It's well-known that diseases of the vascular, or circulatory, system increase the risk of cognitive impairment as we age. This system includes veins, arteries, blood vessels and capillaries that carry blood to and from the heart.
"We cannot see the very small blood vessels in the brain through standard brain imaging techniques" like magnetic resonance imaging, said lead study author Jennifer A. Deal, an assistant scientist in the Department of Epidemiology at Johns Hopkins Bloomberg School of Public Health.
Because blood vessels in the eye and brain are similar, Deal and her fellow researchers hypothesized that looking at blood vessels in the eye might explain what was happening in the brain.
Peering into the future
The study began with more than 12,000 men and women at an average age of 57 who were tested for their memory and thinking skills. The participants took another round of exams about six years later and a third round about 20 years after the initial test.
About three years after the study began, the researchers used a special camera to take photos of each participant's retina, a layer of tissue at the back of the eyes through which light signals are converted to impulses that pass into the brain, enabling us to see.
Deal explained that these photos provided "a snapshot of what is going on in the microvasculature in the eye (and also, we think, the brain) at that point in time, and from that one measure, we can determine if someone has retinal signs indicative of retinopathy," damage to the retinal blood vessels.
Though most of the participants (95%) showed no signs of damage to retinal blood vessels, 365 people, or 3%, had mild retinopathy. Another 256 people, or 2%, had moderate to severe retinopathy.
Analyzing the data, the team found that participants with moderate to severe retinopathy were more likely to score significantly lower on memory and thinking tests than healthy-eyed participants. In fact, these participants saw their average test scores reduced by 1.22 standard deviation units over 20 years; people with healthy eyes saw declines of only 0.91 standard deviation units. Excluding anyone who missed some of the tests, the researchers calculated a 0.57 standard deviation unit difference between the two groups.
"Although our results were stronger in people with diabetes, we also found a strong and significant relationship of retinal signs and cognitive decline in people who did not have diabetes," Deal said.
Unhealthy eyes, unhealthy brain
Dr. Rachel Bishop of the federal National Eye Institute said the study results are "not a surprise at all."
"If the retinal blood vessels are unhealthy, there's every reason to think that the brain blood vessels are unhealthy as well," said Bishop, who was not involved in the new study.
"The blood vessel supply is essential to all function, the function of all organs, and so if the blood vessels are unable to do their job, there's no way that the brain can be functioning as well as a brain that has a good supply."
Bishop said she likes the idea of screening the eye and retina for negative conditions in the brain, "even if I don't know quite where it takes us. I think I share a common hope that we could detect things early enough and have interventions early enough to change the course of a negative event."
After all, she said, the list of diseases and conditions that can be identified by the health of the eye is long -- and growing.
Dr. Justin Bazan, an optometrist and medical adviser to the nonprofit The Vision Council, agrees that it is important to study the blood vessels in the eye.
"They do have a direct link and correlation to systemic disease," he said. As we learn more about the changes that occur with disease, he said, we will be better able to predict cardiovascular events, such as stroke, and "even predict mental changes found in conditions such as Alzheimer's."
People usually come in for an eye exam "just to perhaps check their vision or get contacts or glasses," Bazan said. However, eye doctors can tell them more about their general health.
Diabetes, hypertension, high cholesterol
"Diabetes can be seen in the eyes," Bazan said. Early changes in people with diabetes include fluctuations in vision.
"They may experience periods where their blood sugar is out of control, which causes changes to the lens inside the eye, and when you cause changes to the lens, it ultimately impacts vision," he explained. Specifically, a person with diabetes may have blurry vision for a period, and then it returns to normal.
Sometimes, these changes can be attributed to general aging, but the reality could be "uncontrolled systemic disease" such as diabetes, Bazan said.
High blood pressure -- too much pressure in the blood vessels -- is also "easily observed in the eye," he said. The damaged blood vessels cause hemorrhages and leaking, both visible during routine eye exams.
"High cholesterol is one of the easiest things to pick up in an examination because it's on the front of the eye" or the clear structure known as the cornea, on which a contact lens would sit, Bazan said. He explained that "not a day goes by in my office where we don't detect a buildup of cholesterol."
Sometimes, heart conditions caused by a buildup of plaque will cause concern during an eye exam.
When plaque builds up in the carotid artery, Bazan explained, "sometimes, those little plaques will break off and then travel into the eye, where they clog arteries in the eye, and that leads to very obvious changes to the vascular structure in the back of the eye." A diagnosis would not be made from this little bit of evidence, but the doctor would recommend more tests, including an MRI, to sort out what might be happening.
MS, STDs, thyroid disease and cancer
Multiple sclerosis is often detected when patients have sudden vision loss, Bazan said. Looking into an MS patient's eye, the doctor would see a change in the color and appearance of the optic nerve. This too would lead to more thorough testing to confirm the diagnosis of MS.
Sexually transmitted diseases also make their presence known in the eyes.
"My first patient in optometry school had conjunctivitis," Bazan explained, yet the condition was not cured by standard eye drops. This "unresolved conjunctivitis, or pinkeye, that didn't go away that was unresponsive to typical eye drops used to treat it" was a sign of a bigger problem -- in this case the STD chlamydia, which was found only after further tests.
Different types of herpes infection may also cause conjunctivitis. "I've also seen syphilis," Bazan said, explaining that this STD can cause the pupil to change to "an argyle or gray color." HIV, due to its impact on white blood cells, will also cause visible changes in the retina.
Thyroid disease has several ways of making itself known via the eyes. Because the thyroid controls the hormones responsible, in part, for producing teardrops, many thyroid patients develop dry eye disease, Bazan explained.
Yet thyroid disease also causes the extraocular muscles -- the muscles that control eye movement -- to become enlarged and stiffened.
"So if you've ever seen someone who has bulging eyes, that bulge may be linked to Graves' disease, which is related to thyroid disorder," Bazan said, naming former first lady Barbara Bush as an example of someone with this condition.
"If you have a condition where there is systemic inflammation, it is almost always going to manifest in the eye as uveitis
," he said. Uveitis, red and swollen eyes, would occur as a result of lupus and other autoimmune diseases. In fact, any systemic condition that causes inflammation in the body would cause the same inflammation in the eyes.
Finally, cancers, including of the breast, leukemia and lymphoma, may signal their presence in the eyes, even if most people will detect tumors and symptoms of malignancy before their eye doctors do.
Cancer of the basal cells in the sensitive skin surrounding the eye is one of the most common forms of cancer detected, Bazan said. And ocular melanoma -- cancer in the cells of the eye -- is not as common, but it "definitely does occur, and it is devastating."
"I think the main point is that going to your eye doctor for your annual eye exam is not just about getting your vision checked or getting glasses or contacts," he said. "It's about finding out about the overall health of your eye and in turn the overall health of you."