What are the diagnostic criteria for rhabdomyolysis?

Diagnostic Criteria for Rhabdomyolysis

The definitive diagnosis of rhabdomyolysis requires a serum creatine kinase (CK) level at least 10 times the upper limit of normal, typically >5,000 IU/L, with or without myoglobinuria. 1

Clinical Presentation

Rhabdomyolysis presents with a spectrum of symptoms and findings:

• Classic triad (only present in ~10% of cases):

  • Muscle pain
  • Muscle weakness
  • Dark urine (tea or cola-colored)

• Additional clinical features:

  • Swelling of affected muscles
  • Malaise
  • Fever
  • Tachycardia
  • Nausea and vomiting

Laboratory Diagnostic Criteria

Essential Tests:

• Creatine kinase (CK):

  • Primary diagnostic marker
  • Elevation >5,000 IU/L (or ≥10× upper limit of normal)
  • Peaks 24-72 hours after muscle injury
  • Decreases by approximately 40% per day with adequate treatment

• Myoglobin:

  • Present in urine (myoglobinuria) and serum
  • Prognostic value, especially in non-traumatic rhabdomyolysis 2
  • Appears earlier but clears more rapidly than CK
  • Causes characteristic dark urine when concentration exceeds renal threshold

Supporting Laboratory Tests:

• Electrolytes:

  • Hyperkalemia (from muscle cell release)
  • Hyperphosphatemia
  • Hypocalcemia (early)
  • Hypercalcemia (late, recovery phase)

• Renal function:

  • Elevated BUN and creatinine (if acute kidney injury develops)
  • Urine dipstick positive for "blood" without RBCs on microscopy (myoglobinuria)

• Other markers:

  • Elevated AST, ALT, LDH (released from damaged muscle)
  • Metabolic acidosis
  • Elevated uric acid

Imaging Studies

While not required for diagnosis, imaging can help assess the extent of muscle damage:

• MRI: Most sensitive for detecting affected muscle groups

  • T2-weighted images show hyperintense signal in affected muscles
  • Useful for determining extent of involvement

• CT scan: May show hypodense areas in affected muscles with possible enhancement

• Ultrasound: Can show muscle swelling and decreased echogenicity

Diagnostic Algorithm

1. Suspect rhabdomyolysis in patients with:

   • History of trauma, crush injury, prolonged immobilization
   • Exposure to medications or toxins known to cause rhabdomyolysis
   • Extreme exertion, especially in untrained individuals
   • Muscle pain, weakness, or dark urine

2. Obtain initial laboratory tests:

   • CK level (primary diagnostic test)
   • Complete metabolic panel (electrolytes, renal function)
   • Urinalysis (for myoglobinuria)
   • CBC, coagulation studies

3. Confirm diagnosis if:

   • CK >5,000 IU/L or ≥10× upper limit of normal
   • With or without myoglobinuria

4. Assess for complications:

   • Acute kidney injury (elevated creatinine, decreased urine output)
   • Compartment syndrome (pain with passive stretch, paresthesia, paresis)
   • Electrolyte abnormalities (especially hyperkalemia)
   • Disseminated intravascular coagulation

Special Considerations

Compartment Syndrome Assessment

Monitor for acute compartment syndrome, particularly in traumatic cases:

• Pain out of proportion to injury
• Pain with passive stretch
• Paresthesia
• Paresis
• Pallor and pulselessness (late findings) 3

Compartment pressure measurement:

• Normal compartment pressure is <10 mmHg
• Fasciotomy indicated if:
  • Pressure ≥20 mmHg in hypotensive patients
  • Pressure ≥30 mmHg in normotensive patients with positive clinical findings 3

Differential Diagnosis

Consider other conditions that may mimic or coexist with rhabdomyolysis:

• Myositis
• Malignant hyperthermia
• Neuroleptic malignant syndrome
• Serotonin syndrome
• Inflammatory myopathies

Pitfalls and Caveats

1. Don't rely solely on the classic triad - it's present in only 10% of cases 4

2. Don't wait for dark urine to appear before initiating treatment - myoglobinuria may be absent or may clear rapidly

3. Don't miss compartment syndrome - a potentially limb-threatening complication requiring urgent fasciotomy

4. Don't overlook the underlying cause - identifying and treating the cause is essential for preventing recurrence

5. Don't delay fluid resuscitation - early aggressive fluid therapy is critical for preventing acute kidney injury 3

6. Don't rely on CK alone for monitoring treatment response - clinical improvement and urine output are equally important

By following these diagnostic criteria and being vigilant for complications, clinicians can effectively diagnose rhabdomyolysis and initiate appropriate treatment to prevent serious sequelae such as acute kidney injury and compartment syndrome.