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?

CT Angiography is the Most Appropriate Initial Investigation

For a patient presenting with sudden onset leg pain, paresthesia, and diminished pulse—the hallmark features of acute limb ischemia—CT angiography (CTA) should be performed immediately as the initial diagnostic test. 1, 2

Why CTA is the Correct Choice

Acute Limb Ischemia Requires Urgent Anatomic Diagnosis

• The American College of Radiology explicitly recommends CTA as the most appropriate initial investigation for acute limb ischemia, providing rapid and comprehensive anatomic detail essential for immediate revascularization planning 1, 2
• The principle of "time is tissue" applies here—delays beyond 4-6 hours dramatically increase the risk of permanent tissue damage, limb loss, and death 1
• CTA evaluates the entire arterial circulation in a single study, including the exact level of occlusion, degree of underlying atherosclerotic disease, and patency of below-knee vessels—all critical information needed within hours to plan urgent revascularization 1, 2

This is Acute Limb Ischemia, Not Chronic PAD

• The sudden onset of symptoms distinguishes acute limb ischemia from chronic presentations, which develop over more than 2 weeks 1
• While this patient has known PAD (a risk factor for acute thrombosis), the acute presentation with the classic triad of sudden pain, paresthesia, and diminished pulse mandates urgent anatomic imaging, not screening tests 1, 2

Why the Other Options Are Inadequate

ABI is Insufficient for Acute Presentations

• The American College of Cardiology explicitly states that ABI is indicated for screening and diagnosis of chronic lower extremity arterial disease, not for acute presentations requiring urgent revascularization 1
• ABI only confirms the presence of arterial occlusion but provides no information about the location, cause, or anatomic details needed for treatment planning 1, 2
• In diabetic patients, ABI has additional limitations due to falsely elevated readings from medial arterial calcification 3, 4
• ABI is useful for follow-up after treatment and as a screening tool in stable patients, but not as the initial diagnostic test in acute limb ischemia 1

Doppler Ultrasound is Too Limited for Emergency Evaluation

• Doppler ultrasound is inadequate in emergency situations due to its operator-dependent nature, time-consuming process, and difficulty evaluating deep vessels 1
• Doppler is particularly limited in patients with severe calcification, which is common in diabetic patients with chronic kidney disease 1
• While point-of-care duplex ultrasound (PAD-scan) shows promise for chronic PAD screening with 95% sensitivity 5, this does not apply to acute limb ischemia requiring immediate comprehensive anatomic mapping

Immediate Management Algorithm

Before Imaging

• Start systemic anticoagulation immediately with intravenous unfractionated heparin to prevent thrombus propagation while awaiting imaging 1, 2
• Obtain immediate vascular surgery consultation—even before imaging is complete—as some patients with severe ischemia (paralysis or motor weakness) may need to proceed directly to surgical thromboembolectomy 1, 2

Imaging Protocol

• Obtain CTA of the entire lower extremity, including aortoiliac, femoral-popliteal, and tibial-pedal vessels 2
• The American College of Radiology rates CTA as "usually appropriate" with the highest appropriateness rating for acute limb ischemia 2

Special Consideration for Diabetes and Hypertension

• Despite this patient's diabetes and hypertension (which may indicate chronic kidney disease), the benefit of rapid diagnosis and limb salvage outweighs the risk of contrast-induced nephropathy in acute limb ischemia 1
• Modern reduced tube voltage techniques can minimize contrast dose 1

Critical Pitfalls to Avoid

• Do not delay imaging to obtain an ABI first—this wastes precious time and provides no actionable anatomic information 1, 2
• Do not mistake this acute presentation for chronic PAD exacerbation—the sudden onset with paresthesia and diminished pulse indicates acute arterial occlusion requiring emergency intervention 1
• Assess for the presence of paralysis or motor weakness beyond the toes, which indicates Rutherford Class IIb or III and requires immediate surgical intervention, potentially even before completing imaging 1, 2