Scientists have identified a new type of dementia which has symptoms similar to Alzheimer's disease, but affects the brain differently.

The researchers defined diagnostic criteria and other guidelines for advancing future research into this newly-named dementia, called LATE.

In the past, using the terms "Alzheimer's disease" and "dementia" interchangeably was common.

According to Nina Silverberg, director of the Alzheimer's Disease Centers Program at UK National Institute on Aging (NIA), many of the people who enrolled in clinical trials for Alzheimer's drugs likely did not have amyloid -- the sticky substance that gums up neurons and interferes with thinking -- in their brains.

"Recent research and clinical trials in Alzheimer's disease have taught us two things: First, not all of the people we thought had Alzheimer's have it; second, it is very important to understand the other contributors to dementia," Silverberg said in a statement.

For years, members of the scientific community have noticed that a large number of people who died in advanced age had symptoms of dementia without the telltale signs of amyloid or another common culprit, tau, in their brains at autopsy.

Emerging research seemed to indicate that the protein TDP-43 contributed to that phenomenon.

"More than 200 different viruses can cause the common cold. So why would we think there is just one cause of dementia?" said Peter Nelson of the University of Kentucky in US.

LATE, which tends to appear in the oldest-old, may seem the same as Alzheimer's to the lay person, but the disease inside the brain looks very different.

The incidence of LATE is almost as prevalent among the oldest-old as Alzheimer's.

The study, published in the journal Brain, established that like Alzheimer's disease, LATE affects multiple areas of cognition, ultimately impairing activities of daily life, but it appears that LATE progresses more gradually than Alzheimer's.

However, LATE combined with Alzheimer's -- which is a common combination -- appears to cause a more rapid decline than either would alone.

"By developing a sense of scientific focus around these data, we hope to jump-start a broad field of work to advance our understanding of this form of dementia and, ultimately, to open new opportunities for treatment," said Nelson.

Most importantly, it is time to stop thinking of dementia as a "one-size-fits-all" disease.

"LATE probably responds to different treatments than AD, which might help explain why so many past Alzheimer's drugs have failed in clinical trials," Nelson said.

"Now that the scientific community is on the same page about LATE, further research into the 'how' and 'why' can help us develop disease-specific drugs that target the right patients," he said.