Predicting intraocular pressure using systemic variables or fundus photography with deep learning in a health examination cohort.

The purpose of the current study was to predict intraocular pressure (IOP) using color fundus photography with a deep learning (DL) model, or, systemic variables with a multivariate linear regression model (MLM), along with least absolute shrinkage and selection operator regression (LASSO), support vector machine (SVM), and Random Forest: (RF). Training dataset included 3883 examinations from 3883 eyes of 1945 subjects and testing dataset 289 examinations from 289 eyes from 146 subjects. With the training dataset, MLM was constructed to predict IOP using 35 systemic variables and 25 blood measurements. A DL model was developed to predict IOP from color fundus photographs. The prediction accuracy of each model was evaluated through the absolute error and the marginal R-squared (mR2), using the testing dataset. The mean absolute error with MLM was 2.29 mmHg, which was significantly smaller than that with DL (2.70 dB). The mR2 with MLM was 0.15, whereas that with DL was 0.0066. The mean absolute error (between 2.24 and 2.30 mmHg) and mR2 (between 0.11 and 0.15) with LASSO, SVM and RF were similar to or poorer than MLM. A DL model to predict IOP using color fundus photography proved far less accurate than MLM using systemic variables.

Evaluating Goldmann Applanation Tonometry Intraocular Pressure Measurement Agreement Between Ophthalmic Technicians and Physicians.

PURPOSE: To examine IOP measurement disagreement between technicians and physicians and the impact of an educational intervention on the short and long-term disagreement in IOP measurement using Goldmann applanation tonometry. DESIGN: Prospective study designed to enhance measurement reliability. SETTING: A glaucoma clinic at a university hospital. StudyPopulation: 6 technicians and 2 physicians. INTERVENTION: An educational intervention was implemented for the technicians to improve IOP measurement agreement with physicians. MainOutcomeMeasures: Frequency of IOP measurement disagreement between physicians and technicians, defined as a difference in IOP of >2 or >3 mm Hg and assessed at baseline and immediately and 6 months postintervention. RESULTS: IOP was evaluated for a total of 529 eyes (physician measured mean IOP = 16.4 mm Hg [SD = 5.9]), 30 per technician-physician pair for each data collection period: baseline, immediately postintervention and 6 months postintervention. At baseline, physicians disagreed 17% and 7% of the time when measuring IOP using >2 and >3 mm Hg to define disagreement, respectively, whereas the average disagreement between technicians and physicians was 25% and 13%. Disagreement was greater at IOPs greater than 20 mm Hg. No significant changes were noted in the frequency of disagreement between technicians and physicians immediately or 6 months postintervention. CONCLUSIONS: Two physicians measuring the same patient in the same room disagreed by >2 mm Hg in 17% of patients' eyes, and this amount of disagreement was even higher when comparing physicians to certified technicians. An educational intervention did not improve agreement in IOP measurements between technicians and physicians. This highlights an important limitation of Goldmann tonometry.

Trabeculotomy versus combined trabeculotomy-trabeculectomy for primary congenital glaucoma: study protocol of a randomised controlled trial.

INTRODUCTION: Trabeculotomy and combined trabeculotomy-trabeculectomy (CTT) are major surgical options for primary congenital glaucoma (PCG). However, it is unclear which of these two surgical procedures should be recommended as the optimum first-line treatment for PCG. This trial aims to determine whether the outcomes of trabeculotomy are non-inferior to those of CTT in moderate PCG with a horizontal corneal diameter (HCD) of 12-14 mm. METHODS AND ANALYSIS: This is a 3-year, non-inferiority, prospective, randomised controlled trial. We plan to recruite 248 participants (aged ≤3 years) with PCG with an HCD of 12-14 mm from the Department of Glaucoma, Zhongshan Ophthalmic Center, Guangzhou, China. One eye per participant will be randomly (1:1) assigned to receive trabeculotomy or CTT. The primary outcome is the 3-year postoperative success rate in lowering intraocular pressure (IOP), and the secondary clinical outcomes will include IOP reduction, visual acuity, HCD, central corneal thickness, axial length, cup-disc ratio, refractive error and postoperative complications. Data will be analysed by the intention-to-treat principle. ETHICAL APPROVAL AND DISSEMINATION: The study protocol has been approved by the ethics committee of Zhongshan Ophthalmic Center (2014MEKY023) and the '5010 Plan' evaluation committee at Sun Yat-Sen University, Guangzhou, China. The results will be disseminated in international academic meetings and published in peer-reviewed journals. TRIAL REGISTRATION NUMBER: Chinese Clinical Trial Registry, ChiCTR-IOR-14005588; Date registered: 20 November 2014.

