. It turned out that only 21 % of the patients estimated their walking ability correctly (with an accuracy of ± 10 %). The walking distances were underestimated in 36 % and overestimated in 43 % of the patients. The MS patients incorrectly estimated the walking distance by a mean of 63 %. While 26 % of the patients overor underestimated their walking distance by more than 50 %, 6 % of the patients underestimated the best walking distance even by more then 100 %. In 27 % of the patients the false estimates had consequences on the EDSS measure with a change of at least 0.5 EDSS-points. In 7 % of the cases there was an erroneous estimate by 1.5 points or more (Table 1). The extent of the misjudgement was significantly correlated with the real walking distance but not with the current EDSS. There was no significant difference between women and men (79 % vs. 81 %). It is known that solely the dayto-day variability of the maximum walking distance accounts for a change of up to 1.5 points in some cases [1]. If the EDSS is based on an estimated instead of a measured walking distance, it should be regarded even more critically. The discrepancies were most prominent for actual distances of 100 to 500 meters, corresponding to an EDSS of 4.0 to 6.0. However, EDSSscores between 4.0 and 6.0 are known to change rapidly [11]. The risk of failure to detect an increase of disability has to be considered as well as a misinterpretation of an LETTER TO THE EDITORS