Rhabdomyolysis

• Muscle fiber necrosis and sarcolemmal disruption: The primary pathologic process involves injury to the muscle plasma membrane (sarcolemma) and Z-disc disruption leading to uncontrolled myofilament damage. Physical trauma, extreme heat, electric current, or various toxins cause direct sarcolemmal injury, while hypoxia from crush injury or compartment syndrome triggers ATP depletion. Loss of ATP-dependent Na+/K+-ATPase function results in sodium influx, cellular edema, and further membrane rupture. The damaged sarcolemma becomes permeable to extracellular calcium, leading to uncontrolled intracellular calcium accumulation. This triggers calpain and caspase activation, promoting proteolytic degradation of contractile proteins and acceleration of the necrotic cascade.

• Calcium dysregulation and excitation-contraction uncoupling: Intracellular calcium overload is central to rhabdomyolysis pathophysiology. In normal muscle, calcium is tightly regulated through sarcoplasmic reticulum uptake via SERCA pumps (sarcoplasmic/endoplasmic reticulum calcium-ATPase). With severe injury, calcium influx exceeds reuptake capacity, and calcium accumulates in mitochondria, leading to mitochondrial dysfunction, loss of membrane potential, and impaired ATP synthesis. Elevated intracellular calcium activates phospholipase A2, which cleaves membrane phospholipids into arachidonic acid and lysophospholipids, triggering lipid peroxidation and free radical generation. Calcium-dependent proteases (calpains) degrade myofibrillar and cytoskeletal proteins. Paradoxically, patients develop profound hypocalcemia during acute rhabdomyolysis due to increased bone and soft-tissue uptake of calcium combined with suppressed PTH secretion and reduced 1,25-dihydroxyvitamin D production; hyperphosphatemia from muscle necrosis additionally drives calcium precipitation in tissues.

• Myoglobin release and toxic nephropathy: Massive myoglobin release from lysed muscle cells enters the circulation, exceeding the binding capacity of haptoglobin and hemoglobin. Free myoglobin is filtered by the glomerulus, but owing to its small molecular weight (~17.8 kDa), it is not fully reabsorbed in the proximal tubule. Myoglobin's heme groups generate reactive oxygen species (ROS) in the tubular lumen and epithelium through Fenton chemistry. The acidic milieu of the distal tubule (especially in volume-depleted states) promotes myoglobin precipitation as casts, which physically obstruct the tubule and cause acute tubular necrosis. Myoglobin-induced lipid peroxidation of tubular epithelial cells, direct oxidative injury from heme iron, and inflammatory cytokine release (TNF-α, IL-6) propagate renal injury. The combination of hypovolemia, vasoconstriction, and tubular obstruction creates "pigment nephropathy," a form of intrinsic renal failure that can rapidly progress to oliguria and permanent kidney dysfunction if not aggressively treated.

• Hyperkalemia and arrhythmia risk: Cell lysis releases massive amounts of intracellular potassium, with each kilogram of muscle tissue containing approximately 140 mEq of potassium. In trauma victims with extensive crush injuries, serum potassium can rise to life-threatening levels (>8-10 mEq/L) within hours. Hyperkalemia disrupts cardiac membrane repolarization, causing peaked T waves, prolonged PR interval, widened QRS complex, and ultimately ventricular fibrillation. The risk is amplified in patients with concurrent renal failure, metabolic acidosis, and hypocalcemia, all of which shift potassium from the intracellular to extracellular compartment. In crush syndrome victims entrapped for prolonged periods, reperfusion after extrication can cause sudden hyperkalemia-induced cardiac arrest ("crush syndrome" or "Bywaters syndrome").

• Metabolic acidosis and inflammatory cascade: Tissue hypoxia during ischemia and ATP depletion lead to anaerobic metabolism, lactate production, and systemic acidosis. Acidosis worsens myoglobin precipitation in renal tubules and shifts potassium extracellularly, compounding hyperkalemia severity. Muscle necrosis triggers a potent inflammatory response: damaged tissue releases damage-associated molecular patterns (DAMPs) including heat shock proteins and nucleic acids, activating pattern recognition receptors on innate immune cells. This stimulates release of pro-inflammatory cytokines (TNF-α, IL-1β, IL-6), complement activation, and neutrophil infiltration into affected muscles. The inflammatory cascade perpetuates muscle damage, increases capillary permeability, and contributes to systemic inflammatory response syndrome (SIRS) and multiorgan dysfunction in severe cases.

