Functional exercise capacity evaluated by timed walk tests in myasthenia gravis

Introduction: We sought to evaluate the test–retest reliability and construct validity of the 6‐ and 2‐minute walk tests (6mWT and 2mWT, respectively) in patients with myasthenia gravis (MG). Methods: Thirty‐one patients with generalized MG were enrolled in this study. The 6mWT, 2mWT, MG‐specific quality of life questionnaire Turkish version (MG‐QoL15T), quantitative myasthenia gravis test (QMG), and pulmonary function tests were administered. Results: The intraclass correlation coefficients of 2mWT and 6mWT were 0.894 and 0.932, respectively. The 6mWT and 2mWT had moderate correlations with forced vital capacity, maximal inspiratory pressure, QMG score, and MG‐QoL15T score (ρ for 6mWT: 0.579, 0.539, −0.572, and −0.474; ρ for 2mWT: 0.460, 0.446, −0.532, −0.457). Both tests had similar performances for predicting disease severity (area under the curve = 0.761 for 6mWT and 0.759 for 2mWT). Discussion: The 6mWT and 2mWT have excellent test–retest reliability as well as moderate construct validity for the evaluation of functional exercise capacity patients with MG. Muscle Nerve 59:208–212, 2019

Myasthenia gravis (MG) is an autoimmune disease in which both antibody and cellular immunological attacks block nicotinic acetylcholine receptors, muscle-specific tyrosine kinases, and/or other postsynaptic proteins at the postsynaptic neuromuscular junction. 1,2 Patients with MG may have limited exercise capacity because of proximal muscle weakness, fatigability, and impairment in respiratory muscles. [3][4][5] Assessment and follow-up of functional capacity in patients with MG is important for testing new therapeutic approaches or goal setting in rehabilitation. Because of 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 the physical performances of patients with MG.
As an alternative to laboratory exercise tests, the 6-minute walk test (6mWT) is commonly employed for testing functional exercise capacity. It assesses all of the systems required for exercise and reflects the submaximal level of exercise capacity. 6 It has been validated in cardiopulmonary disorders and neurological disorders as an exercise capacity test. 7-10 The 2-minute walk test (2mWT) has also been used as a short-distance walking test. [11][12][13][14] We sought to evaluate the usefulness of the 6mWT in patients with MG and to compare it with the 2mWT. We hypothesized that both 6mWT and 2mWT have good test-retest reliability and construct validity.

MATERIALS AND METHODS
Participants. This cross-sectional study was carried out in the Department of Neurological Physiotherapy, Hacettepe University; the Hacettepe University Non-Interventional Clinical Research Ethics Board approved this study ( . Forty-five patients with MG were enrolled in this study, but only 31 patients met 2 eligibility criteria: (1) age ranging from 18 to 65 years with a diagnosis of MG by a neurologist and (2) generalized symptoms (having score II or III according to the Myasthenia Gravis Foundation of America [MGFA] clinical classification). Patients with comorbid neuromuscular and cardiorespiratory disorders were excluded. All participants signed the written informed consent form and underwent study-related assessments.

