Eagles quarterback Jalen Hurts' diagnosis with deep vein thrombosis, a type of blood clot that will likely sideline him for the rest of the season, has raised a number of questions about the seriousness of his condition and how it can affect people who face similar medical concerns.

In the span of less than a week, Hurts' condition was upgraded in public statements from superficial venous thrombosis in his calf — a less serious blood clot — to deep vein thrombosis with clots present in both of his lungs. The Eagles and Hurts' agent described him as "clinically stable" Monday and said he will take blood thinners with the expectation that he will make a full recovery.

Hurts was injured when a cleat struck his calf during the Eagles' Oct. 4 game against the Los Angeles Rams, and then he played the following Sunday in London against the Jacksonville Jaguars after the initial clot in his calf was discovered during the team's overseas trip. Further testing this week revealed more extensive clotting.

"There's two sort of families of veins," said Heather Giannini, a pulmonologist at Virtua Medical Group. "There's the superficial veins and there's the deep veins. And first, you know, we were told about the superficial venous clot, which isn't usually something that's as concerning. It can be uncomfortable, but a blood clot in those veins might cause some swelling, some discomfort, some redness."

At the most basic level, blood clots form when blood cells cluster into a sludgy mass that blocks veins or arteries. These clots travel up through the body to be circulated by the heart, potentially causing serious complications. 

"Our body has a lot of factors in the blood that prevent this from happening, but sometimes those can be overcome," Giannini said. "Once you get some clot, more clot wants to form."

Who is at risk for blood clots and what are the symptoms?

Being immobile — especially on long flights or after surgery — is one of the most common risk factors for blood clots. People with recent physical trauma, cancer patients, people receiving hormone therapies, and those with hereditary conditions and family histories of clotting also face increased risks.

Still, about one-third of people who develop blood clots have no obvious triggers for them and may not experience any symptoms at all. They are often detected incidentally during CT scans or other imaging tests.

"There's no blanket screening or evaluation for something like this," said Philip M. Batista, chief of vascular and endovascular surgery at Cooper Norcross Health.

Batista said the rapid shift in Hurts' diagnosis has caused confusion about the dangers of superficial venous thrombosis versus deep vein thrombosis, which affects larger and deeper veins in the lower legs, calves, thighs or pelvis. If clotting spreads to the lungs and is left untreated, chest pain and difficulty breathing are clear warning signs of a pulmonary embolism. In severe cases, people may cough up blood or go into cardiac arrest.

"It can be a significant condition that can be life-threatening," Batista said. "It does require immediate medical attention. If you have one-sided leg pain or swelling, that's a red flag for a deep vein thrombosis. That should be evaluated immediately."

Blood clots are most common in people over 60 years old. The risk generally rises with age, but they can occur in people of all ages and there is no considerable discrepancy between their occurrence in men and women. Otherwise healthy people, like Hurts, can develop blood clots due to trauma or other factors that put them at risk. The American Vein & Lymphatic Society estimates about 300,000 to 600,000 people are affected by deep vein thrombosis in the United States each year, and the CDC says between 60,000 to 100,000 people die annually from related complications, including pulmonary embolism.

Superficial blood clots are generally straightforward to identify and can be treated without major disruptions. The symptoms tend to be easier to recognize when they form lower in the leg.

"Sometimes patients have pain with walking or pain with flexion of the foot, and that's when I see calf clots in the lower part of the leg," Giannini said. "One leg will be a little more swollen than the other, or maybe it'll be a little more red. There'll be some warmth to the skin. I don't think people should discount those symptoms."

If all vital signs are otherwise normal after a visit to the doctor, the most common response may be to simply monitor symptoms or start a trial of blood thinners — known as anticoagulants — that are prescribed for several months. The drugs may increase the risk of bleeding, but they have become easier for people to take without issues.

"They're very well tolerated. We have moved into a new world of anticoagulation where there used to be a lot of struggle and complication with these drugs. You had to get your blood tested. There was a lot of bleeding complications. You had to watch what you eat. There's not usually any you know major side effects now.

Most people are able to fully recover from blood clots without complication, particularly if they can be traced to a specific event that provoked them to form. In cases where the reason is unknown, the risk of recurrence is higher.

"For some of those patients, I recommend indefinite anticoagulation," Giannini said.

When do blood clots become more serious?

Most blood clots are relatively slow moving, Giannini said, but their location can be a factor in how easily and quickly they spread from superficial veins to deeper veins. When a clot breaks off from a vessel in the leg and gets carried upstream to return to the heart, any finding of blood lodged in the lungs can be cause for serious alarm.

"I have seen and I do think that pulmonary embolism is one of the more dangerous diagnoses that can occur and that can be fast," Giannini said. "I have seen it be that fast, and it's on a spectrum."

Some of Giannni's patients show no unusual symptoms in their breathing and heart rate, but a scan reveals a pulmonary embolism. Others are critically ill and require immediate intervention. Emergency surgery, usually performed by a cardiologist or radiologist, involves snaking a catheter all the way up from the thighs into the pulmonary arteries to remove clots that pose an immediate threat to survival.

"The heart has to pump the blood through the lungs, and if you imagine that is normally just going on regularly, but then all of a sudden there's like a clog in the pipe, now the heart is struggling," Giannini said. "The heart is pushing and pounding. They get very high heart rates and palpitations."

While it's fairly uncommon for blood clots to spread as quickly as has been reported in Hurts' case, Batista said any outside interpretation of his situation would be a matter of speculation. It is possible to have superficial vein thrombosis extend into the deep vein system, create a deep vein thrombosis and then provoke a pulmonary embolism, Batista said, but a number of different factors would likely go into that progression taking place rapidly. 

"It is kind of a little peculiar that there was this much advancement of his disease and confusion behind what was diagnosed initially, so I'm not quite certain how much of it's accurate that we were told initially or where (the blood clot) was," Batista said. "It sounds like for him to have a pulmonary embolism now, he definitely had a deep vein thrombosis, and whether or not that was the initial diagnosis or not is unclear given the information that we have."

Dr. Parth M. Rali, a professor of thoracic medicine and surgery at Temple University's Lewis Katz School of Medicine, addressed the factors related to Hurts' injury in an interview with PhillyVoice earlier Monday. Parth noted that the rigors of an NFL schedule and a long flight to London might have contributed to the speedy progression of Hurts' clotting. 

At this point, the information coming out about Hurts all points in a positive direction. He may only need to take blood thinners to make a fully recovery, but Giannini and Batista said he will need to be monitored for a period of time to ensure he does not have any recurrence of clotting.

There are no general guidelines specifically for the prevention of blood clots, but living an active lifestyle promotes healthy circulation and reduces increased risks that come from being overweight. When flying long distances or taking lengthy car rides, Giannini said it's a good idea for people to take some time to walk around and flex their feet. Drinking enough water and avoiding smoking are also good practices to promote overall health and reduce the risk of developing blood clots.

The acute symptoms brought on by a pulmonary embolism are frequently mistaken for a wide range of conditions of the heart, lungs and anatomy of the chest. In some people, even an anxiety attack can cause worrying symptoms. Giannini said that what Hurts is going through serves as a reminder for people to be aware of their bodies and seek medical help if they have concerns.

"It stresses the importance of listening to your body and listening to your symptoms," she said. "And if you feel like something's wrong with respect to breathing, chest pain, swelling of the leg, those types of things — pulmonary embolus is something that is more common than one might think."