Myopathies & Autoimmune Overlap

Inflammatory myopathies: the framework

Presentation. Idiopathic inflammatory myopathies cause subacute, symmetric PROXIMAL muscle weakness (trouble climbing stairs, rising from a chair or lifting the arms) with elevated creatine kinase. Sensation and reflexes are preserved. Dysphagia, neck flexor weakness, respiratory muscle weakness and myocarditis mark severe disease. Isolated polymyositis is now considered rare; many patients once labeled with it prove to have inclusion body myositis, necrotizing myopathy, or antisynthetase syndrome.

Dermatomyositis and malignancy

Skin findings. Dermatomyositis adds Gottron papules (violaceous, scaly plaques over the MCP and PIP knuckles), the heliotrope rash (violet periorbital discoloration, often with eyelid edema), the shawl and V signs over sun-exposed upper back and chest, and mechanic's hands (cracked, hyperkeratotic fingers). The rash can precede the weakness or occur without it (amyopathic dermatomyositis).

Gottron papules sit OVER the knuckles; the lupus hand rash spares the knuckles and involves the skin between them.

Occult cancer. Adult dermatomyositis is strongly paraneoplastic: ovarian, lung, pancreatic, gastric and colorectal cancers and lymphoma, with the risk concentrated in the years around diagnosis. Perform age-appropriate malignancy screening at diagnosis, with a low threshold for CT of the chest, abdomen and pelvis and pelvic imaging in high-risk patients. Anti-TIF1-γ and anti-NXP2 antibodies carry the highest cancer risk. Polymyositis has a weaker malignancy link.

Diagnosis and myositis antibodies

Rule out mimics. Check TSH (hypothyroid myopathy) and review drugs such as statins and glucocorticoids. Obtain PFTs and high-resolution CT, because interstitial lung disease often drives prognosis. Myositis-specific antibodies refine the subtype and prognosis.

Antisynthetase syndrome. Anti-Jo-1, the most common antisynthetase antibody, links myositis with interstitial lung disease (the leading cause of mortality), mechanic's hands, Raynaud phenomenon, nonerosive arthritis and fever. Screen for and treat the ILD aggressively.

Treatment

Immunosuppression. First-line therapy is high-dose corticosteroids, tapered slowly and guided by strength more than by CK, plus a steroid-sparing agent (methotrexate or azathioprine) from the outset. IVIG helps refractory dermatomyositis, and rituximab is another option. Physical therapy, sun protection for dermatomyositis skin, and cancer screening are part of care.

New or worsening weakness with a normal CK during prolonged high-dose steroids suggests steroid myopathy rather than a flare.

Look-alikes and overlap

Inclusion body myositis. IBM is the most common inflammatory myopathy after age 50 and the great steroid non-responder. Men over 50 develop insidious, ASYMMETRIC weakness of the finger flexors (weak grip) and quadriceps (falls), often with dysphagia, while the CK is only mildly elevated and biopsy shows rimmed vacuoles. Management is supportive, centered on physical therapy.

Statin myopathy. Statins cause a spectrum from myalgia with a normal CK, to myositis, to rare rhabdomyolysis; high doses, interacting drugs, hypothyroidism and kidney disease raise the risk. Toxic myopathy fades once the statin stops, but statin-associated immune-mediated necrotizing myopathy (anti-HMGCR) PERSISTS after withdrawal and requires immunosuppression.

Overlap syndromes. Myositis overlaps with connective tissue disease: anti-U1 RNP defines mixed connective tissue disease, and anti-PM/Scl marks polymyositis-scleroderma overlap.