Duchenne muscular dystrophy (DMD) is a severe, progressive, and muscle-wasting disease [1]. It is related to an abnormality in the dystrophin gene that results in a deficiency of the subsarcolemmal dystrophin and causes fibrosis and necrosis of skeletal and cardiac muscles [2-4]. Progressive congestive heart failure caused by dystrophin deficiency in the myocardium is the most common cause of death in this patient population. Thus, regular cardiac function assessment facilitates the prediction of mortality secondary to cardiomyopathy; however, most patients do not experience notable heart failure symptoms until the end stage due to physical inactivity [5]. Thus, early cardiac dysfunction assessment by echocardiography, electrocardiography, and 24-hour Holter monitoring and the reduction of afterload through the administration of angiotensin-converting-enzyme inhibitors are recommended [6]. In addition, early screening for various aspects of DMD are essential for disease management [7,8]. In a previous study, cardiac magnetic resonance imaging (MRI) revealed that 17% of patients aged 6–7 years with DMD showed signs of muscle fibrosis, and myocardial fibrosis is usually advanced before ventricular wall motion anomalies are detected by echocardiography [9]. While cardiac MRI is appropriate for early diagnosis of muscle fibrosis, echocardiograms are primarily used for myocardial function assessment in clinical practice. In general, young patients aged 10 years or younger may show normal systolic function on a conventional echocardiogram [3]. The deficiency of dystrophin facilitated the progression of muscle fibrosis in DMD. As in the skeletal muscle, the same pathological cycle is responsible for myocardial cell death and fibrosis leading to cardiomyopathy [1]. Therefore, if the association between the progression of skeletal muscle fibrosis and cardiac dysfunction is confirmed, it can be assumed to be helpful for early detection and monitoring of cardiac function exacerbation. During the deterioration of the disease, abdominal muscles are recruited to enhance chest wall motion and lower intrathoracic pressure, which improves venous return to the heart during inspiration [10,11]. An animal study confirmed significant associations between respiratory and cardiac functions and myocardial fibrosis [12]. However, no study has examined the relationship between fibrosis of the respiratory and extremity muscles and cardiac dysfunction in human.

By examining the associations of myocardial structural changes and cardiac dysfunction with fibrosis of the respiratory and extremity muscles in patients with DMD, another index that can be used to easily assess the exacerbation of DMD can be discovered. This study aimed to examine the associations between echocardiogram-based cardiac function indices and fibrosis of the abdominal and lower extremity muscles in patients with DMD to facilitate early detection of cardiac dysfunction and identify its predictors.

Twenty-one patients with DMD who gave their informed consent to participate in the study were enrolled. Patients who could not cooperate during the test, those on a mechanical ventilator, and those with a pacemaker or other electrical device were excluded. To determine the association between cardiac dysfunction and fibrosis of abdominal and lower extremity muscles, the following parameters were assessed. The study was approved by the appropriate ethics review board and was conducted in accordance with the principles embodied in the Declaration of Helsinki. All methods were carried out in accordance with relevant guidelines and regulations. Informed consent was obtained from all subjects and/or their legal guardian. The study was approved by the Institutional Review Board of Pusan National University Hospital (IRB No. H-1904-017-077).

Table 1 summarizes the patients’ general characteristics, echocardiographic results including strain ratios and DS scores. All patients were male and non-ambulant. Their mean age was 18.45±4.28 years. From total 84 sites of measurement, 96.4% of DS scores were interpreted as grade 2 or 3. The strain ratio showed a tendency to decrease in the order of rectus abdominis, quadriceps, biceps femoris, and medial gastrocnemius. The strain ratios of the abdominal muscle and quadriceps muscles were significantly higher than those of the medial GCM (p<0.05; Fig. 2). Post hoc comparison using the Mann–Whitney U-test with Bonferroni correction showed a higher strain ratio for the rectus abdominis muscle compared with the biceps femoris muscles and GCM (p<0.05). The strain ratio of the quadriceps muscle was also higher than that of the GCM (p<0.05; Table 2).

Spearman correlation analysis showed that the DS score was not significantly correlated with the cardiac function parameters. The strain ratio of the rectus abdominis muscle has a significant negative correlation with LVEF (p<0.05; Table 3, Fig. 3). Other cardiac function and valvular insufficiency including E/A, E’, LVEDd, and GLS were not significantly correlated with strain ratio of all muscles.

