Alzheimer’s disease and related dementias impact about 5.8 million people in the U.S., but properly diagnosing the condition is tricky. Now, a new study finds that early symptoms of Alzheimer’s may actually show up in the eyes.

The study, which was published in the journal Acta Neuropathologica, analyzed donated tissue from the retina (the light-sensitive layers of nerve tissue at the back of the eye) and brains of 86 people with different degrees of mental decline—the largest study on retinal samples and dementia conducted so far, according to the researchers. This tissue was then compared to tissue from donors who had normal cognitive function.

The researchers found there were increases in beta-amyloid, a hallmark of Alzheimer’s disease, in the tissue of people who had either Alzheimer’s disease or early cognitive decline. Researchers also discovered that microglial cells, which repair and maintain other cells and clear beta-amyloid from the brain and retina, declined by about 80% in people who had cognitive issues.

The researchers concluded that the findings “may lead to reliable retinal biomarkers for noninvasive retinal screening and monitoring of Alzheimer’s disease.”

The study raises a lot of questions about a possible eye test for Alzheimer’s disease—whether it may be coming and why looking at the eyes may help diagnose the condition. Here’s the deal, according to doctors.

Why might Alzheimer’s disease symptoms appear in the eyes?

This actually isn’t the first study to find a connection between Alzheimer’s disease and the eyes. In fact, the American Academy of Ophthalmology (AAO) says online that studies show a clear relationship between brain tissue and eye tissue.

Several studies have shown changes in the retina in people with Alzheimer’s or other forms of dementia—specifically, changes in the layers of the retina or the blood flow within the eye. Another study using fluorescence lifetime imaging ophthalmoscopy (FLIO) has also found that the imaging technique can measure beta-amyloid in the retina. Research has determined, too, that there are changes in eye tissue in patients who have brain diseases like Parkinson’s disease and mad cow disease, the AAO says.

But why? “The retina is considered an extension of the brain,” says study co-author Maya Koronyo-Hamaoui, Ph.D., a professor of neurosurgery and biomedical sciences at Cedars-Sinai. “It is the only central nervous system organ not encapsulated by bone. Hence, it is easily accessible for visualization directly, non-invasively, and affordably, with high spatial resolution and sensitivity.”

The brain and eye have nerves running between them, explains Amit Sachdev, M.D., M.S., medical director in the Department of Neurology at Michigan State University. “Nerves contain two parts: a body and an axon,” he says. “The body contains all-important parts to regulate the nerves maintenance and growth [and] the axon is like a very long tail.”

Axon are long and, if it’s damaged anywhere, it will begin to degenerate “often from the tip back towards the body,” Dr. Sachdev says. “When you look at the eyes, you are looking at the tips of nerves,” he continues. “The nerve runs all the way to the back of the brain. As you can imagine, brain disease might easily interfere with the health of these very long projections.”

Most diseases that impact the brain have some sort of impact on the optic nerve (the nerve that carries messages from the retina to the brain) or retina, says David J. Calkins, Ph.D., vice-chairman and director for research at The Vanderbilt Eye Institute. “This is because these structures as part of the central nervous system use much of the same molecular machinery used in the brain,” he says. “When things go wrong in the brain, there is typically a sign of this in the retina—at least as seen in post-mortem tissue.”

How is Alzheimer’s disease currently diagnosed?

Before the early 2000s, doctors could only suspect someone has Alzheimer’s disease—the only way to know for certain whether a person had Alzheimer’s disease was from an autopsy performed after the patient died, the National Institute on Aging (NIA) noted.

Now, doctors may perform a series of tests to try to look for markers of Alzheimer’s disease. Those include measuring the levels of proteins associated with Alzheimer’s disease and related dementias after collecting cerebrospinal fluid via a spinal tap, and doing brain scans like computed tomography (CT), magnetic resonance imaging (MRI), or positron emission tomography (PET), to support an Alzheimer’s diagnosis or rule out other possible causes for symptoms, the NIA says.

“We have new drugs for the treatment of Alzheimer’s that are here [but] to make the diagnosis and be eligible for these drugs, invasive and expensive tests are done,” Dr. Sachdev says. “New ways of making the diagnosis are needed.”

So, will there be an eye test for Alzheimer’s disease?

Experts say we’re not there—yet. The study was conducted on post-mortem tissue, Dr. Calkins points out. “It is difficult to draw conclusions about whether we can make strong predictions about cognitive status from simply examining the retina at this time,” he says.

Dr. Sachdev agrees. “Practical implications…there are none yet,” he says.

But Dr. Calkins says this study reinforces that brain health and retinal health are linked. “I am hopeful that, as our molecular toolbox becomes more and more sensitive, we will be able to use an ophthalmological exam as a surrogate to help people identify potential cognitive problems even earlier,” he says.