What investigations should be done for a patient presenting with muscle pain?

Investigations for Muscle Pain

For a patient presenting with muscle pain, begin with creatine kinase (CK) and inflammatory markers (ESR, CRP) as initial screening tests, followed by targeted investigations based on clinical context including muscle strength assessment, autoantibody testing, and consideration of imaging or EMG when diagnosis remains uncertain.

Initial Screening Laboratory Tests

The most efficient diagnostic approach starts with blood work:

• Creatine kinase (CK) is the single most useful screening test for identifying muscle pathology and should be measured first 1, 2
• Inflammatory markers (ESR and CRP) help differentiate inflammatory from non-inflammatory causes 1, 2
• Additional muscle enzymes including aldolase, AST, ALT, and LDH provide complementary information, though these may be normal despite active disease 1, 2
• Complete blood count and basic metabolic panel should be obtained 2

These screening tests have the highest sensitivity and specificity for identifying patients requiring further workup 3.

Clinical Assessment Components

Physical Examination Focus

• Muscle strength testing using validated tools like Manual Muscle Test (MMT8) is essential, as weakness is more typical of myositis than pain alone 1, 2
• Neurological examination including assessment of reflexes and sensory function 1
• Skin examination for dermatomyositis features (Gottron papules, heliotrope rash) 2, 4
• Joint examination to exclude arthritis or other rheumatologic conditions 1

Pattern Recognition

• Proximal versus distal weakness helps narrow differential diagnosis 2, 4
• Focal versus diffuse pain guides investigation strategy - focal pain suggests localized trauma while diffuse pain suggests systemic causes 5

Autoantibody Testing

When initial screening suggests inflammatory myopathy:

• Myositis-specific autoantibodies (anti-TIF1-γ, anti-NXP2, anti-MDA5, anti-SRP) provide prognostic information when available 2
• ANA, RF, anti-CCP if symptoms suggest overlap with other rheumatologic conditions 1
• Anti-acetylcholine receptor and anti-striational antibodies if myasthenia gravis is suspected 1

Cardiac Evaluation

Cardiac involvement can be life-threatening and requires early detection:

• Troponin to evaluate myocardial involvement 1
• ECG and echocardiography are required at diagnosis for all patients with suspected inflammatory myopathy 2
• Cardiac MRI may be indicated in specific cases 1

Advanced Investigations (When Diagnosis Uncertain)

These should be performed selectively based on initial results:

Electromyography (EMG)

• Consider when diagnosis is uncertain or overlap with neurologic syndromes (myasthenia gravis) is suspected 1
• Can show muscle fibrillations indicative of myopathy 1

Imaging

• MRI with T2-weighted/STIR sequences is the preferred imaging modality for detecting muscle inflammation and should be interpreted by an expert radiologist 2
• MRI shows increased signal in affected muscles and helps guide biopsy location 1
• Consider ultrasound for joint assessment if arthritis suspected 1

Muscle Biopsy

• Should only be performed as the last step in the diagnostic algorithm 5
• Indicated when diagnosis remains uncertain after non-invasive testing 1
• Most useful when combined with EMG and MRI findings to guide biopsy site 2

Graded Approach Based on CK Elevation

The investigation intensity should match clinical severity:

Grade 1 (Mild pain, normal or minimally elevated CK)

• Complete examination and laboratory workup as above 1
• Consider metabolic causes, medication effects (statins), or fibromyalgia 5

Grade 2 (Moderate symptoms, CK ≥3x upper limit of normal)

• Complete history and examination 1
• Autoimmune myositis blood panel 1
• EMG and MRI imaging of affected areas 1
• Early referral to rheumatologist or neurologist 1

Grade 3-4 (Severe weakness or very high CK)

• As for grade 2 but with urgent referral 1
• Consider hospitalization 1
• Urinalysis for rhabdomyolysis 1

Additional Organ-Specific Evaluations

When inflammatory myopathy is confirmed:

• Pulmonary function tests including CO diffusion capacity 2
• Swallow function assessment using speech/language therapy evaluation, video fluoroscopy, or barium studies 2
• Malignancy screening in dermatomyositis (chest-abdomen-pelvis CT, age-appropriate cancer screening) 4

Common Pitfalls to Avoid

• Do not assume normal CK excludes myositis - muscle enzymes may be normal despite active disease 2
• Do not perform muscle biopsy prematurely before completing non-invasive testing 5
• Do not overlook cardiac involvement - myocarditis can be fatal and requires urgent treatment 1
• Despite exhaustive investigation, specific abnormalities are found in only one-third of patients with myalgia 3
• Consider medication-induced myopathy (especially statins) before extensive workup 5