Consumer Reports Insights: Swelling of the feet poses a diagnostic challenge

Swelling of the feet, ankles and legs, a condition known as peripheral edema, is the reason for countless doctor's office visits each year.

Since the causes of swelling are so numerous, that symptom poses a diagnostic challenge to physicians, according to Marvin Lipman, a clinical professor of medicine emeritus at New York Medical College and the chief medical adviser for Consumer Reports. One way to understand edema is to consider why it doesn't happen all the time.

Edema fluid is a filtrate of blood plasma, the liquid that blood cells float around in, that seeps from the veins and into the tissues of the ankle and foot. Normally, edema doesn't form because the hydrostatic pressure exerted against the walls of the veins isn't high enough to force the fluid out, and large proteins in the blood, mainly albumin, exert osmotic pressure (the opposite of hydrostatic pressure) that prevents leakage from the veins. Anything that upsets the delicate balance between these two opposing pressures can result in the escape of fluid from the veins into the tissues.

Too much pressure

The causes of increased hydrostatic pressure are many and varied. One of the most common is varicose veins. Healthy veins have one-way valves that prevent blood from pooling in the legs and thus raising hydrostatic pressure. Heredity, pregnancy, inflammation or a job that involves a lot of time on your feet (dentist, letter carrier, surgeon) can damage those valves.

Another common cause of increased venous pressure is congestive heart failure, which causes blood to back up in the veins that return blood to the heart. In addition, any abdominal or pelvic mass, such as a tumor or fibroids, can impinge on veins, block the flow of blood toward the heart and raise pressure in the leg veins.

Overexpansion of blood plasma is another way of raising hydrostatic pressure in the venous system. This happens with chronic kidney disease, which impairs the body's ability to excrete fluid. It can also result from the zealous use of intravenous fluids in some hospitalized patients.

Nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen (Advil, Motrin and similar generics) and naproxen (Aleve and generic variations), reduce inflammation by interfering with production of a pro-inflammatory hormone called prostaglandin. Because prostaglandin helps excrete sodium, its absence causes sodium retention and, with it, water retention and blood-volume expansion. Other drugs that can cause volume expansion include two for diabetes, pioglitazone (Actos) and rosiglitazone (Avandia), and one class of calcium-channel blocker antihypertensives such as amlodipine (Norvasc and its generic cousins).

Too little pressure

The second of the two underlying causes of edema is a reduction in the osmotic pressure in the blood due to protein depletion. The poster child for this condition is the protein-starved victim with a potbelly and swollen legs in an otherwise emaciated body. In the United States, edema due to protein depletion is most often seen with certain forms of kidney disease and liver failure.

Finally, some people can develop mild, transient edema without having a disease or taking edema-causing medication. It can happen during the second half of the menstrual cycle or during the third trimester of pregnancy (when it might signal the onset of pregnancy-induced hypertension). It can also happen to someone on an airplane who consumes large amounts of salt-rich foods and remains in a seat in one position for hours.

(c) Copyright 2010. Consumers Union of United States Inc.