What are the diagnostic criteria for rhabdomyolysis?

Diagnostic Criteria for Rhabdomyolysis

The diagnosis of rhabdomyolysis requires a serum creatine kinase (CK) elevation >50,000 IU/L combined with clinical symptoms including myalgia, weakness, and muscle swelling. 1

Key Diagnostic Elements

Laboratory Criteria

• Serum markers:
  • Elevated creatine phosphokinase (CK) >50,000 IU/L (primary diagnostic criterion)
  • Elevated plasma myoglobin
  • Elevated serum potassium (kalaemia)
  • Metabolic acidosis (in severe cases)

Clinical Presentation

• Classic triad (only present in ~10% of cases) 2:
  • Muscle pain
  • Muscle weakness
  • Dark urine (myoglobinuria)

Additional Clinical Features

• Muscle swelling
• Altered mental status (in severe cases)
• Fever (in some cases)
• Signs of acute kidney injury
• Evidence of compartment syndrome in severe cases

Diagnostic Algorithm

1. Assess for elevated CK levels

   • CK >50,000 IU/L is diagnostic
   • Lower levels may still indicate rhabdomyolysis depending on clinical context

2. Evaluate for myoglobinuria

   • Dark tea-colored urine
   • Positive urine myoglobin test

3. Assess for complications

   • Acute kidney injury (elevated creatinine)
   • Electrolyte abnormalities (particularly hyperkalemia)
   • Metabolic acidosis (arterial blood gas)
   • Compartment syndrome (clinical examination)
   • Disseminated intravascular coagulation (coagulation studies)

4. Determine severity based on presence of:

   • Rhabdomyolysis with elevated creatine kinase
   • Metabolic acidosis
   • Elevated serum aminotransferase
   • Renal failure with elevated serum creatinine
   • Seizures
   • Disseminated intravascular coagulopathy 3

Imaging Considerations

While not required for diagnosis, imaging can help evaluate the extent of muscle involvement:

• MRI: Most sensitive for detecting affected muscle groups
• CT: Can identify areas of muscle swelling and edema
• Ultrasound: May show muscle edema and compartment abnormalities
• 18-FDG PET/CT: Can identify metabolically active affected muscle 2

Special Considerations

Distinguishing Acquired vs. Inherited Causes

After establishing the diagnosis of rhabdomyolysis, determine if it is:

• Acquired: Due to trauma, ischemia, drugs, toxins, infections, or exertion
• Inherited: Related to disorders of glycogen metabolism, fatty acid oxidation, or mitochondrial function 4

Exertional Rhabdomyolysis

Most cases are "physiological" without underlying disease and don't require extensive investigation. However, consider underlying muscle disease if:

• Recurrent episodes
• Family history of muscle disorders
• Disproportionate severity relative to exertion
• Persistent CK elevation after recovery 1

Common Pitfalls to Avoid

1. Missing the diagnosis due to absence of the classic triad (present in only 10% of cases)
2. Focusing only on CK levels without considering clinical presentation
3. Failing to monitor for complications, especially acute kidney injury
4. Not considering underlying causes that may require specific treatment
5. Overlooking compartment syndrome which requires urgent surgical intervention

Remember that rhabdomyolysis exists on a spectrum of severity, and early recognition with appropriate management is crucial to prevent life-threatening complications such as acute kidney injury, electrolyte abnormalities, and compartment syndrome.