Dependence of corneal hysteresis on non-central corneal thickness in healthy subjects

OBJECTIVE

To evaluate the dependence of corneal hysteresis (CH) on non-central corneal thickness.

METHODS

Cross-sectional study of 1561 eyes of 1561 healthy volunteers with IOP less than 21mmHg, open angles on gonioscopy and no prior eye surgeries or local or systemic diseases. Pentacam-Scheimpflug technology was employed to segment the cornea into 6 circular zones centered on the apex (zones 1-6) and to determine the mean corneal thickness of these areas. CH was measured with ORA. Univariate and multivariate linear regression models adjusted for age and sex were created to model the dependence of CH on corneal thickness in zones 1 to 6.

RESULTS

In the univariate linear regression models, we found that CH was dependent on mean corneal thickness of zone 1 (B=0,004; R2=0.95%; P<0.001), zone 2 (B=0,004; R2=0.57%; P=0.002), zone 4 (B=0,005; R2=1.50%; P<0.001) and zone 6 (B=0,003; R2=0.92%; P<0.001). Similar results were obtained in the multivariate model (R2=3.46%; P<0.001).

CONCLUSION

This study suggests a significant dependence of CH on non-central corneal thickness. The model of corneal thickness segmentation into circular zones centered on the corneal apex is able to explain 3.47% of the variation in CH measurements.

Repeatability of corneal pachymetry and epithelial thickness measurements with spectral-domain optical coherence tomography (SD-OCT) and correlation to ocular surface parameters

PURPOSE

To assess the repeatability of corneal pachymetry and epithelial thickness measurements with spectral-domain optical coherence tomography (SD-OCT) and identify correlations between epithelial thickness and ocular surface parameters.

METHODS

Adults who happened to have prolonged computer use were recruited, excluding those with conditions interfering with corneal measurements or tear production. All subjects filled in the ocular surface disease index (OSDI) questionnaire. Three consecutive measurements of central and peripheral corneal and epithelial thickness were performed with SD-OCT (RTVue XR). Schirmer test I and tear film break-up time (TBUT) were performed. Repeatability was evaluated with intraclass correlation coefficient (ICC), coefficient of variation and repeatability limit. Spearman correlation was used for non-parametric variables.

RESULTS

113 eyes of 63 subjects were included in the study. ICC was ≥ 0.989 for all corneal and ≥ 0.944 for all epithelial pachymetry segments. The best repeatability was found centrally and the worst superiorly both for corneal and epithelial measurements. Central epithelial thickness was weakly correlated with Schirmer test I (rho = 0.21), TBUT (rho = 0.02), OSDI symptoms and OSDI score (rho <|0.32|). OSDI symptoms and OSDI score were weakly correlated with Schirmer test I (rho <|0.3|) and TBUT (rho <|0.34|).

CONCLUSION

RTVue XR measurements of corneal and epithelial thickness are highly repeatable in all segments. The lack of correlation between epithelial thickness and ocular surface parameters could suggest the assessment of epithelial integrity with reliable methods such as SD-OCT.

Corneal Endothelial Cell Loss after Phacoemulsification in Eyes with a Prior Acute Angle-closure Attack

Purpose

To evaluate endothelial damage after cataract surgery in eyes affected by an angle-closure attack (ACA) and compare it to that in the unaffected fellow eyes (FEs) of patients with ACA and normal eyes (NEs).

Methods

The medical data of eyes affected by ACA, FEs (with no history of acute glaucoma attack), and NEs of patients who underwent cataract surgery with simultaneous intraocular lens implantation were retrospectively reviewed. Endothelial cell density (ECD) and central corneal thickness (CCT) measured before surgery and at 1 week, 1 month, and 3 months after surgery were analyzed, and the percentages of loss in ECD and increase in CCT of the three groups were compared.

