Cystic adventitial disease has been described as consisting of a collection of clear gelatinous material within an aberrant synovial type cyst located in the subadventitial plane of the wall of a major artery. These mucoid cysts may cause luminal compromise of the vessel and produce occlusive symptoms. This disease was first described in the external iliac artery in 1947 and in the popliteal artery in 1954.
The popliteal artery is the most common site but the other sites in a descending order of occurrence include –
Ilio-femoral artery, 
Brachial, radial and ulnar artery and
Venous location - popliteal, lesser saphenous vein.

The theories postulated for the etiology of the adventitial cyst include:
1. Developmental theory – all the cysts develop in non-axial [appendicular] vessels. Therefore it is theororized that during limb bud development, mesenchymal cell rests from the adjacent joints migrate into these adjacent vessel walls. Later in life, these cells are responsible for secreting mucoid material resulting in the formation of a cystic mass within the vessel wall.
2. Repeated trauma – microtrauma damages the adventitia, potentiates mucin production which results in the development of the cyst.
3. Synovial origin – as some of these adventitial cysts have shown a communication with the adjacent joint capsule.
4. Ganglionic origin – this has been postulated because the histological appearance and the chemical composition of the adventitial cyst closely resembles that of a ganglion.
Pathologically, the cysts in the vessel wall may be single or multiple; uni or multilocular and range in size from 1-8 cm. The cyst characteristically does not have a lining and does not communicate with the vessel lumen. Its fluid content is rich in hyaluronic acid.

Clinical presentation – The patient is typically a young, non-smoking male who presents with intermittent claudication. The lesion causes dynamic exercise dependent flow inhibition.

Imaging appearance –
Ultrasound with color Doppler shows characteristic features that suggest the diagnosis:
• An anechoic smooth walled mass seen in the wall of the popliteal artery. It shows good posterior enhancement suggesting that it is fluid filled. Sometimes, the contents of the cystic structure appear hypoechoic.
• Usually a single large cyst is seen, which is eccentric in location.
• Reduction in the diameter of the popliteal artery.
• Color Doppler shows the reduction in vessel lumen, increased systolic and diastolic velocities in the narrowed segment of the artery and post stenotic elevations with flow turbulence. There might be distal flow reduction.

The other modalities that can image this disease are CT and MRI, the latter being used more often. T2 weighted MRI images confirm the diagnosis and eliminate the need for conventional angiography. Arteriography shows a smooth-walled curvilinear narrowing.

Differential diagnosis – the following may be considered:
1. Aneurysm – the wall of the popliteal artery aneurysm is much more echogenic than the the wall of the adventitial cyst, if there is an associated thrombus, atherosclerosis or thickened and calcified intima.
2. Synovial cyst- this is seen in the popliteal fossa and causes external compression of the artery.
3. Popliteal artery entrapment syndrome -this occurs due to external compression of the artery as a result of anatomic deviation from its usual course or by compression from musculotendinous structures in the popliteal fossa.
4. Popliteal arterial occlusive disease – in this case, lack of typical risk factors for atherosclerosis makes this diagnosis less likely.

Management- There have been reports of spontaneous regression and spontaneous rupture of the cyst.  However percutaneous US-guided cyst aspiration, surgical cyst evacuation and, if associated thrombosis of the artery, excision of the segment and by-pass graft are some of the treatment options available.