What are the diagnostic criteria for rhabdomyolysis?

Diagnostic Criteria for Rhabdomyolysis

Rhabdomyolysis is definitively diagnosed when serum creatine kinase (CK) levels are at least 10 times the upper limit of normal, typically accompanied by myoglobinuria. 1, 2

Essential Diagnostic Components

Laboratory Findings

• Creatine Kinase (CK)

  • Primary diagnostic marker
  • Elevation ≥10× upper limit of normal is diagnostic 1, 2
  • Serial monitoring recommended every 6-12 hours in acute phase 1
  • CK typically peaks within 24-72 hours of muscle injury

• Myoglobinuria

  • Presents as tea/cola-colored urine
  • Positive for blood on urine dipstick but negative for RBCs on microscopy 1
  • Plasma myoglobin rises earlier than CK but has shorter half-life (2-3 hours) 1

• Additional Laboratory Tests

  • Serum electrolytes (particularly potassium, calcium, phosphorus)
  • Renal function tests (BUN, creatinine)
  • Liver function tests (AST/ALT often elevated due to muscle breakdown)
  • Coagulation studies if disseminated intravascular coagulation suspected 1
  • Arterial blood gas to assess for metabolic acidosis 1

Clinical Presentation

• Muscle pain and weakness
• Dark urine (myoglobinuria)
• Muscle swelling or tenderness
• Symptoms may range from asymptomatic enzyme elevation to severe systemic illness 3

Diagnostic Algorithm

1. Initial Assessment

   • Evaluate for clinical triad: muscle pain, weakness, and dark urine 3
   • Assess for potential causes/triggers (trauma, exertion, medications, etc.)
   • Check for signs of complications (decreased urine output, altered mental status)

2. Laboratory Confirmation

   • Order CK levels (diagnostic if ≥10× upper limit of normal)
   • Urinalysis (check for myoglobinuria)
   • Comprehensive metabolic panel
   • Consider plasma myoglobin (early marker but shorter half-life)

3. Complication Assessment

   • Evaluate for acute kidney injury
   • Monitor electrolyte abnormalities (particularly hyperkalemia)
   • Assess for compartment syndrome (pain, tension, paresthesia, paresis) 1
   • Check for signs of disseminated intravascular coagulation

4. Underlying Cause Investigation

   • Detailed history for potential triggers
   • Consider specialized testing for recurrent or unexplained cases:
     • Metabolic screening (fasting glucose, lactate/pyruvate, acylcarnitine profile) 1
     • Genetic testing for hereditary muscle disorders
     • Muscle biopsy in selected cases 1

Special Considerations

Recurrent Rhabdomyolysis

For patients with recurrent episodes, episodes triggered by specific factors, rhabdomyolysis with minimal exertion, childhood/adolescent onset, family history of similar episodes, or absence of common acquired causes, consider:

• Comprehensive metabolic screening
• Genetic testing for inherited myopathies
• Muscle biopsy 1

Compartment Syndrome

• Diagnostic threshold: compartment pressure >30 mmHg or differential pressure (diastolic BP - compartment pressure) <30 mmHg 1
• Clinical signs have low sensitivity; pressure measurement recommended in high-risk cases

Common Pitfalls to Avoid

• Relying on single CK measurements (serial monitoring is essential) 1
• Missing genetic causes in adults with first-time rhabdomyolysis
• Attributing recurrent episodes solely to exercise or dehydration
• Failing to perform metabolic studies during acute episodes
• Delaying genetic consultation in recurrent cases 1
• Confusing with myositis (inflammatory markers typically higher in myositis) 1

Management Principles

• Aggressive fluid resuscitation (target urine output >300 mL/hour) 1, 2
• Avoid potassium-containing fluids
• Consider sodium bicarbonate for acidosis
• Monitor electrolytes, renal function, and CK levels every 6-12 hours 1
• Continue IV fluids until CK levels decrease to <1,000 U/L 2
• Nephrology consultation for all cases with acute kidney injury 1

By following these diagnostic criteria and management principles, clinicians can effectively identify and treat rhabdomyolysis, potentially preventing serious complications like acute kidney injury.