At this year’s CRA meeting, Dr. Rachel Shupak presented a workshop on the topic of Idiopathic Inflammatory Myopathies. The following is a summary and pearls of her excellent presentation.
Idiopathic inflammatory myopathies (IIM) are rare with an annual incidence of 5-10 cases per million adults. However, they are the most common acquired muscle disorder.
Idiopathic inflammatory myopathies are a heterogeneous group of acquired muscle disorders that are associated with proximal weakness, have a progressive clinical course, and an inflammatory infiltrate with or without degeneration or necrosis on biopsy.
About 60-90% of cases of IIM are positive for autoantibodies including myositis specific and myositis associated antibodies. The absence of autoantibodies does not rule out the diagnosis.
There are many systemic manifestations of IIM including articular, pulmonary, cardiac, and gastrointestinal. Dermatomyositis and overlap myositis by and far represent the majority of IIM cases (55-95%), polymyositis is rarer (2-10%) and seems to be decreasing (Medicine 92(1) Jan 2013).
Pearl #1- In dermatomyositis, you cannot rely on the CK to make the diagnosis or follow the disease because it correlates poorly. Patients with clinically obvious dermatomyositis may have a normal CK.
Pearl #2- A CK greater than 10,000 occurs with the following conditions: (a) necrotizing autoimmune myopathy (NAM), (b) genetic myopathies, and (c) some paraneoplastic syndromes.
Pearl #3- Normal or minimally elevated CK is seen with: (a) dermatomyositis, (b) juvenile dermatomyositis, and (c) inclusion body myositis.
Pearl #4 - The MRI can distinguish damage (fat - bright on T1) from steroid atrophy (atrophy on MRI) from disease activity (bright on T2-STIR). The MRI can be used to determine response to therapy and to determine the site for biopsy. The best MRI to order is Iliopsoas and Quadriceps.
Pearl #5- Quadriceps is often the best muscle to biopsy. The pathology must be reviewed by an experienced pathologist since the disease can be patchy.
Pearl #6- With dermatomyositis, you see a microangiitis on muscle biopsy and around the nail bed (dilated capillaries, capillary dropout, and periungal erythema/swelling).
Pearl #7- With polymyositis, you see endomysial lymphocytic infiltration.
Pearl #8- Necrotizing Autoimmune Myopathy was previously classified as PM without inflammation. NAM typically occurs in middle-aged females, has sudden onset, is associated with myalgia, proximal muscle weakness, and cardiac involvement, and relapse is common. A simultaneous necrosis-regeneration is seen with no inflammation.
Pearl #9- If you see a young person in their late teens or early 20’s with a very high CK and you are told it is PM - think again! This may be a dysferlinopathy. Dysferlinopathy is an autosomal recessive neuromuscular disorder caused by a deficiency of functional dysferlin protein due to mutations in the dysferlin gene. These patients often have wasted gastrocnemius muscles. They may have a worse prognosis with steroids.
Pearl #10 - Polymyositis is a RARE stand-alone diagnosis and is a diagnosis of exclusion. It affects proximal muscles and evolves subacutely. Other diagnoses must be excluded before settling on this diagnosis.
Pearl #11- With treatment, clinical benefit lags behind biochemical improvement. This becomes difficult in patients who have normal CK on presentation.
Pearl #12- The only two evidence-based treatment recommendations are for combination methotrexate/azathioprine and IVIG. There are many case studies with other immunomodulators and biologics.
Pearl #13- Biopsy is important to establish the diagnosis and to guide therapy.
Pearl #14- If you’re not sure of the diagnosis or the patient is not responding to treatment the way you think they should — Rethink the diagnosis.