Vascular Endothelial Function Testing: A New Frontier in Preventive Eye Care

How retinal vascular findings may help identify systemic cardiovascular risk

Every day in my exam room, I get to do something no other clinician quite can: look directly at living blood vessels, no incision, no dye required for a first look. The retina is the one place in the body where the microcirculation shows itself. And what it shows is often not just about the eye.

When I see arteriolar narrowing, arteriovenous nicking, a small retinal hemorrhage, or a cotton-wool spot, I am looking at vessels — and vessels elsewhere in that same patient are living under the same blood pressure, the same blood sugar, the same cholesterol. Retinal findings like these don't diagnose heart disease. But they are frequently the first visible hint that the vascular system as a whole deserves a closer look.

That is why a recent article from the American Optometric Association caught my attention. In "Endothelial function testing: What doctors of optometry should know," Kerry Gelb, O.D., describes bringing endothelial function testing into his practice for exactly this reason: so many of his patients showed retinal vascular findings that he wanted a way to screen them on the spot, and his results have led to cardiology referrals for patients who might otherwise have waited years for that conversation.

So what is the endothelium, and what does it mean for it to "function"?

The endothelium is the thin layer of cells lining the inside of every blood vessel. It is not wallpaper — it is an active organ. It regulates how vessels tighten and relax, how inflammation behaves, and how blood clots. One of its key tools is nitric oxide, which tells vessel walls to relax and lets blood flow the way it should. When that system falters — less nitric oxide available, more oxidative stress, more inflammation, vessels that react sluggishly — we call it endothelial dysfunction. The important part: it can develop well before a patient feels a single symptom of cardiovascular disease.

How do we measure it? A few ways, and they are not interchangeable:

• Flow-mediated dilation (FMD) uses ultrasound on the brachial artery in the arm. Blood flow is briefly interrupted with a cuff, then released, and the artery's widening in response tells us how endothelium-dependent that vessel is.
• Peripheral arterial tonometry (PAT) reads changes in fingertip pulse amplitude during the same kind of reactive hyperemia — an indirect window on peripheral microvascular function.
• Blood-pressure-based devices — the kind Dr. Gelb describes using — take yet another approach, and other tools use pulse-wave analysis. These differ in what they actually measure and in how well they are validated, so one test's result should never be read as another test's result.

Here is where I want to be careful, because enthusiasm is easy and precision matters. Endothelial function testing is not a routine, universally recommended screening test today. Results shift with age, medications, smoking, caffeine, a recent meal, even room temperature and protocol. An abnormal result is not a diagnosis of coronary artery disease, and it should never be the sole basis for treatment. The evidence that adding these measurements to standard risk assessment consistently improves outcomes is still being built.

What it can do, used properly, is sharpen a referral. In my own practice, the sequence looks like this: I see retinal vascular signs, I check the blood pressure, I weigh the history — diabetes, lipids, smoking, family history — and I communicate with the patient's primary care physician. In selected patients, an added vascular measurement can make that communication richer and the referral better timed. The goal is not to replace the cardiologist or the primary care doctor. It is to catch the patient whose risk is still invisible everywhere except, perhaps, in the back of the eye — and get them evaluated while early still means early.

That, to me, is the quiet promise here. Cardiovascular disease remains the leading cause of death in the United States, and the eye exam is one of the few appointments healthy-feeling people actually keep. If the vessels I see every day can help start the right conversation sooner — with good tests, honest limits, and close collaboration with primary care and cardiology — then the exam room becomes a genuine front door to preventive care.

The technology will keep evolving, and the research needs to keep pace — standardized methods, stronger outcome data. Until then, I will keep doing what optometric physicians are positioned to do: look carefully, measure honestly, and refer wisely.

— Paddy Kalish, OD