In the April 2010 issue of EyeWorld, a panel of eye surgeons discussed the importance of the ambient humidity and temperature in the laser surgery suite and eye hydration before, during and after surgery to achieving predictable, high-quality visual results.

Among the opinions expressed:

Daniel S. Durrie, MD, (Overland Park, Kan.), Emil W. Chynn, MD, (New York, N.Y.) and Joseph Rosenshein, PhD, (Victor, N.Y.) said controlling the temperature and humidity levels in the refractive surgery operating room (OR) are important factors in ensuring consistent outcomes.

Richard L. Lindstrom, MD, (Minneapolis, Minn.) said that in addition to controlling temperature and humidity, the air in the OR should be cleaned with high efficiency particulate-absorbing filters.

Controlling the humidity in the operating room is important, but may be somewhat overstated, said Steven C. Schallhorn, MD, (San Diego, Calif.). Dr. Schallhorn noted that in a large LASIK study he recently completed, he found that the level of humidity in the laser room “did not correlate with the refractive outcome.”

Dr. Chynn, however, said a 2003 study published in the Journal of Cataract & Refractive Surgery found a zero percent LASIK enhancement rate in winter and a 50 percent enhancement rate in late summer, and that the study showed a tendency toward LASIK under-correction when humidity was high.

The surgeons agreed that more significant factors in LASIK outcomes than controlling temperature and humidity in the OR (which most LASIK surgeons do) are the patient’s behavior and environment after surgery.

For the best laser eye surgery outcomes possible, Dr. Chynn recommends that patients drink plenty of water, avoid or reduce consuming alcohol and caffeine, use a humidifier in their home and use all eye drops as directed to keep their corneas well-hydrated during the healing process.

Ambient humidity after surgery is more important for patients that undergo PRK and LASEK than it is for LASIK patients, Dr. Chynn said, because the corneal epithelium must grow back over the treated area of the eye in the former procedures, and this is easier if the corneal surface is kept moist.

The surgeons also agreed that treating a pre-existing dry eye condition prior to LASIK and other laser vision correction surgery is essential in order to achieve predictable results.

Researchers in Canada assessed the safety and effectiveness of LASEK for the treatment of bilateral hyperopia, including cases of unequal farsightedness with associated amblyopia.

A total of 72 eyes of 47 farsighted children were treated in the study. The mean amount of hyperopia prior to surgery was +3.42 diopters (D). All procedures were performed at a single eye surgery center in Calgary, Alberta.

Results of the study were determined by measurements taken one year after LASEK surgery.

Among all study participants:

• The mean amount of hyperopia was reduced to +0.59 D (range -1.25 to +2.00 D).
• 41.7 percent of eyes had improved corrected distance visual acuity (CDVA).
• Mean stereopsis improved 25 percent.

Unequal amounts of farsightedness in the two eyes of a child can cause amblyopia in the eye with a greater degree of hyperopia. This is called hyperopic anisometropic amblyopia.

Among the study participants, 10 children exhibited hyperopic anisometropic amblyopia prior to surgery. The mean difference in the amount of farsightedness in the two eyes of these children was 4.39 D (range of 1.75 to 7.75 D).

Among this subgroup, results one year after LASEK were:

• 83 percent of the anisometropic eyes were within 1.00 D of the fellow eye.
• 64.7 percent of eyes had improved CDVA.
• Mean stereopsis improved 22 percent.

None of the study participants had decreased corrected distance visual acuity after LASEK surgery.

The authors of the study concluded that LASEK is safe and effective and improves the visual acuity of children with hyperopia, with or without associated hyperopic anisometropic amblyopia.

[Ed. note: Laser eye surgery generally is not recommended for children. But in cases of high degrees of stable childhood hyperopia, this study suggests LASEK may be a viable alternative to contacts, especially for children who cannot tolerate contact lens wear.]

SOURCE:  Laser-assisted subepithelial keratectomy for bilateral hyperopia and hyperopic anisometropic amblyopia in children: One-year outcomes. Journal of Cataract & Refractive Surgery. February 2010.

Researchers in Belgium have found just the opposite: LASEK eye surgery appears to reduce pre-operative retinal straylight, which is a potential source of glare. The results of their study suggests patients undergoing LASEK for the correction of myopia may have reduced glare after surgery, especially if they wore soft contact lenses prior to the procedure.

Retinal straylight is light that is scattered by defects in the optical media of the eye and therefore is not available to be focused into a clear image by the retina. This scattered light reduces the contrast of images created by the retina and contributes to glare.

