WASHINGTON An experimental treatment for multiple sclerosis has caused death, strokes, nerve damage and abdominal bleeding and has no proven benefits for sufferers of the disease, the Food and Drug Administration warned Thursday.

Known as liberation therapy, the treatment targets chronic cerebrospinal venous insufficiency or CCSVI a narrowing of the veins in the head and neck. It involves inserting balloons or stents into veins to widen them in an attempt to relieve the symptoms of MS.

The FDA received reports in 2011 of a patient who died from bleeding in the brain after undergoing the treatment and another who was left permanently paralyzed by a stroke. Those incidents prompted the formal warning.

Other serious adverse effects from the procedure have been reported in medical journals.

The FDA has not approved any devices for use in liberation therapy, but doctors can offer the procedure to their patients. Doctors and other experts estimate that thousands of Americans have had the treatment, but the exact number is not known, because there is no central registry of patients.

Tim Coetzee, chief researcher for the National Multiple Sclerosis Society, said the link between the condition and MS was not yet well understood, nor were the potential side effects of liberation therapy.

"We're still determining the risks associated with this," he said.

MS is an autoimmune disease with a wide range of symptoms that include physical disability and chronic pain. Drugs can treat the symptoms, but there is no cure.

Paolo Zamboni, an Italian researcher, first proposed the link between CCSVI and MS in 2009. Zamboni published research showing that poor blood drainage from the brain and spinal cord aggravated nerve damage caused by MS. Surgically widening veins could help blood flow, he argued, alleviating symptoms of the disease.

Zamboni's findings seemed to promise an effective treatment for MS and generated widespread interest among patients and doctors. Subsequent research tempered the enthusiasm, and the potential of liberation therapy is still being investigated.

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FDA warns against multiple sclerosis treatment