Diurnal Variation of Central Corneal Thickness and Goldmann Applanation Tonometry Estimates of Intraocular Pressure

Sleep deprivation and corneal chronobiology: reevaluating overnight corneal changes

This prospective cohort study is aimed to investigate circadian variations in corneal parameters, focusing on sleep-deprived subjects. Sixty-four healthy individuals (age range: 21-76 years) actively participated in this study, undergoing examinations at least five times within a 24-hour timeframe. The analysis encompassed keratometric parameters of the cornea's front (F) and back (B) surfaces, refractive power in flattest and steepest axes (K1, K2), astigmatism (Astig) and its axis (Axis), aspheric coefficient (Asph), corneal pachymetry values of thinnest corneal thickness (Pachy Min) and corneal thickness in the center of the pupil (Pachy Pupil), volume relative to the 3 and 10 mm corneal diagonal (Vol D3, Vol D10) and surface variance index (ISV). Circadian changes were assessed using a hierarchical, mixed-effects linear regression adjusted for age and night shift. A total of 1,636 measurements revealed significant circadian changes in various corneal parameters, including K1 F, K2 F/B, Astig F/B, Asph F/B, Pachy Min/Pupil, Vol D3/10, and ISV (p < 0.0001). Moreover, K1 B exhibited a significant circadian change (p = 0.0002), while Axis F/B remained unchanged. Notably, Corneal thickness peaked before 6 o'clock in the morning and reached its minimum after 12 o'clock. Contrary to previous notions linking corneal diurnal changes with eyelid closure during sleep, our study reveals that these changes persist in the absence of sleep. This research contributes valuable insights into the impact of sleep deprivation on corneal properties, warranting further investigations to deepen our understanding of daily variations in visual quality and guide the planning of refractive eye surgery interventions.

What intrinsic factors affect the central corneal thickness?

The cornea is one of the tissues responsible for covering and protecting the inner structures of the eye. Central corneal thickness (CCT) is defined as the distance between the anterior epithelial surface and the posterior surface of the endothelial layer. This parameter plays a very important role regarding intraocular pressure (IOP) measurement, evaluation of corneal uniformity, selection of a suitable technique for corneal refractive surgery and the planning of surgical procedures to overcome corneal disease. This comprehensive review elucidates the multifaceted factors influencing the central corneal thickness. Recognising the impact of these factors not only enhances our understanding of corneal dynamics but also contributes significantly to the refinement of diagnostic and therapeutic strategies in ophthalmology.

The variation in axial length in office hours causes a diopter change in the intraocular lens power calculation

PURPOSE

To investigate whether diurnal changes in biometric parameters at different times of the day are visible and to analyze whether the variations could have clinical significance in the process of intraocular lens (IOL) power calculation.

METHODS

Phakic eyes measured by IOLMaster 700 above the age of 16 were included, with the exclusion of previous surgery. Measurements were taken between 7:00 and 15:00 and data were treated in hourly groups within this range. Data such as age, sex and biometric parameters (axial length (AL), anterior chamber depth, central corneal thickness, white-to-white distance, keratometry readings, lens thickness) were used besides the hour:minute time of the examination.

RESULTS

Biometric data from 32,596 eyes were used (38.89% males). There were no statistically significant differences in age and biometric parameters between the office-hour groups (p > 0.05), excluding the AL. The AL at the end of the day was 0.198 mm longer for male and 0.197 mm longer for female compared to the beginning of the office day. Accordingly, the results of IOL power calculation varied between 21.0 and 20.0 D for male, and 21.5 D and 20.5 D for the female population.

CONCLUSION

The results suggest that the assessment of AL may be affected by the intraday time of the biometry. This variation is clinically significant and may have implications for the evaluation of AL.

Glaucoma Home Self-Testing Using VR Visual Fields and Rebound Tonometry Versus In-Clinic Perimetry and Goldmann Applanation Tonometry: A Pilot Study

Purpose

This pilot study aimed to assess the feasibility, accuracy, and repeatability of unsupervised, at-home, multi-day glaucoma testing using the Olleyes VisuALL Virtual Reality Platform (VRP) and the iCare HOME handheld self-tonometer.

Methods

Participants were trained to use two U.S. Food and Drug Administration-registered or approved devices before conducting self-tests at home over 3 consecutive days. The iCare HOME intraocular pressure (IOP) measurements were collected four times daily per eye, and VRP visual field tests were performed once daily. The results were compared with one in-clinic Humphrey Field Analyzer (HFA) visual field test performed on the day of device training, iCare HOME measurements by the trainer, and the last five Goldmann applanation tonometer (GAT) results.

