A 58-year-old woman with hypertension and hyperlipidemia presents with acute onset of severe pain and coldness in the left lower limb. On examination, the limb is pale, pulseless, and there is sensory loss. Duplex ultrasonography shows no flow in the left femoral artery. What is the most appropriate next investigation to determine the cause of acute arterial occlusion and guide emergency management?

Explanation

## Investigation of Choice in Acute Arterial Occlusion **Key Point:** CT angiography is the investigation of choice in acute arterial occlusion when the etiology is unclear and emergency revascularization is planned. It rapidly identifies the site of occlusion, determines the cause (thrombosis vs. embolism vs. dissection), and assesses collateral circulation and runoff vessels. ### Why CT Angiography in Acute Arterial Occlusion? 1. **Speed** — critical in acute limb ischemia where time-to-reperfusion determines limb salvage 2. **Comprehensive visualization** — shows aorta, iliac, femoral, and distal vessels in one acquisition 3. **Identifies etiology** — distinguishes: - **Thrombosis** — in situ clot in atherosclerotic vessel - **Embolism** — abrupt occlusion, often from cardiac source (AF, MI, endocarditis) - **Dissection** — intimal flap, aortic or iliac origin - **Aortic aneurysm rupture** — if rupture is suspected 4. **Guides intervention** — assesses runoff vessels, collaterals, and feasibility of revascularization (surgical vs. endovascular) 5. **No delay** — faster than MRA; does not require patient cooperation ### Differential Diagnosis: Thrombosis vs. Embolism | Feature | Thrombosis | Embolism | |---|---|---| | **Onset** | Gradual (hours to days) | Sudden (minutes to hours) | | **Vessel** | Atherosclerotic, stenosed vessel | Normal vessel | | **Contralateral limb** | Often symptomatic (PAD) | Usually normal | | **Source** | In situ atherosclerosis | Cardiac (AF, MI, endocarditis, prosthetic valve) | | **CTA finding** | Tapered occlusion, collaterals | Abrupt cut-off, no collaterals | **Clinical Pearl:** In acute limb ischemia, CTA is preferred over duplex because duplex is operator-dependent and time-consuming in an emergency. CTA provides rapid, definitive diagnosis and guides the choice between thrombolysis, thrombectomy, or surgical embolectomy. **High-Yield:** The **6 P's of acute arterial occlusion** — Pain, Pallor, Pulselessness, Paresthesia, Paralysis, Perishing cold — indicate acute limb ischemia requiring urgent investigation and intervention within 6–8 hours for limb salvage. **Mnemonic:** **ABCDE of acute limb ischemia investigation: - **A**cute onset → think embolism - **B**ilateral symptoms → think aortic dissection or thrombosis - **C**ardiac history → think embolism (AF, MI) - **D**iabetes/smoking → think thrombosis - **E**mergency CTA → first-line imaging