Inaccuracy of intraocular pressure measurement in congenital corneal opacity: three case reports.

BACKGROUND: To report three cases of congenital corneal opacity where intraocular pressure (IOP) readings were high despite the use of multiple anti-glaucoma eye drops and normalized after corneal transplantation. CASE PRESENTATION: Three Korean infants presented with bilateral dense stromal opacification which had been present since birth. IOPs measured by rebound tonometer were high despite administration of multiple anti-glaucoma medications. One eye of each patient underwent penetrating keratoplasty (PK) because corneal opacity impaired visual development. Immediately after PK, IOPs were normalized and maintained normal without medication, whereas they remained high in the contralateral unoperated eye. On histology, stromal fibrosis was observed in the removed corneal button, and molecular assays revealed increased levels of type 1 and 5 collagens. CONCLUSION: The IOP measurement using the conventional applanation-based tonometry can be inaccurate in congenital corneal opacity which is marked by corneal fibrosis. Therefore, IOP values should be interpreted with caution in these patients, and the possibility of false-positive diagnosis of glaucoma considered.

[Corneal Biomechanics and Measurement of Intraocular Pressure Measurement in Pathological Corneas]. / Biomechanik und Augeninnendruck bei Hornhauterkrankungen.

Measuring the intraocular pressure in eyes with corneal diseases or after corneal interventions often results in false values due to abnormal corneal biomechanics. Many different approaches are used to compensate for these abnormal corneal biomechanic properties, with varying measuring methods. There are differences when these alternative measuring methods are compared to the gold standard, applanation tonometry according to Goldmann. These differences vary between the methods and with different corneal changes. There is no clear recommendation on a precise method that is suitable for every pathological cornea. Therefore it is important to know the different variations and to include them in the analysis of every patient's intraocular pressure analysis.

The Outcomes of a Comprehensive Program for Maintenance of Goldmann Applanation Tonometer.

PRECIS: We checked 190 tonometers every month and repaired faulty ones. Calibration error (CE) frequency reduced from 23% to 0.6% at 1 year. Tonometers needing one or >1 CE repair differed in survival but not in age. PURPOSE: The purpose of this study was to report the outcomes of a comprehensive program to maintain calibration status of the Goldmann applanation tonometer. METHODS: This prospective cohort study was carried out at 2 tertiary eye care referral centers. We included 190 slit-lamp mounted Goldmann applanation tonometers (Model AT 900 C/M). Health care providers (error checking and reporting) and clinical engineers (maintenance) participated. The team carried out CE check once a month, and repair of faulty tonometers, if any, within 24 hours. Failure of tonometer was defined as development of unacceptable CE beyond the third repair. The main outcome measures were the frequency of CE and survival function of the tonometer over 1 year. RESULTS: The median age of the tonometers was 10.7 (range, 0.2 to 25.1) years. The total number of repairs was 86. The proportion (95% confidence interval) of faulty tonometers reduced from 23.1% (17.7, 29.6) in the first month to 0.6% (0.1, 3.3) at 1 year (P<0.01). The median age of the tonometer did not differ between those needing (n=63, 9.4 y) and not needing (n=127, 10.7 y; P=0.24) repair. All tonometers requiring 1 CE repair (n=49, 25.7%) survived until 1 year. The survival of tonometers requiring >1 CE repair (n=14, 7.3%) was 40% at 1 year. CONCLUSIONS: Our in-house program maintained 92.6% tonometers error free. Number of repairs rather than age determined the need for replacement/sending back the tonometer to the manufacturer. Our simple and easy to follow maintenance program has the potential for wide application.

Intraocular Pressure Measurement Accuracy and Repeatability of a Modified Goldmann Prism: Multicenter Randomized Clinical Trial.

