Why would you have edema in one leg and not the other?

September 26, 2025 •

4 min read

Swelling in only one leg is a common symptom affecting millions, often pointing to a localized issue rather than a systemic one. Understanding why you would have edema in one leg and not the other is crucial for identifying the underlying cause and seeking appropriate care.

Quick Summary

Unilateral edema, or swelling in only one leg, typically results from a localized problem affecting that specific limb, such as a deep vein thrombosis (DVT), chronic venous insufficiency, or a lymphatic system blockage. Unlike systemic issues causing swelling in both legs, a single swollen leg indicates a targeted disruption in blood flow, lymphatic drainage, or an infection that warrants prompt medical evaluation.

Key Points

Localized Problem: Edema in one leg indicates a specific issue within that limb, unlike systemic conditions that affect both legs, helping to narrow down the cause.
DVT Risk: Sudden, painful swelling in one leg, especially with warmth and redness, could be a deep vein thrombosis (DVT), a medical emergency requiring immediate attention.
Valve Failure: Chronic venous insufficiency, where vein valves in one leg fail, is a common cause of progressive, one-sided swelling that worsens throughout the day.
Lymphatic Blockage: Lymphedema, resulting from a damaged lymphatic system, can cause significant, chronic swelling localized to a single leg, often following cancer treatment.
Infection Watch: Cellulitis, a bacterial skin infection, can cause rapid, unilateral swelling with redness, warmth, and pain, requiring prompt antibiotic treatment.
Anatomical Causes: Less common conditions like May-Thurner syndrome involve a compressed vein in the pelvis, leading to chronic left leg swelling due to obstructed blood flow.

Understanding Edema

Edema is the medical term for swelling caused by fluid buildup in the body's tissues. While it can occur anywhere, it is most noticeable in the arms, hands, feet, ankles, and legs. When edema affects only one leg, it is called unilateral edema. This distinction is significant because it helps narrow down potential causes, which range from minor issues to more serious conditions that require immediate medical attention.

The Common Causes of Unilateral Edema

Deep Vein Thrombosis (DVT)

One of the most serious and common causes of sudden, painful, one-sided leg swelling is a deep vein thrombosis (DVT). A DVT is a blood clot that forms in a deep vein, most often in the leg. The clot obstructs blood flow, causing pressure to build and fluid to leak into the surrounding tissues. This can lead to a swollen, painful, and warm-to-the-touch leg. A DVT is a medical emergency because if the clot dislodges, it can travel to the lungs, causing a potentially fatal pulmonary embolism.

Chronic Venous Insufficiency (CVI)

Chronic venous insufficiency is a condition where the one-way valves in the leg veins are damaged, preventing blood from flowing efficiently back toward the heart. This causes blood to pool in the leg, increasing pressure and leading to swelling. If the valves in only one leg are compromised, swelling will be isolated to that side. CVI is often progressive and can lead to varicose veins, skin discoloration, and leg ulcers over time. Swelling from CVI often worsens throughout the day and improves after elevation.

Lymphedema

Lymphedema is a blockage in the lymphatic system, a network of vessels that carry a clear fluid called lymph throughout the body. The lymphatic system helps remove waste products and excess fluid from tissues. If it is damaged or obstructed, such as after cancer surgery involving lymph node removal or radiation therapy, fluid can build up in the affected limb. This can result in significant, chronic swelling in just one leg. Lymphedema can also be congenital, though this is less common.

Infection

An infection, particularly cellulitis, can cause isolated leg swelling. Cellulitis is a bacterial infection of the deeper layers of the skin and underlying tissue. It can cause the affected area to become red, warm, swollen, and painful. If an infection occurs in only one leg, the swelling will be confined to that leg. Cellulitis can spread rapidly and requires prompt treatment with antibiotics to prevent serious complications.

Injury or Trauma

Localized trauma to a leg, such as a sprain, fracture, or severe bruising, can cause inflammation and fluid accumulation in the affected area. This is a temporary form of edema that typically subsides as the injury heals. While less severe than other causes, a persistent or unexplained increase in swelling after an injury should be evaluated by a healthcare provider.

