A 28-year-old man presents to the emergency department 6 hours after a crush injury to his right leg from a motorcycle accident. Clinical examination reveals severe pain out of proportion to physical findings, pain on passive stretch of the foot, and tense swelling of the anterior compartment. Which of the following is NOT a characteristic feature of acute compartment syndrome?

Explanation

## Acute Compartment Syndrome: Clinical Features and Pathophysiology ### The Five P's and Their Timing **Key Point:** The classic "Five P's" of compartment syndrome (Pain, Pressure, Paresthesias, Pallor, Pulselessness) occur in a specific sequence. Pulselessness is a LATE finding, not an early or sensitive sign. ### Timeline of Ischemic Changes | Finding | Timing | Sensitivity | Notes | |---------|--------|-------------|-------| | Pain out of proportion | 0–2 hours | **Highest** | Often the earliest and most reliable sign | | Pain with passive stretch | 0–4 hours | **High** | Indicates muscle ischemia | | Paresthesias | 2–6 hours | Moderate | Nerve ischemia; may precede motor loss | | Pallor, coolness | 4–8 hours | Moderate | Vascular compromise | | Pulselessness | 6–8 hours | **Low** | LATE finding; indicates irreversible damage | | Paralysis | 6–12 hours | Late | Muscle and nerve necrosis | **High-Yield:** Pulselessness is a **late and insensitive** sign of compartment syndrome. Waiting for pulses to disappear means waiting for irreversible tissue damage. Compartment syndrome is a surgical emergency based on **clinical suspicion and pain**, not on the presence or absence of pulses. ### Pathophysiology of Muscle Necrosis **Key Point:** Muscle is exquisitely sensitive to ischemia. Irreversible damage begins after 6–8 hours of sustained elevated compartment pressure (typically >30 mmHg or within 30 mmHg of diastolic BP). 1. **0–2 hours:** Pain, swelling, pain with passive stretch 2. **2–6 hours:** Paresthesias, early nerve dysfunction 3. **6–8 hours:** Onset of irreversible muscle necrosis 4. **>8 hours:** Established myonecrosis, rhabdomyolysis, acute kidney injury risk ### Clinical Pearl **Clinical Pearl:** The diagnosis of compartment syndrome is **clinical**, not radiological. Do not delay fasciotomy waiting for imaging or compartment pressure measurements if clinical suspicion is high. The mantra is: "When in doubt, cut it out." ### Why Pulselessness Is a Trap Arterial pulses depend on **large vessel patency**. Compartment syndrome compresses tissues within a fascial envelope but does NOT necessarily occlude the main artery until very late. A patient can have a palpable pulse and still be in compartment syndrome with muscle ischemia from microvascular compression. --- ## Summary **Correct Answer:** Pulselessness is NOT an early or sensitive sign — it is a late finding indicating advanced ischemia and irreversible damage. The other three options are all correct features of acute compartment syndrome.