Excimer laser photorefractive keratectomy (PRK) to treat myopia has been used extensively.1 This technique uses an excimer laser to photoblate the corneal surface.2 The possible effect of this technique on the eye's posterior segment is not well known. Retinal detachment after PRK has been reported.3,4
This study evaluated the incidence and characteristics of choroidal neovascularization (CNV) in a consecutive series of patients with myopia corrected by PRK in a clinically controlled study.
Patients and Methods
This study retrospectively evaluated the incidence and characteristics of CNV in 5936 consecutive eyes in which myopia was corrected by PRK performed by 1 surgeon (A.A.) between April 1, 1992, and April 30, 1998, at the Instituto Oftalmológico de Alicante, Spain. All patients provided written fully informed consent in accordance with the Helsinki Declaration.
Selection criteria were age 18 to 55 years; stable myopia between −1.00 and −14.00 diopters (D); contact-lens intolerance; inability to wear glasses because of psychological inhibition; best spectacle-corrected visual acuity of at least 20/200; a normal anterior segment and peripheral retina; areas of lattice degeneration, atrophic holes, and flap tears treated with photocoagulation; and no general health problems. Patients with anisometropic amblyopia and those who had previous radial keratotomy or cataract surgery were included. Patients with corneal disease, glaucoma, or a history of ocular trauma were excluded. Corneal thickness had to be sufficient so that the remaining postoperative thickness was greater than 400 μm.
Preoperative and postoperative examinations included visual acuity (decimal fraction), manifest and cycloplegic refractions, slitlamp microscopy, applanation tonometry, pachymetry, videokeratography, and indirect ophthalmoscopy. Postoperative examinations were done at 1, 3, 6, and 12 months and, thereafter, once a year. The same physician did all follow-up examinations.
The PRK procedures were performed with a 193 mm excimer laser (VISX Twenty-Twenty) using topical anesthesia of oxybuprocaine hydrochloride 0.4% and tetracaine hydrochloride. The ablation was performed according to a published technique.4 Mean follow-up was 38.5 months ± 17.4 (SD) (range 12 to 85 months).
Choroidal neovascularization was diagnosed by fluorescein angiography and was treated by confluent argon-green laser photocoagulation (Nidek AC-230) through a Goldmann contact lens (Ocular Instruments Co.). The following parameters were used: spot diameter 200 μm, time 0.50 seconds, and sufficient power to induce intense burns (white).
The study parameters were incidence and clinical features of CNV, time between PRK and the occurrence of CNV, and the results of CNV treatment. The Kaplan–Meier survival analysis was used to estimate the risk of CNV.
Mean patient age at PRK was 30.5 ± 8.96 years (range 18 to 55 years). Mean preoperative refraction was −4.71 ± 2.86 D (range −1.00 to −14.00 D) (Table 1). In the series, 1925 eyes (32.42%) had more than −6.00 D of myopia.
Mean best corrected visual acuity (BCVA) before PRK was 20/25 (decimal fraction 0.8 ± 0.2) (range 20/200 to 20/20). Postoperatively, mean BCVA improved to 20/24 (decimal fraction 0.83 ± 0.20) (range 20/200 to 20/20).
A 41-year-old woman, under study at the hospital since March 1991 for high myopia in both eyes, developed CNV. In March 1991, BCVA was 20/40 in the right eye (spherical equivalent [SE] −12.50 D) and 20/200 in the left eye (SE −7.50 D).
The patient had PRK in September 1996. Preoperatively, BCVA was 20/50 (SD −12.00 D) in the right eye. The postoperative period was uneventful, and BCVA was 20/32 (SE −1.75) in December 1996.
In November 1998, the patient presented with a loss of vision, metamorphopsia, and a BCVA of 20/125 in the right eye. Fluorescein angiography showed well-defined extrafoveal CNV (Figures 1 to 3). Because of the characteristics of the CNV, it was treated by confluent direct argon-green laser photocoagulation as described in Patients and Methods. In December 1998, BCVA was 20/32 (SE −2.25 D) with complete closure of the CNV (Figure 4).
The cumulative risk of CNV in the myopic patients corrected by PRK in this study was 0.019% at 26 months.
Degenerative myopia (progressive or pathological) is characterized by an increase in the axial length and several pathological changes in the ocular tissues.5 The most important changes in the fundus are chorioretinal degenerations.6 The pathogenesis of degenerative changes in progressive myopia of the fundus is unknown7; however excessive axial elongation is an important factor as it thins the retina and choroid. Choroidal neovascularization is the last stage in the changes caused by degenerative myopia and is associated to the Fuchs' spot.7 The reported incidence of CNV in eyes with high myopia varies from 5% to 10%.8,9 In etiological studies of CNV, myopia higher than −6.00 D has been implicated. High myopia is reported as the principle etiological cause (62%) of CNV in young patients.10
In our study of 5936 eyes consecutively corrected by PRK for myopia, 1925 of which were highly myopic, only 1 case of CNV developed over a mean follow-up of 38.5 months. This incidence of CNV (0.017% total series; 0.051% highly myopic eyes) is lower than the reported incidence in eyes with degenerative myopia (5% to 10%).8,9 According to our results with a limited follow-up, PRK does not increase the risk of CNV in eyes with myopia.
In cases of high myopia, progressive elongation of the axial length and degenerative changes in the choroid cause the appearance of linear breaks in Bruch's membrane that are called lacquer cracks.11 These lacquer cracks have been found to be related to CNV.7,9
In our case of CNV, myopic changes did not previously exist in the fundus in the right eye. The time between the appearance of CNV and PRK (26 months) did not allow us to implicate PRK in the pathogenesis of CNV.
In the case in our study, the risk of developing CNV was primarily the result of myopia.6,7 There could have been an added risk, although minimal, from the trauma of the excimer laser shock wave during PRK.12,13
The natural course of CNV in high myopia is self-limited and evolves toward an atrophic nonexudative scar in 96% of cases.5 Treatment of CNV by photocoagulation in eyes with high myopia is indicated when the exudation reduces the central vision by serous or hemorrhagic detachment of the pigmentary epithelium.5 It is also indicated in cases of well-defined CNV; that is, more than 100 μm from the foveal center.14 In our case of CNV, laser treatment was performed because the distance between the CNV and fovea was associated with metamorphopsia and a marked decrease in BCVA.
The appearance of macular hemorrhage after excimer laser PRK13 and 1 case of cystoid macular edema15 have been published. To our knowledge, however, no cases of CNV after PRK have been reported.
In conclusion, PRK to correct myopia was followed by a low incidence of CNV. The appearance and the treatment of extrafoveal CNV was not followed by a significant decrease in BCVA.
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