When excess fluid builds up in the tissues, it causes swelling, medically termed edema. Swelling in both feet, known as bilateral edema, often points toward a systemic issue like heart, liver, or kidney problems. Swelling in only one foot—unilateral edema—is a distinct concern that is typically localized to the affected limb. This asymmetry means the problem is likely due to an obstruction, injury, or inflammation specific to that one area of the body. Because the underlying causes of one-sided swelling can range from minor injuries to potentially life-threatening conditions, this symptom always warrants prompt medical evaluation to determine its origin.
Immediate Causes: Injury and Infection
The most common reasons for unilateral foot swelling involve a direct physical event affecting the limb, such as trauma or localized infection. An ankle sprain, fracture, or muscle strain causes immediate swelling because the body rushes inflammatory fluid and immune cells to the injured site. This response is a protective mechanism where damaged blood vessels leak fluid into the surrounding soft tissues, leading to noticeable swelling and often bruising or pain. The fluid accumulation is confined to the area affected by the injury, which is why the opposite foot remains unaffected.
Another acute cause is a localized bacterial infection, most often cellulitis. Cellulitis begins when bacteria enter the skin through a small cut, scrape, or insect bite, causing a rapid immune response. The affected foot or ankle typically becomes red, noticeably warm to the touch, and tender or painful, with the swelling quickly spreading. Unlike swelling from a sprain, the heat and redness are signs of an active infectious process that requires antibiotics to resolve.
Vascular and Fluid Return Disruptions
A serious category of causes relates to problems with the circulatory and lymphatic systems that impede the return of fluid from the foot back toward the core of the body. Deep Vein Thrombosis (DVT) involves a blood clot forming in one of the deep veins of the leg, which is a medical emergency. The clot blocks the normal flow of blood returning to the heart, causing pressure to build up in the veins below the obstruction. This increased pressure forces fluid to leak out of the capillaries and into the foot’s tissue, resulting in sudden, often painful, unilateral swelling. There is a risk that a piece of the clot could break off and travel to the lungs, causing a pulmonary embolism.
Even after DVT is treated, damage to the vein’s delicate one-way valves can lead to Chronic Venous Insufficiency (CVI). With CVI, these damaged valves fail to close properly, allowing blood to pool and causing chronic, long-term fluid accumulation in the affected leg. This pooling leads to persistent swelling and can cause skin changes over time.
Lymphedema is characterized by a failure of the lymphatic system to drain protein-rich fluid from the tissues. When its vessels or lymph nodes are damaged or blocked, the fluid cannot be cleared, leading to swelling. Lymphedema can be primary (present from birth) or secondary, resulting from surgery, radiation, or infection. The resulting swelling affects a single limb and is often non-pitting, meaning that pressing on it does not leave a temporary indentation.
Inflammatory Joint Conditions
Localized inflammatory conditions centered around the foot’s joints can present with swelling confined to one side. The most prominent example is gout, a form of inflammatory arthritis caused by an excess of uric acid in the blood. When uric acid levels are too high, the acid can crystallize into sharp, needle-like structures that deposit within a joint, most commonly the joint at the base of the big toe. This triggers an intense inflammatory reaction, causing the joint to become acutely swollen, hot, and exquisitely painful, often starting suddenly.
Gout can also affect the ankle or other joints in the foot. Other forms of arthritis, such as rheumatoid or psoriatic arthritis, may also begin with inflammation and swelling in a single joint. This localized swelling is due to the body’s autoimmune response or crystal deposition within the joint capsule itself, which is a distinct mechanism from the pressure-related fluid leakage seen in vascular issues.
Recognizing Serious Symptoms
While some causes of unilateral foot swelling are temporary, certain accompanying symptoms act as immediate red flags that signal a need for emergency medical care. Swelling that appears suddenly and is accompanied by severe, unexplained pain should be evaluated immediately, particularly if there is no obvious injury. Signs of an infection, such as fever, or the skin over the swelling being hot to the touch, red, or tender, require urgent attention to prevent systemic illness.
Symptoms that suggest a pulmonary embolism (PE), a complication of DVT, are especially serious and necessitate an emergency room visit. These warning signs include:
- Sudden shortness of breath.
- Chest pain that is sharp or worsens with deep breathing.
- A rapid heart rate.
- An unexplained cough, sometimes with bloody mucus.
If the skin on the swollen foot or leg turns pale, blue, or discolored, or if the swelling is dramatic and does not improve with elevation, it points to a significant circulatory compromise. Any concerning unilateral swelling should be promptly assessed by a healthcare professional.