Popliteal Artery Aneurysm

Etiology

Modifiable risk factors include smoking, atherosclerosis, and connective tissue disorders such as Marfan syndrome and Ehlers-Danlos syndrome. Non-modifiable risk factors include advanced age, male gender, White race, and family history of aneurysmal disease.[4][2]

Epidemiology

The exact incidence of popliteal artery aneurysms is unknown, as no population-based study has been published. However, a few studies issued by different institutions suggest that popliteal artery aneurysm prevalence increases with age and peaks in the sixth or seventh decades of life. The incidence of femoral and popliteal aneurysms in persons with abdominal aortic aneurysms appears higher than in the general population. In one study of 225 patients, 28% (n=63) had a popliteal artery aneurysm greater than or equal to 10.5 mm, 19% (n=43) had a popliteal artery aneurysm greater than or equal to 12 mm, and 11% (n=25) had a popliteal artery aneurysm greater than or equal to 15 mm.

Pathophysiology

The etiology of popliteal artery aneurysms and general aneurysms is unknown. Molecular studies suggest that a combination of genetic defects and inflammatory processes are responsible. Atherosclerosis tends to increase flow turbulence distal to a stenosis, leading to a pathological dilation of the artery. According to current research, decreased wall strength associated with inflammatory cell infiltration results in aneurysm formation.[5]

History and Physical

Most commonly, popliteal artery aneurysms are asymptomatic. If patients exhibit chronic symptoms, these are usually secondary to mass effect and compression of adjacent structures, such as the tibial nerve, which causes leg pain or paresthesias. The popliteal vein also may be compressed, which causes calf swelling. Progressive luminal narrowing of the popliteal artery from thrombus formation may cause an insidious onset of claudication—approximately 50% of the patients present with bilateral aneurysms.

When an aneurysm acutely thromboses, the most common symptoms are acute lower extremity ischemia resulting in the acute onset of pain, paresthesias, paresis, pallor, and poikilothermia due to either aneurysmal intraluminal thrombosis or distal embolization of thrombus. Distal embolization can manifest as blue toe syndrome and/or acral cyanosis. Approximately 30% of untreated patients experience acute thrombosis and distal embolization with the risk of possible limb loss. Patients who develop acute thrombosis or embolism have a relatively poor prognosis, with a 15% amputation rate due to the occlusion of runoff vessels. The popliteal fossa should be examined bilaterally with the knee in a semi-flexed position. About 60% of patients with popliteal aneurysms have a palpable pulsatile mass at the knee joint level.

Evaluation

Duplex ultrasonography is the ideal screening and diagnostic imaging modality to detect a popliteal artery aneurysm and estimate the diameter. The duplex ultrasonography also provides detailed information regarding vessel patency, the development of mural thrombus, and the status of outflow vessels (see Image. Ultrasound of Left Popliteal Artery Aneurysm). Computed tomography (CT) or magnetic resonance angiography are other alternatives to duplex ultrasonography. They can accurately measure true lumen diameters or assist in planning surgical repair (see Image. Popliteal Artery Aneurysm Visualized on Magnetic Resonance Angiography). Conventional angiography is reserved for either elective endovascular repair of an aneurysm or in cases of threatening acute limb ischemia for thrombolytic therapy.[6][7][8]

Treatment / Management

All symptomatic popliteal artery aneurysms require repair because of an increased incidence of thrombosis or limb loss. Asymptomatic aneurysms with a diameter greater than 2 cm are considered an indication for elective operation to avoid complications such as limb-threatening ischemia. Greater than 45 angulation in asymptomatic aneurysms warrants surgical intervention because of the risk of acute limb ischemia secondary to kinking of the vessel.[9][10][6]

Conservative Management 

Asymptomatic aneurysms less than 2 cm are safely manageable via duplex surveillance.

Open surgical approach: Traditional open surgery involves ligation of the popliteal artery proximal (above the knee) and distal (below the knee) below the aneurysmal sac and bypassing the excluded segment using either a reversed autologous vein graft (saphenous vein) or prosthetic graft (see Image. Intraoperative Popliteal Artery Aneurysm Repair). An alternative surgical approach is aneurysmectomy, followed by an interposition bypass with a prosthetic graft between the proximal and distal normal segments of the popliteal artery. However, the former method is more common.

