The corneal thickness and intraocular pressure story: where are we now?

Intraocular Pressure and Its Relation to Ocular Geometry: Results From the Gutenberg Health Study

Purpose

The purpose of this study was to investigate the association between intraocular pressure (IOP) and ocular geometry.

Methods

The Gutenberg Health Study is a population-based cohort study in Mainz, Germany. Study participants underwent a comprehensive ophthalmologic examination including noncontact tonometry, objective refraction, optical biometry, and Scheimpflug imaging of the anterior segment at the first 5-year follow-up examination (in 2012–2017). Multivariable linear regression analysis was carried out to determine associations of IOP and geometric parameter of the human phakic eye, namely central corneal thickness (CCT), corneal curvature, anterior chamber depth (ACD), lens thickness, and axial length. In addition, the relationship of IOP and the anterior chamber angle (ACA) width was analyzed.

Results

There were 6640 participants with phakia (age 57.3 ± 10.2 years, 49.1% women) that were included in this cross-sectional analysis. Mean IOP was 14.8 ± 2.9 mm Hg in the right eyes and 14.9 ± 2.9 mm Hg in the left eyes. IOP increased with higher CCT, greater posterior segment length, higher age (all P < 0.001), thicker lens (P = 0.003), and female sex (P = 0.05), whereas the ACD was not associated with higher IOP. The IOP increased with a narrower ACA in univariable analysis (P < 0.001), but not in adjusted analysis in subjects with an open angle.

Conclusions

IOP values are related to ocular geometry, as shown in this population-based study on Caucasian subjects. Thus, knowledge of the architecture of the eye is an important factor when measuring IOP. Longitudinal evaluation will analyze whether some of these parameters are also risk factors for the development of glaucoma.

Comparison of the corrected intraocular pressure by tonopachy with that of Goldmann applanation tonometry in normal and glaucomatous patients

Purpose:

To compare corrected intraocular pressure (IOP) by tonopachy with that of Goldmann applanation tonometry (GAT) in normal and glaucomatous patients.

Methods:

In this cross-sectional study, IOP and central corneal thickness (CCT) were measured in 426 eyes (213 normal eyes and 213 glaucomatous eyes) of 426 patients by tonopachy followed by GAT and ultrasound pachymetry. IOP was corrected for CCT by in-built formula in tonopachy and Ehlers correction factor for Goldmann tonometer. Limits of agreements were assessed using Bland–Altman plots. Intraclass correlation coefficient was calculated to estimate the absolute agreement between single and average measurements of IOP and CCT of tonopachy with that of Goldmann tonometer and ultrasound pachymetry respectively.

Results:

Mean corrected IOP measured with tonopachy and GAT in glaucomatous eyes was 17.63 ± 5.04 mmHg and 19.42 ± 5.83 mmHg, and in controls it was 13.4 ± 2.5 mmHg and 16.2 ± 3.1 mmHg, respectively. Limits of agreement ranged from –4.63 to +9.25 mmHg for total population (mean = 2.31), ˗6.01 to +9.59 mmHg (mean = 1.79) for glaucoma group and ˗2.99 to +8.65 mmHg (mean = 2.83) for controls. Intraclass correlation coefficient for IOP measurement between tonopachy and Goldmann tonometer was 0.84 for total population, 0.85 for glaucoma group, and 0.63 for controls, respectively.

Conclusion:

Corrected IOP obtained by tonopachy showed moderate agreement with GAT and it is more in glaucoma patients than controls. Thus, tonopachy can be used as a screening tool, but cannot replace GAT.

