Researchers have found that the "younger elderly" in their 60s and 70s showed faster rates of brain tissue loss and cognitive decline than Alzheimer patients who were over 80 years old.
The greatest risk factor for Alzheimer's disease (AD) is advancing age. By age 85, the likelihood of developing the dreaded neurological disorder is roughly 50 percent.
The findings by researchers at the University of California, San Diego School of Medicine have profound implications for both diagnosing AD - which currently afflicts an estimated 5.6 million Americans, a number projected to triple by 2050 - and efforts to find new treatments.
There is no cure for AD and existing therapies do not slow or stop disease progression.
"One of the key features for the clinical determination of AD is its relentless progressive course," Dominic Holland, the study's first author, said.
"Patients typically show marked deterioration year after year. If older patients are not showing the same deterioration from one year to the next, doctors may be hesitant to diagnose AD, and thus these patients may not receive appropriate care, which can be very important for their quality of life," he said.
Holland and colleagues used imaging and biomarker data from participants in the Alzheimer's Disease Neuroimaging Initiative, a multi-institution effort coordinated at UC San Diego.
They examined 723 people, ages 65 to 90 years, who were categorized as either cognitively normal, with mild cognitive impairment (an intermediate stage between normal, age-related cognitive decline and dementia) or suffering from full-blown AD.
"We found that younger elderly show higher rates of cognitive decline and faster rates of tissue loss in brain regions that are vulnerable during the early stages of AD," Holland said.
"Additionally cerebrospinal fluid biomarker levels indicate a greater disease burden in younger than in older individuals," he said.
Holland said it's not clear why AD is more aggressive among younger elderly.
"It may be that patients who show onset of dementia at an older age, and are declining slowly, have been declining at that rate for a long time," Linda McEvoy, co-author of the study, said.
"But because of cognitive reserve or other still-unknown factors that provide 'resistance' against brain damage, clinical symptoms do not manifest till later age," she said.
Another possibility, according to Holland, is that older patients may be suffering from mixed dementia - a combination of AD pathology and other neurological conditions.
These patients might withstand the effects of AD until other adverse factors, such as brain lesions caused by cerebrovascular disease, take hold. At the moment, AD can only be diagnosed definitively by an autopsy.
"So we do not yet know the underlying neuropathology of participants in this study," Holland added.
The study has been published online in the journal PLOS One.