Deep venous thromboses (blood clots) form in the deep veins of the legs in approximately 1-in-1000 people in the United States. Anyone can develop a blood clot, however, there are known risk factors that increase the chances. In addition, having more than one risk factor at a time increases the chances even more. Risk factors include:
- Injury or trauma to the legs.
- Surgery to the abdomen, pelvis, hip or legs.
- Congestive heart failure.
- Chronic obstructive pulmonary disease.
- Chronic kidney or liver disease.
- Increased estrogen from birth control pills or hormone replacement therapy.
- Inherited blood clotting disorders.
However, another risk factor for blood clots not often mentioned is rheumatoid arthritis (RA). Rheumatoid arthritis is a chronic, inflammatory disorder in which the immune system mistakenly attacks joints and other tissues of the body including skin, eyes, heart, lungs, kidneys, nerves, and blood vessels. A large clinical study in the United States looked at the risk of developing blood clots in two groups of patients. The first group of patients were newly diagnosed with RA and had not had a blood clot prior to the study. The second group of patients (control group) did not have RA, and also had never previously had a blood clot. The objective of the study was to determine if the patients with RA were more likely to eventually develop blood clots. The patients in each group were matched for age, sex, and pre-existing risk factors for developing blood clots (like those noted above). The researchers found that the patients with RA had a 40% greater risk of developing blood clots than the patients in the control group.
The chronic, systemic inflammation associated with RA initiates many complex events in the blood that promote the formation of blood clots, even when there is no injury or need for a clot. One of the body’s abnormal coagulation responses is to increase production of a blood protein called fibrinogen. Fibrinogen is converted by an enzyme in the blood called thrombin, into another protein called fibrin. Fibrin binds plasma proteins and platelets together to form blood clots. Increasing the level of fibrinogen in the blood increases the risk of forming blood clots.
But, the effects of chronic inflammation are even more complex. Under normal circumstances, the body has strong, natural mechanisms that prevent blood clots from forming when they’re not needed. Endothelial cells that line blood vessel walls normally interact with blood proteins to produce natural anticoagulant substances that prevent blood clots from forming. Damage to the endothelial cells by chronic inflammation inhibits their interactions with the proteins resulting in reduced activity of the natural anticoagulants in the blood, and thereby also increase the risk of blood clots forming.
Blood clots can permanently damage leg veins resulting in circulation issues, and also create a potentially life threatening condition if part of the clot travels to the lung (pulmonary embolus). Rheumatoid arthritis is a risk factor for developing blood clots in the legs. The chronic inflammation of RA alters many mechanisms in the blood stream that control the formation of blood clots, ultimately shifting the balance toward increasing the risk of clot formation. Patients with RA and at least one additional risk factor should talk with their medical provider about their personal risk for developing blood clots, and strategies to prevent possible physical damage and functional impairment.