Inflammatory muscle diseases are a group of autoimmune diseases affecting mostly skeletal muscles and sometimes cardiac and/or smooth muscles, resulting in damage and destruction of these muscles.
It is as rare as it is relentless. Annual incidence of inflammatory myopathy is 5 per 10 million population and is twice as common among females as compared to males.
The autoimmune mechanism of muscle destruction is by T cells in polymyositis and mediated by complemet and antibody in dermatomyositis. Histopathology slides of affected muscles will show infiltration by lymphocytes and damaged muscle fibers.
There are 6 types of inflammatory myopathies as described below.
Type I, Polymyositis:
It starts quietly affecting pelvic girdle muscles followed by shoulder girdle and then neck muscles. Dysphagia and dysphonia result from weakness of muscles of pharynx and larynx. Immunotherapy can lead to remission however exacerbations are also very common. It is also associated with inflammatory arthritis and Raynaud’s phenomenon.
Type II, Primary Idiopathic Dermatomyositis:
As is evident from its name it predominantly affects skin and muscles. There usually is acute onset proximal muscle weakness associated with characteristic heliotrope rash at eyelids and back of hands. It also causes muscle pain as well as fever, fatigue, malaise and weight loss.
Type III, Myositis Associated with Cancer:
This type of myositis predominantly affects men in or after their forties. Its onset is insidious in the form of progressive weakness and myositis is diagnosed a year or two before the diagnosis of cancer. This type of myositis does not respond to steroids or other form of immunosuppression. Along with causing weakness of limb muscles it also weakens the respiratory muscles and causes dysphagia. Death occurs due to respiratory failure.
Type IV, Juvenile Dermatomyositis:
It affects skin and muscles of children. It is sudden in onset and quick in progression. Remissions may be achieved, which may remain for years but relapse can still occur after as long as 7 years. It is associated with muscle weakness, muscle pain, painful contractures, calcification of muscles and skin, dysphonia, dysphagia and respiratory failure.
Type V, Myositis with Collagen Vascular Disease:0
This type of myositis is associated with collagen vascular diseases such as rheumatoid arthritis, systemic lupus erythmetosus and primary systemic sclerosis. Clinical picture is dominated by the features of primary disease while that of myositis remains masked.
Type VI, Inclusion Body Myositis:
This type of myositis affects men in or above their forties. It is relentlessly progressive and seldom responds to treatment leading to weakness and atrophy of skeletal muscles especially shoulder girdle, pelvic girdle and quadriceps. It causes proximal weakness but it may also cause distal weakness in more than half of patients.