

ORIGINAL ARTICLE 

Year : 2020  Volume
: 33
 Issue : 2  Page : 641645 

Role of central corneal thickness in intraocular pressure changes after corneal refractive surgery
Melad N Sadek^{1}, Abd ElKhalik E. ElSaadany^{2}, Moataz F Elsawy^{2}
^{1} Department of Ophthalmology, Faculty of Medicine, Sohag Ophthalmic Hospital, Sohag University, Sohag, Egypt ^{2} Department of Ophthalmology, Faculty of Medicine, Menoufia University, Menoufia, Egypt
Date of Submission  20Dec2019 
Date of Decision  17Jan2020 
Date of Acceptance  21Jan2020 
Date of Web Publication  27Jun2020 
Correspondence Address: Melad N Sadek Department of Ophthalmology, Faculty of Medicine, Sohag University, Qena 83611 Egypt
Source of Support: None, Conflict of Interest: None  Check 
DOI: 10.4103/mmj.mmj_385_19
Objective The aim of the study was to assess importance of central corneal thickness (CCT) in the intraocular pressure (IOP) variation after corneal refractive surgery. Patients and methods This prospective, interventional study contains 75 patients who were divided into three equal groups. Group I had undergone femtosecond laserassisted insitu keratomileusis (femtoLASIK); Group II had undergone laser insitu keratomileusis (LASIK); and group III had undergone photorefractive keratectomy (PRK). The evaluated parameters included age, sex, spherical equivalent refraction (preoperative and postoperative), CCT preoperatively and postoperatively and IOP preoperatively and postoperatively at 1, 3, and 6 months by Goldmann applanation tonometry (GAT) and ocular response analyzer. Result The mean CCT reductions were 56.6 ± 34.1, 52.8 ± 25.5, and 40.5 ± 25.9 μm after femtoLASIK, LASIK, and PRK surgery. The mean IOP reductions by GAT were 2.42 ± 1.9, 2.46 ± 2.56, and 1.14 ± 1.31 mmHg after 1 month in femtoLASIK, LASIK, and PRK; however, after 3 months 3.14 ± 1.87, 3.22 ± 2.99, and 2.7 ± 2.23 mmHg after femtoLASIK, LASIK, and PRK. After 6 months the mean IOP reduction was 3.22 ± 1.83, 3.44 ± 2.91, and 2.84 ± 2.32 mmHg after femtoLASIK, LASIK, and PRK. The mean IOPg reduction was 2.53 ± 1.81, 3.19 ± 1.76, and 3.29 ± 1.87 mmHg at 1, 3, and 6 months after femtoLASIK. However, the mean decrease in IOPg was 2.67 ± 2.08, 3.28 ± 1.95, and3.52 ± 1.89 mmHg at 1, 3, and 6 months after LASIK. The mean decrease in IOPg was 1.26 ± 1.98, 2.85 ± 1.98, and 2.98 ± 1.79 mmHg at 1, 3, and 6 months after PRK. IOPcc is not correlated with CCT. Conclusion The IOP by GAT and IOPg reduction after corneal refractive surgery significantly depends on CCT changes, but IOPcc measurements are not associated with CCT.
Keywords: central corneal thickness, corneal refractive surgery, intraocular pressure
How to cite this article: Sadek MN, ElSaadany AE, Elsawy MF. Role of central corneal thickness in intraocular pressure changes after corneal refractive surgery. Menoufia Med J 2020;33:6415 
How to cite this URL: Sadek MN, ElSaadany AE, Elsawy MF. Role of central corneal thickness in intraocular pressure changes after corneal refractive surgery. Menoufia Med J [serial online] 2020 [cited 2020 Jul 16];33:6415. Available from: http://www.mmj.eg.net/text.asp?2020/33/2/641/287794 
Introduction   
Corneal refractive surgery remodels the cornea by LASER to change the corneal refractive power and improve the visual acuity^{[1]}. The corneal tissue was ablated. So the central corneal thickness (CCT) was reduced. So, the postoperative intraocular pressure (IOP) value by noncontact air tonometry or Goldmann applanation tonometry (GAT) was decreased^{[2]}. The underestimation of IOP retards the diagnosis and management of glaucoma^{[3]}. Ocular response analyzer (ORA) obviates IOP underestimation after corneal refractive surgery^{[4]}. In our study, we evaluate the importance of CCT in IOP changes after corneal refractive surgery.
