Variations In Central Corneal Thickness Research Articles

In Vivo Biomechanical Response of the Human Cornea to Acoustic Waves

The cornea is the optical window to the brain. Its optical and structural properties are responsible for optical transparency and vision. The shape, elasticity, rigidity, or stiffness are due to its biomechanical properties, whose stability results in ocular integrity and intraocular pressure dynamics. Here, we report in vivo observations of shape changes and biomechanical alterations in the human cornea induced by acoustic wave pressure within the frequency range of 50–350 Hz and the sound pressure level of 90 dB. The central corneal thickness (CCT) and eccentricity (e2) were measured using Scheimpflug imaging and biomechanical properties [corneal hysteresis (CH) and intraocular pressure (IOP)] were assessed with air-puff tonometry in six young, healthy volunteers. At the specific 150 Hz acoustic frequency, the variations in e2 and CCT were 0.058 and 7.33 µm, respectively. Biomechanical alterations were also observed in both the IOP (a decrease of 3.60 mmHg) and CH (an increase of 0.40 mmHg).

THE ASSOCIATION OF CATARACT TYPE ON CENTRAL CORNEAL THICKNESS AFTER PHACOEMULSIFICATION CATARACT SURGERY

Objective: To assess any association between the type of cataract and the central corneal thickness at pre-operative and postoperative three-day and one-month appointments after phacoemulsification surgery.
 Study Design: Prospective, comparative study.
 Place and Duration of Study: Eye Department Combined Military Hospital Multan, from Jun 2020 to Apr 2021.
 Methodology: A total of 89 patients requiring cataract surgery by phacoemulsification were recruited. Age and gender were recorded for each patient. Patient’s type of cataract based on maturity was recorded before the surgery. Phacoemulsification cataract surgery using ‘divide and conquer’ technique was carried out. Patients’ central corneal thickness was measured at the pre-operative, three-day post-operative and one-month post-operative appointment using traditional non-contact tonopachymeter.
 Results: There were 49 (55.1%) male and 40 (44.9%) female patients with a mean age of 62.15 ± 12.40 years. No significant association was found between the type of cataract and central corneal thickness at the three-time intervals (p=0.14). Central corneal thickness significantly increased from 0.48 ± 0.04 mm pre-operatively to 0.51 ± 0.04 mm at the three-day post-operative appointment (p<0.001). It then reduced to 0.49 ± 0.04 mm at the one-month appointment (p=0.01).
 Conclusion: The type of cataract is not associated with the variation in central corneal thickness due to cataract surgery. Central corneal thickness significantly increases from the pre-operative to right after the cataract surgery. However, it returns to preoperative levels after a one-month period.

Changes in Corneal Parameters after DMEK Surgery: A Swept-Source Imaging Analysis at 12-Month Follow-Up Time.

Purpose To assess the time course changes in corneal topographic parameters during the one-year follow-up after Descemet membrane endothelial keratoplasty (DMEK) surgery. Materials and Methods Twenty-one patients (24 eyes) who underwent DMEK surgery were evaluated. Best corrected visual acuity (BCVA), endothelial cell count (ECC), central corneal thickness (CCT), mean keratometry (MK), mean astigmatism (MA), astigmatism asymmetry (AA), and higher-order aberration (HOA) were assessed at baseline and 1, 3, 6 and 12 months after the surgery using CASIA2 anterior segment swept-source OCT (Tomey, Japan). Results In patients who underwent DMEK surgery, BCVA improved gradually at the subsequent visits during the 12-month follow-up. A significant reduction in ECC and CCT at the 1st month was noted, which remained stable until the 6th month postoperatively. Anterior and total MK values remained unchanged, whereas changes in posterior keratometry were noticeable until the 6th month after surgery. A significant reduction in the anterior, posterior, and total astigmatism magnitude as well as astigmatism asymmetry was observed during the first 6 months after surgery. A gradual anterior, posterior, and total HOA decrease was documented until the 12th month after surgery. Negative correlations between baseline values of CCT, MK, MA, AA, and HOA and postoperative variations in those parameters at consecutive follow-up time points were observed. Accordingly, negative correlations between baseline CCT and postoperative changes in corneal topographic parameters after surgery were found. Conclusion The stabilization of most corneal topographic parameters takes place within 6 months after the procedure, whereas HOA reduction and BCVA improvement gradually occur during the first year after surgery. Preoperative values of corneal topographic parameters strongly determine their changes detected after DMEK surgery, which may suggest that early therapeutic intervention results in better visual outcomes.

