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Polymyositis/Dermatomyositis

I. Introduction

A. Definitions

1. Polymyositis (PM) - idiopathic nonsuppurative inflammatory disease of striated muscles

2. Dermatomyositis (DM) - myositis with characteristic skin manifestations.

B. Classification of inflammatory diseases of muscle.

1. Infectious

a. Viral

b. Bacterial - C. welchii

c. Nematode - trichinosis

d. Cestode - cysticercosis

e. Protozoa - toxoplasmosis, trypanosomiasis

2. Autoimmune

a. Polymyositis/dermatomyositis complex (PM/DM)

b. Inclusion body myositis

3. Miscellaneous - sarcoidosis, polymyalgia rheumatica

A. Classification of Peter and Bohan, 1975

1. Primary idiopathic polymyositis

2. Primary idiopathic dermatomyositis

3. PM/DM with malignancy

4. PM/DM of childhood with vasculitis

5. PM/DM associated with other connective tissue diseases

6. Inclusion body myositis

III. Epidemiology

B. In adults, mean age of onset is 50. 10-20% of cases start before age 20

C. Sex distribution

1. Adults and Children - F:M, 2:1

D. PM/DM relative incidence

1. Adults - 1/3 PM, 1/3 DM, 1/3 other classes of PM/DM

2. Children - myositis without any cutaneous manifestations is unusual

A. Etiology

1. Viral - acute or subacute PM may follow viral infection.

a. Muscle biopsies from patients have shown viral-like particles on EM.

b. No virus ever isolated from muscle tissue.

c. PM and DM are associated with enterovirus infections (echovirus) in patients with X-linked agammaglobulinemia.

2. Viral infection of muscle may lead to autoimmune response.

a. HIV is associated with acute and chronic myopathy. Chronic myopathy is identical to PM. May be related to reduced CD4+ T cells.

b. HTLV-I associated with PM and DM in endemic regions.

3. Genetic factors

a. HLA-B8/DR3 associated with PM and DM in whites. Autoimmune haplotype. 

b. HLA-DQA*0501 and 0401 associated with PM in whites and blacks. 

c. HLA-DR3/DQA1*0501 and 0401 associated with anti-Jo-I+ myositis. 

d. Inclusion body myositis strongly associated with HLA-DR3 in whites. 

e. Supertypic DR specificity, DR52, is found on DR3, DR5, DR6 and DR8 and is found in haplotypes associated with myositis and anti-synthetase antibodies. B. Pathogenesis

1. Autoimmune disease.

a. Abnormalities of humoral immunity - hypergammaglobulinemia, ANA and anti-skeletal muscle antibodies, muscle biopsies show vascular deposits of Ig and  C'. 

b. Abnormalities of cellular immunity - mononuclear infiltrate of involved muscles, peripheral blood lymphocytes proliferate and are cytotoxic to skeletal muscles, animal model for PM - experimental allergic myositis.

c. In PM, CDS+ T cells predominate - cytotoxicity. In DM, CD4+ T cells predominate with B cells - autoantibodies.

2. Vasculopathy - found very common in children with DM.

a. Small perifascicular vessels show perivascular mononuclear infiltrate with endothelial damage and thrombosis.

b. Cutaneous lesions due to small vessel vasculitis.

V. Clinical manifestations

A. Idiopathic PM

1. Presentation

a. Typically insidious onset of muscle weakness of several months duration.

b. Muscle weakness typically involves proximal muscles of extremities and is symmetrical.

c. Weakness of neck flexors in 50%.

d. Dysphagia - 20%.

e. Dysphonia occasionally seen.

f. Difficulty in lifting objects, raising arms above head, running, rising from chair, climbing stairs.

g. Most patients present with moderately severe disability.

h. Some patients have preceding febrile illness.

i. Muscle pain and tenderness 50-60%.

2. Associated symptoms and signs.

a. Systemic symptoms - malaise, anorexia, weight loss, fever.

b. Arthritis and Raynaud's occasionally.

c. Muscle atrophy is common.

d. Joint contractures - 10%.

e. Cardiac involvement- 30-50%. Usually not symptomatic.