Inflammatory myopathy

Inflammatory Myopathy
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Overview
Classification
Differentiating Inflammatory myopathy from other Diseases

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Sadaf Sharfaei M.D.[2]

Overview

Inflammatory myopathies are a group of disorders that involve muscles and skin. Latest classification by European League Against Rheumatism and American College of Rheumatology (EULAR/ACR) classified idiopathic inflammatory myopathy into 6 subtypes including polymyositis, dermatomyositis, inclusion body myositis, immune−mediated necrotizing myopathies (IMNM), amyopathic dermatomyositis, and juvenile dermatomyositis. It is considered that autoimmunity has a major role on pathogenesis of these diseases. Inflammatory myopathies might present with involvement of other organs including blood vessels, lungs, connective tissue, solid organs, or joints. Inflammatory myopathies are rare and may affect both children and adults. There are no definitive therapy for inflammatory myopathies. However, corticosteroids and symptomatic therapy are the first line treatment for inflammatory myopathies. Immunosuppressants might be used in patients who have recurrent or resistant symptoms. Lifestyle modifications including physical therapy, watching diet, and controlling weight are recommended. The prognosis of inflammatory myopathies varies and it depends on extramuscular involvement, early initiation of treatment, and presence of certain autoantibodies. Diagnosis is usually made by biopsy and detecting inflammation on muscle or skin biopsies.

Classification

• The 2017 European League Against Rheumatism and American College of Rheumatology (EULAR/ACR) classified idiopathic inflammatory myopathy into 6 subtypes:
  • Polymyositis
  • Dermatomyositis
  • Inclusion body myositis
  • Immune−mediated necrotizing myopathies (IMNM)
  • Amyopathic dermatomyositis
  • Juvenile dermatomyositis

The 2017 EULAR/ACR classification criteria for adult and juvenile idiopathic inflammatory myopathy	Score
Variable	No muscle biopsy	With muscle biopsy
Age of onset ≥ 18 and < 40 years	1.3	1.5
Age of onset ≥ 40 years	2.1	2.2
Objective symmetric weakness, usually progressive, of the proximal upper extremities	0.7	0.7
Objective symmetric weakness, usually progressive, of the proximal lower extremities	0.8	0.5
Neck flexors are relatively weaker than neck extensors	1.9	1.6
In the legs, proximal muscles are relatively weaker than distal muscles	0.9	1.2
Heliotrope rash	3.1	3.2
Gottron’s papules	2.1	2.7
Gottron’s sign	3.3	3.7
Dysphagia or esophageal dysmotility	0.7	0.6
Anti−Jo1 autoantibody present	3.9	3.8
Elevated serum levels of CK or LDH* or ASAT/AST/SGOT* or ALAT/ALT/SGPT*	1.3	1.4
Endomysial infiltration of mononuclear cells surrounding, but not invading, myofibers on muscle biopsy		1.7
Perifascicular atrophy		1.9
Rimmed vacuoles		3.1

Probability of IIM including muscle biopsy=1/[1+exponential (5.33–score)]

Probability of IIM without muscle biopsy=1/[1+exponential (6.49–score)]

To use the calculator to assess the probability of IIM, click here.

Classification algorithm for subgroups of IIM

Abbreviation: ACR, American College of Rheumatology; EULAR, European League Against Rheumatism; IIM, idiopathic inflammatory myopathies;

A patient meets the EULAR/ACR classification criteria for IIM (probability of IIM ≥55%)

Age of onset 0-17 years old

No

Yes

Skin manifestations include Heliotrope rash, Gottron's papules, or Gottron's sign

Skin manifestations include Heliotrope rash, Gottron's papules, or Gottron's sign

No

Yes

No

Yes

Clinical features include finger flexor weakness, or no treatment response, or Muscle biopsy includes rimmed vacuoles

Muscle weakness*

Juvenile myositis other than juvenile dermatomyositis

Juvenile dermatomyositis

No

Yes

No

Yes

Polymyositis, or immune-mediated necrotizing myopathy

Inclusion body myositis

Amyopathic dermatomyositis

Dermatomyositis

* Muscle weakness manifested as objective progressive symmetric muscle weakness of proximal upper or lower extremities, neck flexors relatively weaker than extensors, or in the legs proximal muscles relatively weaker than distal

Differentiating Inflammatory Myopathy from other Diseases

Disease	Age of onset	Muscle weakness	Fever	Myalgia	Contractures	Gait abnormality	Neuropathy	Atrophy	Stiffness	Myoglobinuria	Other features	History	Physical Examination	Laboratory Findings	Creatine Kinase	Muscle Biopsy	Electromyogram
Dermatomyositis	40s−50s Can affect children	Proximal	+	+	−	−	−	−	+	−	Rash Dyspnea Weight loss Cough	Viral infections Cancer	Heliotrope rash on face and hands Telangiectasia Erythema Mechanic's hands Gottron's sign ( violaceous scaly eruption )	↑↑ ESR ↑↑ CRP	↑↑	Perimysial mononuclear infiltrate	Myopathic
Polymyositis	> 18 years	Proximal	+	+	−	−	−	−	+	−	Similar to dermatomyositis without mucous and skin involvement	N/A	N/A	↑↑ ESR ↑↑ CRP	↑↑	Endomysial mononuclear infiltrate Patchy necrosis
Inclusion body myositis	50s	Proximal & distal	−	−	−	−	−	−	−	−	Dysphagia Asymmetric weakness	Retrovirus (most common)	N/A	Antibodies to cytoplasmic 5'−nucleotidase	↑↑	Inflammatory cells Invading muscle cells Vacuolar degeneration Inclusions or plaques	Neurogenic
Immune−mediated necrotizing myopathies (IMNM)	30s−70s	Proximal	−	−	−	−	−	−	−	−	Severe symmetric weakness Interstitial lung disease Cardiac involvement	Statin use Viral infections (HIV or hepatitis C)	N/A	Anti−SRP and anti−HMGCoAR autoantibodies	↑↑↑	Minimal or no inflammatory infiltrates and marked muscle necrosis	Myopathic
Amyopathic dermatomyositis	40s−60s	−	−	−	−	−	−	−	−	−	Rash Pruritus Photosensitivity	N/A	Heliotrope rash on face and hands Gottron's sign ( violaceous scaly eruption )	Normal	Normal	Normal	Normal
Fibromyalgia	40−50s	Generalized	−	−	−	−	+	−	−	−	Anxiety or depression features Fatigue Sleep disturbance Numbness Muscle spasms	History of depression	Tenderness in the soft tissue anatomical location	Normal	Normal	Normal	Normal
Polymyalgia Rheumatica	50s	Diffuse	+	+	−	−	−	−	+	−	Weight loss	History of joints stiffness, worse in the morning	Restricted shoulder motion	↑↑ ESR ↑↑ CRP	Normal	Normal	Normal
Disease	Age of onset	Muscle weakness	Fever	Myalgia	Contractures	Gait abnormality	Neuropathy	Atrophy	Stiffness	Myoglobinuria	Other features	History	Physical Examination	Laboratory Findings	Creatine Kinase	Muscle Biopsy	Electromyogram