What are the initial tests and evaluation steps for a patient presenting with muscle aches?

Initial Evaluation of Muscle Aches

Begin with creatine kinase (CK) measurement as the single most useful screening test, along with inflammatory markers (ESR and CRP), to differentiate inflammatory from non-inflammatory causes and guide all subsequent management decisions. 1

Critical First Step: Distinguish True Weakness from Pain Alone

The most important clinical distinction is whether objective muscle weakness is present, as muscle weakness is the hallmark of inflammatory myositis, not pain alone. 2, 1, 3, 4

• Perform formal muscle strength testing using validated tools like the Medical Research Council Manual Muscle Testing scale or MMT8, specifically assessing proximal muscles (standing from chair without arms, lifting arms overhead). 1, 5
• Pain without objective weakness suggests non-inflammatory causes such as polymyalgia-like syndrome, fibromyalgia, medication effects, or musculoskeletal strain. 4
• Normal CK with pain alone strongly argues against inflammatory myositis. 4

Immediate Red Flag Assessment

Screen for serious pathology requiring urgent intervention: 2

• Cardiac involvement: Check troponin and ECG immediately in all patients with suspected myositis, as myocardial involvement requires permanent discontinuation of causative agents and dramatically alters management. 2, 1, 3, 4
• Infection, malignancy, or severe progressive neurological deficit (including cauda equina syndrome). 2
• Medication-induced causes: Review for statins, immune checkpoint inhibitors, fibrates, colchicine, and hydroxychloroquine. 3

Initial Laboratory Panel

Order the following tests based on clinical presentation: 1, 3

• Creatine kinase (CK): The single most useful screening test for muscle pathology. 1
• Inflammatory markers: ESR and CRP to differentiate inflammatory from non-inflammatory causes. 2, 1, 3
• Additional muscle enzymes: Aldolase, AST, ALT, and LDH provide complementary information (though may be normal despite active disease). 1, 3
• Troponin: Essential to evaluate myocardial involvement. 2, 1, 3

Physical Examination Components

Conduct a focused examination targeting: 2, 1

• Neurological screening including reflexes, sensory function, and assessment for Babinski reflex. 1, 5
• Mobility and muscle strength assessment, documenting if weakness limits instrumental activities of daily living. 2, 1, 3
• Skin examination for dermatomyositis findings. 2, 3
• Pattern of weakness: Asymmetric weakness suggests neurologic conditions; symmetric proximal weakness suggests myopathy. 3, 5

Management Algorithm Based on Initial Findings

If Pain WITHOUT Weakness and Normal CK:

• Continue normal activities and provide analgesia with acetaminophen and/or NSAIDs (if no contraindications). 2, 4
• Do NOT initiate corticosteroids. 4
• Consider polymyalgia-like syndrome if inflammatory markers are highly elevated. 4

If Pain WITH Weakness and CK ≥3× Upper Limit of Normal:

• Initiate prednisone 0.5-1 mg/kg/day. 2, 3
• Ensure hydration with urine output >0.5 mL/kg/hour to prevent myoglobin-induced acute kidney injury. 3
• Monitor serum creatinine twice daily if concern for rhabdomyolysis. 3
• Recheck CK, ESR, CRP weekly initially. 3

If Cardiac Involvement (Elevated Troponin):

• Permanently discontinue any causative agents. 3, 4
• Urgent cardiology consultation with consideration of echocardiogram or cardiac MRI. 3
• Initiate methylprednisolone 1-2 mg/kg IV or higher-dose bolus. 2

When Advanced Testing Is Indicated

Radiological imaging is discouraged unless: 2

• Serious pathology is suspected
• Unsatisfactory response to conservative care or unexplained progression
• Imaging is likely to change management

Consider EMG, MRI, or muscle biopsy when: 2, 1, 3

• Diagnosis remains uncertain after initial workup
• Overlap with neurologic syndromes (myasthenia gravis) is suspected
• Need to differentiate myopathic from neurogenic patterns
• MRI with T2-weighted/STIR sequences is preferred for detecting muscle inflammation. 1

Additional Testing for Confirmed Inflammatory Myopathy

If inflammatory myopathy is confirmed, order: 1

• Myositis-specific autoantibodies (anti-TIF1-γ, anti-NXP2, anti-MDA5, anti-SRP) for prognostic information. 1
• ANA, RF, anti-CCP if symptoms suggest overlap with other rheumatologic conditions. 1
• Pulmonary function tests including CO diffusion capacity. 1
• Swallow function assessment. 1
• Age-appropriate malignancy screening (CT chest/abdomen/pelvis, colonoscopy, PSA) for dermatomyositis. 1, 3

Common Pitfalls to Avoid

• Do not assume all CK elevations require corticosteroids: Exercise-induced muscle damage can produce CK levels exceeding 10,000 U/L without renal impairment or need for immunosuppressive treatment. 4
• Do not delay cardiac evaluation: Always check troponin and ECG, as myocardial involvement changes the entire treatment paradigm. 3, 4
• Do not order routine imaging: Radiological studies are discouraged unless specific indications are met. 2