The first signs of Alzheimer's appear in the eyes. Here's what they are

The eyes are more than a window to the soul; they're also a reflection of a person's cognitive health. 

"The eye is the window into the brain," ophthalmologist Dr. Christine Greer, Director of Medical Education at the Institute of Neurodegenerative Diseases in Boca Raton, Florida told CNN. "You can see directly into the nervous system by looking into the back of the eye, toward the optic nerve and retina."

This scientific assumption led to a recent study that looked at how the eye might help diagnose Alzheimer's disease before symptoms start. 

Dr. Richard Isaacson, a neurologist who deals with Alzheimer's prevention and also works at the IND, said that Alzheimer's begins in the brain decades before the first symptoms of memory loss. Isaacson added that if doctors are able to detect the disease in its earliest stages, people will be able to choose a healthy lifestyle and control it by making dietary changes to mitigate risk factors like high blood pressure, high cholesterol and diabetes.

How early can signs of cognitive decline be detected? 

To find out, researchers examined tissues donated from the retinas and brains of 86 people with varying degrees of cognitive decline.

The study's author, Professor Maya Koronyo-Hamaoui, an expert in neurosurgery and biomedical sciences at Cedars-Sinai Medical Center Los Angeles, told the press that this study is the first to provide in-depth analysis of the protein profiles and the molecular, cellular and structural effects of Alzheimer's disease in the human retina and how they correspond with changes in the brain and cognitive function. 

She added that these changes in the retina correlate with changes in parts of the brain called the entorhinal and temporal cortices, which are the centers for memory, navigation and time perception.

They then compared samples from donors with normal cognitive function to those with mild cognitive impairment or late-stage Alzheimer's disease.

The earlier it is, the more accurate it will be

The study, published in the journal Acta Neuropathologica, found significant increases in beta-amyloid protein, a key marker for Alzheimer's disease in people with Alzheimer's and those with early cognitive decline. 

Also, microglial cells decreased by 80% in those with cognitive problems, the study found. These cells are responsible for repairing and maintaining other cells, including clearing beta-amyloid from the brain and retina.

Tissue atrophy and inflammation in cells in the far periphery of the retina were the biggest predictors of future cognitive status, the study found.

Isaacson said that these findings may eventually lead to the development of imaging techniques that will allow doctors to diagnose Alzheimer's disease earlier and more accurately, and monitor its progress non-invasively by looking into one's eyes.