The first common cause is trauma, such as a bruise on the limbs, when the capillaries are traumatized, there will be subcutaneous bruising, accompanied by a painful sensation, which will gradually recover after rest, and there is no obvious effect on the body. The second common cause is simple purpura, common in women of childbearing age, related to changes in the concentration of estrogen in the body, bruising of the skin of the extremities without trauma, pressure will also be accompanied by mild pain, you can take vitamin C treatment, the effect is good. The third common cause is allergic purpura patients, due to allergy to external factors, resulting in increased permeability of capillaries will also appear skin bruising, pressure will also be accompanied by pain, the most effective way is to take glucocorticoid treatment.