You may have heard the phrase “the eyes are windows to the soul”.Recent research has revealed that our eyes may also be windows to our kidneys. At Edinburgh Kidney, we are investigating how subtle changes in the tiny blood vessels of the eye, known as the retinal vasculature, can provide important information about kidney disease.

Kidney disease is a major health concern affecting nearly 850 million people worldwide. It often progresses silently until it reaches an advanced stage which may ultimately require dialysis or transplantation. Kidney disease also affects other organs in our body and is strongly associated with heart disease. Early detection is crucial because it allows for interventions that can slow kidney disease progression, decrease the risk of heart problems and improve quality of life.

Typically, medical professionals look at blood tests to measure creatinine levels and urine tests to detect the presence of proteins to see how well the kidneys function. However, these methods often miss early signs of kidney disease. One often-missed early marker is the dysfunction and damaging of blood vessels in the kidneys, which is thought to occur at the earliest stages of the disease. That is where the eye comes into play.

The retina is the light-sensitive layer at the back of the eye. Its tiny blood vessels are easily visible through a routine eye examination. In fact, retinal blood vessels are the only ones that we can see directly in the entire body, using specialized imaging equipment available in most optician practices or clinics. Advances in imaging technology mean we can now capture high-resolution images of the retinal vasculature in a simple, rapid and noninvasive way. The blood vessel networks in the retina and the kidney have several developmental, structural and functional similarities suggesting that the eye may serve as a proxy for the kidney. In other words, looking into the eye could tell us important information about kidney vascular health.

Recent studies have shown that certain changes in the retinal blood vessels correlate with reduced kidney function. For example, narrowed blood vessels in the retina have been associated with higher levels of protein in the urine, which is a common sign of kidney damage. Similarly, tiny hemorrhages in the retina have been linked to decreased kidney function. My research group has found that people with chronic kidney disease have a thinner retina and choroid (a layer of blood vessels behind the retina) compared with healthy volunteers, and this was associated with decreased kidney function.

Scientists aren’t sure what connects the eye to the kidney, but it’s a new and exciting area of research. Exploring the cellular and molecular factors that cause retinal changes in people with kidney disease may allow us to identify common denominators that cause both kidney and retinal vascular abnormalities. These findings could pave the way for new therapeutic approaches to preserve blood vessel health in people with kidney disease.