An Embolus

Figure 1

An embolus is defined as a material (gas, solid or liquid) that is carried within the circulation and lodges in a blood vessel in another part of the circulation, causing occlusion of the blood vessel. Radiologically the upper border of an embolus is classically concave, known as the meniscus sign (See Figure 1).  An embolus is distinct from a thrombus, which is a blood clot formed in situ within the vascular system of the body and impeding blood flow.

In acute limb ischaemia emboli most commonly arise from the heart (80%) and as such are usually composed of platelets. In the Oxford study, 23.6% of events were cardioembolic; of this group 70.5% had known atrial fibrillation diagnosed before the event. Emboli can arise from proximal arterial disease (either aneurysms or stenoses) and may then contain atheroma. These carry a poorer prognosis for the limb since they are harder to treat and not amenable to thrombolysis.

Bilateral limb ischaemia

This can be more difficult to diagnose, since both legs are abnormal. It may be caused by a saddle embolus at the aortic bifurcation (which carries a high (30%) mortality) [12]. It can also occur as a result of abdominal aortic dissection (this classically presents with back pain and hypotension).