Researchers in San Diego compared changes in simulated night driving performance among patients moderate to high myopia and astigmatism after treatment with either surgical implantation of a toric Visian Implantable Collamer Lens (STAAR Surgical; Monrovia, Calif.) or conventional PRK surgery.

A total of 43 eyes (20 bilateral cases) were treated with the Visian Toric Implantable Collamer Lens (TICL), and 45 eyes were treated with PRK surgery.

Prior to surgery, subjects participating in the study had -6.00 to -20.00 diopters (D) of myopia and 1.00 to 4.00 D of astigmatism.

Before and 6 months following treatment, 27 eyes of 14 TICL patients and 41 eyes of 21 PRK patients underwent a simulated night driving test.  The test required the subjects to detect and identify common road signs and hazards with and without the presence of a glare source.

Examples of a glare source during real-world night driving are the headlights of approaching vehicles.

The results of the night driving simulator testing revealed:

• Without the introduction of a glare source, patients in the TICL group could identify road signs and pedestrian hazards significantly faster than patients in the PRK group.
• With introduction of a glare source, patients in the TICL group could identify pedestrian hazards (but not road signs) significantly faster than patients in the PRK group.
• There was no significant difference in how quickly subjects in the two treatment groups could detect (but not identify) road signs and pedestrian hazards, with and without a glare source.

The researchers concluded that the Visian Toric Implantable Collamer Lens performed better than conventional PRK surgery in the pre- to post-surgical simulated night driving visual performance testing, with and without a glare source present.

A full report of the study appears in the May 2010 issue of Journal of Refractive Surgery.

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.

Also, many refractive surgeons now prefer intraocular refractive procedures such as phakic IOL implantation and refractive lens exchange (RLE) for the correction of high myopia, which may provide better visual outcomes and greater refractive stability after surgery.

Among the results of the 2009 International Society of Refractive Surgery (ISRS) survey of refractive surgeons, when asked their procedure of choice for a 30-year-old patient with –10 D of myopia, 43 percent of surgeons identified phakic IOL implantation as their preferred procedure, compared with 40 percent who said they prefer laser vision correction.

Recently, researchers in Spain conducted a retrospective study of 192 highly myopic eyes that underwent PRK (51 eyes) or LASIK (141 eyes) for vision correction. The degree of pre-operative myopia in all eyes in the study ranged from –10.00 to –18.00 D.

All PRK and LASIK procedures were performed between 1992 and 1995 at Instituto Oftalmologico de Alicante (Alicante, Spain). PRK patients had a mean age of 33.96 years, and LASIK patients had a mean age of 32.44 years.

Ten years after surgery, 31.2 percent of eyes in the PRK group and 45.5 percent of eyes in the LASIK group achieved uncorrected visual acuity (UCVA) of 20/40 or better.

Among eyes treated with PRK, 41 percent were within +/–1.00 D of the desired refractive endpoint 10 years after surgery. Among LASIK-treated eyes, this figure was 42.5 percent.

A total of six eyes (14 percent) in the PRK group and seven eyes (6 percent) in the LASIK group lost two or more lines of best spectacle-corrected visual acuity 10 years after the laser eye surgery.

Eyes in both treatment groups showed statistically similar mean regression of myopia over the 10-year study period: –1.28 D in the PRK group and –1.49 D in the LASIK group.

Corneal haze was a significant long-term problem for PRK-treated eyes in the study, according to the researchers.

The study authors said that, based on the results of this and other studies, the use of LASIK for the correction of myopia equal to or greater than –10 D is no longer “routinely advocated,” and that they no longer recommend PRK for the correction of high myopia.

SOURCE: Comparison of LASIK and photorefractive keratectomy for myopia from –10.00 to –18.00 diopters 10 years after surgery. Journal of Refractive Surgery. March 2010.

Researchers at Emory University School of Medicine (Atlanta, Ga.) performed a retrospective chart review of 40 patients ages 17 to 39 years who underwent bilateral photorefractive keratectomy (PRK) to eliminate their dependence on glasses.

The mean age of patients in the study was 27.9 years, and all patients had mild to moderate hyperopia (mean: +3.06 diopters) and associated accommodative esotropia prior to surgery. Mean pre-operative esotropia at distance and near without glasses was 18.6 prism diopters.

