Popliteal Artery Aneurysm

A popliteal artery aneurysm is a dilatation of the popliteal artery to 20 mm or more (or 1.5 times the adjacent normal diameter). It is the commonest peripheral artery aneurysm.

Men account for over 90% of cases, usually over 60 years. About half to two thirds are bilateral, and 30 to 50% have an abdominal aortic aneurysm.

The main danger is thrombosis or distal embolisation, with limb loss. Rupture is rare, under 5%.

Most are degenerative and atherosclerotic. Other causes are popliteal entrapment, Behçet disease, connective tissue disease, infection and trauma.

Pathophysiology. Turbulent flow and mural thrombus in the sac are the source of emboli. Emboli lodge in the tibial vessels and gradually destroy runoff, so the first acute thrombosis can then cause limb loss.

Large aneurysms compress the popliteal vein and tibial nerve.

• Asymptomatic in about half. A pulsatile popliteal mass, often found on examination or by chance.
• Intermittent claudication, rest pain, blue toes, or acute limb ischaemia from thrombosis (pain, pallor, pulselessness, paraesthesia, paralysis).
• Calf swelling, deep vein thrombosis and venous congestion from compression. Pain and paraesthesia in the calf from nerve compression.
• Rarely rupture, with a painful swollen popliteal fossa.

Red flags.

• Acute limb ischaemia or blue toe syndrome
• Diameter of 20 mm or more, or mural thrombus
• Fewer than two patent tibial arteries (poor runoff)
• Compression of the vein or nerve
• Fever or a painful mass (infection or rupture)

• Duplex ultrasound. Diameter (outer wall to outer wall), thrombus, runoff vessels and the contralateral leg.
• CT angiography for planning: landing zones, runoff, and the whole aorta. Angiography underestimates the aneurysm when thrombus fills the sac.
• Aortic and iliofemoral imaging. Abdominal ultrasound for the aorta and femoral arteries.
• Ankle brachial index and pulse volume recordings.
• Cause. Inflammatory markers when Behçet disease or infection is suspected.

Differential diagnosis.

• Baker cyst and other popliteal cysts or soft tissue tumours
• Popliteal artery entrapment and adventitial cystic disease
• Deep vein thrombosis
• Popliteal pseudoaneurysm after trauma, knee surgery or injection
• Lymphadenopathy and abscess
• Aneurysm of the femoral artery

Medical therapy for all patients.

• Aspirin 75 to 100 mg orally once daily (usual maximum 325 mg daily). Atorvastatin 40 to 80 mg once daily (liver enzymes at baseline and 3 months, avoid in pregnancy). Stop smoking, and keep blood pressure under 130/80 mm Hg.

Asymptomatic aneurysm. Under 20 mm with no thrombus and good runoff: duplex every 6 to 12 months. Repair at 20 mm or more, with mural thrombus, or with poor runoff.

Elective repair.

• Exclusion with bypass using great saphenous vein (medial approach) is the first choice. Five year patency is about 85 to 90% and limb loss is under 2%.
• Endovascular covered stent graft for suitable anatomy and high operative risk. Patency is lower than vein bypass (about 60 to 80% at 5 years), and kinking, stent fracture and endoleak are risks.
• After vein bypass: aspirin long term. After a stent graft: aspirin 75 to 100 mg plus clopidogrel 75 mg once daily for 1 to 3 months, then aspirin.

Acute limb ischaemia. Emergency.

• Unfractionated heparin 80 units/kg intravenous bolus, then 18 units/kg/h, adjusted to anti-Xa 0.3 to 0.7 IU/mL or an aPTT ratio of 1.5 to 2.5, no fixed maximum. Platelets on days 5 to 14.
• Threatened but viable limb: catheter directed thrombolysis to clear the tibial runoff before bypass. Alteplase 0.5 to 1 mg/h through the catheter for up to 24 hours, reassessed at 12 and 24 hours, with fibrinogen, aPTT and haemoglobin every 6 hours. Reduce or stop for fibrinogen under 1.0 g/L or bleeding. Contraindicated with active bleeding, previous intracranial haemorrhage, ischaemic stroke within 3 months, and major surgery or trauma within 3 weeks.
• Immediately threatened limb with sensory loss or paralysis: urgent surgical thrombectomy and bypass, with fasciotomy for reperfusion. Limb loss after acute presentation is about 20 to 30%.
• Irreversible ischaemia (fixed mottling, rigid muscle): amputation.

Special groups. Behçet disease or infection: treat the cause with the specialist team (immunosuppression or prolonged antibiotics) with surgery when the disease is controlled. Screen the contralateral leg, the aorta and the femoral arteries in every patient.

Monitoring. Duplex at 1 month, 6 months and 12 months after repair, then yearly. Aortic ultrasound yearly when there is a small aneurysm. Pulses and symptoms at every visit.

Referral and escalation. Refer to vascular surgery at diagnosis. Emergency for acute limb ischaemia, blue toes or rupture.