Corneal endothelial changes in patients with chronic kidney disease

Purpose The aim of this study was to assess the effect of chronic kidney disease and duration of hemodialysis on the corneal endothelium.Patients and methods This case–control study included 78 participants who were divided into three groups: group I: the dialyzed group, group II: the nondialyzed group, and group III: age-matched control group. The endothelial cell density (ECD), average area of endothelial cells, SD, coefficient of variation, percentage of hexagonal cells, and central corneal thickness (CCT) were compared between the three groups. The effect of blood urea, serum creatinine, and duration of dialysis on corneal endothelium was studied in the dialyzed group.Results The ECD was statistically significantly less in the dialyzed group compared with the nondialyzed and control groups (2345.19±307.34, 2428.46±349.38, and 2481.23±286.16 cells/mm2, respectively, P<0.001). In addition, the percentage of hexagonal cells was significantly less in the dialyzed group compared with the nondialyzed and control groups (41.27±9.73, 69.42±6.16, and 68.54±5.58%, respectively, P<0.001). Meanwhile, the following parameters were significantly higher in the dialyzed group than the nondialyzed and control groups: average area of endothelial cells area (434.23±63.20, 420±66.98, and 408.08±48.80 µm2, respectively, P<0.001), SD (170.96±48.19, 127±36.18, and 117±20.56, respectively, P<0.001), and coefficient variation of size (39.35±7.52, 31.88±7.07, and 30.54±5.82%, respectively, P<0.001). There was no statistically significant difference in the CCT between the three groups (536.19±55.24, 543.42±37.80, and 539.85±38.44 µm, respectively, P=0.84). A significant negative correlation was found between ECD and the blood urea (P=0.005) and creatinine levels (P=0.03), while the correlation with coefficient variation of size, hexagonality, and CCT was not statistically significant.Conclusion Patients with chronic kidney disease have endothelial cell changes that may compromise the results of intraocular surgery. Awareness among physicians about these changes may help better planning of intraocular surgeries when needed.

Update on the genetics of keratoconus

In the past few years we have seen a great acceleration of discoveries in the field of keratoconus including new treatments, diagnostic tools, genomic and molecular determinants of disease risk. Recent genome-wide association studies (GWAS) of keratoconus cases and population wide studies of variation in central corneal thickness and in corneal biomechanical properties confirmed already identified genes and found many new susceptibility variants and biological pathways. Recent findings in genetic determinants of familial keratoconus revealed functionally important variants and established first mouse model of keratoconus. Latest transcriptomic and expression studies started assessing novel non-coding RNA targets in addition to identifying tissue specific effects of coding genes. First genomic insights into better prediction of treatment outcomes are bringing the advent of genomic medicine into keratoconus clinical practice.

Variation in central corneal thickness in diabetes: A comparative prospective study at a tertiary care hospital

