What Causes Leg Swelling? Heart Failure, Blood Clots, and Infections
A clear explanation of leg swelling, why it happens, and how doctors determine whether the cause is heart failure, a blood clot, infection, or something else
Three Patients With the Same Complaint
Three patients arrive during the same shift with the exact same concern. “My legs are swelling.”
The first patient is a 65 year old woman whose legs are swollen on both sides. When she lies flat she becomes short of breath. When she walks even a short distance she feels winded.
The second patient has swelling in only one leg after a long car trip.
The third patient has swelling in one leg as well, but the skin is red, warm, and painful. Same symptom. Very different diagnoses.
Why Leg Swelling Happens
Leg swelling usually means fluid is collecting in the tissues.
Normally the heart pumps blood through the body and the veins return that blood back toward the heart. When something disrupts that balance, fluid can begin to leak into the surrounding tissue.
In medical terms this is called edema. The key question is not simply whether swelling exists. The key question is why the fluid is there in the first place.
When Both Legs Are Swollen
Swelling in both legs often points toward a systemic cause.
One of the most common is congestive heart failure. When the heart muscle cannot pump blood effectively, blood backs up in the circulation.
That pressure causes fluid to leak into the legs and sometimes the lungs. When doctors press on the legs, the skin may temporarily hold the indentation from the fingers. This is called pitting edema. Patients with heart failure often also experience shortness of breath or fatigue.
When Only One Leg Is Swollen
Swelling in one leg raises different concerns.
A blood clot in the deep veins of the leg, called a deep vein thrombosis, can block blood flow returning to the heart. The trapped blood causes swelling, pain, and warmth in that leg.
Long travel, immobility, recent injury, cancer, pregnancy, and certain medications can all increase the risk of clots. Because clots can sometimes travel to the lungs and cause a pulmonary embolism, doctors take this diagnosis very seriously.
When the Skin Is Red and Tender
Sometimes swelling is caused by infection.
A skin infection called cellulitis can develop when bacteria enter through small breaks in the skin. The affected area becomes red, warm, swollen, and painful.
In some cases a pocket of infection called an abscess may form beneath the skin. Patients with diabetes or poor circulation are especially vulnerable to these infections because small wounds may go unnoticed.
THE BOTTOM LINE
• Leg swelling can be caused by heart failure, blood clots, infections, or circulation problems
• Swelling in both legs often suggests a systemic problem like heart failure
• Swelling in one leg with pain or redness may signal a blood clot or infection and should be evaluated quickly
By Dr. Karim Ali, Emergency Physician