A clot that originates as a DVT can also cause a stroke in individuals with a patent foramen ovale (PFO, a hole in the wall between the atria — see "Patent Foramen Ovale" for details about this condition); in such a case, the clot travels through the veins to the right atrium of the heart, passes through the PFO to the left atrium and then travels through the arteries to the brain.
DVT is not related to diving, but divers often travel, and travel is a significant risk factor for DVT. In about half of all cases of DVT, the individual experiences no noticeable symptoms before the onset of the condition. Most often, it starts in the calf. Symptoms may include the following:
- Swelling in the affected leg, ankle or foot
- Pain in the calf that spreads to the ankle or foot
- Warmth in the affected area
- A change in the color of the skin — to pale, red or blue
The incidence of DVT in the general population is one-tenth of one percent, but it is higher in those who have risk factors and those who travel often. Long-distance air travel may double or even quadruple the risk of suffering a VTE. Although DVT is often called the "economy class disease," business-class travelers are susceptible, too. The risk of a DVT occurring on a flight lasting more than four hours is between 1 in 4,650 flights and 1 in 6,000 flights; this is lower than the risk in the general population, but that's because people who take long trips are likely to be healthier than average. The incidence of DVT among travelers with a low to intermediate pre-existing risk for VTE who take a journey longer than eight hours was found to be 0.3 percent for symptomatic cases and 0.5 percent when including asymptomatic cases as well.
Risk factors for DVT include the following:
- Older age (the risk rises after age 40)
- Obesity (defined as a body mass index greater than 30)
- Estrogen use (either hormonal contraceptives or hormone replacement therapy)
- Pregnancy (including the postpartum period)
- Thrombophilia (an abnormally increased tendency of the blood to clot)
- Previous VTE or a family history of VTE
- Active cancer
- Serious medical illness
- Recent surgery, hospitalization or trauma
- Limited mobility
- Central venous catheterization (the presence of a catheter in one's chest, for use in administering medication or nutrients and/or drawing blood samples)
Height is also a factor in one's risk of developing a travel-related DVT. People who are either very short — less than 5 feet, 3 inches (1.6 meters) — or very tall — more than 6 feet, 3 inches (1.9 meters) — appear to be at increased risk as a result of their inability to adjust their seats sufficiently to accommodate their height. In addition to effects of immobility, shorter passengers may suffer greater than usual seat-edge pressure on the backs of their knees, and taller passengers may be cramped due to insufficient leg room. All of these factors can contribute to injury of deep veins, venous stasis and activation of the blood's clotting mechanisms.
Those who are at increased risk of DVT should wear compression socks whenever they fly or drive long distances and should consult their primary-care provider regarding the possible benefit of taking a clot-preventative such as aspirin. Although the risk of DVT for healthy people is small, everybody should be aware of the factors that can precipitate the condition — and avoid long periods of immobility. The best way to prevent DVT is to get up and walk around from time to time. It also helps to flex your feet and calf muscles regularly if you must remain seated for any length of time. Finally, it is also helpful in preventing DVT to stay well hydrated.