Swelling in one leg is most often either the result of a clot blocking the blood flow, an injury in a person's leg, or inflammation from arthritic conditions. There are also some cases in which swelling in one leg might be caused by water retention, which could signal major organ failure. In most cases, swelling in a single leg isn’t as serious as two swollen legs, but if the single leg is swollen because of a blood clot, it can be very serious, as can swelling from water retention in the rare cases when it only affects one leg.
In a typical situation, swelling in one leg usually means that the problem is somehow centered inside that leg. That could potentially be a good or bad thing. Usually a swollen leg is just a sign of injury. If a person hurts his leg by pulling a muscle, or falling and bruising it, a little bit of swelling can be considered normal.
In many cases, swelling in one leg can also be caused by arthritis in the knee joint or the ankle. This is a serious condition that can be debilitating if it isn’t treated. Arthritis can cause severe damage over time because it can eat away at an individual’s cartilage, which can make it harder and harder for someone to get around. Arthritic pain in the knees, ankles, and hips is especially serious because it can limit mobility.
Another serious cause of swelling in one leg is a blood clot inside the leg. This will sometimes cause a lot of other symptoms, such as pain and a noticeable lack of feeling or blood flow in that leg. In other cases, it may be more subtle and difficult to detect. In either situation, the blood clot can be life threatening. The main danger is the possibility that the blood clot will break free and damage another major organ while traveling through the blood stream.
Water retention is one of the most common causes of swollen legs, but it usually causes both legs to swell. Sometimes there may be slightly unequal swelling, with one leg being noticeably bigger than the other. When that happens, a person may think that only one of his legs is swollen because it looks that way to the naked eye. Water retention can be a serious danger because kidney, heart, and liver failure are all possible causes. It is generally considered wise to consult a physician to be sure it's not water retention or a clot if there is any significant leg swelling.