• Trauma and crush injuries: Blunt force trauma, crush syndrome from prolonged entrapment (building collapse, machinery accidents), and compartment syndrome are the prototypical triggers in emergency settings. Crush syndrome specifically refers to injury occurring after prolonged entrapment (typically >4-6 hours) with rhabdomyolysis developing during or after extrication. The ischemic muscle becomes necrotic; reperfusion injury upon release further accelerates myocyte death through ROS generation and calcium influx. High-energy mechanisms (motor vehicle accidents, blast injuries) cause massive muscle fiber disruption.

• Exertional rhabdomyolysis: Intense, unaccustomed physical activity, particularly in hot conditions, precipitates rhabdomyolysis in otherwise healthy individuals. Classic scenarios include military recruits during basic training, marathon runners in heat, and athletes during intense CrossFit or high-intensity interval training (HIIT) workouts. Ambient temperature elevation decreases heat dissipation, raising muscle temperature and exacerbating cellular injury. Even modest exertion can trigger rhabdomyolysis in individuals with underlying muscle disorders (muscular dystrophies, glycogen storage diseases). Risk factors for exertional rhabdomyolysis include male gender, extremes of age, poor physical conditioning, dehydration, and concurrent febrile illness.

• Medications and toxins: Statins cause myopathy and rhabdomyolysis, especially at high doses, in combination with CYP3A4 inhibitors (clarithromycin, ketoconazole, protease inhibitors), or in patients with renal or hepatic impairment; incidence is ~0.15 per 1000 patient-years but increases dramatically with drug-drug interactions. Fibrates independently increase risk and synergize with statins. Corticosteroids can trigger rhabdomyolysis, particularly in acute high-dose regimens. Amphetamines (cocaine, methamphetamine, MDMA/ecstasy) cause intense muscle contractions, hyperthermia, and direct myotoxicity; cocaine additionally causes vasoconstriction, reducing muscle perfusion. Neuroleptic malignant syndrome (from antipsychotics) and malignant hyperthermia (from anesthetic agents like succinylcholine or volatile anesthetics) represent drug-triggered rhabdomyolysis with hypermetabolic muscle activity and uncontrolled calcium release. Alcohol abuse causes both acute rhabdomyolysis (from direct myotoxicity, immobilization with compartment syndrome, seizures) and chronic myopathy. Daptomycin is implicated in myopathy. Antiretrovirals (zidovudine, particularly) and nucleoside reverse transcriptase inhibitors can cause mitochondrial myopathy.

• Hyperthermia and heat-related illness: Exertional heat stroke and environmental heat exposure cause muscle injury through direct thermal protein denaturation and accelerated metabolism. Malignant hyperthermia, though rare (1 in 10,000 to 1 in 50,000 anesthetics), represents a pharmacogenetic disorder of calcium regulation with dramatic rhabdomyolysis onset during general anesthesia.

• Metabolic and endocrine disorders: Hypophosphatemia (especially in refeeding syndrome), hypokalemia, and hypocalcemia can precipitate rhabdomyolysis through disruption of cellular energy metabolism. Diabetic ketoacidosis complicated by infection or hyperosmolar state elevates risk. Thyroid storm increases muscle metabolism and heat generation. Hyperthyroidism generally increases susceptibility to exertional rhabdomyolysis.

• Genetic muscle disorders: Glycogen storage diseases (especially types III, V, VII, and X) predispose to exercise-induced rhabdomyolysis due to impaired energy substrate availability. Carnitine palmitoyltransferase deficiency (CPT I and II deficiency) causes recurrent rhabdomyolysis with fasting, infection, or exertion. Malignant hyperthermia susceptibility and central core disease represent additional genetic muscle channelopathies. Duchenne and Becker muscular dystrophies increase baseline risk.