Diabetes being the greatest challenge of the century and the fact that it affects almost all of the tissues and organs in the body warrants research into the effects of diabetes on every structure. This study was designed with the aim of finding out any association between diabetes and central corneal thickness and to evaluate the effect of diabetes treatment on central corneal thickness. Objectivesa. To find out the differences between CCT values amongst diabetic and non-diabetic patients, if any. b. To assess the presence of any significant change in these measurements after successful control of hyperglycaemia. Methods: A comparative prospective study was conducted in the Dept. of Ophthalmology, Sree Mookambika Institute of Medical Sciences, Kulasekharam to evaluate the effect of diabetes on CCT. A total of 84 subjects were studied which included 42 diabetics. After thorough systemic and ocular CCT was measured using ultrasound pachymeter. The study was initiated after obtaining ethical clearance from the institution’s ethical clearance committee. The collected data was analysed using SPSS software version 20.0. Results: Majority of the study subjects were male in both diabetic group (52.4%) and non-diabetic group (54.8%). The mean CCT among non-diabetics were 529 and among diabetics were 554.5. Students ‘t’ test was used to compare the corneal thickness between diabetic and non-diabetic patients. P value less than 0.05 is considered as significant. In this study it is found that there is significant difference in CCT OD 1 and CCT OS 1 in diabetic and non-diabetic patients. And the difference in CCT was found to be statistically significant. In this study we have also found out that there is strong positive correlation between CCT and diabetic control as per the pearson coefficient evaluation. In this study it is found that there is no significant difference in CCT following diabetic control in diabetic patients. Also, CCT was not correlated with the duration of diabetes. But the effect of control was not evaluated with serial HbA1c and a few patients didn’t have good control of diabetes either. Conclusion: Diabetes is a serious and extremely prevalent systemic illness in today’s scenario which was previously considered a disease of the affluent, diabetes has become a problem of epidemic proportions, contributing to significant morbidity and mortality. And India is moving fast in this race to become a Diabetes Capital of the world. Such being the case, it warrants further research in the field by the medical fraternity, to aid in prevention, early diagnosis, delaying progression and management of the disease. The effects of diabetes on the eye have been studied but as the prevalence of diabetes increases with all the generation, it demands further detailed research activities into the effect’s diagnosis and management of diabetes in the eye. In our study we tried to bring to light the effects of diabetes on corneal thickness, which in turn might affect even the management of glaucoma. We evaluated a group of diabetics and compared them with matched controls. After conducting this study we arrived at a conclusion that central corneal thickness is increased in diabetics and that control of diabetes doesn’t have a significant effect on central corneal thickness.

A multiethnic genome-wide analysis of 44,039 individuals identifies 41 new loci associated with central corneal thickness

Central corneal thickness (CCT) is one of the most heritable human traits, with broad-sense heritability estimates ranging between 0.68 to 0.95. Despite the high heritability and numerous previous association studies, only 8.5% of CCT variance is currently explained. Here, we report the results of a multiethnic meta-analysis of available genome-wide association studies in which we find association between CCT and 98 genomic loci, of which 41 are novel. Among these loci, 20 were significantly associated with keratoconus, and one (RAPSN rs3740685) was significantly associated with glaucoma after Bonferroni correction. Two-sample Mendelian randomization analysis suggests that thinner CCT does not causally increase the risk of primary open-angle glaucoma. This large CCT study explains up to 14.2% of CCT variance and increases substantially our understanding of the etiology of CCT variation. This may open new avenues of investigation into human ocular traits and their relationship to the risk of vision disorders.

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Determination of Corneal Biomechanical Behavior in-vivo for Healthy Eyes Using CorVis ST Tonometry: Stress-Strain Index.

Purpose: This study aims to introduce and clinically validate a new algorithm that can determine the biomechanical properties of the human cornea in vivo.Methods: A parametric study was conducted involving representative finite element models of human ocular globes with wide ranges of geometries and material biomechanical behavior. The models were subjected to different levels of intraocular pressure (IOP) and the action of external air puff produced by a non-contact tonometer. Predictions of dynamic corneal response under air pressure were analyzed to develop an algorithm that can predict the cornea's material behavior. The algorithm was assessed using clinical data obtained from 480 healthy participants where its predictions of material behavior were tested against variations in central corneal thickness (CCT), IOP and age, and compared against those obtained in earlier studies on ex-vivo human ocular tissue.Results: The algorithm produced a material stiffness parameter (Stress-Strain Index or SSI) that showed no significant correlation with both CCT (p > 0.05) and IOP (p > 0.05), but was significantly correlated with age (p < 0.01). The stiffness estimates and their variation with age were also significantly correlated (p < 0.01) with stiffness estimates obtained earlier in studies on ex-vivo human tissue.Conclusions: The study introduced and validated a new method for estimating the in vivo biomechanical behavior of healthy corneal tissue. The method can aid optimization of procedures that interfere mechanically with the cornea such as refractive surgeries and introduction of corneal implants.