PURPOSE: To clinically evaluate a modified surface Goldmann applanation tomometer (GAT) prism for intraocular pressure (IOP) accuracy, repeatability, and safety. DESIGN: Prospective, open-label, randomized, controlled, multicenter reference device reliability and validity analysis. METHODS: A GAT and a modified surface GAT prism measured IOP on 173 unique eyes. The study design and analysis complied with FDA 510(k) and ANSIZ80.10-2014 guidelines. All eyes were randomized to IOP measurement by 1 of 5 standard prisms or 5 modified prisms, each from a different manufacturing lot. Pressures were measured by 6 investigators, 2 times with each prism, for a total of 1384 IOP measurements. Analysis included Bland-Altman difference accuracy, intraoperator and interoperator IOP measurement, and manufactured lot repeatability. RESULTS: Bland-Altman indicated no IOP measurements pairs outside the ±5 mm Hg guidelines. Operator and manufactured lot repeatability F tests and 1-way ANOVAs indicated no statistical difference between the standard and modified prisms (all P > .10). The difference in IOP measurements of the standard and modified prisms correlated well to Dresdner central corneal thickness (CCT) correction (P = .01). CONCLUSION: A modified surface replacement prism is statistically equivalent to a flat-surfaced prism. The modified surface prism indicated statistically significant correction for CCT requiring further testing outside the ANSI standard limits (0.500 mm < CCT < 0.600 mm) to examine its full potential.

Disinfection of the Goldman applanation tonometer: a systematic review.

OBJECTIVES: The Goldmann applanation tonometer is the gold standard for measuring intraocular pressure and a possible vector for the transmission of infectious diseases. The purpose of this study is to consolidate the evidence pertaining to the disinfection of Goldmann tonometers in an effort to facilitate an informed discussion regarding public policy in this important area. DESIGN: Systematic review. METHODS: An exhaustive literature review was undertaken to identify primary-level research that assesses the effectiveness of different agents used in the disinfection of Goldmann applanation tonometer prisms. Seven discrete databases were reviewed by 2 independent researchers, and a symmetrical screening process was used to identify and review all pertinent studies. RESULTS: Our review identified only 19 primary-level studies that relate specifically to the disinfection of Goldmann tonometer prisms. These studies are largely heterogeneous with regard to the pathogens and disinfectants that were tested and the experimental protocols that were employed. Accordingly, definitive conclusions as to the optimal agent cannot be made on this basis. Furthermore, the results of our review reinforce previous suggestions that only adenovirus has been transmitted between patients via the Goldmann tonometer. CONCLUSIONS: The present state of the literature does not permit a definitive conclusion regarding optimal disinfection agent for Goldmann applanation tonometer prisms. Further well-constructed studies are required to better delineate the effectiveness of disinfectants in the specific context of tonometer prisms.

Can Corneal Biomechanical Properties Explain Difference in Tonometric Measurement in Normal Eyes?

SIGNIFICANCE: Corneal biomechanical parameters can affect intraocular pressure (IOP) measurements by different tonometers compared with Goldmann applanation tonometer. This study implies that corneal hysteresis (CH) and corneal resistance factor (CRF) better explain variability in IOP measurements. PURPOSE: The aim of this study was to evaluate the effect of corneal properties on the difference in IOP measured by the Ocular Response Analyzer (ORA), Rebound Tonometer (RBT), Dynamic Contour Tonometer (DCT), and Tono-Pen from the Goldmann applanation tonometer (GAT). METHODS: An observational study was done on healthy participants in a tertiary eye care center. Corneal curvature was measured with a manual keratometer prior to IOP measurements. Intraocular pressure was measured by a single trained examiner with ORA, RBT, DCT, GAT, and Tono-Pen. CH and CRF were measured using the ORA. Central corneal thickness was measured using the ultrasonic pachymeter. Only the right eye was included for analysis. One-way analysis of variance was performed to compare variables, Bland-Altman plots to assess agreement, and regression analyses to study associated factors. RESULTS: We included 82 eyes of 82 participants with a mean age of 40.9 (14.3) years. Mean ± SD DCT IOP (15.22 ± 1.98) mmHg was significantly higher than GAT IOP (13.73 ± 2.42) (P = .01) and ORA Goldmann correlated IOP (13.66 ± 3.16) (P = .003). The limits of agreement between GAT and other tonometers measurements ranged between -5.0 and 2.1 mmHg. With multiple linear regression analyses, CH and CRF were found to be associated with the measured IOP differences between GAT and ORA (corneal compensated IOP and Goldmann correlated IOP) (P < .001) and DCT (P = .014, <.001) whereas differences between GAT and RBT measurements were independently explained by corneal curvature (P = .035) and central corneal thickness (P = .045). CONCLUSIONS: There was good agreement between GAT and other tonometers, but was not good enough for them to be used interchangeably. A combination of CH and CRF may better explain the variability between GAT and tonometers.

