Rhabdomyolysis

1. THE 2-MINUTE PHYSIOLOGY (Rapid Pathophysiology)

Rhabdomyolysis is characterized by the acute necrosis and destruction of skeletal muscle fibers. The core pathophysiological mechanism is driven by ATP depletion and an influx of intracellular calcium, which leads to the breakdown of myocyte membranes.

When the myocyte membrane ruptures, massive amounts of toxic intracellular contents are released into the systemic circulation. These include:

• Myoglobin: A nephrotoxic protein that freely filters at the glomerulus. In the setting of hypovolemia and aciduria, myoglobin precipitates with uric acid to form casts, causing severe renal tubular obstruction and direct oxidative injury (Acute Kidney Injury).
• Potassium & Phosphate: Massive cellular release causes life-threatening hyperkalemia and hyperphosphatemia.
• Creatine Kinase (CK): A diagnostic marker of muscle breakdown.
• Calcium: In the early stages, serum calcium crashes (hypocalcemia) because free calcium rapidly binds to the damaged, necrotic muscle tissue.

2. THE BEDSIDE ACTION PLAN (Rapid ER Management)

• Immediate Stabilization: Assess ABCs, establish large-bore IV access, and place the patient on continuous cardiac monitoring to watch for hyperkalemic dysrhythmias.
• Fluid Resuscitation (The Critical Step): Vigorous volume expansion is the most important treatment to flush the renal tubules and prevent AKI.
• Adults: Administer Isotonic Saline (0.9% NaCl) at 1 to 2 Liters/hour.
• Pediatrics: 20 to 40 mL/kg/hour.
• Fluid Selection: Strictly avoid potassium- or lactate-containing solutions (e.g., Lactated Ringer's) initially, due to the high risk of compounding hyperkalemia and lactic acidosis.
• Resuscitation Goals: Target a state of hypervolemia. You must maintain a strict urine output of 1 to 3 mL/kg/hour (or generally >100 mL/hour in adults).
• Electrolyte & Acid-Base Management:
• Treat hyperkalemia with standard shifting agents (insulin/glucose, albuterol, sodium bicarbonate) and membrane stabilization (IV Calcium).
• Consider adding IV Sodium Bicarbonate (1-2 mEq/kg) to your IV fluids to alkalinize the urine and prevent myoglobin precipitation if severe metabolic acidosis is present.

3. THE DIAGNOSTIC GRID (Differential Diagnosis & Workup)

• "Can't-Miss" Differential Diagnoses:
• Compartment Syndrome: Can be both a cause and a catastrophic complication of rhabdomyolysis.
• Guillain-Barré Syndrome: Can present with diffuse weakness, mimicking myopathy.
• Polymyositis / Dermatomyositis / Acute Myopathies: Inflammatory conditions presenting with profound muscle weakness and pain.
• Prioritized Diagnostic Workup:
• Creatine Kinase (CK): The gold standard diagnostic marker. Levels typically >10,000 U/L serve as a strong marker for clinically significant rhabdomyolysis and risk of AKI.
• Urinalysis & Urine Myoglobin: Send immediately to evaluate for myoglobinuria.
• STAT BMP & Electrolytes: Assess for the classic triad of hyperkalemia, hyperphosphatemia, and hypocalcemia, alongside BUN/Cr for acute kidney injury.
• Coagulation Panel (PT/PTT, D-Dimer, CBC): To evaluate for Disseminated Intravascular Coagulation (DIC), a known lethal complication of severe muscle necrosis.

4. THE VISUAL BOARD (ECG / POCUS / Imaging)

• The Urinalysis Illusion: The classic visual finding is dark amber or "tea-colored" urine. On the urine dipstick, it will test strongly positive for "blood" (due to myoglobin), but the microscopic examination will reveal NO red blood cells.
• 12-Lead ECG: This is mandatory and must be scrutinized for signs of hyperkalemia (peaked T-waves, PR prolongation, widened QRS, sine-wave morphology) resulting from massive potassium release.
• Extremity Assessment: Visually and physically inspect the limbs for tense, woody, or swollen compartments to rule out impending compartment syndrome.