Diurnal variations in central corneal thickness in glaucoma suspects

Aim: To analyse 24 hour variations of central corneal thickness in glaucoma suspects. Materials and Methods: 130 glaucoma suspects were hospitalised and underwent circadian evaluations. Total 6 readings of CCT were recorded over a 24-hour period: Immediately after waking up, 8 am, 11 am, 2 pm, 5 pm, & 8 pm. CCT was measured using an ultrasonic pachymeter. Results: Mean CCT measurement at each time point: Immediate after wake up: 549.21 um; 08 AM: 548.97 um; 11 AM: 545.82 um; 2 PM: 544.41 um; 5 PM: 544.34 um; 8 PM: 543.65 um. The mean CCT was highest just after wake up with a slight decline throughout the day without any statistical significance in both groups (p>0.05). Conclusion: The mean CCT was found to be highest just after wake up in the right eye and at 08:00 AM in left eye with steady decline throughout the day with statistically significance. The average variation in CCT was less than 1.92% in patients with suspected glaucoma from wake up to 8:00 pm. Keywords: Central corneal thickness, Diurnal variation, Glaucoma suspects, Intraocular pressure.

Diurnal variation of central corneal thickness and intraocular pressure in eyes with pseudoexfoliation

Purpose:Pseudoexfoliation syndrome (PXS) is an age related microfibrillopathy characterized by deposition of whitish flaky material over various ocular tissues. PXS eyes are known to have thinner corneas and thus can lead to an underestimation in intraocular pressure measurement. The purpose of this study was to find if there is any variation in central corneal thickness and intraocular pressure in PXS eyes and if there was any relationship between them.Methods:A prospective observational study was done on 141 eyes of 85 patients with PXS without glaucoma between November 2015 to April 2017 in the department of Ophthalmology in a tertiary hospital. CCT and IOP were measured by a handheld ultrasonic pachymeter (DGH Technology INC. Pachette 2, USA) and Goldmann applanation tonometer respectively at 4 different times during office hours.Results:A significant reduction of about 10 μm in mean CCT and 1.4 mmHg in mean IOP was noted over the 4 sessions which was statistically significant (P < 0.001). A significant correlation exists between IOP and CCT in PXS eyes at all times during the day (P < 0.001).Conclusion:The CCT measurements show significant thinning throughout the day, similar drop in IOP was also noted. Our study shows that there is a significant correlation between diurnal variation of CCT and IOP. Hence, it is prudent to measure CCT along with IOP at all times.

Central corneal thickness changes and horizontal corneal diameter in premature infants: A prospective analysis.

To report observations of horizontal corneal diameter (HCD) and central corneal thickness (CCT) changes in premature infants with stable optic disc cupping and intraocular pressures (IOPs). The HCD and CCT at term serve as a baseline for premature infants.Sixty-three premature infants were enrolled in a prospective case series. HCD, CCT, and IOP were measured. RetCam images of the optic discs were used to evaluate the cup-disc ratio (CDR) and read by an independent masked observer. Data were collected at between preterm (32–36 weeks) and again at term (37–41 weeks) postconceptual age. Left eye measurements were used for statistical analysis. Left eye findings were combined to construct predictive models for HCD and CCT.The mean HCD was 9.1 mm (standard deviation [SD] = 0.7 mm) at preterm and 10.0 mm (SD = 0.52 mm) at term. The mean CCT preterm was 618.8 (SD = 72.9) μm and at term 563.9 (SD = 50.7) μm, respectively. The average preterm CDR was 0.31 and at maturity was 0.33. Average IOP of preterm and term was 13.1 and 14.11 mm Hg, respectively. There was significant linear correlation between HCD with the postmenstrual age (r = 0.40, P < .01) and the head circumference (r = 0.33, P < .05). Predictive models were constructed for HCD (R2 = 0.52, 0.2 mm/wk) and CCT (R2 = 0.23, −11.4 μm/wk) with postconceptual ages.The HCD and CCT variation did not affect IOP reading over time. CCT was not correlated with birth parameters and decreased as the infant reached term. Corneal diameter correlated with gestational age at birth and head circumference.