Assessing the True Intraocular Pressure in the Non-human Primate.

SIGNIFICANCE: For glaucoma patients, high intraocular pressure (IOP) is a risk factor for progressive neuropathy. Similarly, animal models used to study the disease are based on an experimental elevation of IOP. Thus, accurate IOP measurements are important in characterizing experimental models and resulting effects. PURPOSE: The purpose of the present study was to investigate IOP measurements in a non-human primate model of experimental glaucoma by comparing clinical tonometry (Tono-Pen and TonoVet) to the true IOP from intracameral manometry. METHODS: A total of 17 rhesus macaque eyes from 12 animals were used for this study. Eleven eyes had no previous experimental intervention, whereas six eyes were at varying stages of laser-induced experimental glaucoma. IOPs were adjusted by inserting a needle in the anterior chamber that was attached to a pressure transducer and syringe pump system. The anterior chamber IOP was adjusted to values between 10 and 50 mmHg and corresponding measures with Tono-Pen and TonoVet were taken. RESULTS: The IOPs by TonoVet and Tono-Pen were linearly related over the range of pressures tested (slope = 0.68 normal/healthy and 0.72 experimental glaucoma). For the most, TonoVet measures overestimated IOP at all anterior chamber pressure settings (mean difference of 3.17 mmHg, 95% CI 12.53 to -4.74 normal and 3.90 mmHg, 95% CI 12.90 to -6.53 experimental glaucoma). In contrast, Tono-Pen measures overestimated IOP at lower IOPs and underestimated at higher IOP (slope = -0.26 normal and -0.21 experimental glaucoma). CONCLUSIONS: The TonoVet and Tono-Pen tonometers that are often used to assess IOP in both clinical and experimental settings generally reflect the status of IOP, but the results from this study suggest that the instruments need calibration with true anterior chamber pressure for accurate measures in experimental models of glaucoma.

Tonometer validation and intraocular pressure reference values in the normal chinchilla (Chinchilla lanigera).

OBJECTIVES: To determine accuracy and precision of three commonly used tonometers (TonoVet® and TonoLab® (ICare Oy, Finland)-rebound tonometers, and Tono-Pen VET™ (Reichert, NY)-applanation tonometer) in normal chinchillas, and to establish a normal intraocular pressure (IOP) reference range in this species. METHODS: The anterior chambers of three chinchilla eyes were cannulated ex vivo and readings obtained at manometric IOPs from 5 to 80 mmHg, using each of the three tonometers in random order. Data were analyzed by linear regression, ANOVA, and Bland-Altman plots. Tonometry was performed in both eyes of 60 chinchillas (age 8 weeks-16.2 years) using the TonoVet® and relationship between age and IOP analyzed using linear regression. For all statistical tests, P < 0.05 was significant. RESULTS: Intraocular pressure values obtained using the Tono-Pen VET™ and TonoVet® (in dog calibration mode;'d') showed strong linear correlation with manometry within the physiologic and clinically relevant range of IOP (0-50 mmHg). The TonoVet® 'd' setting displayed significantly greater precision over the full range of IOP evaluated than the Tono-Pen VET™, and both TonoVet and Tono-Pen VET™ were significantly more accurate than the TonoLab® tonometer. Mean ± SD IOP (TonoVet® 'd') in chinchillas was 9.7 ± 2.5 mmHg, and the 95% reference interval was 4.7-14.7 mmHg. CONCLUSIONS: Both the Tono-Pen VET™ and TonoVet® provided clinically acceptable estimates of IOP in chinchillas. The TonoVet® provides accurate and precise IOP values, while Tono-Pen VET™ derived measurements showed greater variability. Values obtained either with the TonoLab® or TonoVet® used in the 'unspecified' calibration setting were inaccurate in this species.