Results

Of 15 enrolled participants, nine of them (60%) completed the study. The six excluded participants could not self-measure using iCare HOME. There was significant correlation between the average mean deviation (MD) values of the at-home VRP tests and in-clinic HFA test (r2 = 0.8793, P < 0.001). Additionally, the average of the sensitivities in five of six Garway-Heath sectors were significantly correlated. VRP test duration was also shorter than in-clinic HFA testing (P < 0.001). Finally, at-home tonometry yielded statistically similar values compared to trainer-obtained iCare HOME values. The mean and range of at-home tonometry were also statistically similar to those for in-clinic GAT, but at-home tonometry demonstrated higher maximum IOP values (P = 0.0429).

Conclusions

Unsupervised, at-home, multi-day glaucoma testing using two devices resulted in the capture of higher maximum IOPs than in the clinic and good MD correlation of VRP with HFA. However, 40% of participants could not self-measure IOP using iCare HOME.

Translational Relevance

The study findings suggest that at-home remote glaucoma monitoring correlates with in-office testing and could provide additional information for glaucoma management, although patients had more difficulty with the iCare HOME than the VRP.

Comparison of central corneal thickness measured by automatic and manual analysis of optical coherence tomography

PURPOSE

To compare central corneal thickness (CCT) measured with optical coherence tomography (OCT) using an automatic algorithm (A-OCT) vs. manual measurements (M-OCT) with respect to the gold standard ultrasound pachymetry (USP).

METHODS

CCT measurements were performed on both eyes of 28 healthy subjects at four times of the day. A-OCT used an automatic software analysis of the corneal image, M-OCT was performed by two operators by setting a digital calliper on the corneal borders, and USP was performed after corneal anesthesia. Measurements were compared using a three-way repeated measures ANOVA. Bland-Altman plots were used to evaluate the agreement between OCT measurements and USP.

RESULTS

Both A-OCT and M-OCT significantly underestimated the USP measures, with the mean difference, i.e., the systematic error, being larger for A-OCT (- 19.0 µm) than for M-OCT (- 6.5 µm). Good reproducibility between the two operators was observed. Bland-Altman plots showed that both OCT methods suffered from proportional errors, which were not affected by time and eye.

CONCLUSIONS

Measuring CCT with OCT yielded lower values than USP. Therefore, clinicians should be aware that corneal thickness values may be influenced by the measurement method and that the various devices should not be used interchangeably in following up a given patient. Intriguingly, M-OCT had less systematic error than A-OCT, an important outcome that clinicians should consider when deciding to use an OCT device.

Relationship between Diurnal Variation in Intraocular Pressure and Central Corneal Power

SIGNIFICANCE

Relationship between intraocular pressure (IOP) change and central corneal curvature is complicated by measurement techniques and corneal biomechanical parameters. Findings from this study indicate that it is worthwhile to observe the association between diurnal change in IOP and corneal power.

PURPOSE

This study aimed to investigate the relationship between the diurnal change in IOP and central corneal power among eyes with and without myopia.

METHODS

Sixty healthy eyes of 24 emmetropes and 36 myopes were recruited for this cross-sectional study. Both anterior and posterior central corneal powers of the steep (Ks), flat (Kf), mean meridian (Km), best-fit spheres, and central corneal thickness were recorded followed by the IOP (Goldmann-correlated IOP [IOPg] and corneal-compensated IOP) and corneal biomechanics (corneal hysteresis and corneal resistance factor). Measurements were obtained every 3 hours from 9.30 am to 6.30 pm . Linear-mixed model was used to determine the relationship between the change in IOP and the associated change in corneal measurements (adjusted for age, sex, refractive error, central corneal thickness, and biomechanics) among the myopic and nonmyopic eyes.

RESULTS

Group mean, amplitude of change, and the diurnal change in IOPg were (mean ± standard deviation) 15.14 ± 2.50, 3.33 ± 1.44, and 1.81 ± 1.25 mmHg, respectively. Overall, an IOP increase was associated with a decrease in the adjusted anterior corneal powers. Myopic eyes were associated with a decrease of 0.04 D (95% confidence interval [CI], 0.07 to 0.01 D; P = .02) in Ks and 0.03 D (95% CI, 0.06 to 0.001 D; P = .047) in Kf per mmHg increase in IOP, whereas for emmetropes, per mmHg increase in IOP only flattened the Kf by 0.03 D (95% CI, 0.06 to 0.004 D; P = .02).