Results

The study enrolled 140 eyes from 100 patients (50 eyes in the ACA group, 40 eyes in the FE group, and 50 eyes in the NE group). The mean ECD was significantly lower in the ACA group than in the other groups (p < 0.001). However, the percentage of ECD reduction was not significantly greater in the ACA group than in the other groups (p > 0.05). None of the eyes developed corneal edema at 3 months postoperatively. Moreover, the CCTs of the three groups were similar throughout the follow-up period (p > 0.05).

Conclusions

Phacoemulsification was not associated with greater endothelial cell loss in the ACA group than in the NE and FE groups. This finding shows that ACA history may not contribute to the exacerbation of corneal endothelial damage in cataract surgery.

Genetics vs chronic corneal mechanical trauma in the etiology of keratoconus

Both genetic and environmental factors have been considered to play a role in the etiology keratoconus. Eye rubbing, and more recently eye compression due to sleeping position, have been identified to be highly related to the condition, and are present in a high percentage of patients. Today, the predominant model is that these factors can provide the "second hit" necessary to generate the condition in a genetically susceptible individual. In addition, the extremely high prevalence in Arab populations, where endogamy could play a role, the high concordance rate in monozygotic twins, and the presence of family history of the condition between 5 and 23% of cases, support a genetic influence. Segregation analysis studies suggest that keratoconus is a complex non-Mendelian disease. Results from linkage analysis, next generation sequencing studies and genome-wide association studies also have suggested that genetic factors are involved in the condition. Recently, it has been proposed that mechanical trauma (i.e. eye rubbing or eye compression at night), is a sine quanon condition for the onset of keratoconus, and quite possibly its only cause. There are various arguments for and against this hypothesis. Indeed, it is possible, as initially suggested around 55 years ago, that the term "keratoconus" include diverse phenotypically similar conditions, which are actually of different etiology.

[The influence of different corneal diameters on Belin/Ambrósio enhanced ectasia display of Pentacam corneal topography]

Objective: To investigate the effect of corneal diameter on Belin/Ambrósio enhanced ectasia display (BAD). Methods: Retrospective case series study. The subjects were 6 744 myopic patients, including 3 341 males and 3 403 females, who had undergone corneal refractive surgery or had completed preoperative examination but had not undergone corneal refractive surgery at the Army Medical Center and Chongqing Vision Institute in Chongqing from June 2017 to June 2019. Age was (23.74±5.73) years old. No patients had keratoconus. One eye of each patient was randomly included, and the patients were divided into groups according to the corneal diameter measured by Pentacam. Group A included 630 patients (630 eyes) with corneal diameter ≤ 11.1 mm. In group B, there were 4 063 patients (4 063 eyes) with corneal diameter of 11.2 to 11.8 mm. In group C, there were 2 051 patients (2 051 eyes) with corneal diameter ≥11.9 mm. Preoperative BAD parameters of deviation of front elevation difference map (Df), deviation of back elevation difference map (Db), deviation of average pachymetric progression index (Dp), deviation of minimum thickness (Dt), deviation of Ambrósio's relational thickness maximum (Da) and overall deviation value (Do) were measured by Pentacam. One-way analysis of variance was used for preoperative BAD parameters comparison between groups. The distribution of normal, suspicious and pathological results of Df, Db, Dp, Dt, Da and Do in each group was tested by chi-square test. Results: In groups A, B and C, Df was 0.73±1.14, 0.48±1.02, and 0.11±0.91, Db was 1.09±1.07, 0.23±0.83, and-0.34±0.62, Dp was 1.57±0.91, 1.14±0.86, and 0.68±0.75, Dt was -0.11±0.84, -0.2±0.82, and 0.03±0.78, Da was 0.78±0.61, 0.64±0.64, and 0.48±0.64, and Do was 1.65±0.64, 1.24±0.60, and 0.86±0.55, respectively. The BAD parameters of Df (F=129.549), Db (F=829.491), Dp (F=344.373), Dt (F=7.249), Da (F=68.637) and Do (F=524.877) were all significantly different between groups (P<0.01). The proportion of suspicious and pathological BAD parameters [Df (χ²=161.8), Db (χ²=611.75), Dp (χ²=478.84), Da (χ²=44.636), and Do (χ²=553.11)] suggested the distribution in each group was significantly different (P<0.01). Conclusions: Corneal diameter had a significant influence on BAD. Compared with eyes with large corneas, the false positive rate of BAD was higher in eyes with small corneas.(Chin J Ophthalmol, 2020, 56: 761-767).

