The eyes may be the key to diagnosing Alzheimer's disease earlier and more accurately.
Researchers found that loss of cells in the eye's retina may reveal information about whether a person has the neurodegenerative disease and how far it has progressed.
The retina is the layer of tissue found at the back of the eye that converts images from the eye's lens to electric signals before sending them via the optic nerve to the brain, helping us see.
The researchers pointed out that other studies have suggested that glaucoma -- a group of diseases that damage the eye's optic nerve and lead to vision loss and blindness -- may actually be a neurodegenerative disease similar to Alzheimer's.
"The retina is an extension of the brain so it makes sense to see if the same pathologic processes found in an Alzheimer's brain are also found in the eye," co-author Dr. R. Scott Turner, director of the Memory Disorders Program at Georgetown University Medical Center in Washington, said in a press release. "We know there's an association between glaucoma and Alzheimer's in that both are characterized by loss of neurons, but the mechanisms are not clear."
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Prior research on the connection between the two diseases focused on the retinal ganglion cell layer, the part specifically responsible for sending visual information through the optic nerve to the brain.
But, the researchers wanted to focus on the inner nuclear layer which hasn't been studied in connection to the neurodegenerative disease. This layer is responsible for collecting the visual sensory messages that are then sent on to the retinal ganglion cells.
For this study, scientists looked at the thickness of the retinal ganglion cell layer and the inner nuclear layer in mice who had been genetically engineered to have Alzheimer's disease.
They found that those who had Alzheimer's had a significant thinning of both the overall retina and the inner nuclear layer. In mice with Alzheimer's, the retinal ganglion cell layer had a 49-percent loss of nerve cells and the inner nuclear layer had a 37 percent loss, compared to what was seen in healthy mice of the same age.
Retinal thickness can easily be measured in humans using a new process called "optical coherence tomography." Turner said he hopes that this method can be used to do further research and provide a possible test.
"This study suggests another path forward in understanding the disease process and could lead to new ways to diagnose or predict Alzheimer's that could be as simple as looking into the eyes," Turner said.
He added that if Alzheimer's and glaucoma were indeed related, this could also mean that treatments for one disease could potentially work for the other.
The results were presented at the U.S. Society for Neuroscience meeting in San Diego are considered preliminary until they're published in a peer-reviewed journal.
Laura Phipps, science communications manager at the charity Alzheimer's Research UK, told the BBC that she was happy that more research was being done looked at the connection between eye disease and Alzheimer's. She however cautioned that this was just an early stage study conducted only on mice, so further research on humans has to be completed before recommendations can be made.
"Diagnosing Alzheimer's with accuracy can be a difficult task, which is why it's vital to continue investing in research to improve diagnosis methods," she said.
Alzheimer's is the sixth leading cause of death in the U.S. and the most common form of dementia, according to the Alzheimer's Association. More than 5 million people in the U.S. are currently living with Alzheimer's. One in three seniors die with dementia.
Other novel Alzheimer's tests in development include a method created by the University of Florida's McKnight Brain Institute Center for Smell and Taste. Researchers there developed a tool using a scoop of peanut butter and a ruler. People who had more difficulty smelling the peanut butter at shorter distances were linked to a higher incidence of the neurodegenerative disease.