Proximal Muscle Weakness of Lower Limbs: Diagnostic Approach and Management

Diagnostic Priorities

The American College of Rheumatology and European League Against Rheumatism recommend assessing for life-threatening causes, including respiratory muscle involvement, bulbar symptoms, and cardiac involvement, which require urgent intervention and potential ICU admission 1, 2, 3
Checkpoint inhibitor-induced myositis carries a 20% mortality risk due to concurrent myocarditis, requiring immediate withdrawal of immunotherapy and aggressive treatment with high-dose glucocorticoids, IVIG, and/or plasma exchange 3

The European League Against Rheumatism and American College of Rheumatology propose a systematic diagnostic approach, including initial clinical assessment, essential initial laboratory tests, and advanced diagnostic testing, to diagnose idiopathic inflammatory myopathies (IIM) 4, 5, 6
Muscle weakness is more typical of myositis than pain, and distinguishing true weakness from pain-limited movement is crucial 1
The 2017 EULAR/ACR criteria require a score of ≥5.5 for probable IIM and ≥7.5 for definite IIM without muscle biopsy (or ≥6.7 and ≥8.7 respectively with biopsy) 2

The American College of Rheumatology recommends initiating high-dose corticosteroids immediately, typically prednisone 1 mg/kg/day, concurrent with steroid-sparing agents such as methotrexate, azathioprine, or mycophenolate mofetil, for the treatment of IIM 2, 3
Aggressive immunosuppression with high-dose corticosteroids, IVIG, and methotrexate is required for the treatment of necrotizing myopathy 2, 3
Correcting the underlying abnormality, such as treating hyperthyroidism or replacing vitamin D for osteomalacia, is essential for the treatment of endocrine and metabolic causes of proximal muscle weakness 3

The American College of Rheumatology emphasizes the importance of monitoring bone health regularly and prescribing calcium and vitamin D supplements to prevent osteoporosis and compression fractures during corticosteroid therapy 3
Anti-SRP antibody indicates necrotizing myopathy with acute onset, dilated cardiomyopathy, and poor response to standard immunosuppression, highlighting the need for careful diagnosis and management 7