PG - 39: Diurnal Variation in Central Corneal Thickness and Intraocular Pressure in Pseudo Exfoliation Sydrome without Glaucoma

PG - 39: Diurnal Variation in Central Corneal Thickness and Intraocular Pressure in Pseudo Exfoliation Sydrome without Glaucoma

Reproducibility of Central Corneal Thickness Measurements in Healthy and Glaucomatous Eyes.

The purpose of this study is to characterize intraday and interday variation in central corneal thickness (CCT) in healthy and glaucomatous subjects. In this prospective cohort study, 40 healthy subjects and 42 subjects with primary open-angle glaucoma underwent CCT measurements by ultrasonic pachymetry on 5 days over 1 year: first at baseline, then at 1 week and 1, 6, and 12 months after baseline. On 1visit, CCT was measured every 2 hours from 08:00 to 20:00. Intraday CCT variance was compared with interday CCT variance. Mean CCT at all visits and time points ranged from 561 to 574 µm in healthy eyes and from 548 to 563 µm in glaucomatous eyes. The mean intraday CCT range (highest minus lowest readings) was 21±10 µm in healthy participants and 21±10 µm in glaucoma patients. The mean interday CCT range was 27±13 µm in healthy participants and 24±14 µm in glaucoma patients. Within-subject variance was significantly larger among the interday CCT measurements than the intraday CCT measurements in both healthy subjects and glaucoma patients (P<0.0001 for both) demonstrating that measurements taken on separate days are more different than measurements taken on the same day. Interday CCT differences of ≥30 µm were seen in 27.7% of normal subjects and 30% of glaucoma patients. CCT measurements exhibit intraday and interday variation, with the latter being significantly greater than the former. A single CCT assessment inadequately characterizes CCT and may impact risk assessment in patients with suspect and/or diagnosed glaucoma.

Effects of diabetes mellitus on biomechanical properties of the rabbit cornea

To investigate the effects of diabetes on the biomechanical behavior of cornea in alloxan-induced diabetic rabbits. Diabetes mellitus (DM) was induced in 20 rabbits using alloxan, while another 20 age- and weight-matched non-diabetic rabbits served as controls. Eyes were enucleated after 8 weeks of inducing diabetes and the whole cornea was removed with a 3 mm wide scleral ring and tested under inflation conditions with an internal pressure range of 2.0–30.0 mmHg to determine their stress-strain behavior using an inverse analysis process. The blood glucose level (BG), advanced glycosylation end products (AGEs), central corneal thickness (CCT) and intraocular pressure (IOP) increased significantly in the DM group. There were statistically significant correlations between BG and AGEs (r = 0.768, p = 0.00), and between AGEs and CCT variation upon induction of DM (r = 0.594, p = 0.00). The tangent modulus (Et) of the cornea at four stress levels (1–4 kPa, equivalent to approximately IOP of 7.5, 15, 22.5 and 30 mmHg, respectively) was significantly higher in diabetic rabbits than in the control group (p < 0.05). Further, Et at stress of 2 kPa (which corresponded to the average IOP for the control group) was significantly correlated with BG (r = 0.378, p < 0.05), AGEs (r = 0.496, p < 0.05) and CCT variation upon induction of DM (r = 0.439, p < 0.05). IOP, as measured by contact tonometry, was also significantly correlated with both CCT (r = 0.315, p < 0.05) and Et at 2 kPa (r = 0.329, p < 0.05), and even after correcting for the effects of CCT and Et, IOP still significantly increased with both AGEs (r = 0.772, p = 0.00) and BG (r = 0.762, p = 0.00). The cornea of diabetic rabbits showed a significant increase in mechanical stiffness as evidenced by increases in corneal thickness and tangent modulus. The Et increase may be explained by a non-enzymatic cross-linking of collagen fibrils mediated by AGEs due to the high blood glucose levels in diabetes. The study also found significant IOP increases with higher blood glucose level even after controlling the effects of both corneal thickness and tangent modulus.

