(CNN) For an entire week, the man had pain in his left eye as well as blurred double vision. Finally, the 48-year-old, who requested anonymity, visited National Taiwan University Hospital in Taipei.

A brief physical examination by Dr. Wei Li Chen and Dr. Chia-Chieh Hsiao , physicians and researchers in the hospital's Department of Ophthalmology, revealed left eyelid bruising and an artfully, if painfully, distorted left pupil.

The eye's single opening, the pupil is an aperture through which light travels to reach the retina, a thin layer of tissue lining the back wall of the eye. The retina's photoreceptor cells convert light into signals sent to the back of the brain, where the visual cortex assembles the information and decides what is being seen.

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The patient sat behind a slit lamp so his doctors could perform a more thorough inspection of his injured left eye.

The slit lamp, ubiquitous in eye doctor offices, is the clunky tool that requires you to rest your chin on one support and place your forehead against another while your uncomfortably close doctor peers through a microscope into your eyes. The slit lamp focuses a narrow band of light into the eyes, providing a magnified, three-dimensional view of both front and back of the "windows of the soul."

This second examination of the patient's left eye revealed an iris "with the upper portion sagging downward, detached from the 9 o'clock position to the 3 o'clock position," Chen and Hsiao wrote in a case study on the man's injury, published Wednesday in the New England Journal of Medicine.

A separation of the iris like this "can cause multiple pupils, leading to monocular double vision, blurry vision or photophobia" (an inability to tolerate light), they told CNN in an email.

So what caused the rare injury? A rubber bungee cord that the man used to strap goods onto a motorcycle had struck him directly in the eye.

The iris is "what we recognize as the colored part of the eye," explained Dr. Nicholas P. Bell, a professor of ophthalmology and visual science with McGovern Medical School at UTHealth in Houston.

"Muscles in the iris can cause the pupil to open or close to control how much light gets into the eye under different ambient lighting conditions, similar to how you can adjust the aperture on a camera lens," said Bell, who did not treat this particular patient. In a dark room, the pupil opens wide to let in more light, and on a sunny day, the pupil will constrict to reduce the amount of incoming light.

Though Chen and Hsiao saw no evidence of cataract, retinal injury or lens dislocation, the separation of the iris from the band of muscles that sits behind it easily accounted for the patient's blurred and double vision. This damage is known as traumatic iridodialysis.

Bell said the condition is "relatively uncommon," though when it does occur, sports or work-related activities are often the cause. But there are others. "Bungee cord and airbag injuries seem to more frequently result in traumatic iridodialysis because of the focally concentrated strong force," Bell said, adding that airbags save lives, and fear of this extremely rare eye injury is no reason for avoiding them.

Dr. J. Martin Heur, medical director of the USC Roski Eye Institute and associate professor of clinical ophthalmology at the Keck School of Medicine of USC, agreed that this injury is "very rare, but I've had a handful of people who've had this happen to them." "The trauma can be caused by anything that doesn't penetrate the eye" but still causes "the iris to tear at the root like that," said Heur, who was not involved in this patient's care. Balls such as golf balls are likely culprits, he said.

To repair the damage, the Taiwanese patient underwent surgery that restored the pupil shape and improved his vision.

Heur said this surgery, known as iridoplasty, is "usually done in an operating room" and "is relatively short: approximately 30 minutes." The procedure involves McCannel sutures, basically when a surgeon carefully sews the iris back to the inner wall of the eye.

Heur added that after an iridoplasty, a patient has to be careful for about a week or two about lifting heaving objects or getting water in the eye. Once healed, a patient needs to be monitored regularly for life, because glaucoma can develop even years after the injury.

One year later, the Taiwan patient's iris remained attached with only "mild deformity," Chen and Hsiao said. Though there's no evidence of glaucoma so far, the patient now wears glasses to help him achieve 20/50 vision in the injured eye: He needs to be within 20 feet to see what people with healthy vision can spot at 50 feet.

Dr. Jeffrey Goldberg , a professor and chair of ophthalmology at the Byers Eye Institute at Stanford University, said he saw a similarly severe injury "after a fisherman had a steel cable snap back into his eye."

"Because some of the effects of eye trauma can be asymptomatic and appear four to six weeks later, it is important to get a complete eye exam with a professional eye care provider both immediately after severe trauma and a month or two later to be safe," said Goldberg, who did not treat the Taiwanese patient.

Although this level of injury is rare, "less severe iridodialyses happen more often with moderate to severe eye trauma," he said. When small tears occur on the very edge of the iris, they can "lead to scarring on that area that subsequently blocks the drain of the eye, leading to higher eye pressure and glaucoma."

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Dr. Jeff Pettey, an assistant professor at the University of Utah's John A. Moran Eye Center, added BB guns and exercise bands to the list of items that can cause an injury like this: "A young flight attendant used exercise bands when she traveled, and her eye injury resulted in severe eye trauma and a blind eye."