Polymyositis

Related Subjects: |Relapsing Polychondritis |Reactive Arthritis |Raynaud's Phenomenon |Polymyositis |Dermatomyositis |Polyarteritis nodosa |Osteoporosis |Rheumatoid Arthritis |Systemic Sclerosis (Scleroderma) |Rheumatology Autoantibodies |Overlap Syndrome

Inflamed striated muscle with proximal weakness. Skin involved is Dermatomyositis (see topic). May be an association with malignancy

About

• Myositis: inflammation and necrosis of skeletal muscle fibres.
• The skin is spared: Polymyositis
• The skin is involved: Dermatomyositis

Aetiology

• Possibly viral
• Increased with HLA-B8/DR3 phenotypes
• Anti-Jo-1 (anti-tRNA synthetase)
• Affects age 40-60 years old.
• Women > Men 3:1

Clinical

• Systemic: fever, weight loss and malaise
• Proximal Muscle weakness progressive over weeks/months
• Tender and swollen muscles shoudler an pelvis
• Difficult squatting and ascending stairs
• Affects hands, feet, face, bulbar, pharynx, upper oesophagus
• Cardiac involvement seen but rare
• Respiratory muscles can be affected with respiratory failure

Dermatomyositis unlike polymyositis has a heliotrope (purple) discoloration of the eyelids and scaly erythematous plaques over the knuckles (Gottron’s papules). It is associated with an increased incidence of underlying malignancy.

Investigations

• FBC: anaemia, ESR is normal
• ↑↑ CK (x 50-fold), LDH and AST suggests myositis
• Muscle biopsy: chronic inflammatory changes, muscle fibre necrosis
• EMG: fibrillation potentials, short-duration low amplitude polyphasic potentials, bizarre high-frequency discharges
• MRI can show inflamed muscle
• Positive ANA seen in 80% but more specific is Positive anti-Jo-1 (anti-tRNA synthetase) is more strongly associated with acute onset myositis, interstitial lung disease, fever, arthritis, Raynaud's phenomenon, mechanic's hands appearance
• Screen for malignancy when appropriate

Differentials

• Distal rather than a proximal weakness might suggest Inclusion body myositis in an older man

Management

• Prednisolone 0.5-1mg/kg for 1 month and then slowly tapered down. CK falls rapidly but muscle strength takes weeks to improve
• Alternatives are azathioprine, methotrexate, ciclosporin
• Disease tends to burn itself out over time

Revisions

• Last updated 21/07/2017