The U.S. Food and Drug Administration issued a warning about the potential risks of an experimental therapy used to treat a condition -- chronic cerebrospinal venous insufficiency, or CCSVI -- often linked to multiple sclerosis.
CCSVI is characterized by a narrowing of veins in the neck and chest, and some researchers believe that the narrowing can lead to some of the central nervous system inflammation that is a hallmark of multiple sclerosis.
"However, studies exploring a link between MS and CCSVI are inconclusive, and the criteria used to diagnose CCSVI have not been adequately established," the FDA said in a news release.
CCSVI therapy, which has not yet been tested in clinical trials, consists of widening narrowed veins in the chest and neck through the use of balloon angioplasty or stents, both commonly used to treat atherosclerosis, or hardening of the arteries. The intervention is sometimes known as "liberation therapy," or a "liberation procedure," the agency said.
But the FDA added that so far, it has not approved the use of balloon angioplasty devices or stents to treat CCSVI. It also encouraged clinical trials that could solidify the link between the two conditions, and urged patients to discuss the risks and benefits of CCSVI therapy with their doctors.
CCSVI was first identified in 2009 by Dr. Paolo Zamboni, an Italian vascular surgeon. He later tried inflating the veins using the balloon procedure in 65 patients and, despite the fact that the study was unblinded and had no placebo group, Zamboni found some improvement in MS symptoms among most of them.
Sharon Richardson of Arvada, Colo., was one of the first patients to get CCSVI therapy in the U.S.
After she had her therapy in 2009, she told ABC News, some of the persistent pain that plagued her for years not only subsided, but was completely alleviated.
Her eyesight, which had been getting progressively worse, also started to improve.
"It was an enormous quality of life change," Richardson, now 68, said.
She later founded CCSVI Alliance, an organization dedicated to promoting CCSVI education and research. News of the FDA's alert struck her, she said, as "sweet and sour."
"The one thing that's going to be unfortunate is that there are many MS patients that were hoping to get treated in the U.S., and that may have to wait now," she said.
But on the positive side, she added, the FDA alert could bring awareness of CCSVI and lead to more research.
Despite Zamboni's reported success, MS researchers in the U.S. said it's too soon to know for certain how -- and if -- CCSVI and MS are related. Case reports from patients and physicians about how well the therapy works have been mixed.
"We need to define and understand CCSVI first and learn if it's truly related to MS first," said Dr. Robert Fox, medical director of the Mellen Center for Multiple Sclerosis at the Cleveland Clinic. "Is it a separate and distinct entity, and does it have an increased incidence among MS patients?"
Fox is currently involved in research that he hopes will help answer that question.
Fox added that data have been conflicting. Some data have suggested that CCSVI is more common in patients with MS, while other data have not found that to be true.
There have been many patients in the U.S. and Europe who have had CCSVI therapy, though not as part of any clinical trial. Feedback about the therapy, experts said, has been mixed.
"We need to assess whether or not opening up veins has any beneficial effect in changing the course of the disease," said Timothy Coetzee, chief research officer at the National Multiple Sclerosis Society.
"Many people who had the treatment said their fatigue is gone. People who had to stop working were able to go back to work, but others have not seen any improvement," said Richardson.
Fox expressed concern that patients may not fully understand the potential risks of the procedure. The risks include bleeding, blood clots, bruising and infection.
"We have patients who seek out this intervention," Fox said, "but it wasn't clear to me that patients really understood the risks involved, and it wasn't clear to me that patients were reviewing the balance of the data regarding safety and efficacy."
A clinical trial to assess the safety and efficacy of the treatment is about to get underway in Canada, and as research continues, so will the debate over CCSVI.
"There are a variety of opinions about the relationship between CCSVI and MS," said Coetzee. "Some voices will dispute it, some will support it, and some will be in-between. It's a real push-pull between different camps."
ABC News' Katie Moisse contributed to this report.