Patients and Methods   
This was a prospective, interventional study performed on 75 patients, above 18 years of age; the patients were myopic or myopic astigmatism with the spherical equivalent of cycloplegic refraction of less than −8.00 D. The refraction was stable for at least 1 year. The patient was treated for corneal refractive surgery during the period between 2018 and 2019 at The International Eye Center at Luxor with The Ophthalmology Department, Faculty of Medicine, Menoufia University.
Before initiating this study, written informed consent was acquired from each patient after full discussion of the procedure involved, duration of treatment, possible intraoperative maneuvers, and postoperative symptoms.
Each patient knew that participation was voluntary and that he or she might withdraw from the study at any time and without giving any cause. The patient's withdrawal did not affect the medical treatment or relationship with the treating surgeon.
Patients who were less than 18 years, of CCT less than 480 μm or have an ocular disease, previous ocular trauma or surgery, pregnancy, diabetes mellitus, collagen disease, or other systemic diseases that affect the eyes were excluded from this study. Patients were classified into three equal groups according to the clinical indication of each surgery. The patients had the right to choose the type of corneal refractive surgery after explaining the benefit and possible complications of each one. Each group contained 25 patients. Group I was treated with the femtolaserassisted insitu keratomileusis (femtoLASIK) procedure with corneal flaps were created by the VisuMax femtosecond laser (Carl Zeiss Meditec, Oberkochen, Germany). Group II was treated with LASIK surgery with corneal flaps created by a Moria M2 microkeratome (Moria SA, Antony, France). Group III was treated with photorefractive keratectomy (PRK) surgery as the epithelium of the cornea was removed by alcohol. The excimer laser with a Wave Light EX500 Excimer Laser System (Alcon Surgicals, Fort Worth, Texas, USA) was used to ablate the central corneal stroma in all patients.
IOP measurement was performed by GAT (Inami, Rtype) and ORA (Reichert Ophthalmic Instruments Inc., Buffalo, New York, USA) for all patients before and after the operation at 1, 3, and 6 months. CCT and corneal topography were done before and after the operation by Pentacam (Oculus Optikgeräte GmbH, Wetzlar, Germany). Also, the spherical equivalent of cycloplegic refraction was measured preoperatively and postoperatively by Streak retinoscopy (3.6V; Keeler, Clewer Hill Road, Windsor, SL4 4AA, UK), at The International Eye Center at Luxor.
Statistical analysis
Data were assembled, classified, and statistically analyzed using an IBM personal computer with Statistical Package of the Social Sciences (SPSS) version 22 (SPSS Inc., Chicago, Illinois, USA). Quantitative data were approached in the form of mean, SD, range, and qualitative data were presented in the form of numbers and percentages. χ^{2} test was applied to study the relationship between two qualitative variables. A paired ttest was utilized for comparison among two related groups having quantitative variables. Pearson's correlation (r) measures the relationship between quantitative variables. Spearman's coefficient (r) is suitable for both continuous and discrete variables, including ordinal variables. A P value of less than 0.05 was considered statistically significant.
Ethical approval
All procedures were carried out in this study including human participants as per the ethical standards of the Institutional Research Committee of Menoufia University and with 1964 Helsinki Declaration and its subsequent modifications.
Results   
This study includes 75 patients (25 men and 50 women); the mean age was 25.1 years (ranged from 19 to 42 years). The mean difference between the preoperative and postoperative spherical equivalent of cycloplegic refraction was −3.90 ± 2.21 D in the femtoLASIK group, −3.53 ± 1.75 D in the LASIK group, and −2.73 ± 1.21 D in the PRK group. There were no statistically significant differences between the groups.