Promising Approach in the Treatment of Glaucoma Using Nanotechnology and Nanomedicine-Based Systems

Glaucoma is considered a leading cause of blindness with the human eye being one of the body's most delicate organs. Ocular diseases encompass diverse diseases affecting the anterior and posterior ocular sections, respectively. The human eye's peculiar and exclusive anatomy and physiology continue to pose a significant obstacle to researchers and pharmacologists in the provision of efficient drug delivery. Though several traditional invasive and noninvasive eye therapies exist, including implants, eye drops, and injections, there are still significant complications that arise which may either be their low bioavailability or the grave ocular adverse effects experienced thereafter. On the other hand, new nanoscience technology and nanotechnology serve as a novel approach in ocular disease treatment. In order to interact specifically with ocular tissues and overcome ocular challenges, numerous active molecules have been modified to react with nanocarriers. In the general population of glaucoma patients, disease growth and advancement cannot be contained by decreasing intraocular pressure (IOP), hence a spiking in future research for novel drug delivery systems and target therapeutics. This review focuses on nanotechnology and its therapeutic and diagnostic prospects in ophthalmology, specifically glaucoma. Nanotechnology and nanomedicine history, the human eye anatomy, research frontiers in nanomedicine and nanotechnology, its imaging modal quality, diagnostic and surgical approach, and its possible application in glaucoma will all be further explored below. Particular focus will be on the efficiency and safety of this new therapy and its advances.

Pigment dispersion syndrome and pigmentary glaucoma: a review and update

INTRODUCTION

Pigment dispersion syndrome (PDS) is a condition where anomalous iridozonular contact leads to pigment dispersion throughout the anterior segment and the released pigment is abnormally deposited on various ocular structures.

CLINICAL PRESENTATION

The clinical presentation of PDS is defined by the presence of pigmented cells on the corneal endothelium, an increase of pigmentation of the trabecular meshwork, and mid-periphery transillumination defects of the iris. This syndrome, more common in myopes, is usually bilateral and can be associated with ocular hypertension or glaucoma. Secondary open-angle pigmentary glaucoma (PG) can develop due to reduction of the outflow of aqueous humour and consequent increase in intraocular pressure leading to glaucomatous optic neuropathy. Diagnosis of PG is commonly between 40 and 50 years of age, occurring more frequently in men. The advent of ultrasound biomicroscopy and anterior segment optical coherence tomography has contributed to enhancing our knowledge on the condition. Typical alterations of the anterior segment are the posterior insertion of the iris and iris concavity. Treatment of PG should be initiated early to hinder disease progression, glaucomatous damage, and vision loss. Management is based on medical therapy, laser iridotomy, selective laser trabeculoplasty, and filtration procedures.

CONCLUSIONS

The differential diagnosis of PDS with other disorders can be challenging and awareness of the condition together with meticulous ophthalmologic examination allows early diagnosis followed by appropriate management strategies. The present review is a comprehensive report on the clinical characteristics, pathogenesis, current management, and status quo of PDS and PG.

Refractive errors

BACKGROUND

All over the world, refractive errors are among the most frequently occuring treatable distur - bances of visual function. Ametropias have a prevalence of nearly 70% among adults in Germany and are thus of great epidemiologic and socio-economic relevance.

METHODS

In the light of their own clinical experience, the authors review pertinent articles retrieved by a selective literature search employing the terms "ametropia, "anisometropia," "refraction," "visual acuity," and epidemiology."

RESULTS

In 2011, only 31% of persons over age 16 in Germany did not use any kind of visual aid; 63.4% wore eyeglasses and 5.3% wore contact lenses. Refractive errors were the most common reason for consulting an ophthalmologist, accounting for 21.1% of all outpatient visits. A pinhole aperture (stenopeic slit) is a suitable instrument for the basic diagnostic evaluation of impaired visual function due to optical factors. Spherical refractive errors (myopia and hyperopia), cylindrical refractive errors (astigmatism), unequal refractive errors in the two eyes (anisometropia), and the typical optical disturbance of old age (presbyopia) cause specific functional limitations and can be detected by a physician who does not need to be an ophthalmologist.

CONCLUSION

Simple functional tests can be used in everyday clinical practice to determine quickly, easily, and safely whether the patient is suffering from a benign and easily correctable type of visual impairment, or whether there are other, more serious underlying causes.