Central corneal thickness in glaucoma

The purpose of this review is to summarize the role of central corneal thickness (CCT) in the clinical management of a glaucoma patient. The prognostic value of CCT is well recognized in patients with ocular hypertension. However, its predictive value in other glaucoma suspects and patients with established glaucoma is less certain. Tonometry artefacts can result from variations in CCT. However, an adequately validated correction algorithm for Goldmann applanation tonometry measurements does not exist. Newer methods of tonometry are potentially less influenced by CCT but are limited in their clinical use. There may also be biological and genetic associations between corneal thickness and glaucoma. Demographics, environmental factors, glaucoma treatment and the measurement device used have a significant influence on CCT, and should be considered when interpreting the effect of cornea thickness in patients with glaucoma. New measurements of the biomechanical properties of the cornea are likely to be better approximations of the globe biomechanics than CCT, but these require further evaluation. The clinical significance of CCT is well recognized in the context of glaucoma diagnosis and management, though the extent of its importance remains debatable. Corneal biomechanical properties may be more significantly associated with glaucoma than CCT.

Post-Mortem Corneal Thickness Measurements with a Portable Optical Coherence Tomography System: a Reliability Study.

The purpose of this study was to determine the repeatability and reproducibility of post-mortem central corneal thickness (CCT) measurements by using a real-time, portable optical coherence tomography (OCT) system on an animal model, and to prospectively evaluate the time-course of post-mortem changes in CCT. Forty-six ocular globes of sheep (Ovis aries) were analyzed with a portable spectral-domain OCT device by two operators at different postmortem intervals (PMIs) as follows: immediately (i.e. within 10 minutes), at the 30th minute, at the 1st, 6th, 12th, 24th and 48th hour, and later (up to the 96th hour). The coefficient of repeatability ranged from 0.3% to 3.5%, and coefficients of reproducibility ranged from 0.2% to 3.7% in the central region of the cornea. The intraclass correlation coefficients were particularly high at different PMIs, thus confirming good measurement reliability with the portable OCT. The average CCT decreased immediately and then increased thereafter, with two peaks at 6 and 24 hours after death. Our results suggest that portable OCT is a reliable tool for monitoring CCT variations after death and may be useful in characterizing corneas before explantation, detecting quantitative variations during post-mortem corneal degeneration or assessing changes in CCT for forensic implications.

Effect of 2% fluorescein on Scheimpflug central corneal thickness measurements.

To assess central corneal thickness (CCT) changes measured with Scheimpflug device following instillation of 2% fluorescein in normal subjects. This was a prospective randomized study of 60 hospital volunteers. After baseline CCT measurements of both eyes of 40 subjects were obtained using Scheimpflug system, a drop of preservative-free 2% fluorescein, was instilled in one eye and in other eye, one drop of normal saline (control). Measurements were repeated after 1, 2, 5, 10, 20, 30, 40, 50 and 60min (continuous assessment group). Twenty subjects had baseline CCT taken, then fluorescein was instilled in one eye and measurements were taken at 1min. Ten eyes had saline rinse after 1min and 10 other eyes did not, measurements were repeated at 2min (eye rinse group). The mean baseline CCT for continuous assessment group was 546.2 ±32.1 µm (range, 489.0-606.0), control eyes was 546.6±30.7 µm (range, 489.0-602.0). At 1min after fluorescein instillation, CCT significantly increased by 37.0±34.0 µm (P<0.001), then decreased gradually, reaching baseline at 60min. CCT variations were not significant in control group (P>0.05). For eye rinse group, CCT mean differences between baseline and 2min were 18.2 µm (95 % CI: -54.7 to 18.3) with rinse and 26.5 µm (95% CI: -62.9 to 9.9) without rinse; paired sample tests were not significant (P>0.05). The presence of fluorescein increased CCT value to a clinically relevant level of 6.8%. Eye rinse did not significantly reduce the effect at 2min post fluorescein timepoint.

