A 28-year-old woman with a tibia fracture develops compartment syndrome. Which finding best distinguishes acute compartment syndrome from rhabdomyolysis, both of which present with muscle pain, swelling, and elevated creatine kinase (CK)?

Explanation

## Distinguishing Acute Compartment Syndrome from Rhabdomyolysis ### Core Discriminator **Key Point:** Elevated intracompartmental pressure (>30 mmHg) with localized compartment involvement and normal or minimally elevated urine myoglobin is the hallmark discriminator of compartment syndrome, whereas rhabdomyolysis presents with systemic myoglobinuria and marked CK elevation. ### Pathophysiological Basis **Compartment Syndrome:** - Increased pressure within a closed fascial space - Localized to one or more anatomical compartments - Urine myoglobin typically absent or minimal (muscle ischemia, not primary breakdown) - Pressure measurement is diagnostic **Rhabdomyolysis:** - Systemic muscle breakdown (myonecrosis) from various triggers - Massive release of myoglobin into circulation - Myoglobinuria (dark, cola-colored urine) is characteristic - CK typically >5000 IU/L (often >10,000 IU/L) - Systemic complications (hyperkalemia, AKI, DIC) ### Comparison Table | Feature | Compartment Syndrome | Rhabdomyolysis | | --- | --- | --- | | **Intracompartmental pressure** | **>30 mmHg** | Normal | | **Anatomical distribution** | **Localized to 1–2 compartments** | Systemic (multiple muscle groups) | | **Urine myoglobin** | **Absent or minimal** | **Markedly elevated (myoglobinuria)** | | **Serum CK** | Elevated (1000–5000 IU/L) | Markedly elevated (>5000 IU/L) | | **Urine color** | Normal or slightly dark | Dark brown/cola-colored | | **Systemic signs** | Localized; no systemic toxicity | Hyperkalemia, AKI, DIC | | **Trigger** | Trauma, tight cast, crush (localized) | Crush (extensive), heat stroke, statins, sepsis | | **Diagnosis** | Clinical + pressure measurement | CK, myoglobinuria, electrolytes | ### High-Yield Distinction **High-Yield:** The presence of **myoglobinuria** (dark urine, positive dipstick for blood without RBCs on microscopy) strongly suggests rhabdomyolysis, not compartment syndrome. Compartment syndrome does NOT cause significant myoglobinuria because the muscle is ischemic, not actively breaking down. ### Clinical Pearl **Clinical Pearl:** Compartment syndrome is a surgical emergency diagnosed by clinical signs + intracompartmental pressure measurement; rhabdomyolysis is a medical emergency managed with aggressive IV hydration, electrolyte correction, and monitoring for AKI. The two can coexist (e.g., extensive crush injury), but the discriminator is pressure elevation localized to a compartment versus systemic myoglobin release. ### Mnemonic **Mnemonic:** **COMP** = **Compartment** syndrome has **Pressure** elevation; **RHABDO** = **Rhabdo** has **Dark** urine (myoglobinuria). ### Why Pressure Measurement Matters Intracompartmental pressure >30 mmHg (or within 30 mmHg of diastolic BP) is the objective diagnostic criterion for compartment syndrome and is NOT elevated in rhabdomyolysis. This is the single best discriminator when clinical signs overlap.