Can the SENSIMED Triggerfish® lens data be used as an accurate measure of intraocular pressure?

PURPOSE: The SENSIMED Triggerfish® contact lens sensor (CLS) has an embedded micro-sensor that captures spontaneous circumferential changes at the corneoscleral junction and transmits them via an antenna to a device where these measurements are stored. During laparoscopic colorectal surgery, patients are placed in Trendelenburg position which has been shown to increase intraocular pressure (IOP). Laparoscopic colorectal surgery requires both pneumoperitoneum and Trendelenburg positioning; therefore, IOP can vary significantly. We aimed to assess whether circumferential changes in the corneoscleral area can be correlated to IOP changes measured using Tono-pen® XL applanation tonometer during laparoscopic colorectal surgery. METHOD: Patients undergoing laparoscopic colorectal resections were included. On the day of surgery, baseline IOP was taken and the SENSIMED Triggerfish® CLS was then set up in one eye of the patient. During surgery (whilst under general anaesthetic), IOP measurements were taken in the contralateral eye using a Tono-pen® XL applanation tonometer every hour and any time the table was moved to record the fluctuations of IOP during surgery and any association with position change. The timings of these readings were documented. RESULTS: Twenty patients were included in this study (six males, 14 females). Average age was 64.6 years (SD = 16.3). The fluctuation in IOP measured in the reference eye ranged between 6.3 and 46.7 mmHg. The mean correlation coefficient between CLS output measurements and these IOP measurements was r = 0.291 (95% CI). CONCLUSION: Our results showed a weak correlation between the SENSIMED Triggerfish® CLS data output and IOP measurements taken using the Tono-pen® XL applanation tonometer.

Research: Validity of Measurements when Reusing Icare Probes.

BACKGROUND: It has been reported that reusing Icare tonometer probes may not pose a considerable risk of transmission of infection, thereby supporting this practice under extraordinary circumstances, such as mass glaucoma screenings in developing countries. The present study sought to determine whether reusing the probes reduced the validity of pressure readings. METHODS: Thousands of measurements were made with the same Icare probe on a purpose-built calibration device. A second set of measurements was made during which the Icare probe head was wiped with an alcohol pad between each reading. RESULTS: Statistically different readings were observed between the no-wipe and wipe groups for most of the settings evaluated. In addition, statistically significant readings were noted for a few of the settings as the number of readings increased for either group. However, these differences would not be considered clinically significant. CONCLUSION: This study suggests that reusing Icare tonometer probes, even for hundreds of measurements, would not reduce the clinical utility of the pressure readings. For mass screenings in developing countries where cost savings is crucial, considering the reuse of Icare tonometer probes seems reasonable.

Comparison of intraocular pressure as measured by three different non-contact tonometers and goldmann applanation tonometer for non-glaucomatous subjects.

BACKGROUND: To compare the measurement of intraocular pressure (IOP) among the three different non-contact tonometers (NCT) and the Goldmann applanation tonometer (GAT) for non-glaucomatous subjects. METHODS: In 52 eyes of 52 non-glaucomatous subjects, IOP was measured sequentially with the Canon TX-20P, the Nidek NT-530P, the Topcon CT-1P, and the GAT at the same time. We evaluated the IOP-measurement agreement among the tonometers as well as the factors affecting the measurements. RESULTS: A significant positive correlation was shown between the IOP values obtained with GAT and each NCT. The Canon TX-20P showed statistically the most significant agreement with the GAT (ICC 0.906, 95% CI 0.837-0.946). In an analysis of the Bland-Altman plots, the Canon TX-20P also showed the largest mean bias (1.38 mmHg) but the narrowest limits of agreement (LoA) (95% LoA; ± 3.43 mmHg). The Topcon CT-1P showed the smallest mean bias (0.48 mmHg) but the widest LoA (95% LoA; ± 4.16 mmHg). The Topcon CT-1P and Nidek NT-530P both showed a significantly positive correlation between increasing central corneal thickness (CCT) and increasing IOP. CONCLUSION: There was a statistically significant correlation between each of the three different NCT and the GAT measurements. IOP measured with the Canon TX-20P and Topcon CT-1P tended to be higher, and with the Nidek NT-530P lower, than with the GAT. Practitioners need to know the properties of their own NCTs and their respective measurement tendencies.

