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]

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*

Skin manifestations*

No

Yes

No

Yes

Clinical features**, or muscle biopsy***

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

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

**Clinical features include finger flexor weakness, or no treatment response

***Muscle biopsy includes rimmed vacuoles

****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	Symptoms												History	Physical Examination	Diagnosis
	Age of onset	Muscle weakness	Fever	Myalgia	Contractures	Gait abnormality	Neuropathy	Atrophy	Stiffness	Myoglobinuria	Other features	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)
Amyopathic dermatomyositis
Disease	Symptoms												History	Physical Examination	Diagnosis
	Age of onset	Muscle weakness	Fever	Myalgia	Contractures	Gait abnormality	Neuropathy	Atrophy	Stiffness	Myoglobinuria	Other features	Laboratory Findings			Creatine Kinase	Muscle Biopsy	Electromyogram