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Myositis

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Authors

Julia Buddendorff, MD
Emergency Medicine Resident
University of Florida College of Medicine – Jacksonville

John Kiel DO, MPH
Assistant Professor of Emergency Medicine
Assistant Professor of Sports Medicine
University of Florida College of Medicine – Jacksonville

Summary

  • Myositis refers to any pathology that causes muscle inflammation, but most of the literature on musculoskeletal ultrasound for myositis is its application for idiopathic inflammatory myopathies.
  • Myositis causes increased sonographic echogenicity, which is more pronounced as the disease progresses. Chronic cases also include muscle wasting, fibrosis, and eventually lipomatosis.
  • The appearance of echogenicity in IIM can be qualitatively described using adjacent structures or can be quantified using computed grayscale analysis.
  • Sonographic evaluation of myositis is fast and readily available at the bedside, but it can be limited by operator experience.

Introduction

  • Myositis is a broad term for inflammatory muscle changes that may be associated with muscle tissue damage, infection, medication, autoimmune disease, metabolic derangements, and genetic disorders.
  • The use of musculoskeletal ultrasound for myositis has been studied mostly in the context of the idiopathic inflammatory myopathies (IIM) including dermatomyositis (DM), polymyositis (PM), juvenile myositis, and inclusion body myositis.
    • These processes present as bilateral symmetric proximal muscle weakness that is insidious in onset.
    • Muscle-derived enzymes are elevated, including creatine kinase (CK) and lactate dehydrogenase (LDH).
    • Aldolase, aspartate aminotransferase (AST), and alanine aminotransferase (ALT) may also be elevated.
    • CK is the most sensitive laboratory test for myositis and is typically trended throughout the course of illness, which may be acute, subacute, or chronic.
  • Imaging options for IIM include MRI and ultrasound. While MRI has deeper tissue penetration, ultrasound has superior spatial resolution for superficial muscle and fascia.[1]Endo, Y., & Miller, T. T. (2018). Myositis and fasciitis: Role of imaging. Seminars in Musculoskeletal Radiology, 22(3), 286–298. https://doi.org/10.1055/s-0038-1641572

Ultrasound Evaluation

  • The pathologic process of IIM involves replacement of muscle tissue with fat, which leads to an increase in echogenicity on ultrasound.[2]Albayda, J., & van Alfen, N. (2020). Diagnostic value of muscle ultrasound for myopathies and myositis. Current Rheumatology Reports, 22(11), 82. https://doi.org/10.1007/s11926-020-00947-y
    • This finding on ultrasound suggests chronicity and may be accompanied by muscle thinning.
    • With treatment of IIM, these findings may reverse, which makes ultrasound a useful tool in disease monitoring.
  • Several methods of quantifying muscle echogenicity have been described:
    • Heckmatt scoring uses qualitative data for muscle echogenicity, appearance of muscle architecture, and ability to discern deeper structures as 4-point scale to grade severity.[3]Albayda, J., & van Alfen, N. (2020). Diagnostic value of muscle ultrasound for myopathies and myositis. Current Rheumatology Reports, 22(11), 82. https://doi.org/10.1007/s11926-020-00947-y
    • Muscle echogenicity can also be quantified using grayscale analysis on imaging
Myositis US Normal Muscle
Normal vastus lateralis (A, B) and tibialis anterior (C, D) in transverse and longitudinal views. SC, subcutaneous tissue; VL, vastus lateralis; VI, vastus intermedius; TA, tibialis anterior; EDL, extensor digitorum longus; IM, interosseous membrane.[4]Albayda, J., & van Alfen, N. (2020). Diagnostic value of muscle ultrasound for myopathies and myositis. Current Rheumatology Reports, 22(11), 82. https://doi.org/10.1007/s11926-020-00947-y
Myositis US Heckmatt Score

Heckmatt scoring using vastus lateralis muscle (disease examples from established myositis patients); grade 1 is normal; grade 2 shows a slight increase in echogenicity without architecture loss or attenuation. Grade 3 shows clearly increased muscle echogenicity, loss of muscle architecture, and some attenuation causing less visibility of deeper structures. Grade 4 shows a completely white muscle with loss of recognizable features and strong attenuation of the ultrasound signal so no deep structures can be discerned beyond the superficial layer of muscle. VL, vastus lateralis[5]Albayda, J., & van Alfen, N. (2020). Diagnostic value of muscle ultrasound for myopathies and myositis. Current Rheumatology Reports, 22(11), 82. https://doi.org/10.1007/s11926-020-00947-y