To quantify the changes in retinal straylight after LASEK eye surgery, researchers at Antwerp University Hospital (Edegem, Belgium) evaluated straylight values of 86 eyes of 49 patients scheduled for LASEK surgery.

Straylight measurements were taken prior to LASEK surgery and 6 months after the procedure, using a device called the Oculus C-Quant straylight meter (Oculus Optikgeräte GmbH, Wetzlar, Germany).

Straylight values also were compared with values in a “normal” reference database.

Patients were divided into four groups, based on their habitual method of vision correction prior to surgery:

• Rigid contact lenses
• Soft contact lenses
• Spectacles after a period of contact lens wear
• Spectacles only

Analysis of the pre-operative data revealed that age-corrected straylight values were significantly elevated among patients in the study, especially if they wore soft contact lenses full-time prior to LASEK surgery.

Six months after LASEK, straylight values for all patient groups had returned to normal levels, regardless of the amount of nearsightedness and method of vision correction prior to surgery.

The researchers concluded that higher-than-normal retinal straylight values normalized after LASEK refractive surgery among this patient population.

They also said that wearing soft contact lenses causes higher pre-operative retinal straylight, which is eliminated after LASEK.

SOURCE:  Straylight before and after LASEK in myopia: Changes in retinal straylight. Investigative Ophthalmology & Visual Science. Published online ahead of print December 10, 2009.

Thin-flap LASIK (also called sub-Bowman keratomileusis or SBK) is similar to LASIK, but involves creating a significantly thinner flap on the eye to preserve more corneal thickness under the flap. In the thin-flap LASIK procedures evaluated in this study, the corneal flap was created with a femtosecond laser.

LASEK (laser subepithelial keratectomy or laser epithelial keratomileusis) is a modification of LASIK wherein a very thin flap consisting only of the outer epithelial tissue of the cornea is lifted from the eye after being loosened from the underlying tissue with an alcohol solution. Other commonly performed variations of LASIK include Epi-LASIK and Presby-LASIK.

In both thin-flap LASIK and LASEK, the corneal flap is less than 100 microns thick. In conventional LASIK, it is not unusual for the flap to be greater than 150 microns thick.

Thin-flap LASIK vs. LASEK: Study design

This study compared the visual and refractive outcomes of 1,072 eyes treated with thin-flap LASIK and 1,036 eyes treated with LASEK.

Prior to surgery, all eyes were nearsighted, with a mean of -3.93 diopters (D) of myopia in the thin-flap LASIK group and -3.87 D in the LASEK group.

Outcomes were measured one day, one week, one month and three months after surgery, and included:

• Uncorrected visual acuity (UCVA)
• Best spectacle-corrected visual acuity (BSCVA)
• Gain or loss of BSCVA after surgery compared to before surgery

Thin-flap LASIK vs. LASEK: Results

Analysis of the study data revealed:

• The day after surgery, mean UCVA was between 20/20 and 20/25 in thin-flap LASIK eyes and slightly worse than 20/30 in LASEK eyes.
• At one month after surgery, mean UCVA was nearly 20/20 in thin-flap LASIK eyes and between 20/20 and 20/25 in LASEK eyes.
• At three months after surgery, mean UCVA was slightly better than 20/20 in both groups.
• At three months after surgery, BSCVA also was better than 20/20 in both groups.

At three months after surgery, three eyes (0.28 percent) in the thin-flap LASIK group and ten eyes (0.96 percent) in the LASEK group gained two or more lines of BSCVA compared to pre-surgical levels.

Also at three months after surgery, 9 eyes (0.84 percent) in the thin-flap LASIK group and 20 eyes (1.93 percent) in the LASEK group lost two or more lines of BSCVA compared to pre-surgical levels.

Losses in best spectacle-corrected visual acuity diminished with time. At six months after surgery, only three eyes (0.28 percent) in the thin-flap LASIK group and two eyes (0.19 percent) in the LASEK group showed a loss of two or more lines of BSCVA.

Conclusions

The researchers concluded that both thin-flap LASIK and LASEK are safe and effective for the correction of low to moderate myopia, but all-laser thin-flap LASIK provides faster visual recovery and slightly better visual and refractive results in the three months following surgery.

SOURCE:  Comparison between femtosecond laser-assisted sub-Bowman keratomileusis vs laser subepithelial keratectomy to correct myopia. American Journal of Ophthalmology. Published online September 24, 2009.

