Three men with heart failure, caused by the build-up of sticky, toxic proteins, are now free of symptoms after their condition "spontaneously reversed in an unprecedented case", according to researchers from the UK.

Researchers from the University College London and the Royal Free Hospital said that the three men, aged 68, 76, and 82, were diagnosed with transthyretin cardiac amyloidosis -- a form of amyloidosis affecting the heart.

It is a progressive condition and has until now been seen as irreversible, with half of patients dying within four years of diagnosis.

However, in a study, published as a letter in The New England Journal of Medicine, they reported that the men's symptoms improved.

The three men's recoveries were confirmed via blood tests, several imaging techniques including echocardiography (a type of ultrasound), and cardiovascular magnetic resonance (CMR) scans showing that the build-up of amyloid proteins in the heart had cleared.

CMR scans showed heart structure and function had returned to a near-normal state and amyloid had almost completely cleared. Further, scintigraphy (a nuclear medicine bone scan), and, for one patient, an assessment of exercise capacity also proved the improvement.

"We have seen for the first time that the heart can get better with this disease. That has not been known until now and it raises the bar for what might be possible with new treatments," said lead author Professor Marianna Fontana from UCL's Division of Medicine.

The researchers also found evidence of an immune response in the three men that specifically targeted amyloid. The amyloid-targeting antibodies were not found in other patients whose condition progressed as normal.

"Whether these antibodies caused the patients' recovery is not conclusively proven. However, our data indicate that this is highly likely and there is potential for such antibodies to be recreated in a lab and used as a therapy," said Professor Julian Gillmore, head of the UCL Centre for Amyloidosis, based at the Royal Free Hospital.

"We are currently investigating this further, although this research remains at a preliminary stage," he added.

If these antibodies could be harnessed, they could be combined with new therapies being trialled that suppress TTR protein production, enabling clinicians to clear away amyloid as well as preventing further amyloid deposition, the researchers said.