Rhabdomyolysis and Crush Syndrome: Pathophysiology, Diagnosis and Management

Overview and Clinical Context

Rhabdomyolysis is the breakdown of skeletal muscle with release of intracellular contents into the systemic circulation. In the ICU, it ranges from an incidental biochemical finding to a life-threatening syndrome with multi-organ failure. The intensivist must understand the underlying mechanisms deeply enough to anticipate complications, individualise fluid targets, and make timely decisions about renal replacement therapy. In mass casualty settings, crush syndrome - the systemic manifestation of widespread rhabdomyolysis following compressive injury - demands a different operational mindset: triage, resource allocation, and pre-hospital intervention become paramount.

Pathophysiology

Cellular Mechanisms of Muscle Injury

The common final pathway is failure of the $Na^+/K^+$-ATPase pump, leading to intracellular $Na^+$ and $Ca^{2+}$ accumulation. This can result from:

• Direct trauma - compressive crush injury disrupts sarcolemmal integrity
• Energy depletion - ischaemia, prolonged seizures, or extreme exertion exhaust ATP
• Toxin-mediated - statins, alcohol, cocaine, carbon monoxide, snake venoms impair mitochondrial function
• Temperature extremes - hyperthermia (heat stroke, NMS, malignant hyperthermia), hypothermia

Elevated intracellular $Ca^{2+}$ activates phospholipases, proteases, and endonucleases, driving irreversible cellular destruction. Cell contents - myoglobin, creatine kinase (CK), potassium, phosphate, urate, lactate dehydrogenase (LDH), and myocyte enzymes - flood the interstitium and then the vascular compartment.

Renal Injury Mechanisms

Acute kidney injury (AKI) is the most feared complication, occurring in 15-50% of rhabdomyolysis cases. Three mechanisms operate simultaneously:

Aciduria (pH < 5.6) dramatically worsens myoglobin cast precipitation, providing the physiological rationale for urinary alkalinisation.

Systemic Complications

Massive third-space fluid shifts into injured muscle and capillary leak cause profound hypovolaemia. Compartment syndrome can both cause and result from rhabdomyolysis, creating a vicious cycle. Systemic release of muscle contents drives:

• Hyperkalaemia - life-threatening; potassium release from muscle combined with AKI-related impaired excretion
• Hyperphosphataemia - complexes with calcium → hypocalcaemia (early)
• Late hypercalcaemia - as calcium deposited in necrotic muscle is remobilised during recovery
• Metabolic acidosis - lactic acid, organic acids from anaerobic metabolism
• DIC - release of thromboplastin from damaged muscle
• Compartment syndrome - swollen muscle in a rigid fascial compartment elevates tissue pressure above perfusion pressure ($P_{perfusion} = MAP - P_{compartment}$)

Diagnosis

Clinical Features

Presentation varies from asymptomatic CK elevation detected on routine bloods to the classic triad of muscle pain, weakness, and dark ("coca-cola" or "tea-coloured") urine. In sedated ICU patients, diagnosis is often biochemical.

High-risk presentations to actively screen: - Trauma with prolonged immobilisation or crush - Polytrauma, especially after extrication - Severe burns - Status epilepticus or prolonged seizures - Stimulant toxicity (cocaine, amphetamines, MDMA) - Malignant hyperthermia or NMS - Statin use combined with CYP3A4 inhibitors

Biochemical Diagnosis

CK peaks at 24-72 hours then falls with a half-life of approximately 36 hours. A rising CK beyond 72 hours suggests ongoing muscle injury (e.g., ongoing compartment syndrome, evolving myopathy).

Monitoring

Serial 4-6 hourly CK, electrolytes, renal function, and urine output in the acute phase. Urine pH measurement guides alkalinisation strategy. Continuous cardiac monitoring is mandatory given hyperkalaemia risk. Compartment pressure measurement (threshold >30 mmHg, or within 30 mmHg of diastolic pressure) if clinical concern.