Comment on: Intraocular pressure and anetrior segment anatomy after phacoemulsification surgery

Sir, We read the article, “The effect of phacoemulsification surgery on intraocular pressure and anterior segment anatomy of the patients with cataract and ocular hypertension.” by Cetinkaya et al. with a great interest.[1] The authors aimed to evaluate the effect of phacoemulsification surgery on intraocular pressure (IOP), anterior chamber depth (ACD), iridocorneal angle (ICA), and central corneal thickness (CCT) of the patients with cataract and ocular hypertension. They concluded that phacoemulsification surgery decreases IOP and increases ACD and ICA in the short-term, but in the long-term, it does not cause any significant changes. We congratulate the authors for their lightening study and would like to make some contributions and criticism about IOP, ICA, and CCT measurement methods and times of the measurements in the study. We know that CCT value is very important for the exact evaluation of many ophthalmologic diseases such as glaucoma, keratoconus, corneal dystrophy, and kerato-refractive surgery.[2,3,4] Ultrasound pachymetry (USP) is the most frequently used clinical technique and the gold standard of measuring CCT. However, placement of the probe on the corneal center is important and the perpendicularity of the probe in relation to the cornea is often difficult to ascertain. This is subjective and may cause operator-dependent errors due to off-center placement.[3] Usually, topical anesthesia is instilled before USP examination, and this could also induce bias. It is reported that topical anesthesia needed for USP may cause the cornea to swell and may give variations in CCT from −10 μm to +30 μm.[4] The results of the iridocorneal angle evaluation using Goldmann 3-mirror lens is also affected by examiner. We think that if measurements were performed by different examiners, this might be affected the results. The authors did not inform if all measurements were performed by the same examiner or not during 2 years study period. In addition, using the mean value of multiple USP measurements in a period for one person could reduce the risk of error. Another important factor which might be affected the results is a diurnal variation of the CCT and IOP. It is reported that there cornea swells during sleep upon eye opening and deswells about 2 h after eye opening. It is known that the mean circadian fluctuations of CCT are about 15 µm, and the circadian fluctuations may affect the IOP up to about 11 mmhg.[5] The authors also did not give any information about the time of the day during measurements. We think that if measurement were performed in different times of the day, this might be affected the results. We congratulate the authors and thanks for their understanding. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.

Development and validation of a correction equation for Corvis tonometry

Primary objective: This study uses numerical analysis and validation against clinical data to develop a method to correct intraocular pressure (IOP) measurements obtained using the Corvis Tonometer for the effects of central corneal thickness (CCT), and age. Materials and Methods: Finite element analysis was conducted to simulate the effect of tonometric air pressure on the intact eye globe. The analyses considered eyes with wide variations in IOP (10–30 mm Hg), CCT (445–645 microns), R (7.2–8.4 mm), shape factor, P (0.6–1) and age (30–90 years). In each case, corneal deformation was predicted and used to estimate the IOP measurement by Corvis (CVS-IOP). Analysis of the results led to an algorithm relating estimates of true IOP as a function of CVS-IOP, CCT and age. All other parameters had negligible effect on CVS-IOP and have therefore been omitted from the algorithm. Predictions of corrected CVS-IOP, as obtained by applying the algorithm to a clinical data-set involving 634 eyes, were assessed for their association with the cornea stiffness parameters; CCT and age. Results: Analysis of CVS-IOP measurements within the 634-large clinical data-set showed strong correlation with CCT (3.06 mm Hg/100 microns, r2 = 0.204) and weaker correlation with age (0.24 mm Hg/decade, r2 = 0.009). Applying the algorithm to IOP measurements resulted in IOP estimations that became less correlated with both CCT (0.04 mm Hg/100 microns, r2 = 0.005) and age (0.09 mm Hg/decade, r2 = 0.002). Conclusions: The IOP correction process developed in this study was successful in reducing reliance of IOP measurements on both corneal thickness and age in a healthy European population.