[Figure 1] shows CCT changes as the preoperative CCT were 549.1 ± 34.4 μm, reduced to 492.0 ± 46.5 μm postoperatively with the mean reduction of 56.6 ± 34.1 μm in the femtoLASIK group. However, in the LASIK group, the preoperative CCT values were 543.1 ± 25.8 μm which were reduced to 490.2 ± 33.9 μm postoperatively with the mean reduction of CCT being 52.8 ± 25.5 μm. In the PRK group, the preoperative CCT value was 528.4 ± 37 μm preoperatively which was reduced to 487.9 ± 44.9 μm postoperatively with a mean reduction of 40.5 ± 25.9 μm.  Figure 1: Comparison between preoperative and postoperative central corneal thickness μmin the groups.
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[Figure 2] shows IOP measurement by GAT in three groups. In the first month, the reduction in IOP was greater in the femtoLASIK and LASIK groups compared with the PRK group. There was a little IOP difference after 1 month in the PRK group. In the femtoLASIK group, there was little difference in IOP value between the 3 and 6 months but in LASIK and PRK groups there was a significant change in followup period after 3 and 6 months. IOP readings in the femtoLASIK group were 16.8 ± 2.25 mmHg preoperatively, and 14.1 ± 2.19, 13.6 ± 2.29, and 13.2 ± 2.28 mmHg for 1, 3, and 6 months postoperatively, However, the mean IOP reduced from 16.1 ± 2 mmHg preoperatively to13.6 ± 2.49, 12.8 ± 1.84, and 12.6 ± 1.8 mmHg for 1, 3, and 6 months postoperatively, respectively, in LASIK patient and the mean IOP readings were 15.3 ± 2.41 mmHg preoperatively, and 14.1 ± 2.45, 12.6 ± 2.53, and 12.4 ± 2.53 mmHg for 1, 3, and 6 months postoperatively, respectively, in the PRK group.  Figure 2: Comparison between preoperative and postoperative intraocular pressure mmHg measured by Goldmann applanation tonometry among studied groups.
Click here to view 
Thus, the mean decrease in IOP at 1 month was 2.42 ± 1.9, 2.46 ± 2.56, and 1.14 ± 1.31 mmHg after femtoLASIK, LASIK, and PRK, respectively. However, the mean reduction of IOP at 3 months was 3.14 ± 1.87, 3.22 ± 2.99, and 2.7 ± 2.23 mmHg after femtoLASIK, LASIK, and PRK, respectively. At 6 months postoperatively the mean reduction of IOP was 3.22 ± 1.83, 3.44 ± 2.91, and 2.84 ± 2.32 mmHg.
ORA measures the Goldmanncorrelated IOP measurement (IOPg), which simulates IOP measured by Goldmann tonometer and the cornealcompensated intraocular pressure (IOPcc).
The IOPg measurement by ORA had a significant postoperative reduction (P < 0.05), decreased from 16.6 ± 2.15 to 14.1 ± 2.16, 13.5 ± 2.29, and 13.3 ± 2.28 mmHg postoperatively at 1, 3, and 6 months, with a mean reduction of 2.53 ± 1.81, 3.19 ± 1.76, and 3.29 ± 1.87 mmHg in femtoLASIK patients. In the LASIK group preoperatively IOPg was16.3 ± 2.12 reduced to13.5 ± 2.52, 12.95 ± 1.86, and 12.52 ± 1.85 mmHg postoperatively at 1, 3, and 6 months, with the mean reduction of 2.67 ± 2.08, 3.28 ± 1.95, 3.52 ± 1.89 mmHg. However, preoperative IOPg in the PRK patient was 15.4 ± 2.46 which was reduced to 14.3 ± 2.48, 12.7 ± 2.67, and 12.36 ± 2.66 mmHg postoperatively at 1, 3, and 6 months, with a mean reduction of 1.26 ± 1.98, 2.85 ± 1.98, and 2.98 ± 1.79 mmHg, respectively.
