JDM is a chronic disease, where differentiation between activity and remission can be difficult, if evaluated only through clinical and laboratory manifestations. Other tests, such as invasive tests, are difficult to perform in children. For this reason, more accessible exams such as muscle US (cheaper) or MRI (more expensive but considered the gold standard), are potential and important tools, respectively, in determining disease activity. The use of more widely disseminated software, such as US B-Mode and PD, can be useful, both in the assessment of disease activity and in differentiating individuals with or without inflammatory muscle involvement.

This study observed that US was able to differentiate patients from controls, through qualitative and semi-quantitative gray scale, and only in the deltoid and anterior tibialis muscles through PD. We observed that the semi-quantitative gray scale US findings of the proximal musculature were associated with disease activity. Evaluation by SE was not able to differentiate patients from controls, nor to detect disease activity in patients.

Regarding the clinical-laboratory parameters, it was observed that the increase in muscle enzymes and VAS, did not present a strong association with US parameters. However, higher DAS scores ​were associated with loss of normal muscle architecture in all muscles on gray scale, with significant changes in the biceps brachii and forearm flexor muscles on semi-quantitative gray scale, and with greater PD positivity in the anterior tibialis. Regarding the strength and muscle function assessment questionnaires, it was observed that lower CMAS scores ​​were associated with an important change in the semi quantitative gray scale of all muscles, with the exception of the anterior tibialis muscles, while for the MMT, such association occurred only in the deltoid and forearm flexor muscles.

The most recent studies suggest the standardization of videocapillaroscopy due to its ability to provide more accurate results, as it does not require extensive training by the examiner [27]. However, the available device for this study was the conventional stereomicroscope, but its limitation can be considered overcome as the examiner is highly trained to perform this examination. NC can be considered a good marker for skin activity, but it is not always a good marker for muscle activity [11, 12]. In the present study, the SD pattern at NC was not associated with changes in US parameters.

In JDM, US proved to be useful in helping to define the degree of disease activity, and also in patients’ follow-up, demonstrating severity and muscle damage associated with myopathy. Although there are currently no validated criteria for the use of gray scale and PD, recently, a study proposed validation for the use of US only in the rectus femoris muscles 28, 29]. Meng et al. assessed 37 patients with IIM, 17 with DM, and 6 controls, and observed an increase in the echogenicity of patients when compared to controls, suggesting muscle atrophy [22]. Another more recent validation study of US for JDM showed the same results [29]. This present study presented similar results in the qualitative and semi-quantitative gray scale for all muscle groups assessed.

Regarding the evaluation of PD, the results are conflicting. In the study by Meng et al., patients presented higher PD scores, while in the one by Mamyrova et al., no similar results were found [22, 29]. Our study observed increased vascularity in PD only for the deltoid and anterior tibialis muscles. While in the study by Meng et al., it was concluded that the presence of pathological PD was more associated with short-term disease [22], it was not possible to confirm this fact in our study, since no association was found between PD positivity and presence of disease activity. Reimers et al. assessed muscle biopsies from adults with IIM and showed that in acute phase, US tends to show a reduction in echogenicity due to muscle edema, while in chronic phase an increase in echogenicity due to atrophy tends to occur [30]. However, Bhansing et al. [31] assessed 17 patients with JDM, 7 of whom were considered to have disease activity, and concluded that in acute phase there is an increase in muscle echogenicity and an even more significant increase when compared to patients in clinical remission. Habers et al. observed a slight increase in echogenicity at diagnosis, with a more significant increase during the course of the disease, particularly in a patient who developed a chronic course [9]. These findings were attributed to the presence of edema in the initial phase of the disease and to fatty infiltration in the follow up [9]. Although our study showed an increase in echogenicity [21] (through the semi-quantitative gray scale) in deltoids, biceps brachii and quadriceps femoris muscles of patients considered to have disease activity, we cannot infer at which stage of the disease they were. The qualitative gray scale, where the loss of normal muscle architecture was assessed, did not show significant differences between active and inactive disease. US proved to be even more useful in the examination of proximal muscles, where the prevalence of involvement is known.