A structural model for the in vivo human cornea including collagen-swelling interaction

A structural model of the in vivo cornea, which accounts for tissue swelling behaviour, for the three-dimensional organization of stromal fibres and for collagen-swelling interaction, is proposed. Modelled as a binary electrolyte gel in thermodynamic equilibrium, the stromal electrostatic free energy is based on the mean-field approximation. To account for active endothelial ionic transport in the in vivo cornea, which modulates osmotic pressure and hydration, stromal mobile ions are shown to satisfy a modified Boltzmann distribution. The elasticity of the stromal collagen network is modelled based on three-dimensional collagen orientation probability distributions for every point in the stroma obtained by synthesizing X-ray diffraction data for azimuthal angle distributions and second harmonic-generated image processing for inclination angle distributions. The model is implemented in a finite-element framework and employed to predict free and confined swelling of stroma in an ionic bath. For the in vivo cornea, the model is used to predict corneal swelling due to increasing intraocular pressure (IOP) and is adapted to model swelling in Fuchs' corneal dystrophy. The biomechanical response of the in vivo cornea to a typical LASIK surgery for myopia is analysed, including tissue fluid pressure and swelling responses. The model provides a new interpretation of the corneal active hydration control (pump-leak) mechanism based on osmotic pressure modulation. The results also illustrate the structural necessity of fibre inclination in stabilizing the corneal refractive surface with respect to changes in tissue hydration and IOP.

The Conjunctiva in Normal Tension Glaucoma Patients is Thinner Than in Primary Open-Angle Glaucoma Patients: A Comparative Histologic Study

PURPOSE

To compare histologically the thickness of conjunctival specimens of normal tension glaucoma (NTG) patients with primary open-angle glaucoma (POAG) patients.

MATERIALS AND METHODS

In this prospective study, 54 patients scheduled for trabeculectomy were categorized into NTG and POAG based on their maximum untreated intraocular pressure at any time (IOPmax) as measured by Goldmann applanation tonometry. Sixteen patients with NTG (IOPmax≤21 mm Hg) and 36 patients with high tension POAG (IOPmax>21 mm Hg) were included in the study. Biopsies were taken from the superior bulbar conjunctiva during trabeculectomy. The specimens were fixed in formalin, embedded in methacrylate, histologically sectioned, stained with toluidine blue, and analyzed with a light microscope. The stromal conjunctival thickness (CT) was measured in a standardized way and compared between the 2 groups. Intergroup comparisons were performed using the Mann-Whitney U test for continuous variables and the Fisher exact test for categorical variables. The correlation between the central cornea thickness (CCT) and the CT was investigated by the Spearman test.

RESULTS

The stromal CT was significantly thinner in NTG compared with POAG (64±31 vs. 103±44 µm, respectively; P=0.002). Stromal CT of the whole group was positively correlated with IOPmax (r=0.41; P=0.002; 95% confidence interval, 0.15-0.62) but not with central cornea thickness (r=-0.005; P=0.97; 95% confidence interval, -0.28 to 0.27).

CONCLUSIONS

The conjunctiva in patients with NTG was thinner than in POAG patients. This finding is an additional feature in the pattern of thinner ocular structures in patients with NTG.

Correlation between the pressure-to-cornea index and both structural and functional measures of glaucoma

Purpose:

the pressure-to-cornea index (PCI) was proposed in order to integrate intraocular pressure and central cornea thickness as a single-risk factor for glaucoma. The purpose of this study was to correlate the PCI with a structural and two functional measures of glaucoma.

Setting:

University Hospital in South America.

Materials and Methods:

Pressure-to-cornea index was calculated for 70 eyes of 36 subjects (glaucoma and suspects). Cup-to-disc (C/D) ratio, mean deviation (MD) and pattern standard deviation (PSD) as recorded by Humphrey automated perimetry (SITA 24-2) were correlated with PCI (Pearson's correlation coefficient).

Results:

Pearson's correlation coefficient between PCI and C/D was 0.329 (95% confidence interval [95% CI], 0.09–0.526; P = 0.006); between PCI and MD was − 0.356 MD (95% CI, −0.549 to − 0.126; P = 0.003); and between PCI and PSD was − 0.215 (95% CI, −0.433 to 0.025; P = 0.07).