[Figure 3] shows IOPcc measurement by ORA in three groups, as the IOPcc decreased from a mean value of 16.92 ± 2.32 mmHg preoperatively to 15.85 ± 2.63, 15.52 ± 2.42, and 15.39 ± 2.32 mmHg postoperatively at 1, 3, and 6 months in femtoLASIK patients, with a mean reduction of 1.23 ± 0.42, 1.45 ± 0.56, and 1.56 ± 0.45 mmHg. in the LASIK group preoperative IOPcc was16.76 ± 2.05 mmHg which was reduced to 15.55 ± 2.3, 15.05 ± 2.5, and 14.91 ± 2.67 mmHg postoperatively at 1, 3, and 6 months, with a mean reduction of 1.16 ± 0.56, 1.68 ± 0.67, and 1.79 ± 0.57 mmHg. However, preoperative IOPcc in the PRK patient was 15.95 ± 2.11 mmHg which was reduced to 15.32 ± 2.48, 14.31 ± 2.78, and 14.17 ± 2.78 mmHg postoperatively at 1, 3, and 6 months, with a mean reduction of 0.62 ± 0.36, 1.63 ± 0.64, and 1.75 ± 0.68 mmHg, respectively.  Figure 3: Comparison between preoperative and postoperative intraocular pressure mmHg measured by ocular response analyzer among studied groups.
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[Table 1] shows the correlation between postoperative CCT and IOP measured by GAT and ORA among the studied groups. There was a significant positive correlation between postoperative CCT and IOP measured by GAT and IOP g by an o ORA among the studied groups in three groups. IOP measured by GAT r was 0.486. The P value was 0.014 in the femtoLASIK group. However, in the Lasik group r was 0.469, the P value was 0.018 and in the PRK group r was 0.465, the P value was 0.019. However, IOPg measured by ORA r was 0.489. The P value was 0.013 in the femtoLASIK group. However in the LASIK group r was 0.472, the P value was 0.017 and in the PRK group r was 0.466, the P value was 0.018. There was a nonsignificant correlation between the postoperative reduction of IOPcc and CCT in three groups as IOPcc was not affected by CCT, r was 0.217, and the P value was 0.131 in the femtoLASIK group. However, in the Lasik group r was 0.233, the P value was 0.104; and in the PRK group r was 0.247 and the P value was 0.095.  Table 1: Correlation between postoperative central corneal thickness, intraocular pressure by Goldmann applanation tonometry, Goldmanncorrelated intraocular pressure measurement, cornealcompensated intraocular pressure by ocular response analyzer and spherical equivalent among studied groups
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Discussion   
Our study analyzed the change in IOP after corneal refractive surgery due to the CCT changes, as the postoperative IOP by GAT and IOPg measured by ORA are reduced due to a decrease in CCT, but IOP cc is reduced due to changes in corneal biomechanics^{[5]}. Low IOP readings lead to delayed diagnosis of glaucoma and ocular hypertension, So the accuracy of IOP measurement is important for glaucoma diagnosis and followup^{[2]}.
Previous studies have discussed the IOP change after corneal refractive surgery as in a study by Kilavuzoglu and colleagues, which included 214 patients who underwent LASIK for myopia and/or myopic astigmatism, including 81 (37.9%) men and 133 (62.1%) women, and the mean age was 32 ± 7.8 years, The mean SEac was −3.7 ± 1.7 D which shows that the postoperative IOP was significantly lower than the preoperative IOP. The mean reduction in IOP at 1 month postoperatively was 4.6 ± 2.3 mmHg. The IOP decrease after LASIK was significant and was correlated with preoperative IOP, CCT, and the spherical equivalent of the attempted correction. However, IOP reduction was not correlated to mean keratometric values, age, or sex^{[4]}.
This study described a practical formula for predicting the true IOP as follows:
IOP predicted=6.194×0.4489 (preoperative IOP)+0.0129 (preoperative CCT)+0.554 (the spherical equivalent of the attempted correction)−1.009×(optical zone diameter) preoperative IOP−predicted IOP=IOP constant^{[4]}.
However, in another study, Ajaza and colleagues, which analyzes IOP postLasik in a myopic patient, found that the highest reduction of IOP postLASIK was in patients with six or more diopters (6.16 mmHg). An average reduction of IOP was approximately 1 mmHg per diopter. The mean preoperative IOP was 16.4 ± 2.7 mmHg, while the mean postoperative IOP was 11.0 ± 2.4 mmHg. The mean IOP reduction after LASIK was 5.4 mmHg. The average CCT was reduced from 551.9 ± 31.6 μm, preoperatively, to 469.8 ± 45.3 μm postoperatively. So the postoperative IOP is reduced because of the reduced CCT^{[5]}.