The use of elastography in musculoskeletal assessment is still limited and the results are controversial. In this study, SE showed conflicting results when compared with the other clinical-laboratory parameters, and it was not possible to infer associations between increased muscle stiffness in the SE and any other parameter. The SE showed low sensitivity, specificity and PPV, so it was not possible to identify a cut-off from which we would consider the stiffness found to be pathological. However, its high negative predictive value shows that this test was able, in 87.2% of cases, to demonstrate the absence of inflammatory muscle involvement.

In a study that assessed patients with myopathies, one with JDM, another seven with other myopathies, and 20 healthy individuals, revealed that the patients with myopathies showed a reduction in muscle elasticity through a score that comparatively assessed the musculature with the fat around it, in addition to a significant increase in echogenicity only in the biceps brachii muscle [32]. Yet, another study evaluated 18 lesions seen in MRI in 17 patients with different myopathies and showed a reduction in muscle elasticity through a score calculated between the affected musculature and the adjacent musculature, which was more evident in patients with inflammatory myopathies than in other diagnoses (overlapping patients with systemic lupus erythematosus, rheumatoid arthritis and mixed connective tissue disease) [33]. Our study did not find similar results, but a different methodology was used by us.

While US is a valuable tool for muscle assessment, it may not always differentiate between inflammatory myophaties and muscular dystrophies. Muscular dystrophies typically exhibit a uniform increase in muscle echogenicity with significant attenuation of the US signal in deeper layers. In contrast, inflammatory myopathies also show a homogeneous increase in echogenicity without the attenuation seen in dystrophies. However, in advanced stages, inflammatory myopathies can show tissue destruction with fibrosis replacement, leading to severe muscle atrophy and signal attenuation, potentially causing confusion with muscular dystrophies [34].

The only study that assessed only JDM patients, included 18 children with 10 lesions considered active on MRI and used a pre-established score in biceps brachii and quadriceps femoris muscles of healthy children [16, 35]. This study concluded that SE did not correlate with disease activity, clinically or by MRI, and low sensitivity and specificity for detecting active myositis by SE was observed [35]. Our study found similar results for detection of active myositis (clinical and laboratory), however comparisons with MRI have not been made. The use of elastography is consolidated for more restricted lesions within an organ, such as nodular lesions of the breast and liver [26]. As the muscle can be heterogeneously and diffusely inflamed, we consider that this may have been an important factor that limited the detection of alteration using the SE technique. Other non-inflammatory, mechanical and exertion changes that may alter muscle fiber consistency may also have limited SE assessment [36, 37].

The study has some limitations, such as the small power of effect of the sample, due to the small number of patients in activity (27.3%). When analyzing active and inactive patients together, this may have resulted in underestimated cutoff values for quantitative and semi-quantitative changes. Another point was the inclusion of two patients older than 18 years old (although with JDM diagnosed before this age). Other limitations are the lack of standardization for assessment by SE, whose method of measurement varies greatly in the literature, and, additionally, the absence of intra and inter-observer assessments. This was compromised by the difficulty of having another qualified professional in SE to perform dynamic and static measurements of the method, thus preventing the evaluation of the reproducibility index of this tool. An additional limitation is the lack of comparison with the gold standard imaging method, which is MRI. Although this is a limitation, it was not the aim of the study to compare US images with MRI, especially since previous studies have already demonstrated the US effectiveness in detecting muscular abnormalities [29, 38]. This study has strengths such as the quality of the methodology, citing the fact that all the examiners were blind to the other assessments, the execution of the assessments in very short timeframes or on the same day, with the assessment of several muscle groups, the assessment by professionals with expertise in their respective areas, and the use of a control group.

In conclusion, US was able, through qualitative and semi-quantitative gray scale assessment, to differentiate patients with JDM from healthy controls, while PD was able to perform such differentiation only for deltoids and anterior tibialis muscles. However, only a few muscle groups showed an association between the US findings, gray scale, PD and SE, with disease activity markers. SE was not able to differentiate patients from controls, nor to detect disease activity in patients.