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Classification of myositis

Key Points

  • The clinical spectrum of idiopathic inflammatory myopathies (IIMs) has evolved from diseases in which muscle weakness was the main manifestation to systemic inflammatory diseases with multiple organ involvement.

  • The EULAR–ACR classification criteria for adult and juvenile IIMs and their major subgroups capture patients with the typical skin rash of dermatomyositis without muscle weakness (amyopathic dermatomyositis).

  • The myositis-specific autoantibodies (MSAs) are helpful in supporting the diagnosis, classification and subclassification of IIMs, but muscle biopsies with histopathological and immunohistochemical evaluation are still important in many cases to rule out other myopathies.

  • Inclusion body myositis is suggested to be classified on the basis of three features: finger flexor or quadriceps weakness, endomysial inflammation, and invasion of non-necrotic muscle fibres or the presence of rimmed vacuoles.

  • Immune-mediated necrotizing myopathy is characterized by the presence of necrotic muscle fibres, often with scarce or no inflammatory cell infiltrates, typically together with the presence of specific MSAs.

  • Antisynthetase syndrome is characterized by the presence of myositis, interstitial lung disease, mechanic's hand, arthritis and Raynaud phenomenon (albeit some of these features might predominate) together with an antisynthetase antibody.

Abstract

The idiopathic inflammatory myopathies (IIMs; also known as myositis) are a heterogeneous group of disorders in which a common feature is chronic inflammation of skeletal muscle, leading to muscle weakness. Other organs are frequently affected in IIMs, such as the skin, joints, lungs, gastrointestinal tract and heart, contributing to morbidity and mortality. Currently, IIMs are most often subclassified into polymyositis, dermatomyositis and inclusion body myositis, but this subclassification has limitations as these subgroups often have overlapping clinical and histopathological features, and outcomes vary within the subgroups; additionally, subgroups without considerable myopathy are not included. A new way of subgrouping patients could be on the basis of the presence of myositis-specific autoantibodies. These autoantibodies are associated with distinct clinical features and, moreover, can help to identify subsets of IIMs in which extramuscular symptoms, such as skin manifestations, arthritis or interstitial lung disease, might be the presenting or predominant feature when muscle symptoms are mild or absent. The recognition that subphenotypes with single-organ involvement other than muscles exist is important for identifying patients with early disease, for clinical care demanding team management and in designing clinical studies to improve our understanding of this heterogeneous disease to develop new therapies.

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Figure 1: Development of classification and diagnostic criteria for idiopathic inflammatory myopathies over time.
Figure 2: Clinical and histological features of the skin in dermatomyositis and conditions that mimic dermatomyositis.
Figure 3: Muscle biopsy findings of patients with two different idiopathic inflammatory myopathies.

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Acknowledgements

The authors thank A. Tjärnlund for being the research coordinator of the International Myositis Classification Criteria Project (IMCCP) and for the critical reading of this manuscript. The authors also thank the IMCCP consortium. The authors thank V. Leclair for her help with Fig. 1. V.P.W. was involved in the development of the Cutaneous Dermatomyositis Disease Area and Severity Index (CDASI); the copyright for this instrument is owned by the University of Pennsylvania. This work was supported by the US NIH (R21 AR066286 and K24-AR 02207) as well as a US Veterans Affairs Merit Review grant (I01BX000706) (V.P.W.). I.E.L. has received grant support from the Swedish Research Council and the Regional Agreement on Medical Training and Clinical Research between the Stockholm County Council and Karolinska Institutet.

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All authors researched the data for the article, provided substantial contributions to discussions of its content, wrote the article and reviewed and/or edited the manuscript before submission.

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Correspondence to Ingrid E. Lundberg.

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Glossary

Heliotrope

A pink, red or purplish colouring around the eyes and eyelids, with or without oedema.

Gottron sign

A symmetrical, occasionally scaly, erythematous eruption over the extensor surfaces of the metacarpophalangeal and interphalangeal joints of the fingers, knees and elbows.

Gottron papules

Red, and often scaly, papules overlying the extensor surfaces of the metacarpophalangeal and interphalangeal joints of the fingers.

Poikiloderma

Areas of hypopigmentation, hyperpigmentation, telangiectasias and atrophy of the skin.

Telangiectasias

Dilated capillaries that can be seen in areas of poikiloderma or at the proximal nailfold.

Interface dermatitis

Dermatitis in which the primary pathology involves the dermo-epidermal junction, with basal cell vacuolization, apoptotic keratinocytes and a band-like infiltrate of inflammatory cells at the dermo-epidermal junction; other characteristics in dermatomyositis include lymphocytic perivascular infiltrates and interstitial mucin deposition.

Mechanic's hand

The presence of scaling, fissuring, hyperpigmentation and hyperkeratosis, distributed in a characteristic fashion on the lateral fingers and sometimes the palmar surfaces, fingertips and distal skin, as well on the feet.

Palmar hyperkeratotic papules

Describes scaly verrucous papules on the palm.

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Lundberg, I., de Visser, M. & Werth, V. Classification of myositis. Nat Rev Rheumatol 14, 269–278 (2018). https://doi.org/10.1038/nrrheum.2018.41

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