In 2008, Optical Express, the largest provider of laser vision correction in Europe established an online interactive survey of patients who received LASIK or LASEK at Optical Express locations to evaluate the quality of care provided, how well patient expectations were met, and the level of patient satisfaction, according to the company.

At one month after surgery, 13,655 patients completed the survey. Of the respondents, 91 percent had undergone LASIK and 9 percent had undergone LASEK.

Treatments ranged from the correction of up to -11.63 diopters (D) of myopia to up to +6.00 D of hyperopia. The mean spherical equivalent of all laser vision correction procedures was -2.27 D of myopia.

Patients ranged in age from 18 to 71 years, with an average age of 39.4 years.

Laser vision correction survey: One-month results

Results of the surveys completed one month after surgery by this large group of patients revealed:

• 95 percent said they were either “very satisfied” (77.8 percent) or “satisfied” (17.2 percent) with their vision after laser vision correction.
• 74.7 percent said their vision was better than they expected it would be. (19.4 percent said it was equal to their expectations and 5.9 percent said it was worse than what they expected prior to surgery.)
• 82.8 percent said their vision ws better after laser vision correction than it was with glasses or contact lenses prior to surgery.
• 94.2 percent said laser vision correction had changed their life for the better.
• 96.5 percent said they would recommend laser vision correction procedures, such as LASIK, LASEK, Presby-LASIK and PRK eye surgery, to their friends and family.

Conclusions

The researchers concluded the results of the survey show modern laser vision correction surgery by a large corporate provider produces a high level of patient satisfaction.

Regarding limitations of the study, the researchers noted that only 43 percent of 31,979 patients who attended their one-month follow-up visit after LASIK or LASEK completed the online survey.

A comparison of patients who completed the survey and those that did not, however, found no differences in most-pre-operative characteristics.

The researchers said that although the differences between those who completed the survey and those who did not “are unlikely to have true clinical relevance,” it is possible that the patients who completed the survey were selectively more satisfied or less satisfied than those who did not.

SOURCE:  Satisfaction of 13,655 patients with laser vision correction at 1 month after surgery. Journal of Refractive Surgery. July 2009. (Supplement: Clinical Outcomes and Quality of Care in Refractive Surgery, sponsored by Optical Express.)

LASEK is similar to LASIK, but the flap is much thinner and is created in a different way.

The LASEK flap consists only of the superficial layer of corneal cells (called the epithelium). A dilute alcohol solution is first applied to the eye for a brief period to loosen the epithelium from the underlying corneal tissue. The flap is then created by manually scraping the epithelial cells off to the side, leaving a hinge attached.

After the excimer laser treatment is completed, the flap of epithelial cells is replaced and covered with a bandage contact lens to promote healing.

The advantage of LASEK over LASIK is that LASEK often can be safely performed on corneas that may be too thin for LASIK. Also, because the epithelial cells grow back and adhere tightly to the underling corneal tissue after LASEK, there is no risk of delayed flap complications that can sometimes occur after LASIK.

While these advantages are also true of PRK, many surgeons prefer LASEK over PRK because the cornea may heal faster after LASEK, with less post-operative discomfort.

But one potential drawback of LASEK is that, in some cases, corneal haze may develop after surgery. It is believed that this haze, which may or may not affect vision, is due a toxic effect of the alcohol solution on corneal tissue, combined with the scraping motion required to create the epithelial flap.

Study of LASEK versus epi-LASIK

To improve LASEK outcomes and reduce the risk of corneal haze, Dr. Camellin first evaluated the surgical outcomes of LASEK and a similar laser eye surgery called epi-LASIK.

Epi-LASIK, like LASEK, creates an ultra-thin epithelial flap prior to laser treatment. But instead of the flap being created by scraping the alcohol-loosened epithelial cells to the side, in epi-LASIK the epithelial flap is created with a surgical instrument called an epikeratome. No alcohol solution is needed to first loosen the epithelium.

Dr. Camellin found that, though epi-LASIK avoided the toxic effect of alcohol on epithelial cells, the use of an epikeratome frequently would eliminate the hinge on the flap, causing a “free cap” — a complication where the epithelial flap is completely removed from the eye.

When this occurs, the epi-LASIK procedure essentially become the same procedure as PRK, in which the epithelium is intentionally removed and no flap is created. After PRK, the epithelium must grow back without the bandage effect provided by an overlying flap of epithelial tissue. Typically, this does not affect visual outcomes, but it can take a longer time for the eye to heal after PRK compared to LASEK or epi-LASIK.