Conclusion:

In addition to serve as a single-risk factor, PCI can be used to stage glaucoma severity as well.

Numerical study on human cornea and modified multiparametric correction equation for Goldmann applanation tonometer

Glaucoma, a chronic eye disease, is quoted to be a second leading cause of blindness with 66.8 million people suspected to be victimised by the same in year 2000 (Morrison, 2003), and is caused due to an elevated intraocular pressure (IOP). Goldmann Applanation tonometer (GAT), a gold standard (Organization, 2001), suffers from inefficacies to measure intra-ocular pressure (IOP) independent of the biomechanical properties of an eye. Thus, the purpose of this paper is to study the importance of wide range of corneal parameters and to propose a multiparametric correction equation for GAT based on a numerically simulated comprehensive finite element model (Dohadwala et al., 1998; Morrison, 2003; Organization, 2001). Towards this end, with the help of numerical model, the parameters considered in correction equation proposed elsewhere (Elsheikh et al., 2011) were extended to include combined influence of central corneal thickness (CCT), central anterior curvature (R), peripheral corneal thickness (PCT), corneal asphericity (Pa), age, measured IOP itself and, was separately studied for role played by corneal support orientation (θ°) in this aspect. Findings indicate that for a wide range of parameters considered, while increased age is related with gain in corneal stiffness, the maximum (average) individual effects of variations in PCT, Pa, CCT, age and θ° on IOPG were estimated at 0.25mmHg/100µ of PCT, 0.073mmHg/0.1 of Pa, 1.9mmHg/100µ of CCT, 24.3%/decade relative of IOPT and 0.95mmHg/5° increase in θ°. The multiparametric correction equation has been modified accordingly. The GAT correction equation can consider the combined effect of PCT, Pa, CCT, and age on IOPG. Separately the non-linear effect of θ° on IOPG cannot be ignored for reasons of precision.

Dynamic contour tonometry and goldman applanation tonometry: correlation with intracameral assessment of intraocular pressure

Purpose

To compare intraocular pressure (IOP) measured using a dynamic contour tonometer (DCT) and a Goldmann applanation tonometer (GAT) with in vivo intracameral IOP, and establish the relationship between DCT, GAT and central corneal thickness (CCT) in patients with primary open-angle glaucoma (POAG).

Materials and Methods

We examined 50 eyes of 50 patients with POAG scheduled for glaucoma or cataract surgery. Immediately before surgery, CCT, GAT and DCT IOP were assessed, after which manometry of the anterior chamber was performed. A Bland-Altman plot was used to test the agreement among the 3 measurements of IOP, and univariate and multivariate regression analyses were used to evaluate the effect of CCT on DCT and GAT.

Results

On average, the DCT readings were 4.0±1.6 mmHg higher than the GAT readings and 2.3±2.4 mmHg higher than the manometric readings; the GAT measurements were generally a mean 1.7±1.8 mmHg lower than the manometric readings. The CCT had an almost similar influence on DCT and GAT measurements (p=0.84).

Conclusions

The DCT-measured IOP was significantly higher than that measured by means of GAT and anterior chamber manometry. The DCT and GAT readings were both influenced by CCT to the same extent.

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.

Laser refractive surgery in glaucoma patients

Glaucoma may not be an absolute contraindication to Laser-Assisted in situ Keratomileusis (LASIK), but so far it is a relative one. People who are glaucoma suspects or who have glaucoma are just as likely as any other to seek laser refractive surgery. LASIK is a popular ocular procedure, relatively pain free and it is carried out with an extremely precise computer-controlled excimer laser emission. On the other hand, glaucoma is a group of diseases manifested by optic nerve damage with visual field changes. Patients that undergo a transient but significant rise in intraocular pressure during LASIK procedure have risk of further optic nerve damage. Furthermore, steroids which are typically used after refractive surgery can increase intraocular pressure (IOP) especially in steroid responders, who are more prevalent among glaucoma patient. Glaucoma patients interested in LASIK surgery may visit a glaucoma specialist or another LASIK surgeon who has had experience with performing LASIK in glaucoma patients. PRK (photorefractive keratectomy), LASEK (laser epithelial keratomileusis) are good alternatives for glaucoma patients. Refractive surgeons might want to consider giving patients a photo of their optic nerve, or a drawing, or an objective record of their preoperative examination.

