What is rhabdomyolysis?

What is Rhabdomyolysis?

Rhabdomyolysis is the rapid breakdown of skeletal muscle cells with release of toxic intracellular contents—including myoglobin, creatine kinase (CK), potassium, calcium, phosphate, and other proteins—into the bloodstream, potentially causing life-threatening complications including acute kidney injury, severe electrolyte abnormalities, and cardiac arrhythmias. 1, 2, 3

Pathophysiology

The fundamental mechanism involves skeletal muscle cell death triggered by increased intracellular free ionized calcium, which occurs through either:

• Cellular energy depletion leading to failure of ATP-dependent calcium pumps 4
• Direct plasma membrane rupture from trauma or other insults 4

This calcium overload activates proteases, intensifies muscle contractility, induces mitochondrial dysfunction, and increases reactive oxygen species production, culminating in myocyte death and myonecrosis 1, 4.

Clinical Presentation

The classic triad includes:

• Muscle pain, soreness, weakness, or stiffness (though many patients present with non-specific symptoms) 1, 3
• Dark-colored urine (red-to-brown from myoglobinuria) 1, 5
• Elevated creatine kinase levels 1, 2

Early signs of compartment syndrome (which can both cause and complicate rhabdomyolysis) include pain, tension, paresthesia, and paresis, while late signs like pulselessness and pallor indicate irreversible damage 1.

Diagnostic Criteria

A CK level at least 5-10 times the upper limit of normal is considered diagnostic, with most guidelines using ≥5× ULN as the threshold 1, 2, 3. Key diagnostic findings include:

• Urinalysis showing brown/cloudy urine that is positive for blood without RBCs (indicating myoglobinuria) 1
• Elevated plasma myoglobin (though this is rapidly cleared) 1
• Markedly elevated CK levels, often >15,000 IU/L in severe cases 1

CK levels peak 24-120 hours after the inciting event, not immediately, due to lymphatic clearance mechanisms—the large CK molecule (82 kDa) cannot directly enter the bloodstream 1. This delayed peak necessitates repeat CK measurement at 24 hours if clinical suspicion remains high 1.

Major Causes

The etiology can be categorized into four main groups 5:

1. Trauma or direct injury (crush injuries, severe limb trauma, falls) 1, 5
2. Excessive muscle activity (exertional rhabdomyolysis from novel overexertion or unaccustomed exercise volume/intensity) 1, 5
3. Medications and toxins (particularly statins, with incidence of 1.6 per 100,000 patient-years; also cocaine, methamphetamine, MDMA, heroin) 6, 1, 5
4. Other medical causes including:
   • Muscle hypoxia and ischemia 5
   • Metabolic and endocrine disorders (hypothyroidism, electrolyte abnormalities) 1, 5
   • Infections (viral myositis) 1, 5
   • Temperature alterations (heat stroke, malignant hyperthermia) 5
   • Hereditary muscle enzyme defects (CPT2, PYGM, ACADM, AMPD1, VLCAD gene mutations) 1

Life-Threatening Complications

Severe disease progresses to critical complications including 1, 2:

• Acute kidney injury from myoglobin-induced renal tubular obstruction and direct nephrotoxicity (the most serious systemic complication) 1, 2
• Hyperkalemia leading to life-threatening cardiac arrhythmias and arrest 1, 2
• Severe metabolic acidosis 1, 3
• Compartment syndrome requiring urgent fasciotomy 1, 2
• Disseminated intravascular coagulation 1, 2
• Hypocalcemia during acute phase, followed by hypercalcemia during recovery 1

Essential Laboratory Workup

Beyond CK, the following tests are crucial 1:

• Complete electrolyte panel (potassium, calcium, phosphorus, magnesium)—hyperkalemia is immediately life-threatening 1
• Creatinine and BUN to assess renal function 1
• Urinalysis for myoglobinuria 1
• Liver function tests (AST, ALT, LDH, aldolase are commonly elevated) 1
• Coagulation studies to evaluate for DIC 1
• Arterial blood gas to assess metabolic acidosis 1

Serial monitoring of CK, creatinine, and electrolytes daily until CK is declining and renal function is stable is mandatory 1.

Critical Management Principles

Immediately discontinue any causative agents (statins, NSAIDs, supplements like red yeast rice, creatine monohydrate, wormwood oil, licorice, Hydroxycut) 1.

Early aggressive intravenous fluid resuscitation is the cornerstone of treatment, with goals of:

• Urine output ≥300 mL/hour 2, 3
• For severe rhabdomyolysis (CK >15,000 IU/L): volumes >6L may be required 1
• For moderate cases: 3-6L per day is typically sufficient 1
• Use isotonic saline (0.9% NaCl) as the initial fluid of choice 1

Delayed fluid resuscitation is associated with higher risk of acute kidney injury, making early initiation critical 1.

Monitor and emergently correct hyperkalemia to prevent cardiac arrhythmias 1. Renal replacement therapy is indicated for 1:

• Life-threatening hyperkalemia refractory to medical management
• Volume overload despite diuretics
• Uremic symptoms (encephalopathy, pericarditis)

Fasciotomy is indicated when compartment pressure exceeds 30 mmHg or when differential pressure (diastolic BP – compartment pressure) is <30 mmHg 1.

Pain Management Approach

• First-line: Acetaminophen 500-1000 mg (onset 15-30 minutes, max 4-6 g/day) to avoid nephrotoxic NSAIDs 1
• Avoid all NSAIDs (ibuprofen, naproxen, diclofenac) due to renal toxicity in patients already at high AKI risk 1
• For severe pain: Oral morphine 20-40 mg for opioid-naive patients 1
• If GFR <30 mL/min: Use fentanyl or buprenorphine as safest opioid choices 1

Special Considerations

Statin-induced rhabdomyolysis occurs more frequently with:

• Drug interactions (gemfibrozil carries 10-fold higher risk than fenofibrate; avoid this combination) 6, 1
• CYP3A4 inhibitors (cyclosporine, macrolide antibiotics, certain antifungals) 6
• Genetic factors (SLCO1B1 gene mutations) 1
• Patient factors (age, diabetes, renal impairment, hypothyroidism) 1

Impact trauma from falls can drastically increase CK without true muscle breakdown—simple contusion may not carry the same AKI risk as true rhabdomyolysis, but given rising CK trends, assume true muscle breakdown until proven otherwise 1.

In patients with altered mental status and rhabdomyolysis, determine whether AMS is a precipitating cause (seizures, prolonged immobilization, drug intoxication), a consequence (electrolyte disturbances, uremia), or an unrelated complicating condition (neuroleptic malignant syndrome, serotonin syndrome) 1.

Continue intravenous fluids until CK levels fall below 1,000 U/L 2.