One ankle swelling while the other remains normal can be caused by a variety of medical conditions, often related to localized issues affecting blood flow, lymphatic drainage, or tissue integrity in that specific leg. This asymmetrical swelling is medically referred to as unilateral ankle edema and usually indicates a problem confined to one leg rather than a systemic condition affecting the whole body.
The most common cause of swelling in one ankle but not the other is **chronic venous insufficiency (CVI)**. CVI occurs when the veins in the leg cannot efficiently return blood to the heart, leading to blood pooling and fluid leakage into surrounding tissues. This condition often causes swelling, skin discoloration, and sometimes ulcers, typically affecting one leg more than the other if the venous damage is localized[1].
Another critical cause to consider is a **deep vein thrombosis (DVT)**, which is a blood clot in a deep vein, usually in the leg. DVT causes sudden, painful swelling in one leg and can be dangerous if the clot dislodges and travels to the lungs, causing a pulmonary embolism. Because DVT is a medical emergency, any sudden unilateral leg swelling accompanied by pain should prompt immediate medical evaluation[1][2].
**Infections such as cellulitis** can also cause one ankle to swell. Cellulitis is a bacterial skin infection that leads to redness, warmth, swelling, and pain in the affected area. It often occurs after a break in the skin, such as a cut, insect bite, or fungal infection, and can cause swelling localized to one leg or ankle[3]. The skin may appear red and feel hot, distinguishing it from other causes of swelling.
**Lymphedema** is another cause of unilateral ankle swelling. It results from impaired lymphatic drainage, leading to fluid accumulation in the tissues. Lymphedema often causes non-pitting swelling (where pressing the swollen area does not leave a lasting indentation) and thickened skin. It can be primary (due to congenital lymphatic abnormalities) or secondary (due to surgery, radiation, infection, or trauma affecting lymph nodes)[2].
Other causes include:
– **Trauma or injury** to one leg or ankle, such as sprains, fractures, or ruptured Baker’s cysts, which can cause localized swelling and inflammation[4].
– **Medication side effects**, such as from calcium channel blockers or nonsteroidal anti-inflammatory drugs (NSAIDs), which can cause fluid retention and swelling, sometimes asymmetrically[4].
– **Tumors or masses** compressing veins or lymphatic vessels in one leg can cause swelling by obstructing normal fluid drainage[1].
– **Arterial occlusion or poor arterial circulation** can sometimes cause swelling, though it is more commonly associated with pain and color changes[4].
– **Dependent edema** due to gravity is usually bilateral but can appear more pronounced in one leg if the person habitually favors one side or has an underlying localized problem[5].
When a doctor evaluates unilateral ankle swelling, they will perform a physical exam looking for “pitting edema,” where pressing on the swollen area leaves a temporary indentation. Pitting edema is common in venous or systemic causes, while non-pitting edema suggests lymphedema or other tissue changes[1][2]. The doctor will also check for skin changes, ulcers, varicose veins, and signs of infection. Diagnostic tests may include ultrasound to rule out DVT or assess venous reflux, blood tests, and sometimes imaging like lymphoscintigraphy for lymphedema[2].
In summary, swelling in one ankle but not the other is usually due to localized problems such as venous insufficiency, blood clots, infection, lymphatic obstruction, or injury. Because some causes like DVT and cellulitis require urgent treatment, prompt medical evaluation is essential to determine the exact cause and initiate appropriate care.
Sources:
[1] betterhealthwhileaging.net – Leg Swelling in Aging
[2] nortonschool.com – Pitting Edema vs. Lymphedema
[3] legsmatter.org – Cellulitis
[4] droracle.ai – Approach to Unilateral Pedal Edema
[5] droracle.ai – Differential Diagnosis for Swollen Ankles
