TUESDAY, April 15 (HealthDay News) -- The root cause of early-onset dementia is usually not Alzheimer's, but rather another neurodegenerative or autoimmune disorder, new research suggests.

The study authors acknowledge that -- age aside -- the most common forms of dementia are Alzheimer's disease, vascular dementia and the brain damage-associated condition known as Lewy body dementia. However, their current work indicates that among patients below the age of 45, the problem is much more likely to be traced back to diseases such as multiple sclerosis, Huntington's, lupus or HIV infection, among others.

"This is really a novel finding, because there hasn't really been a study that's looked at young-onset dementia in this way," said study author Dr. Brendan J. Kelley, a neurologist at the Mayo Clinic in Rochester, Minn. "And the message is that young-onset dementia is generally not related to Alzheimer's."

The work of Kelley and his team was expected to be presented April 15 at the American Academy of Neurology annual meeting, in Chicago.

The U.S. Administration on Aging highlights 2006 estimates released by the Alzheimer's Association, which indicate that between 220,000 and 640,000 American men and women currently suffer from early-onset dementia. The association specifically defines "early-onset Alzheimer's" as referring to cases that develop before the age of 65.

However, in their study, Kelley and his team focused exclusively on 235 patients diagnosed with a form of dementia diagnosed between the ages of 17 and 45 -- citing statistics suggesting that 12 in 100,000 people develop some form of early-onset dementia before the age of 45.

All the study patients had sought care at the Mayo Clinic between 1996 and 2006, and all had normal cognitive function prior to their dementia diagnosis.

A medical record analysis revealed that despite the fact that most adult dementia is a function of Alzheimer's, less than 2 percent of the cases among the under-45 group was attributable to that disease.

Kelley and his colleagues found that other neurodegenerative conditions -- such as frontotemporal dementia, a group of diseases commonly misdiagnosed as Alzheimer's -- were at play in almost one-third of the cases.

Autoimmune and inflammatory disorders -- such as MS -- accounted for just over 20 percent of the dementia cases. Metabolic abnormalities were cited in just over 10 percent of the diagnoses, while for another 20 percent, no cause for dementia could be established.

Kelley said his work is ongoing. And he added that he and his colleagues are now trying to identify specific disease markers for early-onset dementia to help physicians distinguish those cases prompted by causes other than Alzheimer's.

"Because some of the other disorders linked to early dementia have treatable profiles that allow targeting not just of the symptoms but of the underlying disease process," he noted. "So, we really should be looking to identify them quickly when they are the cause, because the research suggests that treatment could result in a direct improvement of the patient's cognition and behavior."

Greg M. Cole, associate director of the Alzheimer's Disease Research Center at the UCLA David Geffen School of Medicine, described the findings as "interesting, but not completely unexpected".

"We know that Alzheimer's gets rarer and rarer the younger you go," he said. "So, when you're focused as this study is on people between 17 and 45 -- really before middle-age -- it's more likely you'll find some other cause for the dementia, which can be a variety of different things."

"But if you're looking at these other autoimmune causes -- multiple sclerosis, lupus, HIV -- the real question is, can you treat any of this?," pondered Cole. "Because you can get lupus and MS to go into remission. So, in this case, if patients are getting dementia caused by either disease, can the dementia also go into remission? If they can get that to happen, that would be very interesting."

More information

For additional information on early-onset Alzheimer's, visit the U.S. Administration on Aging.