Leg swelling (oedema): causes, red flags and when to seek help

Fluid normally moves out of the smallest blood vessels into the tissues and is carried back by veins and the lymphatic system. Swelling develops when more fluid leaks out, when the veins or lymphatics cannot take it back, or when the body holds on to too much salt and water. Because of gravity, the ankles and feet are usually the first place it shows.

A useful clue is whether pressing a thumb into the swollen skin for a few seconds leaves a dent. Soft, pitting swelling is typical of fluid retention. Firm, non-pitting swelling that does not dent suggests lymphatic or other tissue causes.

The commonest explanations are benign: long periods sitting or standing, hot weather, being overweight, pregnancy, and medicines including some calcium-channel blockers for blood pressure, anti-inflammatories, steroids and some diabetes tablets. Swelling that worsens through the day and settles overnight points to a gravity effect.

The NHS describes DVT as a blood clot in a vein, usually in the leg, and lists throbbing pain in one leg (rarely both), usually in the calf or thigh, swelling in one leg, red, blue or darkened skin around the painful area and swollen veins. It says the risk is higher if you are over 60, live with overweight or obesity, smoke, have had DVT before, take HRT or oestrogen-containing contraception, have cancer or heart failure, are in or have recently left hospital, have had surgery, have been confined to bed for 3 days or more, have taken a long journey of more than 4 hours, are pregnant or have recently had a baby, or are dehydrated.

The NHS describes cellulitis as a skin infection making skin painful, hot and swollen, usually red though less obvious on brown or black skin, sometimes blistered, with possible flu-like illness and swollen glands. It is more common on the lower limbs. You cannot catch it from another person, and the NHS notes lymphoedema, poor circulation and a weakened immune system among the risk factors.

Heart failure means the heart cannot pump blood properly. The NHS lists breathlessness during everyday activity or when lying down, weakness or tiredness, light-headedness, swelling in the feet, ankles, legs or tummy, sudden weight gain, a cough at night and frothy pink phlegm. Swelling from kidney or liver disease tends to be accompanied by other changes in urine, appetite, skin colour or abdominal girth.

The NHS describes lymphoedema as a long-term condition that causes swelling in the body tissues, usually the arms or legs, because the lymphatic system is not working properly. At first the swelling may be soft and dent, may come and go and may be worse during the day. Secondary lymphoedema often follows cancer treatment involving lymph nodes, infection or injury.

The NHS says to call 999 or go to A&E if you have symptoms of DVT, such as pain and swelling, and feel short of breath or have chest pain, because the clot may have travelled to the lungs (pulmonary embolism). Do not drive yourself. For suspected DVT without those symptoms the NHS advises an urgent GP appointment or NHS 111, and says that a person thought to have DVT should be referred to hospital within 24 hours for an ultrasound scan.

For heart failure the NHS advises an urgent GP appointment or NHS 111 if you feel breathless when lying down or from everyday activity, you are coughing up frothy pink phlegm, or you have suddenly gained weight, and 999 for severe difficulty breathing or pale, blue or grey lips or skin.

For cellulitis it advises an urgent GP appointment or NHS 111 if skin is painful, hot and swollen, and 999 or A&E if there is a very high temperature or feeling hot, cold or shivery, a fast heartbeat or fast breathing, purple patches, dizziness or faintness, confusion, cold clammy pale skin or unresponsiveness.

The doctor will ask how long the swelling has been present, whether one or both legs are affected, what medicines you take, and about breathlessness, urine changes, weight, recent travel, surgery, pregnancy and cancer history. They will check pitting, skin temperature and colour, calf tenderness, veins, pulse, blood pressure and listen to the heart and lungs.

Tests are chosen by suspicion: a D-dimer blood test and leg ultrasound for possible clot, blood tests for kidney and liver function, albumin, thyroid and a heart blood marker such as BNP, a urine test for protein, an ECG and an echocardiogram for the heart, and a chest X-ray if heart failure is suspected.

For benign swelling, raising the legs above the level of the heart for periods through the day, walking regularly, avoiding long stillness and not crossing the legs for prolonged periods help. The NHS DVT advice to move around every hour or so, stay hydrated and avoid smoking also supports healthy circulation.

Compression stockings help some causes of swelling but are unsuitable for others, including when an acute infection is present, so ask a clinician before buying them. Reducing salt may help where fluid retention is the issue. Do not stop or change a water tablet or any medicine without medical advice.