Postoperative Rhabdomyolysis

Postoperative rhabdomyolysis is muscle breakdown with release of myoglobin, potassium, phosphate and creatine kinase after surgery, leading to acute kidney injury, electrolyte disturbance and compartment syndrome. Creatine kinase over 5 times the upper limit of normal (over 1,000 units/L) with symptoms is usually accepted for diagnosis.

Epidemiology. It occurs in about 0.1% to 0.3% of general surgery and is higher after bariatric surgery (up to 1% to 7% in obese patients with long operations), spine surgery in prone position, vascular surgery and lithotomy.

Causes. Prolonged positioning and pressure, tourniquet ischaemia, vascular clamping with reperfusion, hypotension, hypothermia, malignant hyperthermia, drugs (succinylcholine, statins, propofol infusion syndrome), electrolyte disturbance (hypokalaemia, hypophosphataemia), infection, and muscle disease.

Risk factors. Body mass index over 40 kg/m2 (especially bariatric surgery over 4 hours), male sex, muscular build, long operation (over 4 hours), lithotomy, statin use, diabetes, renal impairment, vascular disease and dehydration.

Pathophysiology.

ATP depletion or direct muscle injury disrupts sarcolemma and allows calcium influx and release of intracellular contents. Myoglobin causes renal tubular obstruction, direct toxicity and vasoconstriction. Hyperkalaemia and hypocalcaemia cause arrhythmia.

Muscle pain and swelling (often in gluteal, thigh or shoulder muscles), weakness, dark urine ("tea coloured"), reduced urine output, nausea, and fever. Pain may be masked by analgesia. Compartment syndrome and arrhythmia are complications.

Red flags.

• Peaked T waves or arrhythmia.
• Dark urine or oliguria.
• Tense compartment.
• Creatine kinase over 5,000 units/L.

Creatine kinase (peaks at 24 to 72 hours, falls by 40% to 50% daily), urinalysis (positive blood dipstick with few red cells), serum potassium, creatinine, calcium (low early), phosphate, urate, lactate and liver enzymes. ECG. Compartment pressure measurement if limb swelling.

Differential diagnosis.

• Myocardial injury (creatine kinase raised).
• Malignant hyperthermia.
• Propofol infusion syndrome.
• Haemolysis.
• Compartment syndrome.
• Acute kidney injury from other cause.

Remove the cause (pressure, drugs), correct hypovolaemia and start early aggressive fluid resuscitation.

• Crystalloid (0.9% saline or balanced solution): 1 to 2 L IV in the first hour, then 200 to 300 mL/h to maintain urine output of 200 to 300 mL/h (about 1 to 2 mL/kg/h) until creatine kinase is under 1,000 units/L; maximum about 10 L daily; monitor urine output, central venous pressure, potassium, calcium and fluid balance; reduce in heart failure and oliguric renal failure.
• Treat hyperkalaemia: calcium chloride 10% 10 mL IV, insulin 10 units with 25 g glucose IV, and salbutamol 10 to 20 mg nebulised; monitor ECG and glucose hourly.
• Avoid routine calcium for hypocalcaemia unless symptomatic or hyperkalaemic, because rebound hypercalcaemia follows.
• Sodium bicarbonate and mannitol have limited evidence; use bicarbonate 8.4% 50 mmol IV only for severe acidosis (pH under 7.1) or hyperkalaemia.

Renal replacement therapy for refractory hyperkalaemia, acidosis, fluid overload or anuria. Fasciotomy for compartment syndrome. Stop statins and other myotoxic drugs temporarily.

Prevention. Limit operation time, pad pressure points in obese patients, maintain perfusion, avoid hypothermia and monitor creatine kinase after high risk surgery.

Monitoring. Creatine kinase, creatinine, potassium, calcium, phosphate and urine output every 6 to 12 hours until falling.

Referral and escalation. Nephrology and critical care for renal injury. Orthopaedic review for compartment syndrome.