Evan, now a 37-year-old who works in finance, had experienced mild back pain since college but was far more preoccupied with recurring iritis, a painful inflammation of the eye that, if left untreated, can cause blindness.
By the time he was in his 20s, he'd had five bouts. "Every time they treated it, it went away and there was never any talk initially about any other related conditions," he said.
As for his back, his primary doctor thought it was a sciatic nerve. "I never really addressed it," said Evan, not his real name. "OK, I have back pain -- everyone has back pain."
But no one ever connected the dots until 2001.
He was living in New York City and iritis struck again. This time, a new eye doctor asked Evan if he had back pain.
Genetic tests revealed Evan was positive for HLA-B27, a marker for ankylosing spondylitis (AS), a relatively common but incurable form of inflammatory arthritis that strikes young adults, causing chronic pain and sometimes more serious damage to the heart and other organs.
"It was pretty daunting, knowing I would have this for the rest of my life," he said. "But I give my eye doctor credit. He sent me to see a rheumatologist."
Spondyloarthritis -- the name for a family of inflammatory rheumatic diseases that includes AS -- is just one of many systemic diseases that can first be picked up during an eye exam.
Up to 40 percent of all individuals with ankylosing spondylitis will experience at least one episode of iritis during the course of their disease. For Evan, being referred to a rheumatologist for treatment was important.
"Early diagnosis is key to improved outcomes with spondyloarthritis," said Laurie Savage, executive director of the Spondylitis Association of America. "Once diagnosed, an individual can start managing the disease appropriately in partnership with the rheumatologist."
Doctors say dozens, maybe scores of diseases -- from high blood pressure to certain cancers -- can show symptoms in the eye.
Researchers in the United States and in Scotland are even studying how simple eye tests can diagnose illness like heart disease.
"There are many systemic diseases we see in the eye," said Dr. Roy Chuck, chair of the department of opthalmology and visual sciences at Albert Einstein College of Medicine and Montefiore Medical Center in New York City.
Diabetes can also be caught early by looking at the eye, as well as advanced hypertension, which shows up as bleeding in new blood vessels and cause retinal detachment.
"You see very skinny blood vessels where the flow is restricted and, in the worst case, bleeding, or new blood vessels responding to not getting enough oxygen," said Chuck. "You see early narrowing to the point where the vessels cross each other."
And it's not just the inner eye that can reveal disease. In xanthelasma, fatty deposits appear on the eyelids. The condition is usually "fairly normal," he said.
But when they appear on only one lid and not the other, it suggests a blood vessel could be blocked off. "It's not a sign of high cholesterol directly, but a blood flow problem," said Chuck.
Even sickle cell anemia, common in African Americans, is visible during an eye exam.
Jaundice often is more prominent in the whites of the eyes, depending on a person's skin tone, and the color can tip off a doctor to liver disease.
But yellowed eyes can also appear "for no reason at all," especially if a person's eyes are blood shot and before the blood clears, the whites turn a kind of "dirty yellow," according to Chuck.