Functional Assessments in Duchenne Muscular Dystrophy

Functional assessments in Duchenne muscular dystrophy are the standardised motor tests that clinicians and researchers use to track disease progression, determine clinical trial eligibility, and judge whether a treatment is helping. They include the North Star Ambulatory Assessment (NSAA), the six-minute walk test (6MWT), time-to-stand, time-to-climb-stairs, and the Performance of Upper Limb (PUL) scale. Each measures something specific, and each has limits.

This post is a practical overview. Test selection and interpretation belong to the neuromuscular team.

Why functional assessments matter

The visible parts of DMD progression (a child who used to run, then walks slowly, then needs a chair) hide a much finer trajectory underneath. Functional tests turn that trajectory into numbers that can be compared across visits, across patients, and across studies. They make it possible to say a child is changing faster or slower than expected, and to decide whether a clinical trial drug is doing what its sponsors hoped.

The same scores also determine clinical trial eligibility. A patient outside a certain range may not qualify for a particular study, which has practical consequences for families weighing trial participation. For background, see the DMD clinical trial reading guide.

The North Star Ambulatory Assessment (NSAA)

The NSAA is a 17-item functional scale specifically developed for ambulant boys with DMD. It scores activities such as standing, walking, running, climbing stairs, jumping, hopping, getting up from the floor, and standing on one leg. Each item is rated 0 (cannot do), 1 (does with modification), or 2 (does without modification), for a total score of 0 to 34.

Typical use:

• Trajectory across visits, often every 3 to 6 months in trials and routine care.
• Eligibility criteria for clinical trials.
• Endpoint in trials of disease-modifying therapies.

The minimal clinically important difference for the NSAA in boys aged 7 to 10 has been estimated in the range of 2 to 3.5 points, depending on the method used. Disease-modelling studies show NSAA scores typically improve until around age 6 and then decline by approximately 3 to 4 points per year on average after age 7. (Minimal clinically important differences in NSAA, PMC)

The six-minute walk test (6MWT)

The 6MWT measures how far a patient can walk in 6 minutes on a flat course. It captures endurance and is a primary outcome in many DMD therapeutic trials.

What to know:

• A drop of around 30 metres or more between visits is often treated as clinically meaningful, though context matters.
• The test is sensitive to motivation, fatigue, and recent illness.
• Results correlate with NSAA scores but are not identical; the two tests measure overlapping but distinct things. (NSAA, 6MWT, and timed items in ambulant boys with DMD, ScienceDirect)

Timed function tests

A small set of timed tests is widely used alongside NSAA and 6MWT:

• Time-to-stand (Gowers’ time): seconds from a supine position to a standing position.
• 10-metre walk/run: seconds to cover 10 metres.
• Four-stair climb: seconds to climb four standard stairs.

These tests are quick, reproducible, and sensitive to early change. Many trials use them as secondary endpoints. The four-stair climb is the primary endpoint of the Phase 3 EPIDYS trial that supported the FDA approval of givinostat (Duvyzat).

For background, see givinostat explained.

The Performance of Upper Limb (PUL)

The PUL is a functional scale designed for patients who can no longer perform the ambulatory assessments because of advanced disease. It evaluates shoulder, mid-arm, and hand-level activities (lifting the arm, opening containers, tearing paper, picking up coins).

Why it matters:

• It extends measurable function into non-ambulatory stages of DMD.
• It allows research and routine care to track patients across the full disease course, not just while they can walk.
• It is increasingly used as a primary or co-primary endpoint in trials enrolling non-ambulatory patients.

What happens after ambulation ends

When a patient stops walking, the ambulatory tests no longer apply. The care team shifts to upper-limb assessments (PUL), pulmonary function tests (forced vital capacity, peak cough flow), cardiac measures (ejection fraction, cardiac MRI), and quality-of-life questionnaires. The set of metrics evolves with disease stage.

For background, see respiratory care in DMD and cardiac care in DMD.

Limits of functional assessments

These tests are widely used, but they are not perfect:

• They are effort-dependent. A tired or unwilling child performs differently from one who is fresh.
• They measure function on the testing day, not the average week at home.
• Steroid timing can affect results.
• Some patients have stable scores while disease is progressing in non-measured systems (cardiac, respiratory).
• Tests developed for ambulant patients cannot capture upper-extremity decline late in the disease.

Trial endpoints are imperfect proxies for what families actually care about. For background, see accelerated approval and rare disease therapy.

What families can ask

A short list at clinic visits:

• Which functional tests does the centre track, and how often?
• What is the patient’s current trajectory compared to historical natural-history data?
• Are scores affecting trial eligibility for any therapy currently of interest?
• What can the family do to make tests reflect the patient’s real function (rest beforehand, consistent footwear, fed and hydrated)?
• Who in the care team administers the tests?

What is still uncertain

The optimal frequency of testing in routine care, the role of remote and home-based assessments, and how new disease-modifying therapies will change long-term trajectories on standard tests all continue to evolve.

The reasonable framing is that functional assessments are how the disease and its treatments are measured today, with real limits but no available substitute. The decisions about how to use the scores belong to the patient, family, and care team.

For related reading, see the DMD clinical trial reading guide, DMD standards of care, physical therapy in DMD, and the DMD care team explained.

Disclaimer: This post is informational and does not constitute medical advice. Decisions about diagnosis or treatment must be made with a qualified care team.