Duchenne muscular dystrophy (DMD) is the most common congenital muscular dystrophy, affecting approximately one in 5,000 live male births [1]. DMD is characterized by progressive muscle weakness and atrophy, mainly involving the proximal muscles, rather than the distal muscles. As DMD progresses, trunk muscles and respiratory muscles, including the diaphragm, also get affected; therefore, patients experience progressive decline not only in physical, but also in cardiac and respiratory function [2]. Since there is currently no curative treatment to stop disease progression yet [3], regular check-ups for various functional aspects of DMD, including muscle power; bony deformities, including scoliosis or hip dislocation; pain; and pulmonary and cardiac function, are essential for disease management and intervention [3-7]. However, because there is considerable heterogeneity in the rate of progression across patients, a reliable assessment tool that can describe the functional status of patients at each follow-up period in clinical practice is warranted.

The Duchenne muscular dystrophy Functional Ability Self-Assessment Tool (DMDSAT) is a patient-reported outcome scale developed to assess and describe the functional status of patients with DMD throughout the entire disease course [8]. It was created as an easy assessment tool for patients or caregivers and a clinically relevant tool. In a previous study, the English version of DMDSAT demonstrated good validity across the entire disease course and good reliability (Cronbach’s alpha, 0.93) [8]. Consequently, DMDSAT has been used to describe and categorize the functional ability of patients with DMD in the literature [4,9-11].

Furthermore, Landfeldt et al. [12,13] reported that DMD is associated with a considerable cost burden to society and affected families, and interventions for DMD based on DMDSAT were helpful in cost-effectiveness analysis [14]. Considering the worldwide prevalence of DMD [15], translating DMDSAT into languages other than English is necessary to facilitate its implementation. To the best of our knowledge, no study has translated DMDSAT or reported the validity of DMDSAT in a version other than English. Accordingly, this study aimed to systematically translate DMDSAT into Korean and verify the reliability and validity of the Korean version.

This study was designed to translate DMDSAT into Korean and investigate the reliability and validity of KDMDSAT. Results showed that K-DMDSAT had excellent interrater and test-retest reliability and validity, which supports using K-DMDSAT in clinical practice.

Similar results were reported by Landfeldt et al. [8], who developed the original DMDSAT. Among 186 patients from the United Kingdom, DMDSAT showed very good reliability, with a Cronbach’s alpha of 0.93. Moreover, the study demonstrated that a one-point increase in the total score of DMDSAT was significantly associated with a 5.3% change in per-patient annual cost. Although the present study did not investigate the association between K-DMDSAT and clinical outcomes or cost-effectiveness, to our knowledge, it was the first to show a significant association between K-DMDSAT and clinically widely used scales–the Brooke scale and Vignos scale.

Although K-DMDSAT is a patient-reported outcome scale, it shows excellent interrater reliability. There are several possible reasons for this finding. First, the questionnaires are simple and easy to understand since the measure is designed to be completed by patients or caregivers who are not health professionals. Moreover, the questionnaires were about daily activities performed every day, not specific tasks, meaning that the respondents could easily fill out the assessment tool without profoundly thinking. Second, the items for each domain are listed in order of difficulty, which helps recognize the patient’s current function. Finally, because the time to complete the assessment is less than 10 minutes, a more reliable response can be expected with low fatigue and higher compliance.

Since there is a consensus that motor function, which reflects activities of daily living, is more clinically relevant than conventional motor examination, such as manual muscle testing, for assessing patients with muscular dystrophy [18], several functional assessment tools for DMD or other types of muscular dystrophy have been developed. For example, the Brooke scale [16] and Vignos scale [17] were developed to assess each body part, upper and lower limbs, respectively. The North Star Ambulatory Assessment [19], motor function measure [20], and ambulatory functional classification system for DMD [21] were developed to document the ambulatory or motor function of the patient. These scales have demonstrated significant reliability and validity in previous studies. However, since each instrument is related to only one functional aspect, two or more assessment tools should be utilized to comprehensively measure and document the functional status of patients with DMD. Conversely, K-DMDSAT concerns four functional aspects simultaneously, so it appears to be an appropriate tool for quickly identifying and documenting overall functional status.

The Comprehensive Functional Scale for DMD (CFSD) [21], developed as a simplified tool to assess the functional abilities of patients with DMD, includes not only body structures such as joint contracture or Cobb’s angle for scoliosis but also intellectual disability, participation at school or society, and the ability to use personal computers or powered wheelchairs, which reflect the utilization of technology. However, CFSD comprises 21 items in three categories and takes more time to complete owing to several factors, including checking the range of motion, calculating Cobb’s angle, and testing intellectual disability, making it difficult to use in clinical settings and only suitable for research purposes.

The Egen Klassifikation (EK) scale was also designed for the same purpose and extended in 2008 to include more functional abilities, including swallowing function (e.g., food textures) and distal hand function (e.g., ability to control joystick) [22]. However, since it was developed for the functional assessment of nonambulatory patients, there are some limitations in its application in all patients with DMD.

Compared to these two well-validated comprehensive scales, the CFSD and EK scales, the DMDSAT lacks items assessing other aspects of DMD, especially dysphagia and pain. Since dysphagia and pain are symptoms that require careful attention and are strongly associated with quality of life [23], extending to DMDSAT questionnaires on swallowing function and pain would provide more information and help plan the most appropriate intervention. Although DMDSAT is a scoring system, its purpose is to document functional status rather than to provide a quantitative evaluation of physical function. Therefore, we propose adding items on dysphagia and pain to the original format.

In addition, we believe a supplementary item should be considered for the scoring system of the ventilatory support domain of DMDSAT. Most patients typically begin ventilator use at night and eventually increase use during the day if there is dyspnea during the day or elevated daytime carbon dioxide levels despite adequate night treatment [24]. However, variability in ventilator use in DMD patients exists depending on the patient’s comfort and compliance with mechanical ventilation. In fact, some patients use only mouthpiece ventilation during the day, whereas others prefer nasal interfaces for ventilator support and alternate them throughout the day [25]. Additionally, rarely, some patients prefer ventilator support only when they are awake, so they don’t use the ventilator during the night [26]. According to the current DMDSAT items, it is difficult to evaluate such patients because they are not included in any given categories. Although, fortunately, there were no such patients in this study, there may be difficulty in evaluating DMDSAT scores for such patients in future clinical settings. Therefore, we propose assigning 1 point for patients who use ventilatory support only during the daytime.

This study had several limitations. Among the participants, only 11 (12.5%) patients received ventilator support. The mean score of each K-DMDSAT domain for this population was 1.63±1.29 for the arm function domain, 0.27±0.46 for the mobility domain, and 0 for the transfer domain. There might have been a bottom effect, though it was not investigated in the current study. Since there has been considerable improvement in the life span of patients with DMD owing to advances in standards of care, especially in respiratory management [27-29], additional items that could describe the functional status of ventilator users are needed. Moreover, the K-DMDSAT was not designed to consider children’s functional development, meaning that there is no age-specific information or score modification according to the patient’s age. Further studies to investigate the validity of K-DMDSAT in younger patients are warranted.

In conclusion, DMDSAT was systematically translated into Korean, and K-DMDSAT was verified to have excellent reliability and validity. K-DMDSAT is a short, clinically relevant assessment tool for patients with DMD that can be rapidly administered without assistantance. K-DMDSAT can help clinicians easily describe functional levels comprehensively at different stages of disease progression.