Management

Fluid Resuscitation - The Cornerstone

Aggressive early volume replacement is the single most important intervention to prevent AKI. Goals are to restore renal perfusion and maintain tubular flow to flush myoglobin casts.

Targets: - Urine output: 200-300 mL/hr in the acute phase (or 3-5 mL/kg/hr) - This is significantly higher than standard resuscitation targets and requires careful monitoring - Titrate to urine output rather than CVP or MAP alone - Crystalloid preferred: isotonic saline (0.9% NaCl) or Hartmann's/Ringer's lactate; avoid hypotonic solutions

Fluid volume: Can be enormous - 6-12 litres in first 24 hours not unusual in severe crush; up to 1.5 litres/hour may be required initially.

Caution: Overzealous fluid in compartment syndrome worsens oedema and may increase compartment pressure. Once urine output target achieved, titrate down. Monitor for pulmonary oedema, especially in elderly or those with cardiac impairment.

Urinary Alkalinisation

Rationale

Alkalinisation increases urine pH above 6.5, which: 1. Reduces precipitation of myoglobin with Tamm-Horsfall protein (precipitation dramatically increases at pH <5.6) 2. Reduces conversion of myoglobin to the more nephrotoxic ferrihaemate form (favoured in acidic environment) 3. Reduces uric acid cast formation 4. May mitigate vasoconstriction (modest systemic alkalosis improves renal perfusion)

Method

• Add 50-100 mmol of sodium bicarbonate ($NaHCO_3$) to each litre of 5% dextrose or 0.45% saline as an infusion
• Alternatively: 1-1.5 mmol/kg bolus then continuous infusion titrated to urine pH
• Urine pH target: >6.5 (check 1-2 hourly with pH paper or blood gas analyser on urine)
• Continue until myoglobinuria resolves (urine clears) and CK trending down

Limitations and Contraindications

• Worsens hypocalcaemia by increasing calcium-phosphate chelation - correct symptomatic hypocalcaemia cautiously (avoid if asymptomatic, as calcium given to an alkalotic patient with hyperphosphataemia increases risk of calcium-phosphate precipitation in tissues)
• Ineffective and potentially harmful if the patient is oliguric/anuric - alkalinisation requires urine flow to work
• Risk of metabolic alkalosis; monitor serum pH
• Hypernatraemia with bicarbonate load
• Evidence base is limited - no large RCTs demonstrate mortality benefit over aggressive saline diuresis alone, but physiological rationale is strong and alkalinisation remains widely practised for CK >5000 U/L with myoglobinuria

Forced Diuresis with Loop Diuretics

Mannitol (0.25-0.5 g/kg IV) has been used as an osmotic diuretic to increase tubular flow and as a free radical scavenger. It can also reduce compartment pressure (osmotic gradient). However: - Avoid in oliguria (risk of volume overload and osmolar gap toxicity) - No robust evidence of superiority over saline alone - Mannitol is largely falling out of favour as first-line

Frusemide may help maintain urine output in established AKI, but forced diuresis is not beneficial and carries risks of volume depletion if used without adequate resuscitation. It may acidify urine, which is counterproductive in the alkalinisation strategy. Do not use frusemide as primary prevention of myoglobin-induced AKI.

Management of Hyperkalaemia

This is the primary life-threatening emergency in crush syndrome and must be managed in parallel with fluid resuscitation.

Haemodialysis for Refractory Hyperkalaemia

Indications for RRT in rhabdomyolysis:

Intermittent haemodialysis is highly effective for acute hyperkalaemia in established AKI - potassium clearance is rapid (can reduce K+ by 1 mmol/L/hour). CRRT is preferred when haemodynamic instability limits intermittent HD, or in mass casualty settings where prolonged continuous electrolyte control is needed. Note: myoglobin itself (MW ~17,000 Da) is poorly removed by standard HD membranes; high-flux or high-cutoff membranes can achieve limited myoglobin clearance but clinical benefit is unproven.