[Characteristics of corneal topography in parents of keratoconus patients]

Objective: To investigate the corneal topographic characteristics of the parents of patients with keratoconus (KC) and the correlation with their offspring. Methods: Case-control study. Twenty-six patients (49 eyes) with KC, who were 15 males and 11 females, and (18.83±2.74) years old, 48 parents (96 eyes) of patients with KC, who were 23 males and 25 females, and (44.14±1.70) years old, and 44 controls (88 eyes), who were 22 males and 22 females, and (42.81±4.03) years old, were enrolled in Qingdao Eye Hospital. The indexes in the Belin/Ambrósio Enhanced Ectasia Display were acquired with Pentacam topography. The basic indicators were flat K/steep K/maximum K (Kmax) curvature and astigmatism, thinnest pachymetry (TP), and front/back elevation (Ef/Eb); the Belin indexes included deviation of normality of the front/back elevation (Df/Db), deviation of average pachymetry progression/normality of corneal thinnest point/normality of relational thickness (Dp/Dt/Da), overall deviation of normality (Do), minimum/maximum/average pachymetric progression indices (PPImin/PPImax/PPIave), and Ambrósio's average and maximum relational thickness indices (ARTave/ARTmax). All parameters in the groups of parents and controls were compared by Mann-Whitney U test. The ROC curve was used to analyze the differential value of each abnormal index. In addition, the ratio of abnormal indicators in the Belin/Ambrósio Enhanced Ectasia Display was statistically analyzed. Partial correlation analysis was used to find the correlation between the parameters of KC patients and their parents, and binary logistics regression analysis was used to predict the prevalence in children through the parental indicators. Results: Clinical KC was diagnosed in 1 of the 48 parents (2.08%) of patients with KC. There was no statistically significant difference in anterior surface parameters such as K1, K2, Kmax, astigmatism and Ef, but differences in the thickness [522.00 (493.00, 542.75) μm versus 540.00 (523.25, 559.50) μm] and posterior surface elevation value [8.00 (4.00, 11.00) μm versus 5.00 (3.00, 8.00) μm] were statistically significant. Except for Df, the other Belin/Ambrósio indicators such as Do (Z=-4.551, P=0.000), PPImax (Z=-3.959, P=0.000) and ARTave (Z=-4.792, P=0.000) indicated significant difference. The ROC curve analyses showed ARTave had the greatest value in the identification of KC patients (AUC=0.705), PPImax had the best sensitivity (0.845), and Eb had the best specificity (0.837). The ratio of suspicious indicators between the parents' group and the control group was 1.5∶1, and the ratio of pathological indicators was 3∶1. There was a correlation in multiple parameters between KC patients and their parents (all P>0.05). Do/Eb/TP indices of mothers and Do/Kmax indices of fathers were the major influence factors for the disease in the offspring, with a diagnosis rate of 85.6%. Conclusions: The corneal topographic map of the parents of patients with KC showed that the index of the anterior surface was normal, but the thickness was thinner and the posterior surface was higher. According to Belin's analysis, all indicators except Df were abnormal. Moreover, the ratio of suspected and pathological indicators increased. These data suggested that the corneal topography of parents of patients with KC had features of subclinical KC. Compared with the traditional index, ARTave was of the highest value in the identification of subclinical KC. There was a strong correlation between parents of patients with KC and offspring. The Do and Kmax indices of paternal parents and the Do, Eb and TP indices of maternal parents were good predictors of children's disease. (Chin J Ophthalmol, 2020, 56:456-464).