• Infections and sepsis: Bacterial sepsis (particularly from Streptococcus pneumoniae), viral infections (influenza, dengue, HIV, EBV), and fungal infections can trigger rhabdomyolysis through cytokine-mediated muscle inflammation. Polymyositis and dermatomyositis (autoimmune myositis) cause subacute myonecrosis but can progress acutely.

• Electrolyte abnormalities and renal disease: Severe hyponatremia (especially <120 mEq/L), hypokalemia, and hypophosphatemia impair muscle function and can precipitate rhabdomyolysis. Underlying chronic kidney disease impairs potassium excretion, raising hyperkalemia risk in rhabdomyolysis.

• Immobilization and compression: Prolonged immobilization (bedridden patients, anesthesia), external compression (tight casts, tourniquets), or positional compression (prolonged lying on hard surface, "crush syndrome without entrapment") causes muscle ischemia and necrosis.

• Seizures and status epilepticus: Intense, prolonged muscular contractions during prolonged seizure activity generate rhabdomyolysis through mechanical muscle fiber disruption and heat generation.

• Myalgias and muscle weakness: Muscle pain ranging from mild discomfort to severe, exquisite tenderness is the hallmark symptom. Patients typically report pain in the affected muscle groups (often lower extremities in crush injury, widespread in exertional or drug-induced cases). Muscle swelling (myoedema) and palpable firmness reflect cytoplasmic edema and interstitial fluid accumulation. Severe rhabdomyolysis causes muscle weakness proportional to the degree of myonecrosis; in crush injuries, weakness is often localized to compressed compartments, whereas exertional or toxic causes typically produce symmetric weakness. Some patients (particularly with statins) report only mild myalgias before developing severe hypercreatinemia, highlighting the variable symptom-CK relationship.

• Myoglobinuria ("cola-colored" or "tea-colored" urine): The pathognomonic sign is dark, discolored urine due to massive myoglobin excretion. Urine myoglobin concentration may reach 1000 mg/dL (normal <0.02 mg/dL). The urine dipstick characteristically shows positive blood (due to heme groups in myoglobin reacting with the peroxidase-like activity of the dipstick reagent) but few or no RBCs on microscopy—this dissociation (positive blood without hematuria) is a key diagnostic clue. Myoglobinuria typically appears when serum myoglobin exceeds the renal threshold (~0.5-1 g/dL); however, absence of visible myoglobinuria does not exclude significant rhabdomyolysis, as myoglobin may be cleared rapidly or obscured by concurrent hematuria.

• Oliguria and acute kidney injury: Progression to oliguria (urine output <0.5 mL/kg/hour) indicates severe rhabdomyolysis with pigment nephropathy. AKI typically develops 24-72 hours after the inciting event, though hyperkalemia may manifest within hours. Serum creatinine and BUN rise rapidly; in crush syndrome, creatinine can double within 24 hours. Patients may also develop non-oliguric AKI with preserved urine output but rising creatinine, often carrying a better prognosis than oliguric kidney injury. In the most severe cases, acute kidney injury progresses to requiring dialysis, with some patients developing permanent renal insufficiency.

• Hyperkalemia and cardiac manifestations: Severe hyperkalemia (K+ >6-8 mEq/L) causes peaked T waves on ECG followed by prolonged PR interval, widened QRS, and flattened P waves. Symptomatic hyperkalemia may present with palpitations, syncope, or sudden cardiac arrest (ventricular fibrillation) without warning. Some patients report paresthesias, muscle weakness, or paralysis preceding cardiac symptoms. In crush syndrome, particularly after extrication, sudden hyperkalemia-induced cardiac arrest can occur ("crush syndrome death"), necessitating aggressive potassium management and cardiac monitoring.

• Hypocalcemia manifestations: Symptomatic hypocalcemia (typically develops 24-48 hours into illness) causes paresthesias (perioral and in extremities), muscle cramps, tetany, and seizures. Severe hypocalcemia (Ca2+ <6.5 mg/dL) precipitates Chvostek sign (facial muscle twitching with tap on facial nerve), Trousseau sign (carpopedal spasm with blood pressure cuff inflation), and seizures. Hypocalcemia is particularly risky because it lowers the threshold for hyperkalemia-induced arrhythmias; concurrent severe hypokalemia (from treatment or diuresis) exacerbates cardiac risk.

