For the parents of Jameson Finley, there was one thing more alarming than when he coughed up branch-shaped chunks of a substance with the consistency of string cheese.

The thought of what would happen if he did not cough them up.

The boy, then 5, suffered from a condition called plastic bronchitis, which can lead to asphyxiation, pneumonia, and death. It can strike children who have undergone certain complex heart surgeries, as Jameson had, and it occurs on rare occasions in noncardiac patients - some of whom go years before getting the correct diagnosis.

But even physicians who diagnose it properly have struggled to find an effective treatment.

Now, a pair of physicians at the Hospital of the University of Pennsylvania and Children's Hospital of Philadelphia say they have cracked the case.

In the journal Circulation, they reported substantial improvement in 15 out of 17 children who underwent a procedure to block abnormal flow of lymphatic fluid into the lungs. The physicians who developed the technique, Maxim Itkin and Yoav Dori, say they have had similar results in adults.

Clinics from around the world have started to send them patients, at a rate of one or two a week - from the Middle East, Europe, and South America, among other locales.

The exact procedure varies, depending on the case, but generally the team blocks abnormal flow in one or more of the patient's lymphatic ducts by inserting a small metal coil that is then coated with medical-grade glue. Some patients, such as Jameson Finley, are seen at CHOP; others at Penn.

Pulmonologist Frank McCormack, a professor at the University of Cincinnati College of Medicine, said he knew of no other such program.

"Philadelphia should be proud," said McCormack, who has referred two patients to the team. "This is a one-of-a-kind thing."

The main reason the field of medicine has only now successfully tackled the disease is because it involves the lymphatic system - a complicated and poorly understood network of vessels that gets short shrift in medical schools.

Until recently, there was no straightforward way to look at how fluid flowed through these vessels, which, among other functions, serve as a highway for the immune system. The primary imaging method - injecting contrast dye into small lymphatic vessels in the foot - was technically challenging and yielded poor resolution.

"The lymphatic system has been under shadow for almost 50 years," said Itkin, an interventional radiologist at Penn.

But several years ago, Itkin came up with another way: injecting a type of contrast agent into a lymph node in the groin. He then was able to watch the flow of lymphatic fluid using X-ray imaging equipment.

Itkin and Dori, a cardiologist at CHOP, later modified the technique for use with an MRI machine.

The two physicians had met while playing recreational basketball. Dori had told Itkin about his heart-surgery patients who coughed up the whitish substance, and the two surmised that leaking lymphatic fluid might be the culprit.

The new imaging technique proved them correct.

In healthy people, the lymphatic vessels receive the clear fluid, called lymph, from tissues throughout the body. The fluid is channeled into a primary lymphatic duct that drains into a vein near the neck.

But in some heart-surgery patients, lymphatic fluid can flow the wrong way through branches surrounding the lungs, leaking into the bronchial tubes, where it hardens into rubbery "casts."

"You cough up this Jell-O mold of your airways, basically," said Cincinnati's McCormack.

The reason is not fully understood, but Itkin says it seems related to the fact that such patients have higher pressure in their veins, forcing the lymph into the lungs. The lymphatic ducts in such patients may also have an abnormal architecture, he said.

Jameson Finley, born with a severely underdeveloped heart, seemed to be on the mend after the third of three surgeries.

But about a year after the last surgery - a procedure called the Fontan - the boy started coughing up one or two of the casts each day, said his father, Todd, of McKinney, Texas. Eventually, the number rose to four or five.

"It feels like a strong, cooked spaghetti," Todd Finley said. "They look like a white tree branch without leaves."

The young boy grew increasingly exhausted. Todd Finley and his wife, Laura-Marie, who had learned about Itkin's work online, brought him to Philadelphia in 2013 for the procedure to block the abnormal flow.

It involves inserting a catheter through the abdomen into the main lymphatic duct, up to the site of any branches that are leaking fluid into the lungs. Each leak is sealed by inserting a coil, followed by the injection of glue.

"The coil is a matrix for the glue to grab," Itkin said.

In Jameson's case and in most other heart patients since then, that does the trick. The Texas boy, now 8, has coughed up no more casts and recently finished his first season of Little League baseball.

In Itkin and Dori's study, 10 of the 17 patients had complete success like Jameson's, while five more improved but continued to cough up occasional casts. Of the two who did not see improvement, one seemed to get the casts for non-lymphatic reasons, and the other eventually needed a heart transplant.

The main side effects of the procedure were abdominal pain and low blood pressure, both temporary.

Plastic bronchitis affects about 5 percent of patients who have undergone heart surgeries such as Jameson's. It is unclear how often a lymphatic abnormality leads to the condition in noncardiac patients, though it is rare.

In some cases, patients are simply told they have asthma. That happened to Joy Dumire of Adrian, Mich., founder and president of the nonprofit Plastic Bronchitis Foundation.

Dumire said one physician prescribed steroids and did not seek the underlying cause, despite her description of the branchlike "plugs" she coughed up.

"I asked if I could bring in some, and he said 'No, I don't need to see it,' " Dumire recalled.

She eventually found her way to Philadelphia and underwent successful treatment at Penn, though it took several attempts. Dumire, who is in her 50s, said the condition appeared to have arisen in the aftermath of receiving radiation and surgery to treat Hodgkins lymphoma.

Itkin thinks that plastic bronchitis is the first of many lymphatic disorders that will prove to be treatable. The key is the imaging, which reveals the ebbs and flows of the hitherto mysterious vessels.

"It opens up a whole new world of diseases," he said.