Anatomical Obstructions

Certain anatomical issues can cause unilateral edema. For instance, May-Thurner syndrome is a rare condition where the right iliac artery compresses the left iliac vein in the pelvis. This compression restricts blood flow from the left leg, leading to swelling, particularly on the left side. Similarly, cysts or tumors in the pelvic region can press on veins or lymphatic vessels, causing swelling in one leg. For more on venous conditions, visit the American Venous Forum.

A Comparison of Unilateral Edema Causes


Condition	Onset	Associated Symptoms	Key Diagnostic Indicator
DVT	Sudden onset, often acute	Pain, warmth, redness, tenderness	Doppler ultrasound to visualize clot
Venous Insufficiency	Gradual, progressive	Aching, heaviness, varicose veins	Venous duplex ultrasound to check valve function
Lymphedema	Gradual, chronic	Feeling of tightness, thickened skin	Lymphoscintigraphy (lymph system scan)
Cellulitis	Rapid, progressive	Fever, redness, warmth, skin changes	Clinical examination, sometimes blood tests
Injury	Immediate	Pain, bruising, limited mobility	Physical exam, imaging (X-ray, MRI)
May-Thurner Syndrome	Gradual	Primarily left-sided swelling	Pelvic vein ultrasound or venography

When to Seek Medical Attention

While some minor swelling can be a normal part of life, unilateral edema should always be evaluated by a medical professional to rule out serious conditions. Seek immediate medical attention if you experience sudden, severe swelling in one leg, especially if accompanied by pain, warmth, redness, shortness of breath, or chest pain. These can be warning signs of a DVT or pulmonary embolism. For less acute but persistent swelling, a doctor can help determine the cause and recommend an appropriate course of action.

Conclusion: The Importance of Accurate Diagnosis

Swelling in one leg is a clear signal that something is happening within that specific limb. Unlike widespread edema caused by systemic issues, unilateral swelling points toward a local problem affecting blood circulation, the lymphatic system, or a localized injury or infection. By understanding the distinct symptoms and causes, you can take the necessary steps to get an accurate diagnosis. Early diagnosis is key, particularly for serious conditions like DVT, to prevent complications and ensure effective treatment.

Frequently Asked Questions

Edema in both legs often points to a systemic condition, such as heart, kidney, or liver disease, where fluid retention is widespread. In contrast, swelling in only one leg is typically caused by a localized issue affecting that specific limb, such as a blood clot, infection, or vein problem.

You should seek immediate medical attention for sudden, severe swelling in one leg, especially if accompanied by pain, redness, warmth, or difficulty breathing. For persistent but less severe swelling, consult a doctor to rule out underlying conditions like venous insufficiency or lymphedema.

Yes, a localized injury such as a sprain, fracture, or muscle strain can cause inflammation and swelling in the affected leg. The edema is typically temporary and localized to the site of the injury, subsiding as the body heals.

Diagnosis typically involves a physical exam, a review of your medical history, and sometimes imaging tests. An ultrasound (Doppler) can check for blood clots and evaluate venous valve function, while other imaging may be used to look for anatomical obstructions or lymphatic issues.

While heart conditions typically cause bilateral (both legs) swelling, an underlying cardiac issue can sometimes contribute to or exacerbate venous insufficiency in one leg. However, isolated unilateral swelling is less commonly the sole sign of a primary heart problem.

Yes, poor circulation stemming from a specific issue in one leg, like chronic venous insufficiency or a deep vein thrombosis (DVT), is a primary cause of unilateral edema. The problem affects the blood flow in that particular leg, causing fluid to pool.

May-Thurner syndrome is a condition where the right iliac artery crosses and compresses the left iliac vein in the pelvis. This compression restricts blood flow from the left leg, increasing pressure in the vein and leading to chronic swelling in only the left leg.