Following PRK surgery, mean uncorrected visual acuity improved significantly compared with pre-operative levels (20/40 prior to surgery, slightly better than 20/25 after surgery). Mean refractive error after surgery was +0.06 diopter.

Mean best spectacle-corrected visual acuity (BSCVA) remained unchanged after surgery.

All patients had perfect eye alignment (orthophoria) after surgery, with no evidence of residual esotropia at 1 month, 1 year or at their final post-operative evaluations over the course of the 6-year study.

Visual acuity, refractive error and eye alignment remained stable after the 1-year post-operative visit in all cases, and there were no surgical or post-surgical complications.

The researchers concluded that PRK is a safe and effective treatment for low to moderate hyperopia and associated accommodative esotropia in young adults.

SOURCE:  Photorefractive keratectomy for the treatment of purely refractive accommodative esotropia: 6 years’ experience. British Journal of Ophthalmology. February 2010.

Eighty-eight eyes of 44 patients with moderate myopia were randomly selected to undergo either wavefront-guided (custom) PRK or custom LASIK. The mean amount of pre-operative myopia was -3.85 diopters (D) among the PRK eyes and -3.99 D among the LASIK eyes.

For eyes undergoing custom PRK, a 0.002 percent solution of MMC was applied to the exposed corneal tissue for a period of one minute immediately following the excimer laser treatment.

Mitomycin C is a chemical agent frequently used during PRK surgery to decrease the risk of clouding of the cornea during the healing process. This loss of transparency is commonly referred to as corneal haze.

Post-operative corneal haze is potential side effect of PRK surgery that can decrease vision. Though mitomycin C has been proven effective in reducing corneal haze after PRK eye surgery, overexposure to the agent can be toxic to the cornea.

At one year after surgery, the mean uncorrected visual acuity (UCVA) of eyes in both groups was better than 20/20, but more custom PRK eyes (52 percent) than custom LASIK eyes (31 percent) achieved UCVA of 20/12.5 or better.

Evaluation of best spectacle-corrected visual acuity (BSCVA) after surgery revealed more eyes in the PRK group (74 percent) than in the LASIK group (43 percent) gained one or more lines of BSCVA on a standard eye chart, compared with BSCVA of the same eyes prior to surgery.

Eyes in the PRK group also demonstrated better contrast sensitivity under moderate and bright light than eyes in the LASIK group.

When surveyed a year after surgery, more patients in the PRK group (74 percent) than in the LASIK group (64 percent) reported having “excellent vision” as a result of their vision correction procedure.

No eye in the custom PRK group demonstrated clinically significant corneal haze at the end of the study.

The researchers concluded that in this study, wavefront-guided PRK with application of 0.002 percent MMC was more effective than wavefront-guided LASIK for the correction of moderate myopia when evaluated one year after surgery.

They cautioned, however, that further research is necessary to determine the optimal mitomycin C concentration and exposure time, and to evaluate possible long-term corneal side effects of MMC exposure, before 0.002 percent MMC is widely adopted for use in PRK eye surgery.

SOURCE:  One-year outcomes of a bilateral randomized prospective clinical trial comparing PRK with mitomycin C and LASIK. British Journal of Ophthalmology. December 2009.

Twenty-nine eyes of 16 patients underwent LASIK or PRK for the correction of myopia at the Mayo Clinic (Rochester, Minn.) between July 1998 and January 1999. Twenty eyes were treated with LASIK and nine eyes with PRK.

Photographs of the corneal endothelium of all eyes were taken prior to surgery and nine years after the procedures. The photographs were taken with a medical microscope to allow analysis of endothelial cell density and appearance.

Endothelial cell loss can lead to loss of corneal clarity and decreased vision due to corneal edema. However, some minor endothelial cell loss that does not cause symptoms or affect vision occurs as a normal aging change in the eye.

At nine years after surgery, the researchers found that the mean annual rate of endothelial cell loss after LASIK and PRK (0.6 percent) was not different from normal age-related cell loss in corneas that did not undergo refractive surgery.

They concluded that LASIK and PRK have no long-term affect on the corneal endothelium.

The authors of the study also said that based on the findings of this study, eye banks and surgeons can consider corneas that have undergone LASIK or PRK to be acceptable donor tissue for posterior lamellar keratoplasty (a type of corneal transplant surgery) if the patient who had the refractive surgery is an organ donor.

