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Sci Tech
Pupil size crucial for laser vision correction
EXACTLY HOW a person's eyes respond to low levels of light is even more crucial than doctors have thought in deciding who is and who isn't a good candidate to have laser vision correction surgery, according to results announced at the annual meeting of the Association for Research in Vision and Ophthalmology in Ft. Lauderdale.
Earlier after laser surgery some complained of worsened night vision, some reported significant glare from light sources such as headlights, while others saw halos around bright lights.
Occasionally, even now patients undergoing the procedure report such effects.
Ophthalmologist Scott MacRae, professor of Ophthalmology and Visual Science at the University of Rochester Medical Centre, recently studied the role of a patient's pupil size in determining a patient's outcome from the surgery.
In a study he found that generally the larger a patient's pupils, the more likely that person is to have a problem with laser vision correction. MacRae also discussed the results at the recent annual meeting of the Association for Cataract and Refractive Surgery in San Francisco.
MacRae is part of a team that has used a technology known as adaptive optics to discover dozens of previously unknown subtle imperfections in the eye, and he has found that those imperfections loom more important as the pupil size gets larger.
He reports that if a surgeon treats a large enough swath of the eye for such patients, those imperfections are minimised. His study shows that it's essential that doctors treating patients with large pupils consider creating an especially large treatment zone.
He found that doctors must be especially very careful when using a laser to correct vision on people whose pupils dilate to nearly about six millimeters or more, a group that comprises about 40 percent of the population.
If physicians treat too small an area of the cornea, patients are likely to have a problem when their eyes are most dilated — at night.
To understand the problem, think of a window washer who should clean an entire window even when the inside blind is partway down. When the blind goes up, a person looking through the window wouldn't get a clear view unless the previously covered section of the window was washed too.
Likewise, MacRae says, surgeons must treat a large part of the cornea so that when the pupil widens, the entire pupil transmits light clearly.
Pupil size is especially critical if a person is extremely near-sighted. MacRae says that generally, people with pupils larger than seven millimeters should be checked very thoroughly before having the surgery done.
MacRae measures every patient's pupils three different times using three separate instruments. He also makes three measurements of the thickness of the cornea, another crucial element in deciding who is a safe candidate.
Overall he advises against surgery in more than 20 per cent of patients who inquire about the surgery, especially patients who have large pupils and are extremely nearsighted.
"There is tremendous variation, depending on where your eye is looking," MacRae says. "Country roads can be nearly completely dark, while city streets are often brightly lit."
MacRae found that the amount of light entering the eye of a driver on a country road during a rainy night might be nearly 200 times less than what he or she encounters driving through a busy intersection seconds later.
MacRae also found that a typical night driver frequently confronts situations where he or she has less than one-tenth the amount of light that doctors, in their offices, assume their patients encounter at night.
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