CONCLUSIONS

Change in anterior corneal power was inversely related to the change in IOPg, with myopic and nonmyopic eyes reporting a significant but differential impact of IOP. Clinicians must keep in mind the impact of large IOP fluctuation on the anterior corneal power.

Time course of conjunctival hyperemia induced by omidenepag isopropyl ophthalmic solution 0.002%: a pilot, comparative study versus ripasudil 0.4

Objective

We investigated the detailed time course of conjunctival hyperemia induced by omidenepag isopropyl ophthalmic solution 0.002% (omidenepag), a selective prostaglandin E2 receptor 2 agonist.

Methods and analysis

We recruited 34 healthy subjects and administered omidenepag in the right eye and ripasudil 0.4% in the left eye. We evaluated conjunctival hyperemia using slit-lamp photography at baseline and after 15, 30, 60, 120, 180 and 360 min. The conjunctival hyperemia score was graded by three independent observers using a scale from 0 (none) to 3 (severe). We also evaluated conjunctival hyperemia by the pixel coverage of conjunctival blood vessels (per cent coverage) determined using a conjunctival hyperemia-analysing software.

Results

In omidenepag, the conjunctival hyperemia score and per cent coverage peaked at both 30 min (mean score±SD: 1.57±0.67 and 11.90%±3.66%, respectively) and then gradually decreased at 60 min (10.79%±3.32%) and 120 min (1.10±0.52) when they reached a level that was not significantly different from the baseline values. For ripasudil 0.4%, the peak time of the conjunctival hyperemia score and per cent coverage were both at 15 min (score: 2.42±0.54 and 15.26%±3.38%). The degree of conjunctival hyperemia was significantly higher for ripasudil 0.4% than that for omidenepag from 15 to 30 min in both the conjunctival hyperemia score and per cent coverage (p<0.007 by Bonferroni correction).

Conclusion

Conjunctival hyperemia induced by omidenepag gradually peaks to moderate severity, though weaker compared with that induced by ripasudil 0.4%, and subsides relatively quickly.

Diurnal variation of anterior segment parameters handled with Scheimpflug imaging in keratoconus patients

PURPOSE

To define diurnal changes in anterior segment parameters of keratoconus patients by using Scheimpflug imaging.

METHODS

All keratoconus patients had corneal topography measurements 3 times a day (around 09:00 AM, 13:00 PM and 17:00 PM) by the same experienced operator. Three consecutive scans in each measurement session were obtained. The average of three measurements was used for analysis. A repeated measures analysis of variance with one within-subject factor (time of day) was carried out to assess diurnal variation.

RESULTS

This study included 26 eyes of 26 patients with keratoconus. Significant diurnal variation was found in CCT and TCT measurements (P < 0.01, and P < 0.01, respectively). The mean amplitudes of change in CCT and TCT were 4.2 ± 1 µm (95% CI: 1.7-6.8 µm) and 4.1 ± 1.1 µm (95% CI: 1.2-8.0 µm), while the mean of CCT and TCT were 462.4 ± 34.5 µm (95% CI: 448.4-476.6 µm) and 452.9 ± 6.6 µm (95% CI: 439.3-466.5 µm), respectively. The maximum value and the lowest value were observed at 09:00 AM and at 17:00 PM, respectively. Other parameters except CCT and TCT did not show diurnal variation.

CONCLUSION

Statistically significant but clinically insignificant diurnal variation in CCT and TCT measurements of the keratoconus patients were detected over an 8-hour period (09:00 AM-17:00 PM).

Twenty-four hour ocular and systemic diurnal rhythms in children

PURPOSE

Ocular diurnal rhythms have been implicated in myopia, glaucoma, diabetes, and other ocular pathologies. Ocular rhythms have been well described in adults; however, they have not yet been fully examined in children. The goal of this study was to investigate ocular and systemic diurnal rhythms over 24 h in children.

