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50% of patients with giant cell arteritis (GCA), and 15–30% of patients with PMR eventually develop GCA.
   Laboratory Findings
    Laboratory finding are nonspecific.
   ESR is markedly elevated (>40 mm/hour, but values >100 may be seen). However, some patients with mild disease may have only slight elevations of ESR.
   CRP is elevated and considered a more sensitive marker than ESR.
   Serologic tests such as RF, ANA, and anti-CCP antibodies are typically negative.
Suggested Reading
Dasgupta B, Cimmino MA, et al. 2012 provisional classification criteria for polymyalgia rheumatica: a European League Against Rheumatism/American College of Rheumatology collaborative initiative. Ann Rheum Dis. 2012;71(4):484–492.
POLYMYOSITIS, DERMATOMYOSITIS, AND INCLUSION BODY MYOSITIS
   Definition
   Polymyositis (PM), dermatomyositis (DM), and inclusion body myositis (IBM) are related inflammatory myopathies that share common features, including muscle weakness and inflammatory infiltrates on a muscle biopsy.
   PM and DM are characterized by a subacute onset of symmetric proximal muscle weakness, common involvement of other organ systems such as lung and skin, a strong association with autoantibodies, and responsiveness to immunosuppression. Both are widely accepted as having an autoimmune basis. Cutaneous involvement is the primary clinical feature distinguishing patients with DM from those with PM.
   In contrast to PM and DM, patients with IBM typically have slowly progressive weakness in both proximal and distal muscles, rarely have other extra-muscular involvement or autoantibodies, and most often do not respond to immunosuppressive therapies. A muscle biopsy showing the presence of typical inclusion bodies is diagnostic for IBM.
   Who Should Be Suspected?
   Patients with PM and DM typically present with progressive proximal muscle weakness and evidence of muscle inflammation. They may also have constitutional symptoms and evidence of involvement of other organs (e.g., interstitial pulmonary disease, polyarthritis). DM patients can be differentiated by having specific cutaneous signs such as Gottron papules or heliotrope eruption.
   Patients with IBM generally have a more insidious onset compared to PM and DM, and more prominent distal muscle weakness.
   PM and DM can occur at any age, with peak incidence between the ages of 40 and 50, whereas IBM mainly affects individuals >50 years of age.
   Laboratory Findings
    Diagnosis of the three conditions is based on clinical manifestations, serum muscle enzymes, autoantibodies, EMG findings, and muscle biopsy. The latter is the definitive test for establishing the diagnosis of IBM, and in PM or DM patients presenting with atypical clinical or laboratory findings.
   Muscle enzymes:
   Levels of the muscle enzyme creatine kinase (CK) are greatly elevated in PM and DM patients (typically >10 folds the upper limit of normal but may be >50 folds), and to a lesser extent in patients with IBM.
   Other muscle enzymes including lactate dehydrogenase (LDH), aldolase, aspartate aminotransferase (AST) and alanine aminotransferase (ALT) are also elevated. Similar to CK, elevations are less profound in IBM than in PM or DM.
   ANA test is positive in up to 80% of patients with DM or PM.
   Myositis-specific antibodies are positive in 30% of patients with PM or DM. The most common antibodies are those against histidyl-tRNA synthetase (anti-Jo-1), and titers of these antibodies have been found to correlate with disease activity. Other myositis-specific antibodies include anti-Mi-2 and anti–signal recognition particle (anti-SRP) antibodies. Presence of other connective disease conditions associated with myositis is suggested when another type of autoantibodies is positive (e.g., anti-SSA/Ro, anti-SSB/La, anti-Sm, or anti-RNP).
   ESR is normal or mildly increased.
   Myoglobin is elevated in the serum and urine.
Suggested Reading
Mammen