Goldmann applanation tonometry error relative to true intracameral intraocular pressure in vitro and in vivo.

BACKGROUND: Goldmann applanation tonometry (GAT) error relative to intracameral intraocular pressure (IOP) has not been examined comparatively in both human cadaver eyes and in live human eyes. Futhermore, correlations to biomechanical corneal properties and positional changes have not been examined directly to intracameral IOP and GAT IOP. METHODS: Intracameral IOP was measured via pressure transducer on fifty-eight (58) eyes undergoing cataract surgery and the IOP was modulated manometrically on each patient alternately to 10, 20, and 40 mmHg. IOP was measured using a Perkins tonometer in the supine position on 58 eyes and upright on a subset of 8 eyes. Twenty one (21) fresh human cadaver globes were Intracamerally IOP adjusted and measured via pressure transducer. Intracameral IOP ranged between 5 and 60 mmHg. IOP was measured in the upright position with a Goldmann Applanation Tonometer (GAT) and supine position with a Perkins tonometer. Central corneal thickness (CCT) was also measured. RESULTS: The Goldmann-type tonometer error measured on live human eyes was 5.2 +/-1.6 mmHg lower than intracameral IOP in the upright position and 7.9 +/- 2.3 mmHg lower in the supine position (p < .05). CCT also indicated a sloped correlation to error (correlation coeff. = 0.18). Cadaver eye IOP measurements were 3.1+/-2.5 mmHg lower than intracameral IOP in the upright position and 5.4+/- 3.1 mmHg in the supine position (p < .05). CONCLUSION: Goldmann IOP measures significantly lower than true intracameral IOP by approximately 3 mmHg in vitro and 5 mmHg in vivo. The Goldmann IOP error is increased an additional 2.8 mmHg lower in the supine position. CCT appears to significantly affect the error by up to 4 mmHg over the sample size.

Determination of reference values for intraocular pressure and Schirmer tear test results in clinically normal domestic donkeys (Equus asinus).

This study was conducted to establish normal reference range for the Schirmer tear test (STT) and intraocular pressure (IOP) in clinically normal donkeys for use in clinical practice. Sixteen adult Ethiopian domestic donkeys were used in this study. Complete ophthalmic examinations were performed without chemical restraint. STT values were evaluated in both eyes of all donkeys using a commercial STT strip. IOP was measured in both eyes with a Tono-Pen Vet (Reichert. USA) without using regional nerve blocks. STT values for all eyes (n=32) were 22.1±6.9 mm/minute with a range of 13-35 mm/minute. Comparison of mean STT values between right (n=16) and left eyes (n=16) showed no differences (P=0.6). Mean ±sd IOP values by applanation tonometry were 17.8±3.7 mmHg (range 13.5-24.5 mmHg). Comparison of mean IOP values between right (n=16) and left eyes (n=16) showed no differences (P=0.7). This initial survey provides means and ranges for IOP and STT values in donkeys. These data will assist veterinary ophthalmologists in more accurate diagnosis and management of ophthalmic diseases in donkeys, including keratoconjunctivitis sicca and glaucoma.

Measurement of tear production and intraocular pressure in ducks and geese.

OBJECTIVES: The purpose of this study was to establish a reference value of tear production and intraocular pressure in clinically normal ducks and geese. ANIMAL STUDIED: The study population consisted of forty-eight ducks (24 males and 24 females) and fifty-two geese (26 males and 26 females). PROCEDURES: Birds were gently physically restrained in dorsoventral position without any pressure to the head and neck. Tear secretion and intraocular pressure were measured using 5-mm-wide Schirmer tear test strips and TonoVet® rebound tonometer, respectively. RESULTS: Overall mean ± 2SD STT and IOP values of all eyes were 6.2 ± 2.2 mm/min and 10.2 ± 2.2 mmHg, respectively, for the ducks; 5.5 ± 2.6 mm/min and 9.1 ± 2.0 mmHg, respectively, for the geese. There was no statistically significant difference in IOP between left eye and right eye or between males and females in ducks (P = 0.56, P = 0.63) and geese (P = 0.23, P = 0.22). Analysis of variance revealed that there were no significant differences in the STT and IOP between the breeds in ducks (P = 0.49, P = 0.68) and geese (P = 0.14, P = 0.26). CONCLUSIONS: Results of this study may be beneficial for the diagnosis and therapeutic monitoring of ducks and geese affected with Keratoconjunctivitis sicca, glaucoma, and uveitis.

