Previous clinical trials involving excimer laser photorefractive keratectomy (PRK) have used a laser repetition rate of 5 or 10 Hz.1–12 The influence of repetition rate upon the results of PRK is unclear. In this study, we present results of PRK with an increased repetition rate of 15 Hz.
Patients and Methods
Twenty-three eyes of 14 patients had PRK for myopia as part of a clinical trial conducted in accordance with guidelines of the United States Food and Drug Administration under an investigational device exemption granted to Summit Technology, Inc. Informed consent was obtained from all patients after the procedure had been fully explained. All laser treatments were conducted at 1 center by a single surgeon (P.S.H.) using a 15 Hz repetition rate. The mean patient age was 34 years (range 24 to 51 years). The mean preoperative spherical equivalent was –4.62 diopters (D) and the mean attempted correction, –4.41 D (range –2.8 to –5.5 D).
Detailed preoperative and postoperative ophthalmologic examinations included visual acuity measurements under controlled lighting conditions using an Early Treatment of Diabetic Retinopathy Study chart, manifest and cycloplegic refractions, keratometry, and slitlamp examination.
Photorefractive keratectomy was performed using the Summit Technology, Inc., Apex® excimer laser. Laser parameters included a repetition rate of 15 Hz, fluence of 180 mJ/cm2, and pulse duration of 14 nanoseconds, resulting in an estimated ablation rate of corneal stromal tissue of 0.25 μm per pulse. Ablation zone diameter was 6.00 mm in all cases.
Preoperatively, each patient received 2 drops of pilocarpine 1% in the operative eye. After a topical anesthetic agent was applied, the central epithelium was removed manually by gentle scraping. Centration was achieved by asking the patient to fixate on a light coaxial with the laser beam while helium–neon aiming beams were directed at the 3 and 9 o'clock positions, equidistant from the pupillary border. Once reliable centration was achieved, the laser procedure was started. The surgical technique has been described.1 When the procedure was completed, a combination antibiotic–steroid ointment (tobramycin, dexamethasone, and fluorometholone) was applied and the eye patched.
Postoperatively, patients were examined each day and continued using the antibiotic–steroid ointment 5 times daily until the cornea re-epithelialized. They were then examined at 1, 3, and 6 months. Fluorometholone 0.1% was instilled 4 times daily for 1 month, 3 times daily for 1 month, twice daily for 2 weeks, once a day for 1 week, and then discontinued or tapered at the surgeon's discretion.
Patients were evaluated postoperatively for uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity (BSCVA), stability of refraction, and predictability.
Slitlamp examination was performed to evaluate anterior stromal haze, which was graded as clear, trace, mild, moderate, or marked. A clear cornea appeared normal on slitlamp examination and demonstrated no haze. Trace subepithelial haze could be observed only by oblique slitbeam illumination, and mild haze could be seen by direct illumination with a thin slitbeam. Moderate anterior stromal haze partially obscured visualization of the iris, while marked haze almost completely obscured iris detail.10
At the 1 and 6 month visits, patients were given a questionnaire and asked to report postoperative glare and halo symptoms. This subjective assessment of the glare/halo effect was graded on a 0 to 5 scale, with 0 representing no symptoms and 5 indicating a severe effect.
After the data were compiled, they were entered into a spreadsheet program (Excel®). Twenty eyes were available for follow-up at 3 months, and 19 were available at 6 months.
Postoperative refractive stability, or change in refraction over time, was determined by the mean spherical equivalent at each postoperative examination point. Predictability, which indicates the accuracy of the procedure, was defined as the achieved minus the attempted correction.
The Fisher exact test was used to compare our results with those of a study using a 10 Hz repetition rate.13 Significance was set at the 0.05 level.
Uncorrected visual acuity preoperatively and 3 and 6 months postoperatively is shown in Table 1. At 3 months, the UCVA was 20/25 or better in 20 eyes (100%). At 6 months, it was 20/32 or better in 19 eyes (100%) and 20/20 in 14 (73.7%). These results were similar to those in a study using a 10 Hz repetition rate (P = 1.00).