Compartment Syndrome

Urgent surgical fasciotomy is required when: - Compartment pressure >30 mmHg, OR - Within 30 mmHg of diastolic BP ($\Delta P = P_{diastolic} - P_{compartment} < 30$ mmHg) - Clinical signs: pain out of proportion, paraesthesia, pallor, pulselessness, paralysis

Post-fasciotomy wounds create significant ongoing fluid and electrolyte management challenges - massive evaporative losses and ongoing third-spacing.

Crush Syndrome in Mass Casualty Incidents

Definition and Specific Pathophysiology

Crush syndrome is the systemic manifestation of rhabdomyolysis following prolonged compressive injury (typically >1 hour). The key danger is the reperfusion phase after extrication - compressive forces are released and a large bolus of myoglobin, potassium, and acid enters the circulation simultaneously. Cardiac arrest can occur within minutes of extrication.

Pre-Hospital Priorities

• Establish IV access and commence aggressive fluid resuscitation BEFORE extrication where feasible (target 1 litre/hr of normal saline in adults if no contraindication)
• Continuous cardiac monitoring from the outset - VF/VT from hyperkalaemia may be the first clinical event
• Prolonged entrapment >4-6 hours: consider prophylactic treatment of anticipated hyperkalaemia before release (calcium gluconate, bicarbonate, salbutamol pre-administered)
• In truly prolonged entrapment with non-survivable injuries, consideration of field amputation may arise - an extraordinarily difficult decision requiring medical direction and team consensus

Triage Considerations

In mass casualty incidents with multiple crush victims:

Resource scarcity fundamentally alters decision-making - dialysis capacity becomes a critical bottleneck. A single crush victim with AKI may require 12-24 hours/day of RRT; 50 victims overwhelm any health system.

Hospital-Level Mass Casualty Response

• Surge capacity planning for RRT: activate all available HD machines, mobilise CRRT circuits, consider peritoneal dialysis if HD unavailable (less efficient but requires no specialist equipment)
• Electrolyte monitoring at scale - point-of-care testing essential
• Early nephrology and surgical engagement
• Amputation may be lifesaving in the field or hospital if limb loss is inevitable and systemic complications are threatening life
• Psychological support for staff involved in expectant triage decisions

CICM Final Implications

Hot Case / Viva Approach

When presented with a patient with rhabdomyolysis in the ICU:

1. Establish aetiology - trauma, toxin, metabolic, iatrogenic (prolonged immobility, statin + drug interaction)
2. Quantify severity - CK level, trend, urine colour, degree of AKI
3. Immediate threats - hyperkalaemia (ECG), compartment syndrome
4. Fluid strategy - articulate your urine output target (200-300 mL/hr) and how you will achieve it; be explicit about monitoring strategy
5. Alkalinisation - know the rationale, the method, the targets (urine pH >6.5), and the limitations (worsening hypocalcaemia, futile in anuria)
6. RRT indications - do not wait for severe uraemia; refractory hyperkalaemia is the primary indication; justify modality choice (IHD for rapid K+ correction vs CRRT for haemodynamic instability)
7. Compartment syndrome - maintain a low threshold; serial clinical examination in sedated patients

Anticipated Viva Questions

• "What is the mechanism by which myoglobin causes AKI?" - three-part answer: tubular obstruction, direct toxicity (ferrihaemate), vasoconstriction via NO scavenging
• "Does urinary alkalinisation improve outcomes?" - acknowledge limited RCT evidence; strong physiological rationale; widely recommended when CK >5000 + myoglobinuria; ineffective without urine flow
• "When would you start RRT?" - refractory hyperkalaemia > K+ 6.5 mmol/L with ECG changes, oligoanuria refractory to resuscitation, fluid overload, pH <7.1
• "How does mass casualty crush syndrome differ from standard management?" - pre-extrication IV fluids, anticipate K+ surge on release, triage in resource-limited settings, RRT capacity planning

Key Numbers to Know