Expert Perspective: Diagnostic Approach to Differentiating Juvenile Dermatomyositis From Muscular Dystrophy

Abstract

Clinical tools that can aid in the diagnostic differentiation of juvenile dermatomyositis from muscular dystrophy.

Figure 1

A proposed diagnostic algorithm of the approach to the medical examination of a child presenting with proximal muscle weakness and no clear pathognomonic rash of JDM with the goal of differentiating between JDM and MD and ruling out other diagnoses. Throughout the algorithm, purple diamonds indicate highlighted diagnostic tools which, in our expert opinion, may be especially helpful in differentiating diagnostic possibilities. In the first step, clinical history, physical examination, and initial laboratory features more consistent with JDM or MD are in the laterally placed colored boxes. ALT, alanine transaminase; ANA, antinuclear antibody; AST, aspartate transaminase; CK, creatine kinase; dsDNA, double‐stranded DNA; ENA, extractable nuclear antigen; IHC, immunohistochemistry; IMNM, immune‐mediated necrotizing myopathy; JDM, juvenile dermatomyositis; JM, juvenile myositis; LDH, lactate dehydrogenase; MCTD, mixed connective tissue disease; MD, muscular dystrophy; MHC‐I, major histocompatibility complex class I; MRI, magnetic resonance imaging; MSA, myositis‐specific autoantibody; MxA, myxovirus resistance A; NFC, nailfold capillaroscopy; SLE, systemic lupus erythematosus; SSc, systemic scleroderma; UPC, urine protein creatinine; VUS, variant of unknown significance.

Figure 2

Nailfold video capillaroscopy findings from a (A) healthy control, (B) the case study patient with JDM at treatment‐naive visit, and (C) 4 months after treatment initiation and (D–G) examples of key nailfold findings in JDM. (A) A healthy control patient; note the teeth‐on‐a‐comb appearance, regular spacing and organization, no hemorrhage, no dropout (>8 end‐row loops/mm), and lack of dilation (<20mm diameter measured from the apex). (B) Case study patient with JDM at treatment‐naive visit. Note the nailfold capillary drop out (bracket), dilation (triangle), and overall disorganization. (C) Case study patient with JDM 4 months after treatment initiation. The image shows the recovery of normal density, no dilation, and regular spacing. (D) Example of microhemorrhage (plus sign) and decreased density in treatment‐naive patient with JDM. (E) Example of dilation (triangle) and dropout (bracket) in treatment‐naive JDM. (F) Example of abnormal morphology (star) in JDM, defined by a nonconvex tip. (G) Example of abnormal morphology (star) with branched, “bushy” capillary in JDM. JDM, juvenile dermatomyositis.

Figure 3

(A–C) Magnetic resonance imaging scan of the case report patient: a boy aged 4 years with juvenile dermatomyositis. (A) Axial T2‐weighted spectral attenuated inversion recovery image of the midthigh shows diffuse increased signal within muscles of the thigh consistent with edema/inflammation. There is relative sparing of the distal RF muscle. (B) Axial T1 image of the midthigh shows normal muscle signal without fatty infiltration. (C) Coronal STIR image also shows diffuse increased signal within muscles of the thigh consistent with edema/inflammation. (D–F) Magnetic resonance imaging scan of a boy aged 12 years with noninflammatory myopathy. (D) Axial T2‐weighted modified Dixon image of the midthigh with fat signal nulled shows normal signal with no areas of high signal to suggest edema/inflammation. (E) Axial T1‐weighted image of the midthigh shows feathery high signal in muscle consistent with fatty infiltration; the VM, SM, and long‐head of the B muscles are most affected with relative sparing of the VL, ST, G, and S muscles. (F) Coronal STIR image also shows normal muscle signal. B, biceps; G, gracilis; RF, rectus femoris; S, sartorius; SM, semimembranosus; ST, semitendinosus; VL, vastus lateralis; VM, vastus medialis.

Figure 4

Muscle biopsy histopathology findings from the case of JDM described in the clinical scenario (A, C, E, and G) and a case of Becker MD in a 7‐year‐old boy confirmed with genetic testing (B, D, F, and H). (A) JDM hematoxylin and eosin: notable characteristics include perifascicular atrophy (subtle in this case), and perifascicular basophilic fibers. (B) MD hematoxylin and eosin: grouped atrophic, rounded, and basophilic fibers in the center of the image surrounded by abnormally large fibers. Patchy endomysial fibrosis is also present. (C) JDM Major Histocompatibility Complex class I: diffuse sarcolemmal staining with perifascicular accentuation. (D) MD Major Histocompatibility Complex class I: essentially negative for sarcolemmal staining and shows sarcoplasmic staining only in grouped atrophic fibers. (E) JDM C5b9: positive for capillary staining in areas of perifascicular atrophy and perifascicular basophilic fibers. (F) MD C5b9: negative for capillary staining. (G) JDM: positive for perifascicular sarcoplasmic myxovirus resistance A staining. (H) Dystrophin (C‐terminal) staining in MD showing an abnormal mosaic pattern, which is the most common pattern seen in patients with Becker MD. Myxovirus resistance A staining not performed for the patient with MD; expected to be negative and similar in appearance to slide F. JDM, juvenile dermatomyositis; MD, muscular dystrophy.