[Ed. note: Though this study did not evaluate endothelial cell loss occurring after other laser vision correction procedures, such as Epi-LASIK and LASEK, it is likely that these similar procedures would produce similar outcomes. However, additional research that specifically evaluates these procedures is required to be sure.]

SOURCE:  Corneal endothelial cell loss 9 years after excimer laser keratorefractive surgery. Archives of Ophthalmology.

November 2009.

Researchers in Spain recently conducted a 10-year follow-up study of patients who underwent either PRK or LASIK for the correction of -6.00 to -10.00 diopters (D) of nearsightedness.

All procedures were performed with a VISX 20/20 excimer laser at a single surgical center between April 1992 and December 1995.

The study evaluated 34 eyes of 33 patents treated with PRK and 34 eyes of 32 patients treated with LASIK. The two groups were matched for patient age and degree of nearsightedness and astigmatism.

All patients in the study returned to the center for follow-up visits at 3 months, 1 year, 2 years, 5 years and 10 years after surgery for an evaluation their vision and any PRK or LASIK complications.

At 10 years after surgery, the study data revealed:

• 71 percent of the PRK-treated eyes and 88 percent of the LASIK-treated eyes were within 1.00 D of the intended correction.
• Re-treatments were performed on 35 percent of the PRK-treated eyes and 18 percent of the LASIK-treated eyes to achieve satisfactory visual acuity.
• 62 percent of the PRK-treated eyes and 50 percent of the LASIK-treated eyes demonstrated an increase in best spectacle-corrected visual acuity (BSCVA), compared with BSCVA prior to surgery.
• No eye in either group lost more than two lines of BSCVA after surgery.

The most common complication among PRK-treated eyes was mild corneal haze, which was present in one (2.9 percent) of the 34 treated eyes at the end of the study.

The most common complications among LASIK-treated eyes were fine wrinkles (striae) in the corneal flap (2.9 percent) and necrosis or “melting” of the corneal flap edge (2.9 percent). Neither of these problems affected visual acuity at the end of the study.

No eye in either group showed clinical signs of corneal ectasia over the 10-year period of the study.

The researchers concluded that both PRK and LASIK were safe for the correction of -6.00 to -10.00 D of myopia in this group of patients. LASIK demonstrated slightly better predictability and a lower rate of re-treatment than PRK.

They also said that technical improvements in both PRK and LASIK should be taken into account when comparing the results of this study with results of other studies evaluating laser vision correction performed more recently.

Source: Ten years after photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) for moderate to high myopia (control-matched study). British Journal of Ophthalmology. October 2009.

The retrospective study followed 455 eyes that underwent photorefractive keratectomy (PRK) for a period of up to nine years after surgery.

Carole Liernur, MD, presented the results of the study at the annual meeting of the European Society of Cataract and Refractive Surgeons (ESCRS) held last week in Barcelona, Spain.

Prior to PRK, all eyes were myopic, ranging from -1.00 to -10.00 diopters (D) of nearsightedness. In all cases, the PRK procedure was performed with a Nidek 5000 excimer laser.

No patients experienced a detached retina during the follow-up period, and only one patient complained of floaters in one eye. This single case of vitreous floaters was most likely related to the natural history of myopia in this patient rather than the PRK procedure, Dr. Liernur said.

PRK eliminates one LASIK risk

Some clinical studies have reported retinal detachment as an infrequent complication of LASIK eye surgery, especially in patients with high myopia. This study set out to determine if PRK has the same LASIK risks for vitreo-retinal complications (affecting the vitreous and/or retina in the back of the eye).

Since no LASIK-style corneal flap is created during PRK, no suction device is attached to the eye to hold it still for this step. The suction device used in LASIK increases the pressure inside the eye, and when the device is removed after the flap has been created, the internal eye pressure quickly returns to normal.

This significant and sudden change in internal eye pressure may weaken the retina in susceptible individuals, increasing the risk for a retinal detachment during or after LASIK.

This study appears to support the belief that PRK does not pose the same risk of vitreo-retinal complications.

Dr. Liernur also said that risks associated with PRK surgery — specifically, corneal haze and post-operative discomfort — can be alleviated with new surgical techniques and topical pain medications.

