popliteal artery entrapment syndrome
Popliteal artery entrapment syndrome (PAES) is an underreported condition affecting the lower extremities in active individuals. It was first described anatomically in 1879 by a medical student, T.P. Stuart, performing a cadaveric dissection, who noted “The popliteal artery, after passing through the opening in the adductor magnus, instead of, as it usually does, coursing downwards and outwards towards the middle of the popliteal space, so as to lie between the two heads of the gastrocnemius muscle, passes almost vertically downwards internally to the inner head of the gastrocnemius. It reaches the bottom of the space by turning round the inner border of that head, and then passes downwards and outwards beneath it—between it and the lower end of the shaft of the femur .
The reported incidence of popliteal artery entrapment syndrome ranges from 0.6% to 3.5% . However, the prevalence of asymptomatic popliteal artery occlusion is estimated to be as high as 80%. Bilateral disease is found in up to two-thirds of cases  , and 1 study suggested a 15:1 male to female ratio . Most of the current knowledge of popliteal artery entrapment syndrome is based on case studies, making the true incidence of the syndrome difficult to ascertain.
Popliteal artery entrapment syndrome is characterized by two broad categories: (1) congenital or anatomic and (2) functional. Anatomic variants found in popliteal artery entrapment syndrome tend to occur in older men who are greater than 40 years of age with lower functional demands, and the functional variants are due to hypertrophy of a normal medial gastrocnemius muscle and tend to present in highly conditioned women who are 20 to 29 years of age .
The most widely accepted classification system, proposed by Love and Whelan in 1965  and modified by Rich and colleagues  in 1979, divides PAES into six subtypes.
The presenting symptoms can be variable and may change depending on the acuity, chronicity, and extent of the pathology. There is overlap between symptoms with exertional compartment syndrome and one must be aware of both diagnoses when evaluating an individual with leg pain. Individuals may also report concomitant back pain, which can lead the provider to feel symptoms are more related to the lumbar spine. A helpful differentiating factor in the physical examination is the location of pain. In popliteal artery entrapment syndrome, pain and tightness are always in the calf, whereas pain from the exertional compartment syndrome is more common in the anterolateral aspect of the leg, and spinal-based pain is often along the length of the affected leg posteriorly and commonly bilateral . Other symptoms of popliteal artery entrapment syndrome include paresthesia in the lower leg and feet, foot swelling, pallor, cramping, blanching, and cold feet  . Excessive training may result in a hypertrophic mass effect of the gastrocnemius and soleus muscles, which results in compressive sequelae involving the popliteal artery in the posterior compartment.
Many individuals may undergo multiple studies with vague posterior leg pain including lumbar spine, knee and lower leg radiographs, compartment testing, CTs and MRIs before a diagnosis is made in many cases. It is important for sports medicine providers to order imaging in a logical manner when PAES is suspected. Noninvasive studies should be performed first and plain radiographs of the lower leg can rule out osseous or cartilaginous abnormalities such as osteochondromas or exostosis. These can contribute to arterial or venous compression. The ankle-brachial index (ABI) and dynamic duplex ultrasound can be performed with low cost and ease.
The ankle brachial index (ABI), which is a comparison of the blood pressure at the ankle with the upper arm, is performed at rest and then following exercise to the point of reproducing the patient’s symptoms. A normal ankle-brachial index, for reference, may be between 1.0 and 1.4, with lower values suggesting arterial disease and higher values indicating vessel calcification or hardening . Although there is no commonly agreed upon figure, studies have referenced an ankle brachial index decrease of 30% to 50% as a positive test . If there is a drop in the ankle-brachial index with exercise, it is then appropriate to proceed with a dynamic duplex ultrasound of the popliteal artery with provocative maneuvers.
A technique for evaluating popliteal artery entrapment syndrome  has been described as ultrasound of the popliteal artery with the patient in active plantar flexion and then with the knee flexed to 15°. With this provocative maneuver, the peak flow is measured and a decrease is considered to be diagnostic. However, it is important to note that there has been a high rate of false positive findings in athletes. 