Dystrophin, the product of the DMD gene is presented in all types of muscle including skeletal, smooth, and cardiac muscle fibers [12]. Sequence and association of the muscle fibrosis of the different muscle groups during the progression of the disease is not fully investigated. In our study, the muscle fibrosis was most prominent in abdominal muscles to the proximal extremity muscles, followed by the distal extremity muscles. The degree of abdominal muscle fibrosis was significantly correlated with LVEF. The deficiency of dystrophin facilitated the progression of muscle fibrosis in DMD [4]. As in the skeletal muscle, the same pathological cycle is responsible for myocardial cell death and fibrosis leading to cardiomyopathy [1]. In previous study, the markers of abdominal and diaphragm muscle pathology was correlated with cardiac functional measures in mouse model of DMD [12]. Our study was first attempt to find this correlation in human DMD. According to the result of our study, the only skeletal muscle which showed significant correlation with cardiac dysfunction was degree abdominal muscle fibrosis.

Muscle weakness begins from the proximal muscles in the trunk, hip, and shoulder and progresses toward the distal muscles in DMD. Relatively, the weakening of the upper extremities progresses more slowly than that of the lower extremities [20]. The severity of muscle fibrosis was also higher in the proximal muscles than in the distal muscles in this study. The development of fibrosis can be assessed using elastography. The severity of fibrosis related to tissue stiffness is obtained by assessing the propagation of mechanical shear waves through the tissue with ultrasound [21]. Muscle fibrosis and increased intramuscular fat and connective tissue related to reductions in strength and function result in higher echointensity in the ultrasound image [22,23], and such changes in muscle echointensity reflect muscle disruption [24]. A previous study found a significant difference between the ultrasound echointensities of healthy and dystrophic muscles [25]. Muscle pathologies may reflect disease progression [26]; however, predicting disease progression based on the correlations between the degrees of fibrosis of respiratory muscles and skeletal muscles of the extremities and cardiac dysfunction may give additional information regarding the factor to predict the deterioration of the disease. In previous study, we examined echocardiographic parameters to assess early LV dysfunction in patients with DMD; confirming that ultrasound elastography may be inappropriate for the assessment of fibrotic changes in respiratory and cardiac muscles in patients with DMD [3]. In that study, we additionally determined the abnormal parameters for systolic cardiac functions in young patients; however, we examined their correlation with the severity of fibrosis of other muscles to predict the severity of cardiac dysfunction in non-ambulant patients with DMD. It is known that abdominal muscles are used for both expiration and inspiration to compensate weak diaphragmatic function in advanced DMD [10,27], and a previous animal study reported that abdominal muscle fibrosis is weakly associated with LV fibrosis but significantly associated with cardiac function [12]. The present study also confirmed that only the abdominal muscle is significantly associated with LV function. Regarding the direct effect of abdominal muscle fibrosis on cardiac function, the use of the abdominal muscles for inspiration in patients with advanced DMD creates a negative pressure in the thoracic cavity that draws more blood into the right atrium [28]. This is thought to increase venous return to the heart [12]. Based on our findings, the severity of abdominal muscle fibrosis can be used as an indirect indicator of LV function in patients with DMD. Further, the abdominal muscles play an important role in cardiopulmonary function along with other respiratory muscles, including the diaphragm, and their dysfunction should be recognized as an important clinical predictor of disease progression in patients with DMD. However, the degrees of fibrosis of other lower extremity muscles were not significantly associated with other cardiac dysfunction parameters reflecting preclinical myocardial structural changes in our study. The degree of abdominal muscle fibrosis to early predict the cardiac dysfunction before ventricular wall motion anomalies in DMD should be further evaluated. In general, DMD is diagnosed around the ages of 3–5 years, as lower extremity weakness and fatigue progress [20]. Because cardiac dysfunction is usually detected at an older age, examining the associations of myocardial structural changes on MRI before cardiac dysfunction occurs and the respiratory and extremity muscle fibrosis would give valuable clinical information.

One of limitation of this study is that the tests were performed in a small number of patients belonging to a limited age group and it may have affected the statistical significance in some result. In the case of DS score, most of them (96.4%) are grade 2 or 3 may due to the limited number and distribution of the age of the patients, it was judged that they could not reflect the differences between muscles with this qualitative data. Therefore, the result presented comparing the strain ratio between the muscles were mainly discussed as the main result of our study. Given the low prevalence of DMD, long-term follow-up of changes in the parameters associated with age-specific disease progression in a larger number of patients would reveal additional significant correlations. Secondly, the muscle ultrasound findings were not quantified with several techniques. However, using the parameters that are most widely used in clinical practice will make applying the study findings more accessible, and further studies based on these findings are encouraged.

This study is the first to have found a significant association between cardiac dysfunction and the severity of respiratory muscle fibrosis in non-ambulant patients with DMD. Subsequent studies can examine the value of this parameter as an index for early diagnosis and disease progression and use it to explore new treatment targets or therapeutic effects.