METHODS

Subjects, ages 5 to 14 years (n = 18), wore a light, sleep, and activity monitor for one week to assess habitual sleep/wake patterns, then underwent diurnal measurements every 4 h for 24 h. Measurements included blood pressure, heart rate, body temperature, intraocular pressure (IOP), ocular biometry, and optical coherence tomography imaging. Saliva was collected for melatonin and cortisol analysis. Mean ocular perfusion pressure was calculated from IOP and blood pressure. Central corneal thickness, corneal power, anterior chamber depth, lens thickness, vitreous chamber depth, and axial length were determined from biometry. Total retinal thickness, retinal pigment epithelium (RPE) + photoreceptor outer segment thickness, photoreceptor inner segment thickness, and choroidal thickness were determined for a 1 mm diameter centred on the fovea. Subjects' amplitude and acrophase of diurnal variation for each parameter were determined using Fourier analysis, and mean acrophase was calculated using unit vector averaging.

RESULTS

Repeated measures analysis of variance (ANOVA) showed that all parameters except anterior chamber depth exhibited significant variations over 24 h (p ≤ 0.005 for all). Axial length underwent diurnal variation of 45.25 ± 6.30 μm with an acrophase at 12.92 h, and choroidal thickness underwent diurnal variation of 26.25 ± 2.67 μm with an acrophase at 1.90 h. IOP was approximately in phase with axial length, with a diurnal variation of 4.19 ± 0.50 mmHg and acrophase at 11.37 h. Total retinal thickness underwent a significant diurnal variation of 4.09 ± 0.39 μm with an acrophase at 15.04 h. The RPE + outer segment layer was thickest at 3.25 h, while the inner segment layer was thickest at 14.95 h. Melatonin peaked during the dark period at 2.36 h, and cortisol peaked after light onset at 9.22 h.

CONCLUSIONS

Ocular and systemic diurnal rhythms were robust in children and similar to those previously reported in adult populations. Axial length and IOP were approximately in phase with each other, and in antiphase to choroidal thickness. These findings may have important implications in myopia development in children.

Analysis of differences in intraocular pressure evaluation performed with contact and non-contact devices

Background

To evaluate differences of intraocular pressure (IOP) measurements performed with Goldmann applanation tonometer (GAT), dynamic contour tonometer (DCT), rebound tonometry (RT), Ocular Response Analyzer (ORA) and Corvis ST (CST) in eyes screened for refractive surgery.

Methods

One eye, only the right one, of 146 patients was included in this study. Each participant was submitted to a corneal analysis with Scheimpflug camera and IOP evaluation with GAT, DCT, RT, ORA and CST. Differences in IOP values obtained thanks to each instruments were compared and then correlations between these discrepancies and morphological features such as mean keratometry (MK) and central corneal thickness (CCT) provided by Pentacam were studied. Software used to run statistical evaluations was SPSS, version 18.0.

Results

Study participants had a mean age of 33.1 ± 9.2 years old. IOP values observed in this study were 15.97 ± 2.47 mmHg (GAT), 17.55 ± 2.42 mmHg (DCT), 17.49 ± 2.08 mmHg (RT), 18.51 ± 2.59 mmHg (ORA) and 18.33 ± 2.31 mmHg (CST). The mean CCT was 560.23 ± 31.00 μm, and the mean MK was 43.33 ± 1.35 D. GAT provided significant lower values in comparison to all other devices. DCT and RT gave significantly lower intermediate IOP values than those measured with ORA and CST. All the IOP measures and the differences between devices were significantly correlated with CCT.

Conclusions

According to our data, although our findings should be confirmed in further studies, GAT tonometer cannot be used interchangeably with DCT, RT, ORA and CST.

Diurnal variations in intraocular pressure and central corneal thickness and the correlation between these factors in dogs

OBJECTIVE

To study the diurnal variation in intraocular pressure (IOP) and central corneal thickness (CCT) in healthy Beagles by rebound tonometry and ultrasonic pachymetry, respectively, in addition to determining whether a correlation exists between these two variables.

ANIMALS STUDIED

Twenty eyes from 10 healthy Beagle dogs were included in the study.

PROCEDURES

The IOP and CCT were measured by rebound tonometry and ultrasonic pachymetry, respectively, at 2-h intervals over an 8-hour period between 10:00 and 18:00.