• Fever and systemic inflammatory symptoms: Many patients develop fever, malaise, and systemic signs of SIRS (tachycardia, tachypnea, elevated inflammatory markers). In severe cases, this progresses to sepsis-like syndrome with hypotension, altered mental status, and multiorgan dysfunction despite absence of infection. This reflects the intense inflammatory response triggered by massive muscle necrosis (SIRS from sterile myonecrosis).

• Compartment syndrome: In crush injuries, edematous swelling within fascial compartments (particularly anterior and posterior tibial compartments, forearm compartments) can raise intracompartmental pressure above perfusion pressure, causing progressive ischemia. Compartment syndrome classically presents with the "5 P's": Pain (especially pain with passive stretch, the most sensitive sign), Pressure (tense compartment), Pallor, Paresthesias, and Pulselessness (late sign). Pain with passive stretch is the earliest and most sensitive sign and should prompt urgent surgical evaluation.

• Atypical presentations: Rhabdomyolysis can present without classic myoglobinuria if creatinine kinase elevation is mild or transient, or if hematuria masks myoglobinuria. Some patients have predominantly constitutional symptoms (fever, malaise) with minimal myalgias. Severe cases present with shock, altered mental status, or multiorgan failure as the primary manifestation. Drug-induced rhabdomyolysis (especially statins) may be subclinical, detected only on routine laboratory assessment.

• Serum creatine kinase (CK) measurement: CK is the most sensitive marker of muscle injury and the cornerstone of rhabdomyolysis diagnosis. Normal CK is <100-150 IU/L (varies by lab and patient factors); rhabdomyolysis typically elevates CK to >1000 IU/L, often dramatically (5,000-100,000+ IU/L).

Immediate stabilisation

• Cardiac monitoring and ECG first: hyperkalemia, not the muscle injury, kills early. Per the AHA ACLS/ECC guidance, ECG changes or K+ >6.5 mEq/L warrant IV calcium (calcium gluconate or chloride) to stabilise the myocyte membrane, then intracellular shift with insulin plus dextrose, and a nebulized beta-2 agonist (albuterol). Shifting is temporising; potassium removal requires GI binders (sodium zirconium cyclosilicate, patiromer) or dialysis.
• Anticipate reperfusion hyperkalemia in crush syndrome: begin fluids before extrication when feasible, as endorsed by disaster-medicine consensus from the ISN Renal Disaster Relief Task Force.

First-line therapy

• Isotonic crystalloid, aggressively and early: normal saline (or a balanced crystalloid) at high rates, titrated to a urine output commonly targeted at 200–300 mL/hr. Volume expansion restores renal perfusion, dilutes tubular myoglobin, and flushes casts before they obstruct. Earlier fluid initiation correlates with lower rates of dialysis.
• Remove the trigger: stop the statin, fibrate, or interacting CYP3A4 inhibitor (the AHA/ACC/Multisociety cholesterol guideline addresses statin-associated muscle symptoms); dantrolene for malignant hyperthermia; active cooling for heat stroke; benzodiazepines for stimulant toxicity or status epilepticus.

Escalation and unproven adjuncts

• Urinary alkalinisation with sodium bicarbonate and mannitol: theoretically reduce myoglobin cast formation and osmotically promote flow, but neither has demonstrated benefit over saline alone and both carry harm (bicarbonate worsens hypocalcemia; mannitol causes osmotic nephrosis and hyperosmolarity). KDIGO's AKI guidance does not endorse routine use.
• Renal replacement therapy: for refractory hyperkalemia, refractory acidosis, volume overload, or uremia — the standard KDIGO indications. Dialysis clears myoglobin poorly; it is not given to "wash out" pigment.

Definitive/surgical

• Fasciotomy for confirmed acute compartment syndrome — an orthopedic emergency.

