Understanding the Expanded Disability Status Scale (EDSS)

The Expanded Disability Status Scale (EDSS) is a standardized system used to classify the severity and progression of multiple sclerosis (MS)—in other words, to provide a measure of how much disability the disease has caused. It is primarily used in clinical trials to determine who should participate in a given study and to assess the effectiveness of a treatment being researched.

Developed in the 1950s and refined in the 1980s, the EDSS has been "used in virtually every major clinical trial that has been conducted in MS during the last four decades and in numerous other clinical studies," according to the National Multiple Sclerosis Society (NMSS).

Unless you're participating in a clinical trial in which you'll undergo an EDSS assessment, it's unlikely you'll need to understand the scale. However, neurologists sometimes use the EDSS to evaluate the progression of a patient's MS to help make decisions about treatment—for instance, to determine if a change in medication is warranted.

How an EDSS Score Is Calculated

EDSS testing is conducted out by a neurologist or, in some cases, a trained nurse practitioner (NP), who will carry out a neurological exam to assess several distinct "functional systems" (FS)—areas of the central nervous system that control specific functions of the body.

Functional systems in MS evaluation include:

After examining these systems, the neurologist or NP will then score the patient based on his or her findings using an ordinal rating scale ranging from 0 to 10 in half-point increments.

The amount of time it takes to complete an EDSS will depend on the skill of the medical professional who administers it and the condition of the person being evaluated.

Typically, though, it's a relatively quick process: The neurological exam for determining individual FS scores typically takes between 15 minutes and a half an hour; the scores can be determined in a few minutes.

EDSS Rating Scale

Since a practitioner bases scores on what he or she observes during evaluations, some argue that ratings are somewhat subjective. That said, each score is aligned with specific criteria that help guide as consistent an application of the EDSS as possible.

Drawbacks of the EDSS

Although it continues to be used (and useful) for clinical trials, researchers and MS experts note that the EDSS has some shortcomings:

• It places too much emphasis on the ability to walk.
• It is insensitive to cognitive dysfunction in MS (like most scales based on a standard neurological exam).
• The calculation of EDSS scores can be complicated.
• It is insensitive to many clinical changes that a person with MS experiences; patients sometimes seem to “jump” from one part of the scale to another, rather than exhibit a smooth decline.
• One-point differences in scoring are irregular, depending on where they occur within the scale. For example, moving from 8.0 to 9.0 is a big deal, as it means a person essentially has gone from being self-functional to bed-bound. On the other hand, the difference between a score of 1.0 and 2.0 is minimal. These discrepancies can make it difficult to interpret in scientific studies.

A Word From Verywell

Though learning the nuances of the EDSS isn't necessary for most patients (with the exception of those participating in a clinical study), it can be helpful to have general knowledge of the system and to know what the FS scores mean if you tend to read research studies in order to better understand your multiple sclerosis or that of a loved one. Often, the more you understand about a chronic condition, the more control you'll feel you have.