Advanced imaging including computed tomography (CT) and CT angiography or magnetic resonance imaging (MRI) and magnetic resonance angiography (MRA) is important in evaluating for anatomic aberrancies that must be corrected. If planning for surgical intervention based on suspected popliteal artery pathology, catheter-based contrast angiography with provocative maneuvers is the preferred diagnostic imaging used . This allows the surgeon to identify the preferred initial surgical approach for decompression, especially in functional popliteal artery entrapment syndrome.
Image 3: T1 MR findings in a case of popliteal artery entrapment syndrome. 2-A Axial view of the laterally deviated medial head of the gastrocnemius muscle (yellow arrowhead), popliteal artery (white arrow), and popliteal vein (yellow arrow). Figs. 2-B and 2-C Axial views of the popliteal arterial aneurysm (yellow arrow) and popliteal artery (white arrow). Fig. 2-D Sagittal view of the popliteal arterial aneurysm. Adopted from .
MRI can also help a surgeon determine whether the os trigonum has a fibrous, fibrocartilaginous, or cartilaginous attachment to the talus and can detect FHL tenosynovitis or chondral injury. MRI or ultrasonography can be used to assess for FHL tenosynovitis, which can present with fluid in the tendon sheath or nodules. Ultrasound can also be used for dynamic evaluation of the ankle. Dynamic ultrasound assessment of the ankle in active or passive plantar flexion may reveal impingement of the os trigonum between the posterior tibia and calcaneus.
The treatment for popliteal artery entrapment syndrome will vary on the basis of anatomic or functional entrapment and the timing of diagnosis. Depending on the severity of the diagnosis, definitive care may require a joint operation with vascular surgery. In cases of anatomic popliteal artery entrapment syndrome, surgical correction of the anatomic aberrancy should be performed regardless of the severity of symptoms, as the natural history is vascular injury and occlusion over time
Conversely, in patients with functional popliteal artery entrapment syndrome, vascular injury is virtually never seen, and surgical intervention is for symptom relief. In Type I situations, a surgical myotomy is performed with rerouting of the popliteal artery, with or without vascular repair as needed. For Type II situations, a surgical myotomy of the aberrant gastrocnemius insertion is performed. A Type III situation is the most straightforward treatment with resection of the accessory band. In a Type IV situation, a surgical release of the popliteus is performed with rerouting of the popliteal artery, with or without subsequent repair of the popliteus. Type V cases require similar treatments to those of Type I to III cases, depending on presentation but with associated relief of vein compression as well. Results for myotomy are excellent, with 100% patency results at 1 and 10 years postoperatively . When the compression has been a chronic issue, the patient is at risk for associated vascular injury, which may require repair secondary to intimal injury and fibrosis with stenosis.
Treatment of the functional subtype of popliteal artery entrapment syndrome has proven to be more difficult, and no standardized approach has been accepted. The main issue with surgical intervention is the balance of symptom relief with potential functional consequences. Depending on the location of the stenosis seen with provocative maneuvers in patients with functional popliteal artery entrapment syndrome, either a posterior or medial approach can be taken . For the more proximal lesions, a posterior approach is favored, and, for lower lesions, the medial approach should be used .
A relatively novel and promising approach has been described using botulinum toxin A as a means of chemically debulking the gastrocnemius and relieving the compressive effect on the artery. A large case series with 27 subjects reported close to a 60% favorable response rate at a 1-year follow-up with no reported complication or reduction in function . Although there is currently no set of concrete indications for its use, botulinum toxin A may present a valid nonoperative option to those diagnosed with functional popliteal artery entrapment syndrome in which the hypertrophied musculature is the cause of entrapment. Furthermore, those who wish to avoid or delay a surgical procedure or who are not surgical candidates may benefit from this treatment .
In summary, popliteal artery entrapment syndrome (PAES) is a rare, but underdiagnosed, condition that closely mimics other disorders in active, young individuals. It is caused by anatomic variants of the popliteal fossa muscles or transient functional increases in muscle bulk, and long term popliteal artery entrapment syndrome can lead to damage to the regional vasculature. Diagnostic testing should be done in a logical manner and awareness of this condition can lead to a more prompt diagnosis and treatment.
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