Blue-on-yellow perimetry and corneal thickness in patients with ocular hypertension

AIM

To evaluate the frequency of blue-on-yellow perimetry (B/YP) deficits in ocular hypertension (OHT) patients and to correlate these findings with central corneal thickness (CCT), and to investigate the influence of age, refraction and gender on the B/YP results in OHT patients.

METHODS

The B/YP and CCT were checked respectively in 72 OHT patients with normal white-on-white perimetry(W/WP) and normal optic nerve head. The B/YP was tested by Octopus 101 automated perimetry using G2 strategy, while the CCT was checked with DGH-550 ultrasound pachymeter. All patients were chosen randomly one eye for statistical analysis, a binary regression model was used to determine the independent contribution of variables included in the model, and the differences of the intraocular pressure (IOP), CCT, age, refraction and gender between the normal B/YP group and abnormal B/YP group were compared.

RESULTS

Forty-nine out of 72 patients with OHT showed normal B/YP results, whereas 23 of 72 patients(31.9%) demonstrated abnormal B/YP results. CCT showed a correlation with the B/YP results (B=-0.038, SE=0.019, P=0.044), whereas none of the IOP, age, refraction and gender was found to be correlated with the B/YP results. The mean CCT in OHT patients with abnormal B/YP group was lower than that with normal B/YP group (t=2.066, P=0.043).There was a significant positive correlation between IOP and CCT (R(2)=0.513, P=0.000).

CONCLUSION

The mean CCT in OHT patients with abnormal B/YP results was lower than that with normal B/YP results. There was a significant positive correlation between IOP and CCT in OHT patients. The age, refraction and gender didn't influence the B/YP results in OHT patients.

Goldmann Tonometry Correction Factors Based on Numerical Analysis

With the world’s aging population, it is expected that the number of people affected by glaucoma, the second most common cause of irreversible blindness, will increase considerably. Current knowledge on glaucoma progression relates elevation of the intraocular pressure (IOP) to optic nerve damage and hence visual impairment. For this reason, IOP measurement in tonometry has become an essential part of routine eye examinations needed for the diagnosis and management of the disease. The accuracy of the current reference standard in tonometry, the Goldmann applanation tonometer, is known to be affected by the natural variations in corneal thickness, curvature, and material properties. Earlier studies attempted to quantify these effects and produced correction factors that considered the variations in each one of these parameters separately, and no guidance was given as to how to combine the effects of variations in more than one parameter. The present research attempted to address this gap by conducting a multidimensional numerical study that considered variations in thickness, curvature, material properties, and IOP, and used the results to develop a single correction equation that considered these parameters simultaneously. The results of the analysis and the correction equation were validated successfully against the outcome of earlier clinical and mathematical studies on the effect of individual parameters, and the correction equation was presented in a simple form suitable for clinical application.

Comparison of dynamic contour tonometry and Goldmann applanation tonometry and their relationship to corneal properties, refractive error, and ocular pulse amplitude

BACKGROUND

Accurate intraocular pressure (IOP) measurement is essential in diagnosing and managing glaucoma. Dynamic contour tonometry (DCT) is less dependent on corneal properties, such as thickness, elasticity, and rigidity, than Goldmann applanation tonometry (GAT). This study examined the relationship between GAT and DCT as well as their relationship with corneal properties and ocular pulse amplitude (OPA).

METHODS

GAT, DCT, OPA, pachymetry, refractive error, and corneal curvature measurements were obtained on 115 healthy volunteers.

