What blood tests are recommended for a patient with muscular dystrophy (MD) presenting with foot swelling?

Blood Tests for Foot Swelling in Muscular Dystrophy

For patients with muscular dystrophy presenting with foot swelling, a comprehensive panel of blood tests should include creatine kinase (CK), liver function tests, renal function tests, electrolytes, complete blood count, inflammatory markers, and thyroid function tests to evaluate for both disease-related complications and alternative causes of edema. 1

Initial Laboratory Evaluation

Essential Blood Tests

• Muscle enzymes:

  • Creatine kinase (CK) - primary marker for muscle damage
  • Lactate dehydrogenase (LDH)
  • Aspartate aminotransferase (AST)
  • Alanine aminotransferase (ALT)
  • Aldolase (if available)

• Renal function tests:

  • Blood urea nitrogen (BUN)
  • Creatinine
  • Estimated glomerular filtration rate (eGFR)

• Inflammatory markers:

  • Erythrocyte sedimentation rate (ESR)
  • C-reactive protein (CRP)

• Complete blood count (CBC) with differential

• Electrolytes:

  • Sodium, potassium, chloride
  • Calcium, phosphorus
  • Magnesium

Additional Important Tests

• Thyroid function tests (TSH, free T4) - to rule out hypothyroidism as cause of edema
• Albumin and total protein - to assess for hypoalbuminemia
• Vitamin D levels - often deficient in muscular dystrophy patients with limited mobility

Specific Considerations Based on MD Type

Different types of muscular dystrophy have characteristic serum enzyme profiles that may help differentiate them 2:

• Duchenne MD: Marked elevation of CK, ALT, AST, LDH, and ALP
• Becker MD: Moderate elevation of CK, ALT, AST, and LDH
• Limb-girdle MD: Variable elevation of CK, ALT, AST, and LDH (LGMD2B patients may have higher ALP levels)
• Facioscapulohumeral MD: Often normal or mildly elevated enzyme levels
• Emery-Dreifuss MD: Often normal enzyme levels

Cardiovascular Assessment

Since foot swelling may indicate cardiac involvement:

• B-type natriuretic peptide (BNP) or NT-proBNP - to assess for heart failure
• Troponin - to rule out myocardial injury

Cardiac involvement is common in muscular dystrophies, particularly in Duchenne, Becker, and myotonic dystrophy, and can manifest as cardiomyopathy leading to heart failure and peripheral edema 3, 4.

Vascular Assessment

For patients with foot swelling, vascular evaluation is critical:

• Ankle-brachial index (ABI) measurement should be considered to assess for peripheral arterial disease 1
  • ABI >1.30: Poorly compressible vessels, arterial calcification
  • ABI 0.90-1.30: Normal
  • ABI 0.60-0.89: Mild arterial obstruction
  • ABI 0.40-0.59: Moderate obstruction
  • ABI <0.40: Severe obstruction

Additional Tests to Consider

• Myositis-specific and myositis-associated antibodies - to rule out inflammatory myopathies that may coexist 1
• Urine dipstick - to evaluate for proteinuria which may indicate renal involvement 1
• Metabolic profile - branched chain amino acids, acetate, glutamine, and other metabolites may show characteristic patterns in different types of muscular dystrophy 5

Common Pitfalls to Avoid

1. Misinterpreting liver enzyme elevations: Elevated AST and ALT in muscular dystrophy patients often reflect muscle damage rather than liver disease. This can lead to unnecessary liver biopsies 1.

2. Missing cardiac involvement: Heart failure can present with peripheral edema in muscular dystrophy patients even with minimal cardiac symptoms 6.

3. Overlooking alternative diagnoses: When evaluating foot swelling, consider other causes such as venous insufficiency, medication side effects, or systemic conditions beyond the muscular dystrophy itself 1.

4. Failing to recognize acute complications: Sudden increases in CK levels in a patient with foot swelling may indicate rhabdomyolysis, which can lead to acute kidney injury and worsen edema 6.

By systematically evaluating these laboratory parameters, clinicians can better determine whether foot swelling is related to the primary muscular dystrophy pathology, cardiac complications, renal involvement, or other causes requiring specific interventions.