Corneal Biomechanical Changes After Trabeculectomy and the Impact on Intraocular Pressure Measurement.

PURPOSE: To evaluate corneal biomechanical changes induced by trabeculectomy and their impact on intraocular pressure (IOP) measurements. MATERIALS AND METHODS: In total, 35 eyes of 35 consecutive glaucoma patients undergoing first-time trabeculectomy with mitomycin C were enrolled in this prospective interventional case series. Goldmann applanation tonometry (GAT) IOP, central corneal thickness, axial length, and Ocular Response Analyzer measurements [Goldmann-correlated IOP (IOPg), corneal-compensated IOP (IOPcc), corneal hysteresis (CH), and corneal resistance factor (CRF)] were assessed before and 6 months after uncomplicated trabeculectomy. Linear mixed models were used to compare the parameters before and after surgery. RESULTS: IOP, central corneal thickness, and axial length showed a strong correlation with CH and CRF preoperatively and postoperatively. After adjusting for these influencing factors, CH changed from 7.75±1.46 to 7.62±1.66 mm Hg (P=0.720) and CRF from 8.67±1.18 to 8.52±1.35 mm Hg (P=0.640) after trabeculectomy, but these changes were not statistically significant. IOP decreased statistically significantly with all IOP measurements (P=0.001). IOPcc was statistically significantly higher than GAT (4.82±5.24 mm Hg; P=0.001) and IOPg (2.92±1.74 mm Hg; P=0.001) preoperatively and postoperatively (GAT, 3.29±3.36 mm Hg; P=0.001; IOPg, 3.35±1.81 mm Hg; P=0.001). The difference between IOPcc and GAT (P=0.5) and IOPcc and IOPg (P=0.06) did not change significantly before or after trabeculectomy. CONCLUSIONS: Despite a marked IOP reduction and a possible weakening of the ocular walls after trabeculectomy, corneal structural tissue properties are not altered, and therefore, the accuracy of IOP measurements is not changed postoperatively. It seems likely, however, that Goldmann-correlated IOP measurements are underestimated in glaucoma patients before and after surgery.

Mathematical Discrepancies of the Tono-Pen Applanation Tonometer.

PURPOSE OF THE STUDY: The purpose of the study was to determine if Tono-Pen tonometers use simple average and coefficient of variation (CV) algorithms to calculate intraocular pressure (IOP). MATERIAL AND METHODS: IOPs were measured as part of routine ocular examination in 152 client-owned dogs. Using 11 Tono-Pen's, a total of 778 averaged readings were collected. Individual IOP readings, and average IOP and CV displayed by the instrument, were recorded. Average IOP and CV were then manually calculated from individual readings and compared with those displayed by the instrument. RESULTS: The mean absolute difference between the calculated and displayed average IOP was 1.37±2.01 mm Hg (P<0.001). In 6% of cases, the calculated average IOP was 5 to 15 mm Hg different from the displayed average IOP. The difference between the displayed and calculated average IOP was significantly higher in hypertensive eyes with displayed IOP≥25 mm Hg. Calculated CV was equal to, lower than, or greater than displayed CV in 28.6%, 1.5%, and 69.7% of cases, respectively. In 17.6% of cases, calculated CV was >20%, but displayed CV was <5%. Receiver operating characteristic analysis could not correlate number of individual IOP readings with magnitude of difference in average IOP. CONCLUSIONS: Calculated average IOP and CV differ significantly from values displayed by the instrument, especially at higher IOPs. A difference of ≥5 mm Hg between calculated and displayed average IOP seen in 6% of cases may impact clinical judgement. Displayed CV<5% does not correlate with accurate IOP measurement based on individual results.