Peripheral edema happens when fluid leaks out of small blood vessels and accumulates in the surrounding tissues, most commonly in the legs, ankles, and feet. The underlying cause can range from something as simple as sitting too long to serious conditions involving the heart, kidneys, or liver. Whether the swelling affects one leg or both, and whether it appeared suddenly or gradually, tells a lot about what’s driving it.
How Fluid Normally Stays in Balance
Your smallest blood vessels, the capillaries, constantly exchange fluid with the tissue around them. Two opposing forces keep this process in check. Hydrostatic pressure (the physical push of blood against vessel walls) drives fluid out into the tissues. Oncotic pressure, created mostly by proteins like albumin dissolved in your blood, pulls fluid back in. When these forces are balanced, your tissues stay their normal size.
Peripheral edema develops when something tips that balance. Higher pressure inside the veins pushes more fluid out. Lower protein levels in the blood reduce the pull that draws fluid back. A damaged lymphatic system fails to drain the excess. Or capillary walls become leakier than they should be. Most causes of peripheral edema work through one or more of these four mechanisms.
Heart Failure
Heart failure is one of the most common causes of bilateral leg swelling. When the heart can no longer pump blood efficiently, blood backs up in the veins. As blood lingers there, pressure builds. That increased venous pressure forces fluid out of the capillaries and into the surrounding tissues of the legs and feet.
The body makes things worse by trying to compensate. Sensing reduced blood flow, the kidneys hold onto sodium and water in an attempt to increase blood volume. This extra fluid has nowhere to go efficiently, so more of it ends up in the tissues. Swelling from heart failure typically worsens through the day and improves overnight when the legs are elevated during sleep. It almost always affects both legs.
Kidney Disease
The kidneys contribute to edema through two separate pathways. In nephrotic syndrome, damaged filters in the kidneys allow large amounts of protein to spill into the urine. The resulting drop in blood protein levels lowers oncotic pressure, which means less force pulling fluid back into the bloodstream. Fluid leaks into the tissues and stays there.
There’s also a direct kidney mechanism at work. Researchers have found that proteins leaking through damaged kidney filters activate sodium channels in the kidney itself, causing the organ to aggressively retain salt. This intrarenal salt retention expands fluid volume independent of what’s happening with blood protein levels. Both pathways, low protein and excess sodium, compound each other to produce widespread swelling in the legs, face, and around the eyes.
Liver Disease
The liver produces albumin, the protein most responsible for keeping fluid inside blood vessels. In cirrhosis, the liver’s ability to manufacture albumin declines, lowering oncotic pressure and allowing fluid to seep into tissues. But low albumin alone doesn’t fully explain the swelling.
Cirrhosis also causes blood to pool in the vessels of the digestive organs, a consequence of portal hypertension. This pooling reduces the effective volume of blood circulating through the arteries, which triggers the same hormonal cascade seen in heart failure: the renin-angiotensin system ramps up, the sympathetic nervous system activates, and the kidneys retain sodium and water. The result is fluid accumulation in both the abdomen (ascites) and the legs. Intravenous albumin can temporarily expand blood volume and even improve peripheral edema, but because the underlying vascular changes persist, fluid tends to reaccumulate.
Chronic Venous Insufficiency
Veins in the legs rely on one-way valves to push blood upward against gravity. When those valves fail, blood flows backward and pools in the lower legs. This can happen after a deep vein thrombosis damages the valves, or it can develop gradually from prolonged standing, obesity, or aging. Weak calf muscles make the problem worse, since the calf acts as a pump that squeezes blood upward with each step.
The sustained high pressure in leg veins gets transmitted down into the capillaries, where it damages the vessel walls and increases their permeability. Fluid, proteins, and even red blood cells leak into the surrounding tissue. Over time, this produces characteristic skin changes around the inner ankle: a reddish-brown discoloration from iron deposits left by broken-down red blood cells, thickened and hardened skin, and eventually venous ulcers that are slow to heal. Venous insufficiency is the most common cause of chronic swelling in one leg, though it frequently affects both.
Lymphedema
The lymphatic system acts as a secondary drainage network, collecting excess fluid and proteins from tissues and returning them to the bloodstream. When lymphatic vessels are damaged or blocked, protein-rich fluid builds up and produces a distinctive type of swelling.
Early lymphedema can look similar to other types of edema and may even pit when you press on it, because the protein-rich fluid hasn’t yet triggered tissue changes. Over time, however, the skin becomes thickened, firm, and difficult to indent. A useful clinical sign is the Kaposi-Stemmer sign: if you can’t pinch and lift the skin on the top of the second toe, lymphedema is likely. Causes include surgical removal of lymph nodes, radiation therapy, infections (especially in tropical regions), and congenital abnormalities of the lymphatic system.
Medications That Cause Swelling
Several common medications cause peripheral edema as a side effect. Calcium channel blockers, frequently prescribed for high blood pressure, are among the most well-known culprits. These drugs relax the arteries but not the veins, creating a pressure imbalance that pushes more fluid into the tissues. The swelling is dose-dependent: at low doses, ankle edema occurs in roughly 1 to 15% of patients, but at high long-term doses the rate can exceed 80%.
Other medication classes that commonly cause leg swelling include NSAIDs (which promote sodium retention), corticosteroids, certain diabetes medications, and some hormonal therapies like estrogen or testosterone. The swelling is typically bilateral and resolves after the medication is stopped or the dose is reduced.
One Leg vs. Both Legs
The pattern of swelling is one of the most useful clues to its cause. Sudden swelling in one leg raises concern for a deep vein thrombosis (blood clot), especially if accompanied by pain, warmth, or redness. Other causes of acute one-sided swelling include infection, a ruptured cyst behind the knee, or injury. Chronic one-sided swelling points more toward venous insufficiency, lymphedema, or compression of a vein higher up in the pelvis.
Bilateral swelling that comes on suddenly suggests heart failure, a new medication, or acute kidney problems. When both legs have been swollen for weeks or months, the most common explanations are chronic heart failure, liver disease, nephrotic syndrome, venous insufficiency, medication effects, or pregnancy. Less common causes include pulmonary hypertension, thyroid disease, and obstructive sleep apnea.
How Pitting Edema Is Graded
When you press a finger into swollen tissue and an indentation remains, that’s called pitting edema. Clinicians grade its severity on a 1 to 4 scale based on how deep the pit goes and how long it takes to spring back:
- Grade 1: A 2 mm pit that rebounds immediately
- Grade 2: A 3 to 4 mm pit that rebounds in under 15 seconds
- Grade 3: A 5 to 6 mm pit that rebounds in 15 to 60 seconds
- Grade 4: An 8 mm pit that takes two to three minutes to rebound
Pitting edema generally indicates that the fluid in the tissues has a relatively low protein concentration, which is typical of heart failure, kidney disease, liver disease, and venous insufficiency. Non-pitting edema, where the skin feels firm and doesn’t hold an indentation, is more characteristic of advanced lymphedema or thyroid-related swelling. However, early lymphedema can still pit, so this distinction isn’t absolute.