Polymyositis (PM), dermatomyositis (DM)
Polymyositis and dermatomyositis are two out of the four major subtypes of inflammatory myopathies. The other two are immune-mediated necrotizing myopathy and inclusion body myositis. They are characterised by insidious onset of progressive symmetrical proximal muscle weakness and autoimmune-mediated striated muscle inflammation.
Inflammatory myopathies are a heterogenous group of acquired disorders in which the immune system plays a pathogenic role.
Polymyositis is thought to be marginally more prevalent than dermatomyositis, but both are rare. It is thought that both dermatomyositis and polymyositis are the result of loss of blood vessels to the muscle, and injury to myofibers at the edges of the fascicles, secondary to this damage. In dermatomyositis (where there is increased skin involvement) the picture is similar, with seemingly a loss of capillaries to the skin.
Some patients present with autoantibodies such as type 1 IFN-inducible protein MDA5, but its significance is unclear.
The two forms of inflammatory myopathy have different clinical features and therefore different clinical diagnostic criteria:
- Dermatomyositis - think myositis + skin involvement
- Diagnostic skin involvement (heliotrope rash (periorbital rash), Gottron’s papules - pictured on hand below) OR diagnostic muscle biopsy finding of perifascicular atrophy OR
- All of the following
- Suggestive skin involvement
- Subacute or chronic proximal or distal weakness
- Muscle biopsy showing perimysial or perivascular inflammation without features suggesting another disorder OR skin biopsy showing interface dermatitis along with clinical exclusion of SLE
- Anti-Mi-2 antibodies are highly specific but only seen in 25% of patients
- Polymyositis
- All of the following
- Subacute or chronic proximal weakness
- Elevated serum creatine kinase
- Muscle biopsy showing invasion of endomysial inflammation
- Response to immunotherapy OR appropriate consideration and exclusion of limb-girdle muscular dystrophies