5. THE SCORING MATRIX (Risk Stratification & Guidelines)

• The McMahon Score: A validated risk prediction score used to predict mortality or the need for renal replacement therapy (dialysis) in rhabdomyolysis.
• Criteria Components:
• Initial Creatinine and Calcium levels.
• Initial CK (e.g., >40,000 U/L adds 2 points).
• Phosphate and Bicarbonate levels.
• Demographics (Age, Sex).
• Etiology (1 point is added if the etiology is NOT seizures, syncope, exercise, statins, or myositis).
• Definitive Cutoffs:
• Score $\le$ 5: Indicates a low (3%) risk of requiring dialysis or death.
• Score $\ge$ 10: Indicates a high (52%) risk of requiring dialysis or death, mandating early nephrology consultation and ICU admission.

6. THE DANGER ZONE (Pitfalls & Critical Actions)

• Cognitive Trap (Treating the Calcium): Reflexively administering IV Calcium to correct asymptomatic hypocalcemia. Correction: Hypocalcemia is common early on as calcium binds to necrotic muscle. If you administer IV calcium when it is not strictly needed for hyperkalemic ECG changes or severe symptoms, it will compound the dangerous "rebound hypercalcemia" that occurs in the late recovery phase when muscle releases the stored calcium.
• Cognitive Trap (Lactated Ringer's): Grabbing Lactated Ringer's for high-volume resuscitation. Correction: You must avoid potassium-containing solutions (LR) until you have definitively proven the patient is not hyperkalemic. Use 0.9% Normal Saline.
• Critical Action: You must aggressively push IV fluids to achieve a target urine output of >100 mL/hr (or 1-3 mL/kg/hr) to mechanically flush the renal tubules of obstructing myoglobin.

7. MCQ MASTERCLASS (Written Exam Tips)

• Buzzwords: "Urine dipstick positive for blood, but no RBCs on microscopy," "Dark amber/tea-colored urine," "Crush injury," "Statins," "CK > 10,000."
• Classic Distractor (The Electrolyte Trap): A question describes a patient with rhabdomyolysis, a CK of 45,000, Potassium of 4.9, and Calcium of 7.2 mg/dL. A distractor option will suggest "Administer IV Calcium Gluconate to correct the hypocalcemia." Explanation: This is a trap. You only administer Calcium for severe hypocalcemia or to stabilize the cardiac membrane in the setting of hyperkalemia. Treating mild/moderate hypocalcemia in rhabdo will cause lethal late-phase hypercalcemia. Choose the option for aggressive IV hydration.
• Classic Distractor (The Fluid Trap): An option will suggest initiating resuscitation with Lactated Ringer's. Explanation: LR contains potassium. In a disease characterized by massive cellular potassium release, introducing exogenous potassium is contraindicated until labs result. Choose 0.9% Normal Saline.

8. THE BOARDROOM SCRIPT (OSCE & Oral Board Tips)

• The Initial Approach: "Given the patient's history of prolonged immobilization and presentation with dark urine and muscle pain, I am highly concerned for acute rhabdomyolysis. I will immediately place the patient on continuous cardiac monitoring to assess for hyperkalemic dysrhythmias and order a stat 12-lead ECG."
• The Resuscitation: "I will secure two large-bore IVs and immediately initiate aggressive fluid resuscitation with 0.9% Normal Saline at 1 to 2 Liters per hour. I am explicitly avoiding Lactated Ringer's until I confirm the patient's potassium level. I will place a Foley catheter to strictly monitor urine output, targeting greater than 100 mL per hour to flush the renal tubules."
• The Workup & Disposition: "I am ordering a stat serum Creatine Kinase, a basic metabolic panel to assess for hyperkalemia and acute kidney injury, and a urinalysis to check for myoglobinuria. I will calculate a McMahon score to predict their risk for renal replacement therapy. I am admitting the patient to a monitored bed for continuous hydration and serial CK/renal function checks, and I will consult Nephrology if the hyperkalemia becomes refractory."