There are two main complications of deep vein thrombosis (DVT) - pulmonary embolism and post-thrombotic syndrome.
Pulmonary embolism
Occasionally, if you have a DVT, part of the blood clot can break off and travel in your bloodstream. This travelling blood clot is known as an embolus. The embolus will eventually be carried into the heart. From there it will be transported into the main branches of the arteries to the lung, where it will block the blood flow. The blockage, known as a pulmonary embolism, is a common cause of sudden, unexpected death. A severe pulmonary embolism will cause the lungs to collapse, and result in heart failure.
If the pulmonary embolism is small, it might not cause any symptoms. If it is medium sized, it can cause breathing difficulties and chest pain. A large pulmonary embolus can be fatal. About 10% of people with an untreated DVT develop a pulmonary embolism that is severe enough to cause symptoms or death.
Post-thrombotic syndrome
If you have had a DVT, you may develop long-term symptoms in your calf. This is known as post-thrombotic syndrome, and commonly affects people with a history of DVT.
If you have DVT, the blood clot in the vein of your calf can divert the flow of blood to other veins, causing an increase in pressure that can affect the tissues of your calf. The symptoms can include calf pain, discomfort, swelling and a rash. In severe cases, an ulcer may develop on the calf.
If DVT develops in your thigh vein, there is an increased risk of post-thrombotic syndrome occurring. It is also more likely to occur if you are overweight, or you have had more than one DVT in the same leg.