The Schallhorn and colleagues study found a postoperative decrease in IOP value in the myopic patient that was strongly linked to the amount of myopia corrected, 0.40 mmHg per diopter of myopic correction for both PRK and LASIK. For a conventional ablation profile, this equals 0.32 mmHg per 10 mm of tissue removal^{[6]}.
The study by Lin et al.^{[7]}, which compared the postoperative IOP between LASIK and femtoLASIK procedures noticed that CCT and ablation depth was important in predicting IOP change in the LASIK group, while CCT, ablation depth, and flap thickness were important predictors in the femtoLASIK group.
In the femtoLASIK patient, the mean CCT was 535.1 ± 34.6 μm, the mean depth of ablation was 87.5 ± 21.7 μm, the mean thickness of the prepared flap was 103.9 ± 6.1 μm, the mean IOP was 14.2 ± 3.2 mmHg preoperatively and 8.07 ± 2.49, 7.48 ± 2.42, 7.49 ± 2.4, and 7.56 ± 2.46 mmHg after 1 week, 1, 6, and 12 months, respectively^{[7]}.
In the LASIK patient, the mean CCT was 549.2 ± 34.0 μm; the mean ablation depth was 82.8 ± 24.8 μm; the mean prepared flap thickness was 126.4 ± 8.2; the mean IOP was 15.5 ± 2.9 mmHg preoperatively; and 10.23 ± 2.88, 8.8 ± 2.45, 8.23 ± 2.41, and 8.31 ± 2.51 mmHg postoperatively at 1 week, 1, 6, and 12 months, respectively. In both groups, the postoperative IOP value at week was higher than others at 1, 6, and 12 months^{[7]}.
In the postoperative IOP, the reduction was found to be 0.045 mmHg for every micron increase of flap thickness; however, the postoperative IOP decreased by 0.035 mmHg for every more micron in the depth of ablation. So, the flap dissection per micron effect on the IOP lowering was a little stronger than the stromal ablation^{[7]}.
In our study, we found a significant positive correlation between the reduction in CCT and IOP reduction by GAT. In the femtoLASIK patient, the mean decrease in IOP was 2.42 ± 1.9, 3.14 ± 1.87, and 3.22 ± 1.83 mmHg at 1, 3, and 6 months, respectively; however, the mean decrease in CCT was 56.6 ± 34.1 μm. In the LASIK patient the mean decrease in IOP was 2.46 ± 2.56, 3.22 ± 2.99, and 3.44 ± 2.91 mmHg at 1, 3, and 6months, respectively; however, the mean decrease in CCT was 52.8 ± 25.5 μm. In the PRK patient, the mean decrease in IOP was 1.14 ± 1.31, 2.7 ± 2.23, and 2.84 ± 2.32 mmHg at 1, 3, and 6months, respectively; however, the mean decrease in CCT was 40.5 ± 25.9 μm. Also, we found a significant positive correlation between the reduction in CCT and IOPg reduction by ORA among the studied groups as in the femtoLASIK patient, the mean decrease in IOPg was 2.53 ± 1.81, 3.19 ± 1.76, and 3.29 ± 1.87 mmHg at 1, 3, and 6 months respectively, However, in the LASIK patient, the mean decrease in IOPg was 2.67 ± 2.08, 3.28 ± 1.95, and 3.52 ± 1.89 mmHg at 1, 3, and 6 months, respectively. Although, in the PRK patient, the mean decrease in IOPg was 1.26 ± 1.98, 2.85 ± 1.98, and 2.98 ± 1.79 mmHg at 1, 3, and 6 months, respectively. There was a nonsignificant correlation between the postoperative reduction of IOPcc and CCT in all groups.
Conclusion   
The IOP by GAT and IOPg by ORA after corneal refractive surgery were underestimated due to reducing the CCT. But changes in IOPcc measurement by ORA independent is of CCT changes. IOP is not correlated with age, sex, or spherical equivalent refraction.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1]