[Corneal topography in practice]

Corneal surface analysis is now more and more accurate. Corneal topography is a gold standard in corneal pathology follow-up: keratoconus, corneal grafts, orthokeratology. In refractive surgery, cornea ectasia post-Lasik must be avoided. Analyzing anterior and posterior surface can detect forme fruste keratoconus FFKC. Topography allows also better predictability of premium intraocular implants surgery. Topography is key examination and its interpretation is essential.

Corneal topography in clinical practice

Developments in corneal topography allow for increasingly precise, detailed analysis of the corneal surface. This test is becoming indispensable in the treatment of complex corneas: keratoconus, corneal transplants, orthokeratology, etc. In refractive surgery, the combined analysis of the anterior and posterior cornea has permitted better screening for forme fruste keratoconus at risk for post-LASIK ectasia. Topography also assists in the calculation of premium intraocular lenses. Topography is an indispensable test for analyzing and following corneal disease.

POSTOPERATIVE THINNING OF LAMELLAR DONOR GRAFT AFTER CONVENTIONAL DESCEMET'S STRIPPING AUTOMATED ENDOTHELIAL KERATOPLASTY

- The purpose of this study was to evaluate postoperative deturgescence of lamellar donor graft after conventional Descemet's stripping automated endothelial keratoplasty (DSAEK). It was a prospective study that included 55 eyes of patients (mean age 70.9±9.4 years; female 61.8%, male 38.2%). Preoperative thickness of lamella was compared with postoperative thickness six months after surgery. Central lamellar graft thickness decreased from 142±27 µm preoperatively to 124±20 µm 6 months postoperatively (p<0.01). After performing conventional DSAEK corneal transplantation, surgeons should expect deturgescence of corneal graft and reduction in thickness of lamellae by about 12% of initial thickness according to our results. We found this information important for better planning of surgical procedures and knowing what to expect after surgery, as well as for better cooperation with eye banks when ordering pre-cut corneal tissue.

Accuracy of Cirrus HD-OCT and Topcon SP-3000P for measuring central corneal thickness

PURPOSE

To compare and analyze the interchangeability of three measuring systems, each based on a different technique, for central corneal thickness (CCT) analysis.

METHODS

CCT measurements were measured using optical coherence tomography (OCT), non-contact specular microscopy (NCSM), and ultrasonic pachymetry (USP) in 60 eyes of 60 healthy patients with a mean age of 66.5±15.0 years and a mean spherical equivalent of 0.43±1.14 D. Analysis of variations in measurement concordance and correlation among the three different methods were performed. Comparison of CCT measurements were done using Bland-Altman plots (with bias and 95% confidence intervals), intraclass correlation coefficient (ICC), and paired t-student analysis.

RESULTS

Mean CCT values were: 549.20±26.91μm for USP (range 503-618μm), 514.20±27.49μm for NCSM (range 456-586μm) and 542.80±25.56μm for OCT (range 486-605μm). CCT values obtained with NCMS were significantly lower than those obtained with OCT and USP methods. NCMS CCT value was 36.08±10.72μm lower than USP value (p<0.05), and NCMS CCT value was 7.88±8.86μm lower than OCT value (p<0.05). ICC between USP-NCSM pair was 0.488 and 0.909 between USP-OCT pair.

CONCLUSION

OCT and UPS offered highly comparable results, whereas NCSM offered lower mean CCT values compared to the other two methods. Therefore, NCSM should not be considered a reliable method for measuring CCT and should rather be considered for assessing longitudinal changes in the same patient.

