What is the best initial test for a patient with chronic limb ischemia (CLI), a history of intermittent claudication relieved by rest, who presents with acute leg pain and diminished pulses?

Best Initial Test for Acute-on-Chronic Limb Ischemia

CT angiography (CTA) is the best initial test for this patient presenting with acute leg pain and diminished pulses superimposed on chronic limb ischemia. 1, 2

Why CTA is the Correct Answer

The American College of Radiology explicitly recommends CTA as the preferred initial imaging modality for acute limb ischemia because it provides rapid, comprehensive anatomic detail essential for immediate revascularization planning. 3, 1, 2

CTA offers several critical advantages in this acute setting:

• Reveals the exact nature and level of both thrombosis and underlying atherosclerotic plaque to plan an appropriate treatment strategy 3, 1
• Evaluates the entire arterial circulation including the level of occlusion, degree of atherosclerotic disease, and patency of below-knee vessels—all critical information needed for immediate revascularization planning 3, 1
• Allows immediate diagnosis and treatment planning in a single study, which is essential given the "time is tissue" principle where delays beyond 4-6 hours increase risk of permanent damage and limb loss 1
• Has near-equivalent accuracy to diagnostic angiography while being faster and more widely available 2

Why Other Options Are Inadequate

Ankle-Brachial Index (ABI) - Option A

The American College of Radiology explicitly states that ABI is insufficient as an initial diagnostic test in acute limb ischemia because it only confirms arterial occlusion but provides no information about location, cause, or treatment planning. 1

• The American College of Cardiology states that ABI is indicated for screening and diagnosis of chronic lower extremity arterial disease, not for acute presentations requiring urgent revascularization 3, 1
• ABI can be quick when performed at bedside, but it does not provide further detail on the anatomic location of the occlusion 3
• Given the acute nature of this presentation and urgency in prompt initiation of treatment, screening tools such as ABIs are not ideal if there is an anticipated delay in obtaining the necessary information 3

Doppler Ultrasound - Option B

Doppler ultrasound is too time-consuming, operator-dependent, and limited in scope for acute limb ischemia evaluation in the emergency setting. 1

• The American College of Radiology explicitly states that duplex ultrasound is limited by the need for operator expertise, poor accessibility of vessels, heavy calcification, and poor overall accuracy if multilevel disease is present 1
• Ultrasound cannot provide the comprehensive anatomic mapping of the entire arterial tree needed for revascularization planning in this emergency 1
• While duplex ultrasound has good accuracy for chronic aortoiliac disease (92% sensitivity, 96% specificity), this is not applicable to acute presentations requiring urgent intervention 3

MR Angiography - Option C

MRA is not the best initial test despite being an alternative imaging option, primarily due to longer acquisition times and limited availability in emergency settings. 3

• The American College of Radiology rates MRA with IV contrast as appropriate (rating of 7), but CTA receives higher ratings (7-8) for acute presentations 3, 1
• MRA may help reduce total contrast dose, but this consideration is secondary to the need for rapid diagnosis in acute limb ischemia 3

Critical Management Principles

All patients presenting with acute limb ischemia should be initiated on anticoagulation therapy immediately (usually intravenous unfractionated heparin) to prevent thrombus propagation while awaiting imaging. 3, 1

The clinical presentation described—acute leg pain with diminished pulses in a patient with chronic claudication—represents acute-on-chronic limb ischemia, which requires:

• Immediate vascular surgery consultation, even before imaging is complete, as some patients with severe ischemia should proceed directly to surgical thromboembolectomy 1
• Assessment of the Rutherford classification, where the presence of motor weakness or sensory loss indicates Class IIb (immediately threatened) or Class III (irreversible), requiring urgent intervention within 6 hours 1
• Revascularization within 6 hours for marginally or immediately threatened limbs, as skeletal muscle tolerates ischemia for only 4-6 hours before permanent damage occurs 1

Common Pitfalls to Avoid

• Do not delay treatment for screening tests like ABI when clinical presentation clearly indicates acute limb ischemia 3, 1
• Do not assume that a history of claudication means this is just worsening chronic disease—the acute presentation with diminished pulses represents a vascular emergency 3, 1
• Do not order echocardiography as part of the acute workup, as it should not delay revascularization 1
• In patients with motor deficits, do not delay intervention for extensive imaging—proceed directly to treatment as this indicates Rutherford Class IIb requiring intervention within 6 hours 1