RESULTS

The mean values (± SD) of IOP obtained were 11.45 ± 2.96 at 10:00, 10.00 ± 1.89 at 12:00, 8.25 ± 1.62 at 14:00, 7.05 ± 1.05 at 16:00, and 6.55 ± 1.36 at 18:00. The mean values (± SD) of CCT obtained were 554.95 ± 72.41 at 10:00, 549.20 ± 69.10 at 12:00, 566.15 ± 80.56 at 14:00, 545.45 ± 70.19 at 16:00, and 538.30 ± 73.33 at 18:00. The IOP and CCT of dogs were found to decrease progressively from the first to the last measurement. There were statistically significant differences between the IOP (P = 0.000) and CCT values (P = 0.032) measured at different times of the day. There was no effect or interaction between gender and eye with the dependent variables. The IOP and CCT were found to be positively correlated (r = 0.213, P = 0.034). The regression equation demonstrated that for every 100 μm increase in CCT, there was an elevation in IOP by 0.8 mmHg.

CONCLUSIONS

The CCT and IOP values were lower in the afternoon/evening than in the morning, and these were positively correlated. Both findings are important for the diagnostic interpretation of IOP values in dogs.

Reproducibility of Central Corneal Thickness Measurements in Healthy and Glaucomatous Eyes

PURPOSE

The purpose of this study is to characterize intraday and interday variation in central corneal thickness (CCT) in healthy and glaucomatous subjects.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Diurnal Variations in Intraocular Pressure, Central Corneal Thickness, and Macular and Retinal Nerve Fiber Layer Thickness in Diabetics and Normal Individuals

Purpose:

To evaluate diurnal variations in intraocular pressure (IOP), central corneal thickness (CCT), and macular and retinal nerve fiber layer (RNFL) thickness in diabetic patients and normal individuals.

Methods:

This study included 11 diabetic patients with macular edema and 11 healthy individuals. IOP, CCT, and macular and RNFL thickness were measured every 3 hours on a single day between 9 AM and 6 PM. Diurnal variations in IOP, CCT, total macular volume (TMV), central macular thickness (CMT), average macular thickness (AMT), and RNFL thickness were measured.

Results:

None of the parameters showed a significant absolute or relative change over the course of the day. However, the following non-significant changes were observed. In the control group, all parameters demonstrated the highest values at 9 AM. The lowest IOP, TMV and AMT occurred at 12 PM; lowest CCT and RNFL at 6 PM; and the lowest CMT at 3 PM. Diabetic subjects had the highest values of RNFL, CMT and TMV at 9 AM, and that for IOP, CCT and AMT at 6 PM. The lowest RNFL and CMT values occurred at 6 PM; lowest IOP at 12 PM; and the lowest CCT, TMV and AMT were observed at 3 PM. In the diabetic group, TMV, CMT, AMT and CCT were significantly higher and RNFL was significantly lower than the control group at all time points (all P- values < 0.05).

Conclusion:

While there were slight decreases in IOP, RNFL thickness and CMT during the day, these changes were not significant between 9 AM and 6 PM and probably do not affect the interpretation of measurements.

High intercorneal symmetry in corneal biomechanical metrics

Backgroud

To evaluate the symmetry of corneal biomechanical metrics, measured using an ocular response analyzer (ORA) and self-built corneal inflation test platform, in bilateral rabbit corneas and to investigate their relationship with physical intraocular pressure (IOPp).

Methods

Twenty fresh enucleated eyes from ten rabbits were used for ex vivo whole ocular globe inflation. IOP was increased from 7.5 to 37.5 mmHg with 7.5 mmHg steps and biomechanical metrics were acquired using the ORA. At least 3 examinations were performed at each pressure stage. Two biomechanical metrics, corneal hysteresis (CH) and corneal resistance factor (CRF) were recorded and analyzed as a function of IOPp. Corneal specimens were then excised from the intact ocular globe and tested under inflation conditions up to 45.7 mmHg posterior pressure. The experimental pressure-deformation data was analyzed using an inverse modeling procedure to derive the stress-strain behavior of the cornea.

Results

A comparison of corneal shape parameters showed no statistically significant difference (P > 0.05) between bilateral eyes. Similarly, there were no statistically significant differences in values of CH, CRF and corneal stiffness (as measured by the tangent modulus, Et) between bilateral eyes (CH: F = 0.94, P = 0.54; CRF: F = 4.42, P = 0.35; Et: F = 3.15, P = 0.12) at different pressure levels. IOPp was highly correlated with CRF while the relationship with CH was less pronounced.

Conclusions

An obvious interocular symmetry in biomechanical metrics is found in this research. IOP has been shown to have important influences on the value of CRF provided by ORA.