Outcome Measurements
First Admission.
Demographic data (age, sex, body mass index) and disease course (time elapsed since the last myasthenic crisis, disease duration and medication history, and comorbid diseases) were recorded. Patients underwent an examination by the physiotherapists, with both performance-based and self-report assessments. The evaluation included quantitative myasthenia gravis score (QMGS), pulmonary function tests, 6mWT, 2mWT, and the myasthenia gravis-quality of life questionnaire Turkish version (MG-QoL15T). Timed walk tests were performed at 3-h intervals on the same day and were performed in the same order for all patients (6mWT followed by 2mWT). Patients did not do any practice walk before the first recorded trial. Each timed walk test was conducted approximately 1-2 h after pyridostigmine intake. Patients were taking 240-360 mg/day pyridostigmine.
Second Admission. Three to 7 days after the first admission, the 6mWT and 2mWT were repeated with 3-h intervals in between the 2 tests.
Quantitative Myasthenia Gravis Score. A thirteen-item performance-based scale was used to rate patients' performance from 0 to 3 (total score range 0-39). Higher scores indicate worse function. 15 Pulmonary Function Tests. The spirometry test was performed by a physiotherapist specialist in cardiopulmonary rehabilitation who followed the European Respiratory Society guidelines for using a Fitmate MED spirometer (Cosmed, Rome, Italy). 16 Forced expiratory volume in the first second (FEV1), vital capacity (VC), forced vital capacity (FVC), and FEV1/FVC were recorded. The highest score among the 3 trials was used.
Respiratory Muscle Strength Tests. Respiratory muscle strength tests were conducted with a portable electronic mouth pressure device ((Micro MPM; Micro Medical, Kent, England). Maximal inspiratory and expiratory pressures (MIP and MEP, respectively) were measured. 17 The predicted values of MIP and MEP were calculated from the same age-and sexmatched references. 18 Respiratory muscles were considered as weak if they were less than 60% of predicted MIP and MEP values. 19 Timed Walk Tests. Both the 6mWT and the 2mWT were performed in a 30-m hallway. The distances traveled were separately recorded within 2 and then 6 min. Patients were asked to rate their fatigue according to the Borg scale before and after the tests. Arterial oxygen saturation also was measured via pulse oximetry (Veron; Shenzhen Aeon Technology, Shenzhen City, Guangdong, China). The American Thoracic Society guidelines were followed for 6mWT and 2mWT. 6 The total distances walked in meters in 6 and 2 min were also calculated as predicted percentages. 20,21 Myasthenia Gravis-Specific Quality of Life Assessment, Turkish Version. All patients completed the MG-QoL15T at the first admission. 22,23 Statistical Analysis. Numeric variables were expressed as mean and standard deviation, and categorical variables were expressed as counts. Test-retest reliability and validity analyses were conducted according to the Consensus Based Standards for the Selection of Health Status Measurement Instruments (https://www.cosmin.nl/) guidelines. 24 Test-retest Reliability. The 6mWT and the 2mWT intertest reliabilities were assessed with intraclass correlation coefficient (ICC) analyses.
Predictive Validity. Receiver operating characteristic (ROC) curves and area under the ROC curves (AUC) analyses were used for predictive validity. Patients were divided into 2 groups according to the MGFA clinical classification: (1) IIa and IIb were grouped as 2 and referred to as minimal disability and (2) IIIa and IIIb were grouped as 3 and referred to as moderate disability. Sensitivity and specificity of 6mWT/2mWT to disease severity was determined.
Power Analysis. A post hoc power analysis was performed in G*Power 3.0.10 (Heinrich Heine University, Düsseldorf, Germany http://www.gpower.hhu.de/). 26 Analysis was carried out by using the 6mWT data (the first trial) with the results of Gibbons et al. 27

RESULTS
Thirty-one patients with MG were included in the study. No patient prematurely terminated the 6mWT or the 2mWT test due to fatigue or dyspnea, and no patient had substantial oxygen desaturation, which is defined as ≥4% decrease in arterial oxygen saturation from baseline. The mean changes in the fatigue perception during the first trials of 6mWT and 2mWT were 3.06 ± 2.15 and 1.74 ± 2.09, respectively. Patient demographic data and disease courses are displayed in Table 1. Test-Retest Reliability. There was excellent test-retest reliability for both the 2mWT and the 6mWT (ICC = 0.894 and 0.932, respectively). In addition, the ICC values of predicted 2mWT and 6mWT were 0.863 and 0.956, respectively.
Hypothesis Testing. The 2mWT and 6mWT tests showed a high correlation (ρ = 0.879). The other analyses of construct validity of both tests demonstrated moderate correlation and ranged from 0.446 to 0.579 (Table 2).
Predictive validity. According to the AUC values, the 6mWT and 2mWT had similar performances for discriminating different levels of disease severity (AUC= = 0.761 and 0.759, respectively; Fig. 1).
Power analyses. According to the post hoc power analyses, the power of the study (1-β) was > 99%.

