What is the most appropriate initial investigation for a patient with Diabetes Mellitus (DM), Hypertension (HTN), and Peripheral Arterial Disease (PAD) presenting with sudden onset leg pain, paresthesia, and diminished pulse?

Acute Limb Ischemia: CT Angiography is the Most Appropriate Initial Investigation

In a patient with diabetes, hypertension, and known PAD presenting with sudden onset leg pain, paresthesia, and diminished pulse, CT angiography should be performed immediately as the initial investigation to diagnose acute limb ischemia and plan urgent revascularization. 1

Clinical Context: This is Acute Limb Ischemia, Not Chronic PAD

The presentation of sudden onset leg pain with paresthesia and diminished pulse represents acute limb ischemia (ALI), not chronic stable PAD. 1 This distinction is critical because:

• ALI is a vascular emergency where "time is tissue"—delays beyond 4-6 hours significantly increase risk of permanent damage and limb loss 1
• The sudden nature of symptoms distinguishes this from chronic limb-threatening ischemia (CLTI), which develops over >2 weeks 2
• Patients with known PAD have increased risk of ALI, particularly with comorbid atrial fibrillation and diabetes 1

Why CT Angiography is the Correct Answer

CT angiography is the recommended initial imaging modality for acute limb ischemia because it provides:

• Rapid diagnosis with immediate availability in most emergency departments 1
• Complete anatomical detail of the entire arterial circulation, including the level of occlusion, degree of atherosclerotic disease, and below-knee arteries 1
• Immediate revascularization planning in a single study, allowing vascular surgery consultation to proceed without delay 1
• High diagnostic accuracy with an appropriateness rating of 7-8 out of 9 for acute arterial obstruction 1

The American College of Radiology specifically recommends CTA as appropriate initial imaging for acute limb ischemia, and the American College of Cardiology endorses CTA for providing excellent anatomic detail for revascularization planning. 1

Why ABI is Incorrect in This Context

While ABI is the primary screening test for chronic stable PAD 2, 3, it is inadequate for acute limb ischemia because:

• ABI only confirms arterial occlusion exists but provides no information about the location, extent, or cause of the occlusion 1
• ABI cannot guide revascularization planning, which requires anatomical detail that only imaging can provide 1
• ABI is useful for screening and follow-up, not for emergency diagnosis when immediate intervention is needed 1
• In diabetic patients, medial arterial calcification can make ABI falsely elevated (>1.40), rendering it unreliable 2, 4

The ESC guidelines clearly state that ABI is indicated for screening and diagnosis of chronic LEAD, not for acute presentations requiring urgent revascularization. 2

Why Doppler Ultrasound is Insufficient

Doppler ultrasound has significant limitations in acute limb ischemia:

• Operator-dependent with variable accuracy in emergency settings 1
• Difficulty evaluating deep vessels and assessing multilevel disease 1
• Severely limited by calcification, which is common in diabetic patients with PAD 1
• Cannot provide the comprehensive anatomical mapping needed for urgent surgical or endovascular intervention 1

While duplex ultrasound is recommended as first-line imaging for chronic stable PAD when revascularization is being considered 2, it is inadequate for the time-sensitive evaluation required in ALI.

Immediate Management Algorithm

1. Start systemic anticoagulation immediately (usually unfractionated heparin) to prevent thrombus propagation while awaiting imaging 1
2. Obtain CT angiography emergently to define anatomy and plan revascularization 1
3. Consult vascular surgery immediately, even before imaging if clinical findings suggest Class IIb or III limb threat 1
4. Assess Rutherford classification: presence of motor weakness or sensory loss beyond the toes indicates Class IIb (immediately threatened) or Class III (irreversible), requiring urgent intervention even before imaging 2, 1

Critical Pitfalls to Avoid

• Do not delay imaging to obtain ABI—this wastes precious time in a "time is tissue" emergency 1
• Do not rely on Doppler alone—it cannot provide the anatomical detail needed for revascularization 1
• Do not wait for symptoms to worsen—the presence of paresthesia already indicates significant ischemia 2
• Do not assume chronic PAD management applies—acute presentations require different diagnostic and therapeutic approaches 1

The presence of diabetes with known PAD increases both the likelihood of ALI and the risk of poor outcomes, making rapid diagnosis and intervention even more critical. 4, 5