A new form of dementia has been identified by scientists that mimics Alzheimer's but affects the brain in a completely different way.
Lack of understanding about the disorder, given the name Late, could be one reason why so many trials of Alzheimer's drugs have failed, according to the researchers.
Late stands for limbic-predominant age-related TDP-43 encephalopathy.
Studies suggest it is caused by an abnormal version of the TDP-43 protein, which in its healthy form helps regulate gene activity in the brain.
New guidelines for recognising and diagnosing the disease are reported in the journal Brain.
Late does not share a common feature of Alzheimer's - beta-amyloid peptide, a harmful protein building block which accumulates in the brain in sticky clumps.
It also has nothing to do with another Alzheimer's hallmark, knots of protein in nerve cells known as tau tangles.
The abnormal "misfolded" protein involved in Late is also thought to play a causative role in motor neurone disease, a relatively rare condition.
But misfolded TDP-43 is very common in older adults. Roughly a quarter of individuals over the age of 85 have enough abnormal TDP-43 to affect memory and thinking ability, say the scientists.
Dr Peter Nelson, from the University of Kentucky, who co-chaired the group of international experts who drew up the Late guidance, said: "More than 200 different viruses can cause the common cold, so why would we think there is just one cause of dementia?
"Late probably responds to different treatments than AD (Alzheimer's disease), which might help explain why so many past Alzheimer's drugs have failed in clinical trials.
"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."
The group established that Late progresses more gradually than Alzheimer's and tends to affect people near the end of their lives - the "oldest of the old".
However when Late occurred in combination with Alzheimer's - which was common - the two diseases together caused a more rapid decline than either would alone.
Dr James Pickett, head of research at Alzheimer's Society, said: "Dementia is an extremely complex condition that may be caused by many different underlying diseases. Though at an early stage, this research is taking a real step forward by proposing a new sub-type of dementia.
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