Clinical Characteristics

During embryonal development, the sciatic artery is the main blood supply to the lower extremity until the 6th week of gestation. Beyond this period, the artery begins to regress, and its remnants contribute to the formation of the inferior gluteal, deep femoral, popliteal, peroneal, and pedal arteries. The sciatic artery can fail to involute in either complete or incomplete forms.

A complete sciatic artery, occurring in 63% to 79% of cases, provides the main blood flow to the lower extremity. It originates off the internal iliac artery, courses through the greater sciatic foramen, runs along the adductor magnus muscle, and joins the popliteal artery distally lateral to the insertion of the adductor magnus. The superficial femoral artery is often hypoplastic and terminates in the thigh.

The incomplete PSA is hypoplastic and ends in the thigh, and the superficial femoral artery provides the main blood supply to the popliteal artery (Figure 1). Bilateral PSAs occur in about 20% of cases. Associated venous anomalies are present and can include a persistent sciatic vein or large communicating veins between the deep femoral vein and the popliteal vein.

PSAs are asymptomatic in up to 40% of patients. However, they are prone to early atherosclerosis and aneurysm formation. This is thought to be partially caused by congenital hypoplasia of the wall as well as repetitive traumatic forces to the artery during sitting and hip flexion and extension. The presence of a mass in the buttock, claudication, acute ischemia, gangrene, and sciatica are all reported symptoms that can develop.

Aneurysms are the most common cause of symptoms in patients with a PSA, and patients with aneurysms have a high associated rate of complications and limb loss. The estimated incidence of PSA aneurysm formation is between 15% and 44%. They form posteriorly at the level of the greater trochanter and are often palpable. Distal embolization, thrombosis, and rupture into the gluteal region are possible complications. With larger aneurysms, compression of the adjacent sciatic nerve can lead to sciatica, foot drop, or progressive loss of function of the extremity. Progressive atherosclerosis can also occur in a sciatic artery as in any lower extremity artery. The resulting arterial insufficiency can range from claudication to tissue loss and gangrene.

Diagnostic Considerations

Detection of a PSA by physical examination alone can be difficult. Cowie’s sign, the classic finding, is the absence of a femoral pulse with intact popliteal and pedal pulses. It has only been reported in five cases. In patients with an aneurysm of the sciatic artery, a palpable pulsatile mass can sometimes be felt over the buttock with an associated audible bruit.

When a persistent sciatic artery is suspected, arteriography has been the primary diagnostic modality. The course of the artery, presence of aneurysmal or atherosclerotic disease, and the status of distal runoff to the lower extremity can be assessed with such studies. Characteristic associated findings include enlargement of the internal iliac artery with or without a hypoplastic superficial femoral artery. Aortography with lower extremity runoff should be performed, because unilateral femoral arteriography can fail to identify a PSA. A selective injection with the catheter tip placed in the orifice of the internal iliac artery should also be performed for best visualization of the sciatic artery.

The use of computed tomography angiography (CTA) and magnetic resonance angiography (MRA) has recently been reported for diagnostic use and surgical planning. A detailed examination of the PSA is possible, allowing evaluation for aneurysm size, intramural thrombus burden, visualization of the contralateral side, and demonstration of totally occluded vessels that cannot be visualized on conventional arteriography. Another advantage of CTA and MRA is visualization of the PSA in relation to surrounding nerves, venous anomalies, and the musculoskeletal structure. These modalities are particularly helpful when planning an operative approach.