What is acute limb ischemia?

Definition of Acute Limb Ischemia

Acute limb ischemia (ALI) is a rapid or sudden decrease in limb perfusion that threatens tissue viability, typically occurring within 14 days of symptom onset, requiring emergent evaluation and revascularization to prevent limb loss. 1

Clinical Presentation and Diagnosis

ALI is characterized by the classic "5 Ps" (some clinicians include a 6th "P"):

• Pain: Sudden onset, severe, often extending above the ankle
• Pallor: Initially skin pallor, progressing to cyanosis over time
• Pulselessness: Absent pulses in the affected limb
• Paresthesias: Numbness and sensory deficits
• Paralysis: Motor weakness or loss of function
• Polar (cold): Temperature difference compared to contralateral limb 1

Classification of Severity

ALI severity is categorized based on clinical findings:

Category	Description	Sensory Loss	Muscle Weakness	Arterial Doppler	Venous Doppler
Viable	Not immediately threatened	None	None	Audible	Audible
Threatened (Marginally)	Salvageable if promptly treated	Minimal (toes) or none	None	Often inaudible	Audible
Threatened (Immediately)	Requires immediate revascularization	More than toes; with rest pain	Mild, moderate	Usually inaudible	Audible
Irreversible	Major tissue loss inevitable	Profound anesthetic	Profound paralysis	Inaudible	Inaudible

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Etiology

The two principal causes of ALI are:

1. Arterial embolism (30-50% of cases):

   • Typically from cardiac sources (atrial fibrillation, cardiomyopathy, ventricular aneurysm)
   • Characterized by sudden onset without prior claudication
   • Normal pulses in contralateral limb
   • Emboli typically lodge at arterial bifurcations 1

2. Arterial thrombosis (40-60% of cases):

   • Acute thrombosis superimposed on atherosclerotic plaque
   • Common in superficial femoral artery
   • May have history of claudication
   • Often bilateral pulse abnormalities 1, 2

Less common causes include:

• Trauma
• Iatrogenic complications
• Popliteal aneurysm thrombosis
• Hypercoagulable states
• Aortic or arterial dissection 3

Diagnostic Approach

For patients with suspected ALI and a potentially salvageable limb:

1. Immediate anticoagulation with heparin to prevent thrombus propagation 1
2. Vascular imaging to define anatomic level of occlusion:
   • CT angiography (fastest option)
   • MR angiography (alternative if time permits)
   • Catheter-directed angiography (typically at time of intervention) 1
3. Laboratory evaluation:
   • Complete blood count
   • Coagulation studies
   • Renal function
   • Cardiac biomarkers 1

Management Algorithm

1. Initial assessment: Determine limb viability using clinical categories above

2. For viable or threatened limbs (Class I recommendation):

   • Immediate systemic anticoagulation
   • Emergent evaluation to define anatomic level of occlusion
   • Prompt revascularization 1

3. Revascularization options:

   • Surgical: Embolectomy, bypass, or endarterectomy
   • Endovascular: Catheter-directed thrombolysis, mechanical thrombectomy
   • Hybrid approaches: Combining surgical and endovascular techniques 2, 3

4. For irreversible ischemia (Class III recommendation):

   • Avoid attempts at revascularization
   • Primary amputation may be necessary 1

Prognosis and Outcomes

Without prompt revascularization, most patients with CLI will require amputation within 6 months 1. Even with appropriate treatment, ALI carries significant morbidity and mortality:

• Amputation rates: 10-30%
• Mortality rates: 15-20% at 30 days
• Higher mortality in patients with cardiac embolism 4

Important Considerations

• Time is tissue: Delay in diagnosis and treatment may lead to irreversible ischemic damage 2
• Reperfusion injury: Restoration of blood flow can cause additional tissue damage through inflammatory mechanisms 5, 6
• Underlying conditions: Identify and treat the source of embolism or thrombosis to prevent recurrence 4
• Systemic complications: ALI can lead to metabolic derangements, rhabdomyolysis, and multiorgan failure 6

ALI represents a true vascular emergency requiring immediate recognition, assessment, and intervention to preserve limb viability and prevent life-threatening complications.