To compare the results of LASEK versus epi-LASIK, Dr. Camellin evaluated the surgical outcomes of 67 patients. With each patient, LASEK was performed on one eye and epi-LASIK was performed on the other.

Study results showed:

• Flap quality was typically better in the epi-LASIK eyes, but frequently there were hinge problems and the formation of free caps with epi-LASIK.
• In the first two months after surgery, the LASEK eyes had less astigmatism.
• Though no clinically significant corneal haze developed in either group, a slight amount of haze was found three months after surgery in a minority of the LASEK eyes. This haze did not influence visual outcomes.
• Corneal topography and wavefront measurements indicated the epi-LASIK eyes had more irregularities in the central cornea in the first eight weeks after surgery, but this difference disappeared as more time passed.
• Some patients found the suction applied to the eye during the use of the epikeratome in the epi-LASIK procedure caused minor discomfort.

New epi-LASEK procedure combines best features of LASEK and epi-LASIK

To improve surgical outcomes, Dr. Camellin decided to develop a hybrid procedure that combines the best features of traditional LASEK and epi-LASIK. In this new procedure, which he calls “epi-LASEK,” Dr. Camellin soaks the cornea in a 20 percent alcohol solution for 20 seconds, and then uses an epikeratome to create the epithelial flap.

In a study of 45 patients, in which one eye was treated with traditional epi-LASIK and the other eye was treated with the new epi-LASEK procedure, results showed:

• The epi-LASEK eyes had significantly less astigmatism in the first eight weeks after surgery.
• The corneal surface was smoother in the epi-LASEK eyes.
• Flap quality was better in the epi-LASEK eyes, and no free caps occurred.
• Uncorrected visual acuity was slightly better in the first month in the epi-LASEK eyes. (Thereafter, the results were comparable).
• There was no significant difference in haze formation between the epi-LASEK and epi-LASIK eyes.

Because there was no significant corneal haze created by the epi-LASEK procedure, Dr. Camellin believes the corneal haze that sometimes occurs in traditional LASEK is likely due to the mechanical scraping of the eye that occurs in that procedure after the alcohol soak, rather than because of the alcohol solution itself.

Dr. Camellin concludes that the new epi-LASEK procedure is an improvement over traditional LASEK and epi-LASIK procedures and offers advantages over LASIK and PRK as well.

SOURCES:

1. Epi-LASEK aims to minimize drawbacks of LASEK and epi-LASIK. Ocular Surgery News. February 10, 2009.
2. Epi-LASIK versus epi-LASEK. Journal of Refractive Surgery. January 2008.

In the study, 998 eyes with nearsightedness ranging from -1.00 to -8.00 diopters (D) were treated at the university eye center using one of the following laser eye surgery procedures: LASEK, Epi-LASIK, PRK and Epi-PRK.

LASEK, Epi-LASIK and PRK: Study design

In the LASEK and PRK groups, the corneal epithelium was first loosened from the underlying corneal tissue with a 20 percent alcohol solution for 20 seconds. In the LASEK group, the flap was reapplied to the eye after the wavefront-guided excimer laser treatment; in the PRK group, the flap was removed and discarded.

In the Epi-LASIK and Epi-PRK groups, a hand-held surgical tool called a MicronEdge Separator, (Gebauer Ophthalmic Instruments, North Palm Beach, Fla.) was used to create the epithelial flap. In the Epi-LASIK group, the flap was reapplied to the cornea after the laser treatment; in the Epi-PRK (also called flap-off Epi-LASIK) group, the flap was removed and discarded.

In all procedures, the wavefront-guided laser ablation was performed with a VISX Star S4 IR excimer laser (Abbott Medical Optics, Santa Ana, Calif.).

All eyes were evaluated for a period of one year after surgery, and measures of uncorrected visual acuity (UCVA) and best spectacle-corrected visual acuity (BSCVA) were taken before surgery and throughout the post-operative period.

Results

Analysis of the data one year after surgery revealed that there was no statistically significant difference in proportion of eyes achieving UCVA of 20/20 or better and BSCVA of 20/20 or better among the four surgical groups.

Refractive results were found to be very stable in all surgery groups, and outcomes were within 0.50 D of the intended correction in at least 96 percent of eyes in all groups.

Sadhana Kulkarni, MD, presented the results of the study at the annual meeting of the American Society of Cataract and Refractive Surgery (ASCRS) this week in San Francisco.

SOURCE:  Large retrospective study explores best technique for wavefront-guided surface ablation. Ophthalmology Times. Published online April 5, 2009.