What is the appropriate workup and treatment for a patient with arm muscle pain of unknown etiology?

Workup and Treatment for Arm Muscle Pain of Unknown Etiology

Begin with a focused history to distinguish between tendinopathy (activity-related, localized pain) and inflammatory myositis (weakness predominates over pain), as this fundamentally determines your diagnostic and treatment pathway. 1, 2

Initial Clinical Assessment

History - Key Distinguishing Features

• Onset pattern: Insidious onset with increased or new activity suggests tendinopathy, while subacute onset without clear mechanical trigger raises concern for inflammatory or systemic causes 1
• Pain characteristics: "Sharp" or "stabbing" pain that worsens with activity but may improve after warm-up indicates tendinopathy 1
• Weakness vs. pain: Muscle weakness is the hallmark of inflammatory myositis, NOT pain - if pain predominates without objective weakness on examination, inflammatory myositis is unlikely 1, 2
• Medication history: Specifically ask about statin use, as statin-associated myalgia occurs in 5-10% of patients in clinical practice and presents as bilateral, proximal muscle pain without weakness 2
• Immune checkpoint inhibitor exposure: If patient is on cancer immunotherapy, this changes the entire workup algorithm 1

Physical Examination - Critical Elements

Perform a systematic examination focusing on:

• Muscle strength testing: Assess proximal muscle groups (standing from chair without arms, lifting arms overhead) - objective weakness indicates potential myositis requiring urgent workup 1, 2
• Inspection: Look for muscle atrophy (suggests chronicity), asymmetry, swelling, and erythema 1
• Palpation: Well-localized tenderness that reproduces activity-related pain supports tendinopathy 1
• Range of motion: Often limited on symptomatic side in tendinopathy 1
• Skin examination: Check for dermatomyositis signs (Gottron's papules, heliotrope rash, V-sign, shawl sign) 3
• Multiple tender tendons: Should prompt consideration of rheumatic disease 1

Laboratory Workup Algorithm

If Pain WITHOUT Objective Weakness (Most Common Scenario)

Initial screening tests:

• Creatine kinase (CK): Normal CK with pain alone strongly argues against inflammatory myositis 2, 4
• ESR and CRP: Elevated inflammatory markers with pain but no weakness suggest polymyalgia-like syndrome rather than myositis 2

If CK and inflammatory markers are normal: Tendinopathy or non-inflammatory cause is most likely - proceed to imaging for structural assessment 1

If Pain WITH Objective Weakness or Elevated CK

Comprehensive inflammatory myositis panel:

• CK, AST, ALT, LDH, aldolase: Can be elevated even when CK is normal in some myositis cases 1, 3
• Troponin: Must be checked to evaluate for potentially life-threatening myocardial involvement 1, 3, 2
• ESR and CRP: Essential for diagnosis and monitoring 1, 3
• Autoimmune myositis panel: If weakness present 1

Imaging Strategy

For Suspected Tendinopathy (Pain Without Weakness)

• Plain radiography first: May reveal bony abnormalities, loose bodies, osteoarthritis, osteophyte formation at epicondyles, or calcific tendinosis 1
• Ultrasonography: Useful for detecting tendon thickening, heterogeneous echogenicity, and moderate to full-thickness tears 1
• MRI: Reserve for recalcitrant pain despite conservative management or when diagnosis remains unclear - provides detailed soft tissue and anatomic information 1

For Suspected Myositis (Weakness Present)

• MRI of proximal limbs: Can reveal inflammatory changes even when muscle enzymes are normal, more sensitive than ultrasound 1, 3
• Consider EMG: May demonstrate myopathic changes despite normal CK levels 1, 3
• Muscle biopsy: When diagnosis uncertain or overlap with neurologic syndromes suspected 1, 3

Treatment Algorithm

For Tendinopathy (No Weakness, Localized Pain)

Conservative management:

• Relative rest: Reduce activity to decrease repetitive loading of damaged tendon 1
• Analgesia: Acetaminophen or NSAIDs (ibuprofen 400 mg every 4-6 hours as needed) if no contraindications 1, 5
• Activity modification: Avoid provocative movements while maintaining general fitness 1

Common pitfall: Imaging studies should be reserved for when diagnosis remains unclear after thorough history and physical examination, or for recalcitrant pain despite adequate conservative management 1

For Pain Without Weakness and Normal Labs

Do NOT initiate corticosteroids - continue normal activities and use simple analgesia 2

Critical caveat: Do not assume all CK elevations require corticosteroids, as exercise-induced muscle damage can produce CK levels exceeding 10,000 U/L without renal impairment or need for immunosuppressive treatment 2

For Myositis (Weakness Present or CK ≥3× Upper Limit Normal)

Grade 2 (moderate symptoms):

• Hold any causative medications 1
• Initiate prednisone 0.5-1 mg/kg if CK elevated ≥3× normal with weakness 1, 2
• Early referral to rheumatology or neurology 1

Grade 3-4 (severe weakness):

• Consider hospitalization for severe weakness 1
• Prednisone 1 mg/kg or methylprednisolone IV 1-2 mg/kg 1
• Urgent rheumatology/neurology referral 1
• Permanently discontinue causative agents if myocardial involvement present 1, 2

Red Flags Requiring Immediate Escalation

Immediate action required if:

• Objective weakness develops on examination (difficulty standing from chair, lifting arms overhead) 2
• CK rises to ≥3× upper limit normal with any degree of weakness 2
• Cardiac symptoms or troponin elevation - represents medical emergency 1, 3, 2
• Dysphagia or respiratory muscle involvement 3

Key Clinical Pitfalls to Avoid

• Do not order extensive workup for isolated pain without weakness or elevated CK - this typically represents tendinopathy or non-inflammatory causes 2, 4
• Always evaluate cardiac involvement with troponin and ECG when myositis is suspected, as myocardial involvement requires permanent discontinuation of causative agents 2
• Do not use corticosteroids for pain alone without elevated CK or objective weakness 2
• Joint effusions are uncommon with tendinopathy and suggest intra-articular pathology requiring different evaluation 1