Best spectacle-corrected visual acuity preoperatively and 3 and 6 months postoperatively is shown in Table 2. Two eyes (10.5%) lost 2 or more Snellen lines of BSCVA at 3 and 6 months (Figure 1). However, BSCVA was 20/25 or better in all eyes (100%) at both 3 and 6 months and 20/20 or better in 18 eyes (94.7%) at 6 months.
Mean spherical equivalent refraction preoperatively and 1, 3, and 6 months postoperatively is shown in Figure 2. Myopic regression occurred after 1 month.
At 6 months, predictability was within ±0.5 D in 15 eyes (78.9%) and within ±1.0 D in 19 (100%) (Figure 3). No eyes were undercorrected or overcorrected by 1.00 D or more.
At 6 months, 4 eyes (21.0%) had clear corneas without any corneal haze. Fourteen eyes (73.7%) demonstrated trace subepithelial haze. Fifteen eyes (78.9%) had no glare/halo effect, and 4 (21.0%) had minimal glare/halo effect (1 on the scale).
The 4 patients (17.4%) lost to follow-up and the patients who completed the 6 month study had similar preoperative and postoperative characteristics; i.e., there were no differences in preoperative UCVA or BSCVA, manifest refraction spherical equivalent, or attempted correction. At 2 months postoperatively, 3 of the patients (75%) had a UCVA of 20/25 or better compared with 94% of the patients who completed the study. They were within ±1.0 D of attempted correction; 100% of the patients who completed the study were within ±1.0. The remaining patient was lost to follow-up 3 days postoperatively. Efforts to contact all patients lost to follow-up were unsuccessful.
Previous studies of the repetition rate in excimer laser PRK have found that modifying the rate may influence the ablation effect on the corneal surface. In a study using variable repetition rates in pig eyes, Fasano and coauthors14 suggest that lower repetition rates may lead to decreased corneal surface irregularity.
We hypothesize that variable repetition rates may influence the ablation effect on the corneal surface through corneal hydration changes. Several investigators have presented theories about corneal hydration changes during PRK. The shock-wave theory proposes that acoustic shock waves create ripples in the corneal fluid, which accumulate centrally.15 The differential-hydration model suggests that hydration increases centrally during PRK because ablation occurs closer to the endothelium in the central cornea than in the periphery.16
By acting as a barrier to excimer laser pulses, these hydration changes may influence ablation and play a role in the formation of central islands and other topography patterns.17 Central islands are small areas—1.0 to 3.0 mm in diameter and 1.0 to 3.0 D steep—of relatively underablated tissue in the central cornea detected by corneal topography.15,17–19 Increasing the laser repetition rate from 10 to 15 Hz may influence corneal ablation, predictability of refractive correction, and surface morphologic and topographic architecture by effecting corneal hydration changes during the procedure. However, our study showed no salient differences between the 2 repetition rates.
Preoperative patient characteristics and clinical outcomes in this 15 Hz study are similar to those reported in a previous study using a 10 Hz repetition rate.13 The mean age of patients was 34.0 years in the 15 Hz study and 39.0 years in the 10 Hz study and the mean preoperative spherical equivalent, –4.62 ± 0.86 D and –4.99 ± 1.43 D, respectively. Follow-up in both investigations was 6 months. At that time, UCVA was 20/32 or better in 100% of patients in the 15 Hz study and in 98% of those in the 10 Hz study. The Fisher exact test showed a statistical similarity between the 2 groups. As shown in Table 3, other refractive parameters between the 2 studies were similar.
Comparison of the 2 studies is limited because of the larger sample size in the 10 Hz investigation. In addition, 2 patients in this study (10.5%) lost 2 or more Snellen lines of visual acuity, whereas no patient in the 10 Hz study lost 2 or more lines. Of these 2 patients, however, one had a preoperative BSCVA of 20/10 and a postoperative BSCVA of 20/20 at 3 and 6 months. The other patient had a preoperative BSCVA of 20/16 and a postoperative BSCVA of 20/25 at 3 and 6 months. There was no evidence of haze or topographic changes in these patients that would account for this loss of BSCVA. Thus, the significance of this loss is questionable and should be addressed in a larger study of this kind.
Continued follow-up will be necessary to confirm our initial findings. In addition, corneal topography studies may help in analyzing the effects of an increased repetition rate in PRK given the potential influence of hydration and procedure length on surface irregularities.20
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