Walk Tests a Reliable Measure of Exercise Capacity in MG Patients, Study Suggests

Two timed walk tests were found to be highly reliable for the measurement of exercise capacity in patients with mild to moderate generalized myasthenia gravis (MG), a study reports.

The six-minute walk test (6MWT) and the two-minute walk test (2MWT), both of which are commonly used to evaluate physical capacity and endurance, also correlated moderately with lung function, respiratory muscle strength and patient-reported quality of life, and may be used as secondary efficacy measurements in clinical trials, the researchers say.

The study, “Functional Exercise Capacity Evaluated by Timed Walk Tests in Myasthenia Gravis,” was published in the journal Muscle & Nerve.

Myasthenia gravis may limit the ability of patients to exercise due to symptoms such as muscle weakness, a tendency to tire easily, and malfunctioning of the muscles that control breathing.

“Due to the difficulty of measuring the maximum oxygen consumption in clinical settings and the absence of valid and reliable physical performance tests, there are shortcomings when monitoring MG patients’ physical performances,” the researchers said.

To take a closer look at this, researchers at the Faculty of Health Sciences, Hacettepe University in Turkey conducted a study to evaluate how useful the 6MWT and 2MWT would be for testing exercise capacity in MG patients. They also explored if patients’ performance on these tests could be a reliable measure of disease outcomes.

The 6MWT was developed by the American Thoracic Society, and officially introduced in 2002. It measures the distance a person is able to walk over six minutes. The 2MWT is the same, except it assesses walking distance over just two minutes.    

Both tests have been widely used and validated as sub-maximal measures of physical capacity in people with cardiopulmonary disorders and neurological disorders, including chronic obstructive pulmonary disease (COPD), multiple sclerosis, and pulmonary fibrosis.

For the study, 31 generalized MG patients, with a score of II or III (mild to moderate muscle weakness) according to the Myasthenia Gravis Foundation of America clinical classification, were enrolled.

Patients completed the 6MWT, the 2MWT, an MG-specific quality of life questionnaire (MG-QoL15T), the Quantitative Myasthenia Gravis test, plus spirometry tests to measure lung function and a test to measure the strength of breathing muscles (maximal inspiratory and expiratory pressures).

Test-retest analysis, which compares the results of two measures, repeated with a couple of days in between, showed that both the 6MWT and 2MWT had excellent reliability. Repeated tests were highly correlated, with coefficients of 0.894 for the 6MWT and 0.932 for the 2MWT.

A better performance on the tests was moderately associated (correlation coefficients ranged between 0.58 to 0.45) with greater breathing capacity, namely forced vital capacity and maximal inspiratory pressure, as well as with better MG-specific quality of life and lower disease severity scores.

Sensitivity and specificity results showed that both walk tests had comparable performances for discriminating between different levels of disease severity.

“The 6mWT and 2mWT have excellent test-retest reliability as well as moderate construct validity for the evaluation of MG patients’ functional exercise capacity,” the researchers concluded.

Given that both tests are easy to perform in the clinical setting and that both correlate moderately with MG-specific outcomes, the researchers also suggest they “can be used as secondary efficacy measurements in clinical trials.”

However, further studies are needed to see if these tests are also reliable measures in MG patients with more severe levels of disability and must be further validated by cardiopulmonary exercise tests.