Unexplained bruising on the legs can occur when a person bumps or hits her leg and does not remember hitting it. In some cases, a bump that doesn't cause any pain or barely registers with a person can lead to a bruise on the leg. Some people are more prone to bruising as they age, because the skin becomes thinner and the blood vessels weaker. Unexplained bruising can be a sign of something more serious, though.
Generally, unexplained bruising on the legs, particularly on the shins, isn't a cause for concern. A bruise forms when the capillaries, or blood vessels near the surface of the skin, are damaged. Blood leaks out of the vessels, causing discoloration and some tenderness. As the blood returns to the vessels, the bruise heals.
The strength of the capillaries varies from person to person. Some people may need to receive a hard blow to receive a bruise, while bruises will seem to magically appear on others. Older women are usually more prone to unexplained bruising on the the legs than younger women, children, and men.
Many people have a layer of fat between the skin and the capillaries. Some people lose this layer, particularly as they age, which makes bruising more likely. In addition to aging, excess sun exposure and some medicines can make the skin thinner.
Certain medications can thin the blood and make bruising more likely. Aspirin, warfarin, and clopidogrel are designed to thin the blood or reduce clotting. If a person is on the medications, she may bruise more easily, as the blood will not stop flowing very quickly. Fish oil and ginkgo also thin the blood and can lead to unexplained bruising on the legs. Corticosteroids, either oral or topical, can thin the skin.
In some cases, unexplained bruising is a sign of a serious condition. It can point to a bleeding disorder, especially if accompanied by petechiae, or red spots under the skin, bleeding in the mouth, and frequent nosebleeds. A person may have a low platelet count or other platelet disorder or an infection in the blood.
Other causes of unexplained bruising on the legs include vitamin deficiency, leukemia, or an auto-immune disorder. Bruises caused by an auto-immune disorder, such as lupus, are usually raised and firm to the touch. Typically, they appear without any injury to the leg.