Having Lasik? Check forecast
Some people who undergo Lasik procedures find afterward that their vision isn’t as sharp as they’d hoped. With their dreams of tossing away their glasses and contact lenses deferred, an estimated 10% must undergo follow-up procedures to fine-tune their sight.
Now an eye surgeon in a region with extremely muggy summers says he’s found a reason for some of the less-than-perfect results, at least among his own patients.
Humidity in the operating room, and even outdoors, he says, may affect how well the laser procedure corrects vision.
Dr. Keith Walter, an ophthalmologist at Wake Forest Baptist Medical Center in Winston-Salem, N.C., noticed a few years ago that he was busy performing follow-up surgeries in November and December on patients who had undergone Lasik in summer.
Suspicious that climate conditions might be playing some role, he reviewed Lasik procedures he had performed on 368 eyes throughout 2000, analyzing a dozen potential influences, including the patient’s age, sex, the curvature of the cornea and environmental factors such as indoor and outdoor humidity and temperature.
Lasik, short for Laser-Assisted In Situ Keratomileusis, uses pulses of laser light to reshape the cornea, the clear, front part of the eye, in nearsighted people.
The percentage of his surgeries requiring “enhancements,” the term eye surgeons use for vision-correcting follow-ups, ranged from none in winter to 50% in September, when morning humidity in Winston-Salem peaks at 90%.
Walter found that during drier months, he tended to over-correct, leaving patients a bit farsighted. In humid months, he tended to under-correct, leaving them still slightly nearsighted.
Writing in the April issue of the Journal of Cataract and Refractive Surgery, Walter reported that indoor humidity was the strongest influence on the need for redos: For every 10% increase in operating room humidity, he found that nine of every 100 patients required another Lasik procedure. Outdoor humidity in the two weeks before surgery, and to a lesser degree outdoor temperatures, also affected results.
“It’s something that ophthalmologists and refractive surgeons are going to be paying attention to,” said Dr. Stephen Obstbaum, director of ophthalmology at Lenox Hill Hospital in New York City.
“It’s fair to say that since ophthalmologists who are doing a fair amount of refractive surgery are trying to make this a more perfect operation, this may be one of the variables that folks should look at as well,” Obstbaum said.
Laser manufacturers instruct doctors to keep operating room humidity within a range of 35% to 65% for good results, although many doctors use humidifiers or dehumidifiers to keep humidity within a much narrower range.
But Walter’s findings suggest that doctors also may need to adjust the depth of the corneal cutting with the laser. They already adjust their formulas for older patients and extremely nearsighted patients.
When the air is more humid, corneal tissue takes on more moisture and absorbs less laser energy, so less tissue gets removed, Walter theorized. As a result, he adds more laser pulses to remove more tissue. In dry months, fewer pulses are needed. Since he began making adjustments, he said he’s reduced his average redo rate from about 15% to a range of 3% to 4%.
“What this tells us is that humidity in some fashion has an effect upon the amount of correction that is achieved for any given programming of the laser,” said Dr. Douglas D. Koch, editor of the journal, published by the American Society of Cataract and Refractive Surgery and the European Society of Cataract and Refractive Surgery.
Some eye surgeons who practice in particularly dry or humid climates already have figured out that “we have to adjust our lasers differently,” said Koch, an ophthalmology professor at Baylor College of Medicine in notoriously humid Houston. Koch tries to keep humidity at 40% to 42% in his operating rooms.
But in cities where temperature and humidity vary widely between seasons, such as Winston-Salem, doctors may not be aware of climate’s effect.
Koch said Walter’s findings suggested that machines might have to be adjusted seasonally, although that would have to be borne out in larger studies.
In addition, he said Walter’s findings were the first to suggest that the climate conditions to which a patient has been exposed before coming into a climate-controlled medical office also might be a factor.
“The patients live in the world outdoors, and that affects the water content of their corneas. Even if you keep your laser center tightly controlled, the patient is going to walk in from whatever environment he or she was in and that will probably influence your treatment rate.”
Dr. Edward Manche, director of refractive surgery at the Stanford Eye Laser Center in Palo Alto, suggested that people who are considering Lasik surgery “make sure to inquire about the operating suite and whether they pay attention to temperature and humidity.”
