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

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

For a patient with diabetes, hypertension, and known PAD presenting with sudden onset leg pain, paresthesia, and diminished pulse—findings consistent with acute limb ischemia (ALI)—CT angiography is the most appropriate initial investigation. 1, 2

Why CT Angiography is the Correct Choice

The sudden onset of symptoms distinguishes this as acute limb ischemia, not chronic PAD exacerbation, making this a vascular emergency where "time is tissue." 2 The American College of Radiology specifically recommends CT angiography as the initial diagnostic test for acute limb ischemia, providing rapid, comprehensive anatomic detail including the exact level of occlusion, degree of atherosclerotic disease, and visualization of the entire arterial circulation needed for immediate revascularization planning. 1, 2

Key Clinical Features Supporting ALI Diagnosis:

• Sudden onset (not gradual worsening) of leg pain and paresthesia 2
• Diminished pulse on examination (one of the classic "5 Ps": pain, paresthesia, pulselessness, pallor, polar/cold) 2
• High-risk patient profile: DM, HTN, and pre-existing PAD significantly increase thromboembolism risk 1, 2

Why NOT Ankle-Brachial Index (ABI)

ABI is insufficient for acute limb ischemia because it only confirms arterial occlusion exists but provides no information about the location, cause, or extent of disease—all critical for planning urgent revascularization. 2 While ABI is the appropriate initial test for chronic PAD screening and diagnosis 1, 3, it has no role in the acute setting where delays of more than 4-6 hours increase permanent damage and limb loss risk. 2

Additionally, ABI is notoriously unreliable in diabetic patients due to arterial calcification (medial wall sclerosis), often producing falsely elevated readings ≥1.3 that mask severe ischemia. 1, 4

Why NOT Doppler Ultrasound Alone

Doppler ultrasound is operator-dependent, time-consuming, and limited in evaluating deep vessels—particularly problematic in emergency situations requiring immediate anatomic detail. 2 While Doppler can assess waveforms and is useful for chronic PAD evaluation 1, it cannot provide the comprehensive arterial mapping needed for urgent surgical or endovascular intervention planning in ALI. 2

Immediate Management Algorithm

1. Start systemic anticoagulation immediately (usually heparin) to prevent thrombus propagation while arranging imaging 1, 2
2. Obtain CT angiography urgently to define anatomy and plan revascularization 1, 2
3. Consult vascular surgery immediately, ideally before or concurrent with imaging 2
4. Assess Rutherford classification: presence of motor weakness or sensory loss beyond the toes indicates Class IIb (immediately threatened) or Class III (irreversible), requiring emergent intervention even before imaging completion 2

Critical Pitfalls to Avoid

• Never delay imaging to obtain ABI first—this wastes precious time in a "time is tissue" emergency 2
• Never assume adequate perfusion based on "diminished" rather than "absent" pulses—even skilled examiners can detect pulses despite limb-threatening ischemia 1, 4
• Never rely on ABI in diabetic patients due to falsely elevated readings from arterial calcification 1, 4
• Never wait for symptoms to worsen—any delay beyond 4-6 hours significantly increases amputation risk and mortality 2

Special Consideration for Chronic Kidney Disease

While this patient has diabetes (increasing CKD risk), the benefit of rapid diagnosis and limb salvage with CT angiography far outweighs the risk of contrast-induced nephropathy in this emergency situation. 2 Modern reduced-dose contrast protocols minimize nephrotoxicity risk. 2