Using thickness measurements done with confocal microscopy, investigators at the Mayo Clinic demonstrated that the mean central ablation depth after LASIK is significantly greater than the predicted ablation depth. They showed that the difference between the measured and the predicted ablation depths increased as the ablation depth increased. In contrast, this study showed no significant difference between the measured and the predicted ablation depths after PRK performed with the same excimer in the same environment.
Twenty-five eyes (-4.23 ±1.38 D) of 15 patients (mean age 39 ±7 years) underwent PRK. LASIK was performed on 25 eyes (-6.64 ±
2.39 D) of 15 patients (mean age 33 ±10 years). All patients were white. Contact lens use was discontinued two weeks preop for soft lenses and three weeks preop for GP lenses.
Surgeons used the Visx Star S2 laser in all procedures, and used the automated Hansatome microkeratome for the LASIKs.
In LASIK, the measured ablation depth (81 µm ±34) was 25 percent greater than the predicted ablation depth (65 µm ±13; P = .007). The difference between the measured and predicted depths was positively associated with the mean ablation depth (r = 0.81, P < .001).
In PRK, there was no difference between the measured ablation depth (48 µm ±19) and the predicted ablation depth (47 µm ±18; P = .84).
(J Cataract Refract Surg 2004; 30:321-325)
Erie J, Hodge D, Bourne W
Corneal Effects of 30-day Lenses
In a Turkish study, the mean central and peripheral corneal thickness increased in patients wearing 30-day Focus Night & Day continuous-wear silicone hydrogel contact lenses (CIBA Vision) despite the high oxygen permeability of the lenses.
 |
Researchers fit 26 eyes of 13 patients (mean age 26 ±6 years) with Night & Day lenses (refractive power = -1.25 to -4.50 D, diameter = 14 mm, base curve = 8.60). None of the patients had worn contact lenses before. The physicians measured the thicknesses of the central corneal area and eight peripheral areas with ultrasonic pachymetry, and captured the corneal topographies and simultaneous keratometric values with an automated keratorefractometer. Patients with keratometric values greater than 780 mm were excluded from the study.
The researchers evaluated the subjective complaints and slit-lamp appearances of patients 24 hours after lens wear and every five days during a one-month period. They measured corneal thicknesses before CL fitting and at day one, day 10 and one month after lens wear—always between 10 a.m. and noon.
Two patients developed complications (punctate epitheliopathy, excessive increase in corneal thickness) and were excused from the study. Visual acuity (20/20 with contact lenses in all patients) did not change during the one month of the study. There were no differences in mean values of corneal topography. However, extended wear of lenses did cause local corneal thinning or thickening in one month on the same regions of the cornea: superior, inferior, superior peripheral, nasal peripheral and inferior peripheral regions.
(Eye & Contact Lens 2004; 30:1: 6-9)
Yeniad B, Seidu Adam Y, Közer Bilgin L, Gözüm N
Formula to Predict LASIK Retreatment
Researchers at Northwestern University used a retrospective study of LASIK enhancement in 130 of 720 eyes to develop a formula to predict a patient's need for enhancement. They found that the factors of older age, higher preoperative cycloplegic sphere and the surgeon himself significantly influenced a patient's likelihood for LASIK retreatment. The formula is presented in the table below.
Researchers explain that to use their formula, individual surgeons will have to evaluate their existing retreatment rates and determine their surgeon factor between 0 and 1. Conservative surgeons can use the coefficient 0 and calculate the score based on age and preop cycloplegic sphere to estimate the retreatment probability. More aggressive surgeons can use the surgeon coefficient 1. They note that surgeons using this formula will notice that its strength lies in its high negative predictive value (93 percent). They say that for patients with probability scores below the cut point (0.15), the likelihood of not requiring retreatment is 93 percent.
(J Cataract Refractive Surg 2004; 30:363-368)
Hu D, Feder R, Basti S, Fung B, Rademaker A, Stewart P, Rosenberg M
One-Eye Glaucoma Drug Trials
Uniocular trials of glaucoma medications don't adequately predict second-eye intraocular pressure responses to the same medications, says a team of researchers from West Virginia, New Jersey and Arkansas. They reviewed 52 eligible charts from glaucoma patients seen in university-based glaucoma specialty practices. Seventy-three percent of the patients were female, 71 percent were white and the mean age was 66 ±14 years.
Among the 52 first-treated eyes undergoing uniocular trials with various medications, intraocular pressure dropped from a mean pretrial baseline of 22.4 ±5.2 mmHg to a mean IOP of 16.7 ±4.3 mmHg. This represented a 5.7 mmHg, or 24-percent, reduction in IOP. When the same medication was then added to the second eye of each patient, the mean IOP in the second eye dropped from a pretreatment baseline of 19.7 ±4.2 mmHg to a mean IOP of 16.9 ±3.7 mmHg, a 2.8-mmHg, or 14.2-percent, reduction. A calculation of the IOP reduction in the first and second eyes demonstrated essentially no correlation between the magnitude of IOP responses of fellow eyes.
Researchers examined subsets of data to rule out the possibility of the effects of other variables on the predictive ability of the uniocular drug trial. To account for contralateral crossover effect, they analyzed a subset of 26 latanoprost users. To take the type of glaucoma into account, they analyzed a subset of 31 patients with POAG. To find out if a history of glaucoma surgery had an effect, they analyzed a subset of six patients who had undergone unilateral or bilateral laser trabeculoplasty or trabeculectomy before the uniocular trial. In all cases, essentially no correlation was demonstrated between IOP reduction responses of fellow eyes.
(Ophthalmology 2004; 111:421-426)
Realini T, Fechtner R, Atreides S, Gollance S
Equipment Cleaning and Silicone Oil Emulsification
German investigators report that during routine cleaning and sterilization of vitreoretinal instruments and accessories, remnants of silicone oil and detergents can persist. These impurities may trigger emulsification of the silicone oil, which comes into contact with these contaminated devices during instillation of the endotamponade.
Investigators determined the contaminants on a ready-to-use standard set of vitreoretinal instruments that had been cleaned and sterilized in an eye clinic. The determination of detergents was carried out according to a standardized procedure that first uses ultrapurified water to rinse the equipment, then measures conductivity. Researchers used Fourier transformed infrared spectroscopy to determine silicone oil remnants. In addition to a scrupulous cleaning regimen, authors recommend disposable instruments and well-trained staff.
(Retina 2004;24:110-115)
Dresp J, Menz D-H
