Abdominal aortic aneurysms are generally asymptomatic in their early stages, or when they do not reach a certain size. For abdominal aortic aneurysms under 4 or 5 cm in diameter, the risk of aneurysm rupture is relatively small, and the patient has no obvious symptoms and may only occasionally feel a throbbing mass in the abdomen. Early detection is usually not easy because the aneurysm does not obviously cause much pain to the patient. As the aneurysm increases in size, other symptoms will appear. The first of these is pain. When the pain increases, it is often a sign that the aneurysm is about to rupture, or even has ruptured, before it is too late. Sometimes the patient may feel a tinge of vague pain because the mass is large and uncomfortable to hold there. In larger diameter abdominal aortic aneurysms, the shell becomes thin and painful when pressed. Another symptom is arterial embolism. Because of the local expansion of the blood vessel, a vortex is created when the blood flows, and this vortex causes a thrombus to form in the inner layer of the arterial wall, which is medically known as an appendage thrombus. Once these thrombi are dislodged, they will flow with the blood to the lower extremities, causing embolism of the lower extremity arteries. Patients may experience sudden pain in the lower extremities and may even be unable to walk. In some cases, abdominal aortic aneurysms are discovered because of lower extremity ischemia. Therefore, most aneurysms are discovered occasionally by the patients themselves, while some patients have aneurysms discovered during normal physical examinations, such as a checkup for ultrasound of the liver, gallbladder, kidney and spleen, or a CT examination of the abdomen or pelvis, and a large bulge in the aorta is found by chance.