Reductions in intraocular pressure after acute aerobic exercise: a meta-analysis

OBJECTIVE

Aerobic exercise is known to reduce elevated intraocular pressure (IOP), a primary risk factor for a disease of the eye known as primary open-angle glaucoma. Given the disparate nature of experimental protocols across the literature, an analysis of studies on the effect of acute aerobic exercise on IOP is necessary to verify the influence of participant characteristics, exercise intensity, and duration.

DATA SOURCES

The electronic databases PubMed, Web of Science, and Embase were searched, producing 35 empirical studies for review. Ten studies producing 14 independent groups were chosen as per the criteria of the analysis.

MAIN RESULTS

Random effects model was used to produce subgroup analyses, and meta-regressions were used to verify the impact of group allocation, intensity, and duration on the interstudy variability of the effect size (ES). The outcome variable of postexercise change in IOP produced a significant effect of exercise, almost 2-fold greater for sedentary populations than for normally active populations (ES = -4.198; confidence interval, -5.151 to -3.245); ES = -2.340; confidence interval, -3.305 to 1.375, respectively). The significant effect of exercise on IOP is potentially mediated by group allocation. Intensity and duration do not contribute to the overall ES nor do they explain the difference between sedentary and normally active populations.

CONCLUSIONS

There is a robust effect of exercise on IOP for sedentary participants. However, the heterogeneity across study parameters, such as exercise protocol, IOP measurement, and participant selection, prohibited the inclusion of studies in this analysis that may have influenced the results. The current analysis makes clear the need for standardization of protocol across this field of research.

Intraocular pressure fluctuation and glaucoma progression: what do we know?

While mean intraocular pressure (IOP) has long been known to correlate with glaucomatous damage, the role of IOP fluctuation is less clearly defined. There is extensive evidence in the literature for and against the value of short-term and long-term IOP fluctuation in the evaluation and prognosis of patients with glaucoma. We present here the arguments made by both sides, as well as a discussion of the pitfalls of prior research and potential directions for future studies. Until a reliable method is developed that allows for constant IOP monitoring, many variables will continue to hinder us from drawing adequate conclusions regarding the significance of IOP variation.

Correction factors for Goldmann Tonometry

PURPOSE

The purpose of the study was to evaluate the in vitro accuracy of correction factors in decreasing the error in the intraocular pressure (IOP) measurements obtained using the Goldmann Applanation Tonometer (GAT).

METHODS

Nineteen donor corneas, from individuals aged between 57 and 99 years (mean 75.7 years, standard deviation±11.4 years) were subjected to posterior pressure simulating in vivo true IOP (IOPT) using an inflation test rig. Central corneal thickness and corneal curvature were measured. The posterior pressure was set at 25 different pressure levels between 5 and 45 mm Hg and IOP was measured using the GAT. Five different correction equations were applied to the IOP measurements obtained using the GAT to determine corrected IOP. The multiparameter correction equations applied were derived by Elsheikh, Ehlers, Chihara, Shimmyo et al, and Orssengo and Pye. The differences between IOPT and the IOP measured using the GAT were recorded as uncorrected errors, whereas the differences between IOPT and each of the corrected IOP were the tonometry errors after correction.

RESULTS

The mean and standard deviation of error in tonometry before correction was +2.25±0.62 mm Hg. The mean errors in tonometry after correction using the Elsheikh and Chihara equations were +0.78±0.62 and +1.08±0.61 mm Hg, respectively. The mean errors in tonometry for the Ehlers, Shimmyo et al, and Orssengo and Pye equations were negative, indicating an overcorrection; the values were -0.75±2.28, -1.27±1.85, and -0.77±1.83 mm Hg, respectively.

CONCLUSIONS

The Elsheikh and the Chihara et al's equations considerably decreased error in IOP measurements obtained by the GAT when compared with IOPT and were more consistent than other correction equations. The 2 equations may be of clinical utility in obtaining estimates of IOPT.

Clinical evaluation of multiparameter correction equations for Goldmann applanation tonometry

PURPOSE

To evaluate and compare the multiparameter equations in correcting intraocular pressure (IOP) measurements obtained using the Goldmann applanation tonometer (IOPG) for the effects of central corneal thickness (CCT), corneal curvature (R), and age in different ethnic populations.

