A person is sitting on top of a bar chart in the middle of the edss disability scale.

Deconstructing Disability Measures in People With MS: The EDSS

Chief concern following an MS diagnosis: the likelihood the disease will lead to disability.

Disability is a broad term, however. Losing the ability to walk safely is perhaps what first comes to mind – who doesn’t imagine a future spent in a wheelchair upon being diagnosed with MS? But other losses can be equally significant, like the inability to swallow or control one’s bladder.

How is MS disability measured? Does an Expanded Disability Status Scale (EDSS) score matter? And can patients measure disability without a doctor?

What is the Expanded Disability Status Scale (EDSS)?

The Kurtzke Disability Status Scale, created in the 1950s, specifically targeted disability severity in people with MS. Our doctors have used some version of this instrument to accurately measure MS disability since then.1

After several modifications, it was renamed the Kurtzke Expanded Disability Status Scale in 1983, now known commonly as the EDSS.2

How does EDSS work?

It measures levels of impairment on a 0-10 scale. A chief target for EDSS observation: ambulatory ability (being able to walk safely without assistance). However, a subscale known as Functional System Scores (FSS) also determines the status of eight functional systems in a person living with MS.1

FSS and EDSS: What’s the difference?

The EDSS combines scores identifying gait impairment with all other impairments as defined by the FSS. The EDSS final score ranks on a scale of 0-10.

FSS, as a subscale of the EDSS, tabulate scores on a scale from 0 to 5-6. The systems observed when calculating the FSS include:1

  1. Pyramidal (motor function)
  2. Cerebellar
  3. Brainstem
  4. Sensory
  5. Bowel and bladder
  6. Visual
  7. Cerebral or mental
  8. Other

So, while the EDSS ranks with a strong bias toward ambulation, the separate FSS score focuses on other functional limitations, including problems with sensory overload, balance, incontinence, brain fog, and more.

Is EDSS accurate?

It’s certainly easy to use, a global application that requires no specific equipment. For a general overview of someone’s disability, or to track functional changes, it can offer insights. However, while the EDSS remains the gold standard for measuring disability in MS, it’s criticized for:3,4

  • poorly representing non-walking disabilities such as cognitive issues
  • interrater variability: two or more doctors may arrive at different scores for the same patient or use different criteria that may vary as much as 20 percent between doctors when giving patients the same score
  • problems linking changes in scores to actual disability severity
  • being an unreliable measure of small changes
  • subjective scores in certain areas such as continence and cerebral function

Ambulatory versus nonambulatory disability

What about those with MS for whom gait isn’t a concern? Many can walk without assistance but struggle with other concerns, such as:

  • Bladder and/or bowel dysfunction
  • Memory issues
  • Speech problems
  • Inability to swallow
  • Relentless fatigue

This explains why the FSS was developed: not to de-emphasize gait, but to maximize observations about disruptions in cognitive and other functions that bring disability.

Other instruments are being developed to more fairly represent MS disability. Among them is the newer Multiple Sclerosis Functional Composite (MSFC) system.5

Why measure disability?

Even if the EDSS is imperfect, knowing one’s MS disability can inform therapy decisions. For instance, doctors use this number at diagnosis to choose therapies to help their patients manage and improve gait problems. The EDSS is also useful in measuring trends in MS severity across large patient populations over time, data that's critical for research planning.6 Finally, those planning to apply for social security disability insurance (SSDI) will need an EDSS score as objective evidence of disability.7

Calculating EDSS

Can anyone calculate their EDSS score? Sure, but there are dozens of parameters to consider. Unless you’re a medical professional, you may not understand the language used by the instrument. For most, getting an EDSS score is a task best left up to the neurologist.

Yes, there’s an app for that, but…

Online EDSS calculators and smartphone apps do exist. They claim to do the work for you. Whether these applications are reliable remains a common concern.8 Also, people living with MS may not be objective about how it impacts daily living. Fatigue, depression, and other symptoms can color judgment, skewing EDSS results that are far worse—or far better—than the reality.9

If you really need a solid measure, it’s best to leave the scoring to your MS specialist, who ideally should be charting more than just your ability to walk anyway.

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