Treatment of Proximal Myopathy

Immediate Priority: Identify Life-Threatening Causes

Checkpoint inhibitor-induced myositis carries a 20% mortality risk due to concurrent myocarditis, and immediate withdrawal of immunotherapy and initiation of high-dose glucocorticoids, IVIG, and/or plasma exchange is recommended for life-threatening manifestations, according to the European League Against Rheumatism 1, 2
Assessing for bulbar symptoms and respiratory involvement, such as dysphagia, dysarthria, dysphonia, or dyspnoea, is crucial, as these indicate severe disease requiring urgent high-dose methylprednisolone, as suggested by the American College of Rheumatology 1

Cushing's disease-related myopathy is multifactorial, involving protein degradation through the FOXO3 pathway and intramuscular fat accumulation, and definitive treatment requires addressing the hypercortisolism through transsphenoidal surgery, according to the Endocrine Society 3
Post-remission growth hormone deficiency may contribute to persistent myopathy, and lower 6-month postoperative IGF-I levels strongly predict more severe long-term muscle atrophy, as reported by the Endocrine Society 3

For newly diagnosed uncomplicated inflammatory myopathy, initiating high-dose corticosteroids immediately, typically prednisone 1 mg/kg/day, is recommended, as suggested by the American College of Rheumatology 4, 5
First-line treatment includes high-dose corticosteroids concurrent with methotrexate, azathioprine, or mycophenolate mofetil as steroid-sparing agents, according to the American College of Rheumatology 4, 5
Severe myositis requires administration of high-dose methylprednisolone PLUS IVIG, cyclophosphamide, rituximab, or cyclosporine, as recommended by the American College of Rheumatology 4, 5

Corticosteroids are a major cause of morbidity in long-term management, with primary adverse outcomes being osteoporosis, compression fractures, and avascular necrosis, and preventive measures such as monitoring bone health, prescribing calcium and vitamin D supplements, and adding bisphosphonate if osteoporosis develops are necessary, as suggested by the American College of Rheumatology 4, 5

The American College of Cardiology recommends that statins are the most common drug cause of proximal myopathy, and can produce two distinct patterns, including statin-associated myopathy and immune-mediated necrotizing myopathy (IMNM), which requires aggressive immunosuppression with high-dose corticosteroids, IVIG, and methotrexate 6
Corticosteroids, alcohol, and SGLT2 inhibitors can also cause proximal myopathy 6

The American Academy of Neurology recommends that polymyositis (PM) is characterized by symmetric proximal weakness with CD8+ T cell invasion of muscle fibers on biopsy 7
Dermatomyositis (DM) is characterized by proximal weakness with pathognomonic skin findings, elevated CK, and requires immediate cancer screening in adults due to substantial malignancy risk 7, 6
Immune-mediated necrotizing myopathy (IMNM) is a severe myopathy with minimal inflammatory infiltrate, triggered by statins, viral infections, or malignancy 7, 6
Inclusion body myositis (IBM) is the most prevalent acquired myopathy above age 50, with vacuolization and abnormal protein accumulation 7
Juvenile dermatomyositis (JDM) occurs in children <18 years with calcinosis cutis, cutaneous vasculitis, and potential gastrointestinal vasculopathy 7

Immune checkpoint inhibitor (ICI)-induced myositis is rare but potentially fatal, more common with anti-PD-1/PD-L1 agents, presenting with proximal weakness, markedly elevated CK, and can involve the myocardium with high mortality when combined with myocarditis or myasthenia gravis 8, 9, 10

Initial laboratory evaluation must include creatine kinase (CK) levels, thyroid function tests (TSH, free T4), and inflammatory markers (ESR, CRP) if inflammatory myopathy is suspected 6, 8
Myositis-specific antibody panels (anti-Jo-1, anti-SRP) and electromyography (EMG) are essential for detecting myotonic discharges even when clinical myotonia is absent 6
Muscle MRI identifies inflammation and guides biopsy site, and muscle biopsy is necessary to distinguish IIM subtypes and confirm diagnosis 7, 6