• adema
• mri
• muscular dystrophies
• myositis

Purpose
The differential diagnosis between idiopathic inflammatory myopathies (IIM) and muscular dystrophies (MD) in the absence of muscular biopsy may be challenging, as the two diseases share very similar clinical and laboratory features. For both the diseases, MRI has been recognized as a useful tool for the identification and estimation of the extent of muscular edema, the amount of fat infiltration/substitution and therefore to define active/quiescent disease phases. The aim of the study is to assess the role of muscular MRI in the differential diagnosis between IIM and MD evaluating the patterns of muscular involvement.
Materials and methods
From 2008 to 2018, 135 MRI scans have been included: 92 patients diagnosed with IIM and 43 patients with MD. All the exams have been performed on a 1.5Tesla or a 3Tesla scanner with the following protocol: STIR (short tau inversion recovery), DUAL ECHO and DWI (diffusion-weight images) sequences, all on the axial plane, all including paravertebral lodges, buttocks and thighs. The images were evaluated in all the muscles singularly for the presence of edema and muscular atrophy according to a 0-3 Likert scale for severity (0 absent, 1 mild, 2 moderate, 3 severe). Additionally, adipose infiltration/substitution was evaluated in each muscle according to a 0-2 Likert scale for severity (0 absent, 1 infiltration, 2 substitution).
Results
Compared to MD, edema was significantly more prevalent in IIM in all muscular groups evaluated, such as pelvis’ muscles (p<0.001), thigh’s anterior lodge (p=0.044) and medial lodge (p=0.044), except for the posterior lodge (p= 0.137). In particular, the muscles in which the difference was more present were: gluteus (maximus, medium and minimus), iliopsoas, piriformis, pectineus, tensor fasciae latae, adductor longus, adductor brevis, obturator externus and obturator internus (p<0.001). On the contrary, adipose infiltration/substitution and muscular atrophy were significantly more prevalent in MD compared with IIM. In particular, adipose infiltration/substitution had a significant prevalence in MD in all pelvis’ muscles (p=0.05) except for iliopsoas (p=0.741), in all the muscles of the anterior lodge (p=0.001) except for rectus femoris (p=0.216),in all the muscles of the medial lodge (p=0.018) except for obturator externus (p=0.307), obturator internus (p=0.090) and pectineus (p=0.086) and in all thigh’s posterior muscles (p<0.001). Atrophy had a significant prevalence in MD in all muscles examined (p<0.001).
Conclusions
A significant difference in the distribution of muscular involvement between IIM and MD may be identified according to the parameters evaluated with muscular MRI, as well as the different patterns of muscular alterations, varying from edema to fat infiltration and substitution. In conclusion, muscular MRI may help in the difficult differential diagnosis between the two diseases and may be a useful tool to identify in reducing the number of muscular biopsies that may be reserved only for doubtful cases.