Endovascular approach: Recently, endovascular techniques have gained popularity in popliteal artery aneurysm repair as an alternative to the open surgical approach. This technique excludes the aneurysmal sac with stent-graft implantation. Recent studies show that popliteal artery stenting is a safe alternative technique for the treatment of popliteal artery aneurysms, especially in high-risk patients. The advantages of the endovascular technique include a shorter hospital stay and an abbreviated surgical time compared to open surgical intervention. Disadvantages include higher 30-day graft thrombosis rates (9% in the endovascular treatment group versus 2% in the open surgical treatment group) and higher 30-day re-intervention rates (9% in the endovascular treatment group versus 4% in the open surgical treatment group). The 2 repair groups have no significant difference in mortality or limb loss rates. Other reviews have also concurred that endovascular and open repair of popliteal artery aneurysms is safe.[11] Open repair is associated with greater limb salvage, fewer reintervention rates, and decreased short-term thrombosis.[12](A1)

Management of Acutely Thrombosed Aneurysms

Management varies according to the presentation. Asymptomatic patients with completely thrombosed popliteal artery aneurysms and those who are non-ambulatory patients or have acutely thrombosed aneurysms with a poor runoff despite thrombolysis are not candidates for surgical intervention. Only patients with disabling symptoms or critical limb ischemia are treated with surgical bypass as the first line of therapy. Patients presenting with acute limb-threatening ischemia should undergo urgent bypass. Acute thrombosis should be treated with intravenous heparin and continuous heparin infusion to limit the extension of the thrombus. However, some surgeons advise that threatening ischemia is better treated with thrombectomy, followed by bypass surgery.

Intra-Arterial Thrombolytic Therapy

This method is indicated when angiography demonstrates no target vessel or multiple distal thrombi are noted. The decision analysis suggested that open repair with great saphenous vein (GSV) bypass might be considered the preferred treatment for older, asymptomatic patients with popliteal artery aneurysms. The incidence of popliteal artery aneurysms in the general population is low, ranging from 0.1% to 2.8%. According to the rarity, screening unselected patients or patients undergoing abdominal aorta aneurysm is not cost-effective. However, patients with a known popliteal artery aneurysm have a significantly higher risk of having a contralateral popliteal artery aneurysm or an abdominal aortic aneurysm. The literature review suggests that younger age, the availability of a single-segment GSV, complicated runoff vessels, and lower surgical risk favor the option of open surgical repair. However, with advancing age and insufficient GSV, satisfactory landing zones, tibial runoff, and higher surgical risk might favor the endovascular options. 

Therefore, in patients with poor tibial or pedal runoff, open surgical repair (OPAR) with GSV is considered preferential for moderate-risk patients. High-risk patients, defined as those with high cardiovascular risk or adverse anatomic criteria such as severe venous stasis or lymphedema, pose a challenge. For patients with a poor life expectancy and an asymptomatic popliteal artery aneurysm, conservative management is preferred. If treatment of a popliteal artery aneurysm is indicated for such patients because endovascular treatment is less taxing with limited postsurgical complications, endovascular popliteal artery aneurysm repair (EPAR) is preferred. The option is offered regardless of the GSV's availability or the runoff vessels' quality. Recent literature review suggests that younger age, the availability of single-segment GSV, challenged runoff vessels, and lower surgical risk favor the choice of OPAR.[13](A1)

Special Considerations

In patients with connective tissue disorders, a lack of consensus is apparent on whether an open or endovascular approach is better for repairing a popliteal aneurysm. Endovascular interventions are relatively contraindicated because of the risk of damaging an inherently weak artery [14] and subsequent aneurysmal degeneration.[15] However, most reports using the endovascular approach have suggested caution with promising results.[16] The best conduit for open reconstruction is unknown; synthetic grafts have inferior long-term patency rates with concerns that vein grafts may lead to aneurysms.[14](B2)

Differential Diagnosis

The entities that should be considered in the differential diagnosis include Baker cyst, lymph node, varicosity, and cystic adventitial disease.

Complications

Limb loss is considered a major complication.