METHODS

Data of IOPG, CCT, R, and age were collected from three clinical centers. The sample size consisted of 945 eyes of 945 glaucoma patients or suspects (669 Europeans, 127 African Americans, and 149 Indians). The 'corrected IOP' was calculated using five multiparameter equations to decrease the association of CCT, R, and age with measured IOP. Regression analyses were performed to calculate variance (r(2)) and determine the association of CCT, R, and age with IOPG and corrected IOP (residual association).

RESULTS

Overall, CCT accounted for the majority of variance in IOPG, while R and age had a much smaller effect, with the combined effect on IOPG ranging from 4.7 to 7.5% in the three data sets. The residual association of CCT, R, and age with corrected IOP in the three groups ranged from 0.2 to 1.3% and 0.5 to 1.8% with the application of the Elsheikh and the Chihara equations, respectively. The residual association of CCT, R, and age with corrected IOP calculated using the Ehlers, Orssengo and Pye, and Shimmoyo equations were 7-11.5, 1.8-11.7, and 4.6-8.3%, respectively.

CONCLUSION

The Elsheikh and the Chihara equations better decreased the association of CCT, R, and age with measured IOP than the Ehlers, Orssengo and Pye, and Shimmoyo equations.

Comparison of the influence of nonpreserved oxybuprocaine and a preserved artificial tear (thera tears) on human corneal thickness measured by two pachymeters

PURPOSE

To compare the effect of nonpreserved oxybuprocaine and preserved artificial tears on central corneal thickness (CCT) obtained by 2 pachymeters.

METHODS

In this prospective, placebo-controlled study, involving a random sample of 100 eyes of 50 subjects, aged 24±2.3 years, CCT readings were obtained in 2 separate sessions with the Topcon SP-3000P and ultrasound pachymetry (USP), respectively, before, 5 and 10 min after instillation of a drop each of either oxybuprocaine hydrochloride (oxybuprocaine HCl) (group 1) or carboxymethylcellulose sodium, thera tears (group 2), and placebo.

RESULTS

The baseline mean CCT for SP-3000P was 509±38 μm and 542±36 μm for USP. No statistical significant differences between baseline CCTs (P>0.05 for both devices) in both groups. In both group experimental eyes, neither SP-3000P nor USP-measured CCTs varied significantly from the control eyes at 5 (P>0.05) and 10 (P>0.05) mins postinstillation of drops in both sessions. In group one, the 95% confidence intervals (CIs) for the SP-3000P CCTs were similar at 5 (-16 to 17 μm) and 10 min (-16 to 17 μm), but in the USP-measured CCTs, it was wider at 10 min (-41 to 46 μm) than at 5 min (-30 to 41 μm) postinstillation. In group two, the 95% CIs at 5 and 10 mins postinstillation, respectively, ranged between -20 and 47 μm, -21 and 43 μm (SP-3000P) and -29 and 23 μm, -26 and 23 μm (USP). Within groups and between groups, variations in CCT were similar at both times intervals in all comparisons.

CONCLUSION

Although oxybuprocaine HCl and thera tears consistently did not affect the mean CCT obtained by both devices at both time intervals, variation in SP-3000P measured CCT was more consistent in both sessions and narrower in relation to USP-measured CCT. It may be reasonable to suggest that measurements of CCT in normal patients be taken before examinations requiring instillation of anesthetics or such measurements when obtained postinstillation of either oxybuprocaine or preserved artificial tears be interpreted with caution.

Multiparameter correction equation for Goldmann applanation tonometry

PURPOSE

To develop a correction factor to improve the accuracy of intraocular pressure (IOP) measurements made by the Goldmann applanation tonometer (GAT), which considers the combined effects of variations in central corneal thickness (CCT), central anterior curvature (R), age, and the IOP level itself.

METHODS

Nonlinear numerical simulations based on the finite element method were used to represent corneal behavior under the effect of IOP and external tonometric pressure. The simulations considered various biomechanical corneal properties including the cornea's nonuniform thickness, elliptical topography, weak stromal interlamellar cohesion, low epithelial and endothelial stiffness, and hyperelastic and hysteretic material behavior. The simulations were used to model the GAT procedure on corneas to obtain a correction equation based on the values of CCT, R, age, and IOP measured using GAT (IOPG). The efficiency of the equation in reducing the effects of corneal parameters on IOPG measurements was also assessed using an independent clinical database.

RESULTS

The individual effects of variations in CCT, R, and age were estimated at 1.66 mm Hg/100 μ of CCT, 0.89 mm Hg/1 mm of R, and 0.12 mm Hg/decade of age. The correction equation reduced the association between clinical IOP measurements and corneal parameters with r2 reducing from 11.8 to 0.02%.