  • Muscle appearance differs depending on specific type and duration of IIM:
    • DM has focal increases in muscle and subcutaneous echogenicity, and fascia is thickened compared to normal in both DM and PM.
    • IBM has diffuse increases in muscle echogenicity with thinning of muscle. IBM has been described as having a “moth-eaten” appearance.
    • Acute cases of myositis less than one year in duration do not typically involve loss of muscle thickness, and have less echogenicity.
    • Muscle lipomatosis, which is a late finding, produces more echogenic changes than fibrosis.[6]Weber, M.-A. (2009). Ultrasound in the inflammatory myopathies. Annals of the New York Academy of Sciences, 1154(1), 159–170. https://doi.org/10.1111/j.1749-6632.2009.04390.x
  • While ultrasound is a useful tool in evaluating focal areas of pain and weakness, ultrasound evaluation of specific muscles has better diagnostic value.
    • Soft tissue ultrasound can only penetrate superficially, so evaluating muscles of the forearm like the flexor digitorum profundus increases sensitivity and specificity of IIM.[7]Karvelas, K. R., Xiao, T., Langefeld, C. D., Walker, F. O., Pathak, S., Caress, J. B., Baute, V., & Cartwright, M. S. (2019). Assessing the accuracy of neuromuscular ultrasound for inclusion body … Continue reading
Myositis US Greyscale Analysis
Quantitative grayscale analysis in a healthy (A) and diseased muscle (B). The mean echogenicity (EI) can be read from the histogram output. In (B), the muscle can be seen as having an increase in echogenicity compared with overlying subcutaneous tissue and normal muscle in (A). Mean grayscale level in the normal vastus lateralis is 91 (A) versus 130 in the diseased (B). VL, vastus lateralis; VI, vastus intermedius[8]Albayda, J., & van Alfen, N. (2020). Diagnostic value of muscle ultrasound for myopathies and myositis. Current Rheumatology Reports, 22(11), 82. https://doi.org/10.1007/s11926-020-00947-y
Increased echogenicity in a “ground glass” pattern (A), and a “moth-eaten” pattern (B)[9]Albayda, J., & van Alfen, N. (2020). Diagnostic value of muscle ultrasound for myopathies and myositis. Current Rheumatology Reports, 22(11), 82. https://doi.org/10.1007/s11926-020-00947-y
Dermatomyositis showing increased echogenicity in the subcutaneous tissue, focally altered and “see-through” echogenicity within the muscle (A), as well as calcinosis (*) causing posterior shadowing (B). SC, subcutaneous tissue; Br, brachialis; Hum, humerus; VI, vastus intermedius; Fem, femur[10]Albayda, J., & van Alfen, N. (2020). Diagnostic value of muscle ultrasound for myopathies and myositis. Current Rheumatology Reports, 22(11), 82. https://doi.org/10.1007/s11926-020-00947-y
Myositis US Muscle Atrophy
Affected flexor digitorum profundus FDP (B) and gastrocnemius (D) muscles in IBM showing markedly increase echogenicity and atrophy, in comparison with normal (A, C). fdp, flexor digitorum profundus; fcu, flexor carpi ulnaris; SC, subcutaneous tissue[11]Albayda, J., & van Alfen, N. (2020). Diagnostic value of muscle ultrasound for myopathies and myositis. Current Rheumatology Reports, 22(11), 82. https://doi.org/10.1007/s11926-020-00947-y

Pearls & Pitfalls

  • Ultrasound is a quick tool that can be used at the bedside to evaluate focal areas of pain for patients, especially in the emergency department when MRI is not feasible
  • Sonographic appearance of muscle can be evaluated at the bedside to guide diagnostic evaluation.
  • Experienced sonographers have excellent sensitivity and specificity to diagnose IIM using ultrasound. It also has great potential to be used to follow progression of disease.
    • A known limitation of MSK ultrasound is sonographer experience, and specialized training can be used to establish proficiency.
  • MSK ultrasound is limited in its penetration of deeper tissues, and MRI may ultimately be required in some cases.

References

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