Short-term effect of Nd : YAG laser capsulotomy on refraction , central macular thickness and retinal nerve fiber layer thickness

Objective: The aim of this study is to examine the effect of Nd: YAG laser capsulotomy on refraction, central macular thickness (CMT) and retinal nerve fiber layer (RNFL) thickness. Methods: 42 eyes of 42 patients who treated with Nd:YAG laser capsulotomy were included in thisprospective study. Spherical equivalent (SE), cyclindrical power refraction, CMT and RNFL thickness were evaluated preoperatively and at postoperative first day, first week and postoperative first month. Results: Spherical equivalent values and central macular thickness values did not significantly change in the first month after treatment. Average and nasal retinal nerve fiber layer thickness values significantly increase and cyclindrical power refraction significantly decreased during the visits. Conclusion: Nd:YAG laser capsulotomy is confident and reliable treatment option of the posterior capsula opacification (PCO). After Nd: yag laser capsulotomy cyclindrical power refraction and RNFL thickness values significantly change.


IntRoductIon
O ne of the most common long-term postoperative complication of the cataract surgery is posterior capsule opacification(PCO).PCO causes several problems such as low visual acuity, contrast sensitivity and uniocular diplopia.The main reason of PCO is remaining lens epithelial cells.These cells proliferate and migrate into the optic axis between intraocular lens and posterior capsule.Some factors associated with PCO, these are; patient age, intraocular lens (IOL) type, surgery technique, any ocular or systemic disease.For example, PCO rate is higher in children or young patients than in geriatric patients.Any patient who has diabetes mellitus or uveitis, PCO development rate increases.3) Standard treatment of the PCO is neodymium-doped yttrium aluminium garnet (Nd:YAG) laser capsulotomy.This procedure is effective but it has several complications such as postoperative intraocular pressure (IOP) elevation, IOL damage, inflammation, cystoid macular edema, and retinal detachment. (4,5)Macular edema is caused by inflammatory mediators due to the damage of blood-aqueous barrier; elevated IOP is associated with an increased amount of aqueous particles following Nd:YAG laser capsulotomy. (6,7)Elevated IOP can affect retinal nerve fiber layer (RNFL) thickness.If IOP elevation does not detect, RNFL and visual field damaging may occur.But the IOP elevation is usually transient.We can easily measure macular and RNFL thickness with optical coherence tomography (OCT).Nd:YAG laser capsulotomy may affect refraction.There is no consensus about the effect on refractive status.Some authors claimed that large capsulotomy size may cause posterior movement of the IOL and result in hyperopic shift. (8)On the other hand, some authors said that Nd:YAG laser capsulotomy does not affect spherical equivalent (SE). (9)his study aims to examine the short-term effects of Nd:YAG laser capsulotomy on refraction status, macular and RNFL thickness.

methods
Approval was obtained from the local ethics committee for the study.The study conformed to the tenets of the Declaration of Helsinki.
This prospective study enrolled 42 patients with PCO.All patients treated with Nd:YAG laser capsulotomy between August 2014 and July 2015.The minimum period between the cataract surgery and Nd:YAG capsulotomy was 6 months.The inclusion criteria were previous uncomplicated phacoemulsification, presence of PCO, visual loss (at least two lines compared to the last visit) and a capsulotomy opening approximately 4.0 mm in diameter.The exclusion criteria were glaucoma, corneal or retinal disease, a history of ocular inflammation, previous laser treatments, ocular trauma, or surgery during the follow-up period, anterior capsule contraction and implantation of a toric or multifocal IOL.
All subjects underwent a comprehensive eye examination including refraction and best corrected visual acuity (BCVA), slit-lamp biomicroscopy, intraocular pressure measured using Goldman tonometry, and fundus examination.Refraction power was measured using a Topcon KR-8100 autorefractor (Topcon Corp, Nagoya, Japan).The spherical equivalent RE values were calculated as the sum of the sphere plus half the cylindrical power.
All measurements were repeated 3 times.VA was measured with an office-based Snellen system.The VA values were converted to logMAR units for statistical analysis.
All patients underwent OCT measurements preop¬eratively and postoperatively (at 1 and 7 days and 1 month).Before OCT measurement, full mydriasis was obtained with topical tropicamide 1% and phenylephrine hydrochloride 2.5%.Data on retinal nerve fiber layer (RNFL) thickness and macular thickness were obtained using the OCT (Spectral OCT SLO; Opko/OTI, Miami, FL).We also divided patients into 2 groups according to the energy levels.Group A was lower than 60 joules, group B was higher than 60 joules and we compared the OCT values.Signal strength was rated on a ten-point scale; signal strength values of ≥ six were considered acceptable.Multiple images were taken from each eye by an experienced operator and the scan with the best signal was chosen for the study.
Tropicamide 1% and phenylephrine 2.5% were administered for pupillary dilatation prior to the procedure.A standard contact lens was used to enhance power density at the level of the posterior capsule and an approximately 4.0 to 4.5 mmdiameter circular area of the central posterior capsule was cleared by emitting laser energy on the capsule.Energy levels starting from 1.0 mJ and up to 2.5 mJ were applied to the capsule.The Nd:YAG laser was posterior defocused by 0.50 mm in every eye.The energy level, total spot count, and total energy use of each patient were recorded.After capsulotomy, prednisolone acetate 1% four times daily and brimonidine tartrate 0.2% two times daily for 1 week were prescribed.All the procedures were performed by one ophthalmologist (S.C) and using an ophthalmic Nd:YAG laser (Visulas YAG II, Zeiss, Germany) with an Abraham capsulotomy lens (Ocular Instruments Inc, Bellevue, Washington).
Data analysis was performed using Statistical Package for Social Sciences for Windows software (SPSS version 16.0, SPSS Inc. Chicago, USA).Normality distribution of variables was tested by Kolmogorov-Smirnov test.Mean ± standard deviations (SD) were used in the presentation of the data.Variables distributed normally were compared using Student's T test and variables distributed abnormally were compared using Mann-Whitney U test.Comparison of the parameters before and after the surgery between groups was performed using general linear model (repeated measure).Categorical variables were presented as frequency (%) and compared between the groups using chi-square test and Fisher's exact test.Differences were considered statistically significant when p value was<0.05.
The mean foveal thickness was 214.10 ±30.67μm preoperatively, 217±33.9μmat 1 day, 217.67 ±34.17μm at 1 week and 218.26±32.44μmat 1 month, postoperatively.There was not found any statistically significant difference between preoperatively and postoperatively foveal thickness (p value:0.769).We also compared foveal thickness according to the energy levels in two group (Group A was lower than 60 joules, group B was higher than 60 joules), there was not any significant difference (p value: 0.259).