CONCLUSIONS

The GAT correction factor can consider the combined effect of variations in corneal thickness, curvature, age, and IOP. The factor could significantly reduce the reliance of IOPG measurements on corneal stiffness parameters.

Assessment of the Ocular Response Analyzer as a Tool for Intraocular Pressure Measurement

The ocular response analyzer (ORA) is a new indentation tonometer that subjects the cornea to an increasing then decreasing air pulse, and uses the corresponding two applanation pressures P1 and P2 to estimate the intraocular pressure (IOP). The present study aims to improve the accuracy of IOP estimation through representative numerical simulation of the ORA procedure. A parametric study has been carried out to consider the effect of corneal thickness, curvature, age, and true IOP on the P1 and P2 measurements. Based on the obtained database of input and output parameters, an equation has been suggested relating the P1 and P2 measurements to the value of IOP. The equation is intended to make IOP estimates independent of corneal stiffness, which varies with size and age. The equation has been tested against a clinical data set obtained at Moorfields Eye Hospital involving 144 subjects, and found to produce IOP estimates that are less dependent on corneal size and age than published equations.

No correlation between intraocular pressure and intracranial pressure

A recent study has reported an excellent correlation between intraocular pressure (IOP) and intracranial pressure (ICP), suggesting that measurement of IOP may serve as a noninvasive means to determine ICP. To reexamine the relation between IOP and ICP, we reviewed the medical records of all patients who underwent lumbar puncture between 1991 and 2007 in the neuro-ophthalmology clinic at the University of California San Francisco. Data for IOP and ICP were available for 55 patients. There was no correlation between IOP and ICP (Pearson correlation coefficient, r = 0.07; p = 0.59). IOP measurement is not a useful substitute for ICP measurement.

Age and racial variation in central corneal thickness of preschool and school-aged children

PURPOSE

The purpose of this study was to determine variations in central corneal thickness (CCT) of preschool and school-aged African American and white children. Secondary aims were to assess possible correlations between CCT measurements and gender, axial length, intraocular pressure (IOP), family history of glaucoma, or history of prematurity.

METHODS

Contact ultrasound was used to measure CCT and axial length in 76 white and 60 African American children between the ages of 7 months and 18 years. A questionnaire was completed by the parents or guardians, including medical and family history. Statistically significant associations and differences were assessed using the independent t test, analysis of variance, and linear regression. All associations were defined as significant when the alpha value was less than 0.05 (two-tailed).

RESULTS

Mean CCT was thinner in African American children (535 +/- 35 microm) compared to white children (559 +/- 38 microm) (P < .001). The corneal thickness in children ages 10 to 18 years was significantly higher than in all other age groups in both African American (P = .03) and white (P < .005) children. No association was found between CCT and gender, axial length, IOP, or family history of glaucoma. Premature children had thinner CCT (536 +/- 40 pm) than full-term children (552 +/- 38 microm) (P = .009).

CONCLUSIONS

African American children have a thinner CCT compared to white children at all ages. Children of both racial groups have an increasing value of CCT with increasing age after approximately age 10 years. Children born prematurely have a thinner CCT than full-term children.

The effect of contact lens induced oedema on the accuracy of Goldmann tonometry in a mature population

AIM

To determine the effect of contact lens induced oedema on the accuracy of Goldmann tonometry measurements of intraocular pressure (IOP) in mature subjects.

METHODS

22 healthy subjects aged between 50 and 60 years were recruited. Corneal curvature, IOP, and central corneal thickness (CCT) were measured before and after two hours of monocular closed eye wear of a thick hydroxyethyl methacrylate (HEMA) contact lens. Measurements were then repeated at 20 minute intervals for one hour after lens removal.

RESULTS

Both CCT (+54.1 mum) and IOP (+2.7 mm Hg) increased significantly after lens wear (p<0.001, paired t test with Bonferroni correction). For the hour following lens removal, the measured IOP was correlated to the increase in CCT (r = 0.84, p<0.001), at a rate of 1.0 mm Hg/10 mum (95% confidence interval, 0.8 to 1.2 mm Hg/10 mum, linear mixed model analysis).

CONCLUSIONS

A relatively small increase in CCT from contact lens induced corneal oedema caused an overestimation error in Goldmann tonometry measurements of IOP in healthy mature subjects.