The disease, called LATE, may often mirror the symptoms of Alzheimer's disease, though it affects the brain differently and develops more slowly than Alzheimer's. Doctors say the two are frequently found together, and in those cases may lead to a steeper cognitive decline than either by itself.
In developing its report, the international team of authors is hoping to spur research -- and, perhaps one day, treatments -- for a disease that tends to affect people over 80 and "has an expanding but under-recognized impact on public health," according to the paper.
"We're really overhauling the concept of what dementia is," said lead author Dr. Peter Nelson, director of neuropathology at the University of Kentucky Medical Center.
Still, the disease itself didn't come out of the blue. The evidence has been building for years, including reports of patients who didn't quite fit the mold for known types of dementia such as Alzheimer's.
"There isn't going to be one single disease that is causing all forms of dementia," said Sandra Weintraub, a professor of psychiatry, behavioral sciences and neurology at Northwestern University Feinberg School of Medicine. She was not involved in the new paper.
Weintraub said researchers have been well aware of the "heterogeneity of dementia," but figuring out precisely why each type can look so different has been a challenge. Why do some people lose memory first, while others lose language or have personality changes? Why do some develop dementia earlier in life, while others develop it later?
Experts say this heterogeneity has complicated dementia research, including Alzheimer's, because it hasn't always been clear what the root cause was -- and thus, if doctors were treating the right thing.
What is it?
The acronym LATE stands for limbic-predominant age-related TDP-43 encephalopathy. The full name refers to the area in the brain most likely to be affected, as well as the protein at the center of it all.
"These age-related dementia diseases are frequently associated with proteinaceous glop," Nelson said. "But different proteins can contribute to the glop."
In Alzheimer's, you'll find one set of glops. In Lewy body dementia, another glop.
And in LATE, the glop is a protein called TDP-43. Doctors aren't sure why the protein is found in a modified, misfolded form in a disease like LATE.
"TDP-43 likes certain parts of the brain that the Alzheimer's pathology is less enamored of," explained Weintraub, who is also a member of Northwestern's Mesulam Center for Cognitive Neurology and Alzheimer's Disease.
"This is an area that's going to be really huge in the future. What are the individual vulnerabilities that cause the proteins to go to particular regions of the brain?" she said. "It's not just what the protein abnormality is, but where it is."
More than a decade ago, doctors first linked the TDP protein to amyotrophic lateral sclerosis, otherwise known as ALS or Lou Gehrig's disease. It was also linked to another type of dementia, called frontotemporal lobar degeneration.
LATE "is a disease that's 100 times more common than either of those, and nobody knows about it," said Nelson.
The new paper estimates, based on autopsy studies, that between 20 and 50% of people over 80 will have brain changes associated with LATE. And that prevalence increases with age.
Experts say nailing down these numbers -- as well as finding better ways to detect and research the disease -- is what they hope comes out of consensus statements like the new paper, which gives scientists a common language to discuss it, according to Nelson.
"People have, in their own separate bailiwicks, found different parts of the elephant," he said. "But this is the first place where everybody gets together and says, 'This is the whole elephant.' "
What this could mean for Alzheimer's
The new guidelines could have an impact on Alzheimer's research, as well. For one, experts say some high-profile drug trials may have suffered as a result of some patients having unidentified LATE -- and thus not responding to treatment.
In fact, Nelson's colleagues recently saw that firsthand: a patient, now deceased, who was part of an Alzheimer's drug trial but developed dementia anyway.
"So, the clinical trial was a failure for Alzheimer's disease," Nelson said, "but it turns out he didn't have Alzheimer's disease. He had LATE."
Nina Silverberg, director of the Alzheimer's Disease Research Centers Program at the National Institute on Aging, said she suspects examples like this are not the majority -- in part because people in clinical trials tend to be on the younger end of the spectrum.
"I'm sure it plays some part, but maybe not as much as one might think at first," said Silverberg, who co-chaired the working group that led to the new paper.
Advances in testing had already shown that some patients in these trials lacked "the telltale signs of Alzheimer's," she said.
In some cases, perhaps it was LATE -- "and it's certainly possible that there are other, as yet undiscovered, pathologies that people may have," she added.
"We could go back and screen all the people that had failed their Alzheimer's disease therapies," Nelson said. "But what we really need to do is go forward and try to get these people out of the Alzheimer's clinical trials -- and instead get them into their own clinical trials."
Silverberg describes the new paper as "a roadmap" for research that could change as we come to discover more about the disease. And researchers can't do it without a large, diverse group of patients, she added.
"It's probably going to take years and research participants to help us understand all of that," she said.
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