[Effect of optical zone diameter on corneal biomechanical properties after small incision lenticule extraction]

Objective: To investigate the effect of optical zone diameter on corneal biomechanical properties after small incision lenticule extraction (SMILE). Methods: In this prospective case-control study, 62 eyes of 42 patients with myopia and myopic astigmatism that had undergone SMILE procedure were divided into 2 groups according to the optical zone (OZ) diameter: group A, 31 eyes of 19 patients, OZ= 6.5 mm; group B, 31eyes of 23 patients, OZ=6.0 mm. These patients were examined for corneal hysteresis (CH) and corneal resistance factor (CRF) with the Ocular Response Analyzer (ORA) preoperatively and at 1 week, 1 month and 3 months postoperatively; at the same time, the central corneal thickness-corrected indices of DifCH and DifCRF were calculated. Pearson correlation was applied to analyze the correlation between CH and CRF and central corneal thickness. Independent-samples t test was utilized to compare CH and CRF between the two groups. Repeated ANOVA was used to compare the difference of corneal biomechanical properties at different follow-up timepoints. Results: The preoperative mean CRF and CH values were (10.13±1.19) mmHg and (9.65±1.12) mmHg in group A and (10.34±0.89) mmHg and (10.04± 1.05) mmHg in group B, respectively, and no significant differences were found in CRF and CH between the two groups (CRF: t=-0.807, P=0.423. CH: t=-1.405, P=0.165). The mean CRF values in group A at postoperative 1 week and 3 months were (6.62 ± 1.09) mmHg and (6.83 ± 1.07) mmHg, respectively, which were significantly lower than the relevant mean CRF values in group B (t=-2.703, P=0.009. t=-3.733, P= 0.001). Meanwhile, the mean CH values at 1 week, 1 month and 3 months postoperatively in group A were (7.31±1.06) mmHg, (7.37±0.96) mmHg and (7.82±0.97) mmHg, respectively, which were significantly lower than those in group B (t=-2.415, P=0.019. t=-2.113, P=0.039. t=-2.67, P=0.01). Moreover, the DifCRF and DifCH values in group A were significantly lower than those in group B at 3 months postoperatively (t=-3.409, P=0.001. t=-2.064, P=0.044). The CRF and CH values in both groups showed a significant reduction at 1 week, 1 month and 3 months postoperatively (P<0.05). Conclusions: The size of optical zone has some effects on corneal biomechanical properties after SMILE. The smaller optical zone diameter has a relatively smaller effect. (Chin J Ophthalmol, 2017, 53:182-187).

Validity of Tono-pachymetry for Measuring Corrected Intraocular Pressure in Non-surgical and Post-photorefractive Keratectomy Eyes

Purpose

To assess the validity of central corneal thickness (CCT) and corrected intraocular pressure (IOP) values obtained by tono-pachymetry in non-surgical and post-photorefractive keratectomy (PRK) eyes.

Methods

For the study, 108 young healthy participants and 108 patients who had PRK were enrolled. Measurements were randomly performed by tono-pachymetry, ultrasonic (US) pachymetry, and Goldmann applanation tonometry (GAT). CCT measurement by tono-pachymetry was compared to that of US pachymetry. The corrected IOP value obtained by tono-pachymetry was compared to that obtained by US pachymetry and GAT. The corrected IOP from US pachymetry and GAT was calculated using the identical compensation formula built into the tono-pachymetry. Bland-Altman plot and paired t-test were conducted to evaluate the between-method agreements.

Results

The mean CCT measurement using tono-pachymetry was significantly greater by 7.3 µm in non-surgical eyes (p < 0.001) and 17.8 µm in post-PRK eyes (p < 0.001) compared with US pachymetry. Differences were significant in both Bland-Altman plotand paired t-test. The mean difference of corrected IOP values obtained by tono-pachymetry and calculated from measurements by US pachymetry and GAT was 0.33 ± 0.87 mmHg in non-surgical eyes and 0.57 ± 1.08 mmHg in post-PRK eyes. The differences in the Bland-Altman plot were not significant.

