The patients' brains had the familiar spongy tissue make-up
A new form of Creutzfeldt-Jakob disease (CJD) may have been uncovered in a handful of patients in the US.
Ten people have so far died from a fast-advancing form of fatal dementia called PSPr, New Scientist reports.
Patients develop the trademark brain damage associated with CJD - the type not linked to BSE - but scientists believe there may be a genetic cause.
Experts in the UK are now checking records to see if any cases have happened across the Atlantic.
There are between 50 and 100 new cases of so-called sporadic CJD diagnosed in the UK every year.
Unlike "variant CJD", the human form of BSE in cows contracted by eating contaminated brain tissue in the 1980s and 1990s, the cause of most cases of sporadic CJD is unknown.
The new cases were referred to CJD surveillance units in the US because they were a suspiciously fast-advancing form of dementia with additional symptoms such as the loss of the ability to speak and move, even though traditional tests that normally help diagnose CJD proved negative.
Post-mortems on those who died revealed the familiar "spongy" brain tissue, covered with tiny holes.
These are thought to be caused by the accumulation of "prions", a misshapen version of a normal brain protein.
Dr Pierluigi Gambetti, director of the US National Prion Disease Pathology Surveillance Center, in Ohio, said that he believed the newly discovered type had probably "been around for years, unnoticed".
He suggested one interesting common factor was that the patients came from families with a history of dementia, suggesting a genetic cause, but did not carry the gene traditionally associated with a small number of sporadic CJD cases.
Dr Mark Head, from the UK's National CJD Surveillance Unit, in Edinburgh, said the finding had prompted scientists to start reviewing cases of sporadic CJD in this country to see if there were any of the newly discovered version.
He said: "What is interesting about this is that it may mean there are other genes out there waiting to be found which are associated with prion disease, and looking at these patients in the US could help find them."