RESULTS

Participants with thicker corneas (>or=580 microm) had higher IOP measurements with GAT than DCT (P = 0.005). Those with thinner corneas (<or=520 microm) had lower IOP with GAT versus DCT (P = 0.008). GAT and DCT readings did not differ significantly in corneas with average thickness (521 to 579 microm). A clinically significant IOP difference between DCT and GAT was found in 18.2% of subjects. A correlation was found between OPA and both refractive error and IOP (R(2) = .343, P < 0.0001). OPA was higher with increased IOP and decreased myopia.

CONCLUSION

DCT provides IOP measurements that are less dependent on corneal factors than GAT, aiding in diagnosis and treatment of patients with ocular hypertension and glaucoma. Additional studies are necessary to examine the relationship between OPA, refractive error, and IOP and its possible association with increased incidence of glaucoma in myopic patients.

Pascal dynamic contour tonometry versus goldmann applanation tonometry in gas and air-filled eyes after vitrectomy surgery

PURPOSE

To compare Pascal Dynamic Contour Tonometry with Goldmann Applanation Tonometry in eyes after vitrectomy surgery with intraocular tamponade of air, silicone oil or perfluorocarbon gas.

METHODS

Prospective clinical comparative study. Eighty-two consecutive patients undergoing vitrectomy surgery with postoperative air, gas or oil tamponade were recruited. Intraocular pressure was measured with both devices.

RESULTS

Mean Goldmann intraocular pressure was 16.6 mmHg (range, 1.0-46.0; SD = 8.80) and the mean Pascal intraocular pressure was 21.70 (range, 4.7-58.5; SD = 9.8) The mean difference between the Pascal and Goldmann readings was 5.09 mmHg (range, -14.7 to +12.9; 95% CI = 4.2-6.0; SD, 4.0; P < 0.001). Mean differences for the different tamponades were 5.09 mmHg for silicone oil, 4.02 mmHg for air, and 5.38 mmHg for perfluorocarbon gas.

CONCLUSION

Pascal dynamic contour tonometry gives readings that are highly correlated with Goldmann applanation tonometry, but on average 5 mmHg higher in eyes after vitrectomy surgery with air, gas or silicone oil tamponades. The difference between Goldmann and Pascal readings does not appear to be altered by the presence of a scleral buckle, or the size of the intraocular gas bubble.

Comparison of intraocular tonometry using three different non-invasive tonometers in children

BACKGROUND

In childhood glaucoma, the correct determination of intraocular pressure (IOP) is crucial in clinical decision-making. We therefore investigated how intraocular tonometry readings correlate with readings from commonly used tonometers.

METHODS

IOP was measured unilaterally in 20 children suffering from congenital (n = 7) or secondary glaucoma (n = 13), 10 minutes after the induction of general anaesthesia. The children were aged from 1 month to 17 years (mean age 4.3 years, median age 1.3 years). Non-invasive applanation (Tono-Pen XL, Perkins tonometer) and indentation tonometry (Schiötz tonometer) were performed in random order prior to intraocular tonometry with a 26-gauge needle connected to a pressure sensor. Linear regression analysis and the coefficients of variance (CV) were used to compare the data obtained from the various tonometers.

RESULTS

Compared with intraocular pressure, the CV was 10% for the Tono-Pen XL, 17% for the Schiötz, and 19% for the Perkins tonometer. The coefficient of determination (r(2)) was 0.74 for Tono-Pen XL, 0.60 for Schiötz and 0.78 for Perkins tonometry. The IOP values obtained with the Tono-Pen XL scattered homogeneously around the intraocular IOP, while the Perkins and Schiötz tonometers underestimated intraocular measured IOP.

CONCLUSION

Of the three tonometers evaluated in this series of paediatric glaucoma patients, the Tono-Pen XL most closely reflected true IOP.