Conclusions

The CCT measurement determined using tono-pachymetrywas significantly thicker than that of US pachymetry. The difference in CCT was greater in post-PRK eyes than in non-surgical eyes. However, the corrected IOP value obtained by tono-pachymetry showed reasonable agreement with that calculated from US pachymetry and GAT measurements.

Oxidative Stress Levels in Aqueous Humor from High Myopic Patients

Purpose

To compare oxidative stress status in the aqueous humor of highly myopic eyes and control eyes.

Methods

Aqueous humor samples were collected from 15 highly myopic eyes (high myopia group) and 23 cataractous eyes (control group) during cataract surgery. Central corneal thickness, corneal endothelial cell density, hexagonality of corneal endothelial cells, and cell area of corneal endothelial cells were measured using specular microscopy. Axial length was measured using ultrasound biometry. 8-Hydroxydeoxyguanosine (8-OHdG) and malondialdehyde levels were measured using enzyme-linked immunosorbent assay.

Results

8-OHdG level was lower in the aqueous humor of myopic patients than in that of control group (p = 0.014) and was positively correlated with central corneal thickness and negatively correlated with axial length (r = 0.511, p = 0.02; r = -0.382, p < 0.001). There was no correlation between 8-OHdG level and corneal endothelial cell density, hexagonality, or cell area. Malondialdehyde level did not show any correlation with any parameters evaluated.

Conclusions

8-OHdG might be a sensitive biomarker for evaluating oxidative stress status in the eye. Oxidative stress level was lower in the aqueous humor of highly myopic eyes compared to that in control eyes, which indicates lower metabolic activity in these eyes.

Structural analysis of different incision sizes and stromal hydration in cataract surgery using anterior segment optical coherence tomography

Purpose

To analyze healing changes of corneal wounds of different corneal incision sizes with or without stromal hydration in cataract surgery using anterior segment optical coherence tomography.

Methods

Cataract surgeries were performed by a single surgeon and 2.2- and 2.8-mm corneal incisions were made using a diamond blade (ME-759; Meyco, Biel-Bienne, Swiss). Patients were divided into four groups according to incision size (2.2 and 2.8 mm), and with/without stromal hydration. Fifteen eyes were assigned to each group and incision wounds were measured using anterior segment optical coherence tomography at 2 hours, 1 day, 1 week, 1 month, and 3 months postoperatively. Corneal thickness, incision length and incision angle were measured and existence of epithelial, endothelial gaping and Descemet's membrane detachment was evaluated.

Results

Incision thickness was greater in the group with stromal hydration than in the group without on operation day (p < 0.05). Stromal hydration exerted greater influence in the 2.2-mm incision group than in the 2.8-mm incision group. Corneal thickness decreased more rapidly in the stromal hydration group than in the group with no hydration (p = 0.022). Endothelial gaping was greater in the 2.2-mm incision group than in the 2.8-mm incision group 1 day, 1 month, and 3 months after surgery (p = 0.035, p = 0.009, and p = 0.008, respectively). No other statistical significance was observed between the two groups (2.2 and 2.8 mm) during follow-up regarding corneal thickness, epithelial gaping and Descemet's membrane detachment.

Conclusions

Corneal wounds with a smaller incision could be more vulnerable to external stimuli such as stromal hydration and are less stable than those with a larger incision.

Bilateral coexistence of keratoconus and macular corneal dystrophy

Bilateral coexistence of keratoconus and macular corneal dystrophy is a very rare clinical entity. Further elaboration on the possible genetic, histopathologic, pathophysiologic and biochemical correlation is required to study the nature of the condition.The authors hereby report a 21-year-old female who presented with the typical signs and topographic evidence of keratoconus in association with macular corneal dystrophy. Histopathologic evaluation from the excised corneal button after corneal transplant confirmed the diagnosis.To our knowledge, there is only one previous report in the literature linking the association of keratoconus and macular corneal dystrophy in the same eye bilaterally.