Contraindicated/avoid

• Loop diuretics for oliguria, NSAIDs, iodinated contrast, and other nephrotoxins.
• Routine calcium repletion for asymptomatic hypocalcemia — reserve calcium for tetany, seizures, or hyperkalemic ECG changes.

Emergencies

• Hyperkalemia with malignant arrhythmia: lysed myocytes dump potassium; acidosis and hypocalcemia amplify membrane instability. Signalled by peaked T waves progressing to PR prolongation, QRS widening, a sine wave, and then ventricular fibrillation or pulseless VT or asystole. Classic in Bywaters (crush) syndrome at the moment of reperfusion after extrication.
• Acute kidney injury (pigment nephropathy): combined hypovolemic vasoconstriction, heme-iron oxidative tubular injury, and cast obstruction. Signalled by rising creatinine 24–72 hours in, oliguria, and pigmented granular casts; a disproportionately high creatinine relative to BUN reflects creatinine release from muscle.
• Acute compartment syndrome: muscle edema — often worsened by the very fluid resuscitation used to treat the kidney — raises intracompartmental pressure above perfusion pressure. Signalled by pain out of proportion and pain on passive stretch; pulselessness is late. Requires fasciotomy.
• DIC: necrotic muscle releases thromboplastin-like material. Signalled by falling platelets and fibrinogen with rising D-dimer and PT/aPTT; carries very high mortality.

Metabolic

• Early hypocalcemia: calcium deposits in injured muscle and precipitates with released phosphate; do not over-correct.
• Late rebound hypercalcemia: during the recovery/diuretic phase, deposited calcium remobilises while 1,25-vitamin D rebounds — a trap for clinicians who gave aggressive calcium early.
• Hyperphosphatemia and hyperuricemia from purine and phosphate release; high-anion-gap metabolic acidosis from lactate and organic acids.

Treatment-related

• Volume overload and pulmonary edema from massive crystalloid, especially once AKI is established — monitor for oxygen requirement and rising urine-output/intake mismatch.
• Bicarbonate therapy: alkalemia lowers ionised calcium and can precipitate tetany.
• Mannitol: osmotic nephrosis, hyperosmolar hyponatremia, and worsened AKI if given to an oliguric patient.
• Dialysis catheter complications: bleeding, pneumothorax, catheter-related bloodstream infection.

• The single best next step is almost always aggressive IV isotonic crystalloid, started before confirmatory labs return. Fluids precede bicarbonate, mannitol, diuretics, and dialysis in every stem.
• Dipstick positive for blood with no red cells on microscopy is the classic dissociation. The dipstick peroxidase reaction cannot distinguish myoglobin from hemoglobin — the distractor is calling it hematuria and hunting for a stone.
• CK, not serum myoglobin, is the diagnostic test. Myoglobin has a short half-life and clears within about a day, so a normal myoglobin never excludes rhabdomyolysis; CK peaks roughly 1–3 days after injury and falls slowly. Trend CK to gauge resolution.
• Check the ECG before the CK comes back. Peaked T waves in a crush-injury or found-down patient mean IV calcium immediately for membrane stabilisation, then insulin/dextrose.
• Do not treat asymptomatic hypocalcemia. Calcium given early redeposits in muscle and returns as rebound hypercalcemia in the recovery phase. Reserve calcium for tetany, seizures, or hyperkalemic ECG changes.
• The association examiners love: statin plus a CYP3A4 inhibitor or a fibrate (clarithromycin, azole antifungals, protease inhibitors, gemfibrozil) in a patient with reduced renal function. Also malignant hyperthermia after succinylcholine/volatile anesthetic → dantrolene, and McArdle disease (myophosphorylase deficiency) with exercise intolerance and a second wind phenomenon plus no rise in lactate on ischemic forearm testing.
• Pain on passive stretch beats pulselessness. In a swollen, tense limb after fluid resuscitation, the answer is compartment pressure measurement/fasciotomy, not more fluid.
• Common distractors: sodium bicarbonate or mannitol as the "correct" first-line answer (neither is guideline-supported over saline); furosemide for oliguria (avoided per KDIGO); and an elevated CK-MB or troponin attributed to MI when skeletal muscle is the source — troponin I is the more specific discriminator.