SOURCE:  No vitreoretinal complications reported after PRK in long-term case series. Ocular Surgery News website (OSNsupersite.com). September 21, 2009.

Lasers and pacemakers: Study design and methods

Dr. Sher evaluated the effect of operating a number of lasers used in eye surgery, including a popular excimer laser used in LASIK and PRK, on popular cardiac pacing devices manufactured by Medtronic (Minneapolis) and St. Jude Medical (St. Paul).

To simulate the devices being implanted in a human subject, the pacemakers and defibrillators were submerged in salt water in a waterproof container to simulate resistance around the heart.

The lasers were then fired in close proximity to the devices to see if electromagnetic fields generated by the lasers would cause the pacemakers and defibrillators to malfunction.

Lasers and pacemakers: Study results

The study revealed that operation of the ophthalmic lasers caused no malfunction in any of the cardiac pacing devices tested.

The experiment was repeated several times without the occurrence of a single adverse event.

Conclusions

Dr. Sher concluded that the results of the testing suggest laser vision correction and other forms of laser eye surgery pose little cardiac risk to patients who have an implanted pacemaker or defibrillator.

He noted that the worst-case scenario was used in the study, with the use of the highest laser energy, the most pulses, the widest laser beams, and the most sensitive settings on the cardiac devices.

Dr. Sher recommends surgery centers reconsider the posting of warning signs about lasers and pacemakers on the doors of their laser suites, and he believes lasers commonly used in eye surgery pose no risk to the normal operation of cardiac pacing devices.

SOURCE:  A beat beyond: pacemakers and lasers. EyeWorld. June 2009.

Recently, a variation of LASIK called sub-Bowman keratomileusis (SBK) or “thin-flap LASIK” has been gaining popularity among LASIK surgeons. A thinner corneal flap is created in SBK, which may reduce certain LASIK risks — particularly dry eyes and corneal ectasia.

To compare the outcomes produced by thin-flap LASIK with those produced by PRK, researchers Stephen G. Slade, MD, Daniel S. Durrie, MD, and Perry S. Binder, MD, recently conducted a study of 50 patients, performing thin-flap LASIK on one eye and PRK on the other eye of each patient.

Thin-flap LASIK vs PRK: Study design and methods

Of the 100 eyes in the study, the mean pre-operative refractive error was -3.66 diopters (D) of nearsightedness and -0.66 D of astigmatism.

Eyes in the thin-flap LASIK group had a corneal flap created with a 60-Hz IntraLase femtosecond laser (Abbott Medical Optics, Santa Ana, CA). The intended flap thickness was 100 microns, which is about 20 to 40 percent thinner than conventional LASIK flaps.

All eyes in both groups underwent a customized laser ablation with a LADARVision 4000 CustomCornea excimer laser (Alcon Laboratories, Fort Worth, TX)

Follow-up exams were performed one, three and six months after surgery. Measurements performed at each visit included uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity (BSCVA), contrast sensitivity, corneal topography and wavefront aberrometry.

Thin-flap LASIK vs PRK: Study results

Data gathered during the follow-up visits yielded the following results:

• One month after surgery, 88 percent of thin-flap LASIK eyes achieved UCVA of 20/20 or better, compared with 48 percent of PRK eyes.
• Six months after surgery, there was no significant difference in UCVA of eyes in the two groups: 92 percent of thin-flap LASIK eyes and 94 percent of PRK eyes had UCVA of 20/20 or better.
• At one and three months, the thin-flap LASIK eyes had less significant higher-order aberrations (including spherical aberration) than the PRK eyes.
• Six months after surgery, there was no significant difference in higher-order aberrations between the two groups.

Thin-flap LASIK vs PRK: Study results

The researchers concluded that quality of vision recovered faster in eyes undergoing thin-flap LASIK compared with eyes undergoing PRK, but at six months there were no significant visual differences between the two groups.

The results of the study show that both thin-flap LASIK and PRK produce excellent visual outcomes.

One implication of this study if you are considering laser vision correction and want no possible risk of LASIK flap complications, PRK provides comparable visual acuity to thin-flap LASIK, but you probably will have to wait a few weeks longer after surgery for optimum visual acuity.

SOURCE:  A prospective, contralateral eye study comparing thin-flap LASIK (sub-Bowman keratomileusis) with photorefractive keratectomy. Ophthalmology. June 2009.