Understanding Neurological Rating Scales
Important features of the EDSS
The varied nature of symptoms in MS complicate the use of the EDSS. Care is needed when comparing scores because the EDSS scale is not linear. Differences between the steps are not the same. Symptoms at step six are not, for example, twice as bad as those at step three.
The EDSS is also non-linear in terms of the time spent at each stage of the scale. This can be seen from the following graph, which shows the average time that a person with MS stays at a given EDSS score, and how this can vary considerably from step to step.
For many years after diagnosis, people with MS often experience little change in their symptoms. They may, however, still progress between the first few steps of the EDSS during this time because the early part of the scale is very sensitive to small changes. This feature of the scale creates an incorrect impression of greater disease activity in the early stages of MS. People with MS may worry unnecessarily if they progress fairly quickly through the lower EDSS scores when, in reality, it is the higher scores that have more of an impact on mobility. This can be misleading when assessing the activity of a study drug. If one drug slows disease progression through the very sensitive early parts of the EDSS scale, it can appear, wrongly, to give greater benefit than one that slows progression through later parts of the scale.
When comparing the results of different clinical trials, it is important to notice the patients’ EDSS scores. Comparisons between two study drugs are difficult if the study population had low EDSS scores in one trial but higher scores in another.
Ideally, the trial should include patients with a range of EDSS scores to assess the effects of the study drug on a wide population.