[The risk of glaucoma and corneal thickness]

Intraocular pressure is still the most important risk factor for the development of glaucomatous optic nerve damage. There is growing evidence that corneal thickness is a risk factor for the development of glaucoma. This might be caused by the effect of corneal thickness on intraocular pressure (IOP) measurements. Goldmann applanation tonometry measurements are correlated with corneal thickness. Thick corneas lead to false high readings whereas thin corneas lead to false low readings. If corneal thickness as a risk factor for glaucoma is only related to the dependency of IOP measurements on corneal thickness or is related to possible different biomechanical tissue properties in glaucomatous eyes is not known. However, a large proportion of the corneal thickness effect seems to be related to the effect on IOP readings by applanation tonometry. Neglecting corneal thickness can lead to false measurements of IOP with consequent misdiagnosis and false treatment. Therefore, measurements of corneal thickness should be performed in glaucoma patients and suspects.

Frequency of Goldmann Applanation Tonometer Calibration Error Checks

PURPOSE

To investigate how quickly Goldmann applanation tonometers used in clinical practice develop calibration errors, and to determine the frequency of checks required to detect these errors.

MATERIALS AND METHODS

Prospective check of the calibration error of all Haag-Streit Goldmann applanation tonometers in the department at month zero, month one, and month four. The tonometers were checked according to the Haag-Streit method using a standard calibration check weight bar by two independent observers. Calibration errors were classed as +/-0.5 to 2.5 mm Hg, +/-3 to 4 mm Hg, or >+/-4 mm Hg. Tonometers with a calibration error greater than +/-2.5 mm Hg were returned to the manufacturer for re-calibration.

RESULTS

At month zero 2 of 34 (5.9%), at month one 3 of 29 (10.3%), and at month four 0 of 33 (0.0%) tonometers fell within the manufacturer's recommended calibration range of +/-0.5 mm Hg. A total of 14 of 34 (41.2%) tonometers at month zero, 10 of 29 (34.5%) tonometers at month one, and 17 of 33 (51.5%) tonometers at month four were identified to have calibration errors greater than +/-2.5 mm Hg.

CONCLUSIONS

Goldmann applanation tonometers are not as accurate as the manufacturer's recommended calibration error tolerance of +/-0.5 mm Hg would suggest. Calibration error of less than +/-2.5 mm Hg is clinically acceptable. Calibration error checks should be carried out once monthly and tonometers with calibration error greater than +/-2.5 mm Hg returned to the manufacturer for re-calibration. Additional checks should be made if tonometers suffer specific damage. Ideally individual ophthalmologists should check calibration before each session.

Relationship between corneal thickness and optic disc damage in glaucoma

BACKGROUND

Central corneal thickness (CCT) has been found to be a powerful predictor for the development of glaucoma in patients with ocular hypertension. This study aimed to determine whether CCT was also related to glaucoma severity.

METHODS

The vertical cup/disc ratio (VCDR) was used as a single objective measure of glaucoma severity. Vertical cup diameters and vertical disc diameters were measured using a graticule attached to a 60 D Volk lens, in consecutive patients presenting to a single ophthalmologist. Measurements were corrected for magnification. The difference between corrected VCDR and 95% probability of VCDR normality was compared to CCT, which was measured with an ultrasonic pachymeter.

RESULTS

During the study period 554 eyes from 281 patients with glaucoma were reviewed. The mean CCT was 519.0 microm, with the mean corrected VCDR being 0.71. A univariate linear regression model revealed that an increase of 10 microm in CCT resulted in a 0.009 decrease in the difference between true VCDR and the predicted normal VCDR. The R2 value for the regression was 0.04 (P < 0.0001).

CONCLUSION

There is a highly statistically significant, albeit small, negative correlation between corneal thickness and glaucoma severity. This study supports the notion that CCT should be measured in the assessment of patients with glaucoma. Progressive thinning or presence of a thin cornea may have pathogenic or prognostic roles in some types of glaucoma.

Application of physical principles in the development of tonometry

A review of the development of applanation tonometry is undertaken from the perspective of the physical principles involved. Original articles that have contributed to this process are discussed. The result of this endeavour leads to a conclusion that corneal thinning procedures have no effect on original intraocular pressure. Also concluded is a methodology that would allow the direct measurement of original intraocular pressure.