dIscussIon
Standard treatment of the PCO is Nd:YAG laser capsulotomy.This method is reliable but it has several complications such as IOP changes, IOL damage and dislocation, iridocyclitis, vitreous hemorrhage and vitritis, retinal detachment, macular edema and refraction change.In our study, we did not find significant change in spherical equivalent but we detected that cylindrical power refractions significantly decrease during the first month.Our results are similar to the study of Khambhiphant et al.They analyzed 47 eyes and they found no significant change in SE but they found significant decrease cylindrical change after 3 months (10) .In 2014, Oztas et al. found significant decrease in cylindrical power refraction (11) .Vrijman et al. reported that Nd:YAG laser capsulotomy resulted in unchanged refraction for 93% of eyes. (12)hey did not find significant change in either SE or cyclindrical power refraction.Ozkurt et al. and Hu et al. also reported no significant change in SE after treatment. (13,14)Zaidi et al. found hyperopic shift in SE. (15) They found that yag laser capsulotomy causes anterior movement of the IOL.For the satisfaction of the patient, it is important to prescribe the optimal corrective lenses after the yag laser capsulotomy.Because of the decreasing cylindrical refraction, we suggest the prescription after 1 month.
In general, yag laser capsulotomy improve visual functions.We found a statistically significant improvement in BCVA.Sometimes BCVA can not increase as we expect.One of the reasons of the worse BCVA is cystoid macular edema.Yag laser capsulotomy damage the vitreous and cause increasing inflammatory mediators.These mediators increase permeability of the perifoveal capillaries and result in cystoid macular edema.Karahan et al. examined patients according to the yag laser capsulotomy size(group 1‹3.9mm, group 2›3.9mm).They found that CMT thickness increased 1 week after treatment but 4 weeks after treatment CMT decreased to preoperative levels in both groups. (16)Ari et al. who examined 30 patients according to the energy levels, they found significant increase in CMT in high energy group. (17)Another study from Turkey, Yılmaz et al. studied about long term changes in macular thickness, they did not find significant change in CMT. (18)Casas et al. found insignificant change in CMT at 3 months after treatment. (19)Steinert et al. found that the rate of cystoid macular edema was 1.23% in 897 patients. (20)In our study, we evaluated 42 eyes, we did not find significant change in CMT during the first month.We also compared CMT according to the energy levels, we did not detect any significant difference in short follow-up period.We did not observe cystoid macular edema in any patient.
The other complication of the yag laser capsulotomy is IOP change.Brimonidine tartrate was used after yag laser capsulotomy two times daily for 1 week.We checked IOP every visit during the follow-up period and we did not find significant increase.The RNFL measurement is a helpful diagnostic method in optic disc diseases especially in glaucoma.After yag laser capsulotomy glaucoma may become.In glaucoma, RNFL thickness get thinner.In the analysis of RNFL, we found increase in average RNFL and nasal quadrant of RNFL thickness.Our findings is correlated to study of Kara et al.They found statistically significant increase in average RNFL thickness which was measured 90.24± 15.9 μm preoperatively and 98.27±14.1 μm postoperatively.(21) Although we find statistically significant increase in RNLF thickness, the result is not clinically significant.We think that preoperative RNFL thickness was measured low because of the influence of PCO on OCT image quality so we think that preoperative RNFL thickness is of low reliability.After the yag laser capsulotomy image quality improves and reliability of the RNFL scans increase.This differentiation may affect the measuring of the CMT.In our study we took multiple images from each eye and the scan with the best signal was chosen for the study.Gonzales et al. showed that yag laser capsulotomy improve signal strength, but they did not find significant difference in CMT between preoperative and postoperative measurements. (22)Further evaluation about this issue is needed.

conclusIon
We searched the short time effect of the yag laser capsulotomy on spherical equivalent, cylindrical power refraction, central macular thickness and retinal nerve fiber layer thickness.Nd: YAG capsulotomy is a safe procedure and produces a statistically small increase in average RNFL thickness that is not clinically significant and leads to an improvement in BCVA and reduction in cylindrical power.