Polymyositis (Autoimmune Myopathies)

Shoulder girdle weakness: Patients struggle to raise their arms above their head, for instance to comb their hair. Pelvic girdle weakness: Patients struggle to stand from a chair or climb stairs. Dysphagia that can occur from pharyngeal or esophageal involvement. Interstitial lung disease (ILD) manifests with dyspnea; there is characteristic ground-glass radiographic appearance on chest imaging. Congestive heart failure that can occur from myocarditis. There is an increased risk of malignancy in inflammatory myopathies (more-so in dermatomyositis than polymyositis). Note that the CPKs listed below vary widely within individuals and within the published literature. Antisynthetase syndrome is now a recognized subtype of polymyositis (or we can just think of it as classic polymyositis, as the term polymyositis is on its way out, entirely). It is anti Jo-1 antibody positive, referring to aminoacyl transfer ribonucleic acid (RNA) synthetase antibodies (hence: antisynthetase syndrome). Patients have clinical findings of interstitial lung disease and mechanic's hands: hands with hyperkeratotic lesions (cracking and thickening) on both surfaces. It can also manifest with arthritis and Raynaud's phenomenon, amongst other symptoms. Histopathology shows pronounced endomysial inflammation and invasion of non-necrotic muscle fibers: meaning inflammation invades the myocytes but they do not appear necrotic. Average CPK: 1,500 (U/L) In immune-mediated necrotizing myopathy (aka autoimmune necrotizing myopathy), the weakness can develop rapidly and, as indicated earlier, the CPK can be much higher. Toxins/Triggers include statins, which are HMG-CoA reductase inhibitors (eg, atorvastatin); HMG-CoA reductase stands for 3-hydroxy-3methylglutaryl coenzyme A reductase.

• The immune response in statin myopathy is believed to either stem from the increased expression of HMG-CoA reductase levels in muscle or a conformational change within the enzyme that's brought on by the statin drug. The onset of weakness can be delayed for almost a decade after the initiation of statin therapy and can continue for several months after discontinuation, which complicates the diagnosis of it.

Laboratory tests include:

• Anti-HMG-CoA reductase antibody
• Anti-SRP (signal recognition particle), which is its own clinical phenotype but similarly both produce a necrotizing myopathy and both respond to immune therapy to varying degrees.

Histopathology shows many destroyed, necrotic muscle fibers with minimal inflammation (rather just some invading macrophages). Average CPK: 10,000 (U/L) Inclusion body myositis is a mixed inflammatory and degenerative disorder. It involves distal musculature (show wasting of the forearm flexors, especially the deep ulnar finger flexors) as well as proximal lower extremity muscles (show wasting of the quadriceps and tibialis anterior muscles) often in asymmetric (rather than symmetric) pattern.

• Thus, as opposed to typical inflammatory myopathies, patients may complain of tripping and falling before complaining of climbing stairs and they may complain of trouble buttoning a shirt before complaining of raising their arms above their head.

IBM presents predominantly in men (rather than women) and in a chronic, insidious manner (meaning over years) rather than acutely/subacutely like the other disorders. Dysphagia is common due to weakened pharyngeal muscles. Histopathology shows inflammatory changes: endomysial inflammatory infiltrates, and degenerative changes: characteristic rimmed vacuoles (vacuolated muscle fibers) and inclusion bodies. Average CPK: 1,000 (U/L) Normal Muscle Fiber Histology