DISCUSSION
This study has potentially useful findings because both timed walk tests were found to be reliable between test-retest conditions. Moreover, it was found that they had good construct validity. Both timed walk tests presented similar performances for discriminating among different levels of disease severity.
In previous studies, the 6mWT was shown to be an exercise capacity test in neuromuscular diseases. 14,28,29 McDonald et al. 29 found that 6mWT test-retest reliability was excellent in patients with Duchenne muscular dystrophy. However, modifications were used in this study, such as addition of an orientation video prior to testing, continuous verbal encouragement from the testing staff to maintain attention to the task, or a "safety chaser" to walk behind the participant during testing. We did not require modification. In our study, the tests were performed 1-2 h after pyridostigmine intake. Pyridostigmine may improve symptoms and maximize both reproducibility and distances traveled during the tests. However, reproducibility cannot be directly attributed to pyridostigmine intake unless the 2 conditions (with or without medication) are compared.
The normal values of the mean distance traveled in the 6mWT have been well defined in previous studies. 21,27,[30][31][32] Gibbons et al. 27 reported that the healthy population's normal 6mWT was 655 ± 84 m (in the first and unfamiliar trial) for people in the age range 20-40 years. Furthermore, Troosters et al. 30 reported normal values as 631 ± 93 m for people in the age range 50-85 years. The age range for patients in our study was 18-65 years, and the mean distance traveled in the 6mWT was significantly less than in both healthy young and healthy elderly participants. Likewise, the mean distance traveled by patients with MG in the 2mWT was less than the normal healthy participants' values reported by Bohannon et al. 20 Although that study was not designed to obtain normal 6mWT values from patients with MG, results from this study might be useful for future studies. Primary and accessory inspiratory muscle involvement is essential for functional activities, and this relationship has been demonstrated in obstructive and restrictive diseases. 33,34 We found a moderate correlation between MIP and 6mWT; this positive correlation indicates that a reduced exercise capacity in patients with MG may be related to inspiratory muscle weakness.
Anderson et al. 14 reported that the 2mWT is a potential alternative to 6mWT in neuromuscular diseases (not included in MG) for describing walking capability. We also found high correlations between the 6mWT and 2mWT. Clinicians should choose 2mWT for patients who cannot complete the 6mWT because of fatigue or dyspnea. However, it should be taken into consideration that 2mWT had lower correlations with other parameters such as pulmonary function and strength, and it was not sensitive to fatigability.
Patients with generalized MG were included in this study, and predictive validity was tested between MGFA classes II and III. Classes I and IV/V were not included in our study. Moreover, classes II a/b and III a/b were combined. The cutoff of values of timed walked tests are applicable to differentiate only between classes II and III. This was a limitation of our study because the results are not generalized for all patients with MG. To increase reproducibility, patients with ocular MG (class I) should be included in future studies. Class IV should also be included, but severe muscle weakness involvement may reduce reproducibility. The Borg scale was used for assessing fatigue perception. An additional fatigue scale could be used to assess fatigue in detail.
In conclusion, the 6mWT and 2mWT are performed easily in clinical settings and well tolerated in patients with MG. Both tests correlate moderately with disease specific outcomes and can be used as secondary efficacy measurements in clinical trials. However, the present study was carried out with a narrow spectrum of patients. Thus, the 6mWT and/or 2mWT may be integrated into routine assessments only in patients with mild and moderate generalized MG. However, additional studies that include different disability levels are required. Our results must also be confirmed by comprehensive studies that use cardiopulmonary exercise tests for criteria validation.
This study was presented at the National Cardiopulmonary Rehabilitation Congress, November 2017, Hacettepe University, Ankara, Turkey; and at the National Neurology Congress, November 2017, Antalya, Turkey.
Ethical Publication Statement: We confirm that we have read the Journal's position on issues involved in ethical publication and affirm that this report is consistent with those guidelines.