Central and paracentral corneal pachymetry in patients with normal tension glaucoma and ocular hypertension

[Influence of corneal curvature and peripheral thickness on tonometry readings]

PURPOSE

This study investigates the influence of peripheral corneal thickness (PCT) and its curvature on tonometry readings.

MATERIAL AND METHODS

The study included 49 patients (49 eyes) who were indicated for glaucoma surgery. Using bidirectional applanation tonometry, the following parameters were obtained: IOPcc, IOPg - intraocular pressure (IOP) corrected for corneal compensation, taken as the most reliable indicator; IOP converted to Goldmann measurement, taken as the result of applanation tonometry, ΔIOP (IOPcc-IOPg), CH and CRF (corneal hysteresis and corneal resistance factor). During corneal topography, the corneal thickness was studied in the center, PCT at 1.5; 2, 3, 4 and 5 mm from the center in four meridians, as well as ΔPCT (PCT 3 mm - PCT 1.5 mm), the curvature of the anterior and posterior surfaces of the cornea and the depth of the anterior chamber. Aberrometry was used to obtain refractometry data and the curvature of the anterior surface of the cornea. The influence of the studied parameters on ΔIOP was evaluated.

RESULTS

ΔIOP correlated with CRF (r= -0.652), CH (r= -0.873), central corneal thickness (r= -0.293), PCT at all distances except 5 mm (r= -0.297; -0.287; -0.302; -0.303), with the strong and weak meridians of the anterior surface of the cornea (r=0.328; r=0.315), with the strong and weak meridians of the posterior surface, as well as the average curvature of the posterior surface (r=0.307; r=0.332; r=0.328). After step-by-step selection of the above parameters for creating a linear regression model for ΔIOP calculation, CH, CRF and PCT1.5mm remained in the model. The model describes ΔIOP with high accuracy (R2=0.974).

CONCLUSION

Biomechanical parameters of the cornea are the leading factor of applanation tonometry error. Individual linear dimensions of the cornea (thickness, curvature) have a lesser effect.

[Normal Tension Glaucoma]

Normal tension glaucoma (NTG) is a subvariant of primary open angle glaucoma (POAG) or a separate entity. NTG is defined as chronic open angle glaucoma without elevated intraocular pressure or normal intraocular pressure. Normal intraocular pressure is between 10 and 21 mmHg and is defined as two times the standard deviation of the mean intraocular pressure in the normal population. In addition to the absence of elevated intraocular pressure, all the classic symptoms of glaucoma are otherwise present. These include a conspicuous glaucomatous optic disc excavation, nerve fibre bundle defects, and corresponding visual field defects. Papillary rim haemorrhages are frequently found. The visual field defects in NDG are usually more central compared to POAG and are therefore described as more disturbing by the patient. The anterior chamber angle is open and there are no other changes suggestive of secondary glaucoma (pigment dispersion, pseudo-exfoliation). The exact pathophysiology of NDG is not well understood. Pathophysiologically, circulatory disturbances in the sense of arterial hypo- as well as hypertension may play an essential role or at least increase the susceptibility of the optic nerve to intraocular pressure fluctuations as well as blood pressure dips. Therefore, this requires not only a purely ophthalmologic but also interdisciplinary treatment of the patient with confirmed NDG. The primary goal of treatment is the reduction of intraocular pressure, which can stop the disease. This article gives an overview of epidemiology, aetiology, clinical findings and therapies.

Normaldruckglaukome

Das Normaldruckglaukom wird definiert wird als primär chronisches Offenwinkelglaukom, bei dem kein erhöhter Augeninnendruck vorliegt. Trotz normalem Augeninnendruck ist das primäre Ziel der Behandlung die Augeninnendrucksenkung, mit der es gelingen kann, die Erkrankung aufzuhalten. Dieser Artikel soll eine Übersicht über Epidemiologie, Ätiologie, Pathogenese, klinische Befunde sowie Therapien geben.

Biomechanical Glaucoma Factor and Corneal Hysteresis in Treated Primary Open-Angle Glaucoma and Their Associations With Visual Field Progression

Purpose

To investigate the relationship between biomechanical glaucoma factor (BGF) measured with Corvis ST and glaucomatous visual field (VF) progression, compared to corneal hysteresis (CH) measured with ocular response analyzer using a longitudinal dataset of primary open-angle glaucoma (POAG). The discriminative powers of BGF and CH were also compared using a cross-sectional dataset.

Methods

The longitudinal dataset included 166 POAG eyes. The rate of VF change during the follow-up period was evaluated using the mean of 52 pointwise total deviations in the Humphrey 24-2 field test. Variables associated with the VF progression rate were identified from BGF, CH, age, baseline VF severity, and intraocular pressure during the VF follow-up period by identifying the optimal model. The cross-sectional dataset included 68 POAG eyes and 68 healthy eyes. Using this dataset, the area under the curve (AUC) values of the receiver-operating curve were compared between CH and BGF.

Results

The optimal multivariate linear mixed model to describe the VF rate included age and CH, but not BGF. Between POAG and healthy eyes, CH was statistically different (P < 0.001), although this was not the case with BGF. The AUC values were 0.61 and 0.71 for BGF and CH, respectively (P = 0.027).

Conclusions

CH, but not BGF, was associated with VF progression in POAG patients under treatment. BGF was not useful to discriminate POAG between treated and normal eyes.

A new biomechanical glaucoma factor to discriminate normal eyes from normal pressure glaucoma eyes

BACKGROUND

To test the ability of the newly calculated Dresden biomechanical glaucoma factor (DBGF) based on dynamic corneal response (DCR) deformation and corneal thickness parameters, to discriminate between healthy and normal pressure glaucoma (NPG) eyes.

METHODS

Seventy healthy and 70 NPG patients of Caucasian origin were recruited for this multicentre cross-sectional pilot study, which included both eyes for analysis. Logistic regression analysis with generalized estimating equation (GEE) models to account for correlations between eyes and a threefold cross-validation were performed to determine the optimal combination of Corvis ST parameters in order to separate normal from NPG eyes.

RESULTS

The DBGF was calculated using 5 Corvis ST parameters, which showed the best discrimination power: deformation amplitude ratio progression, highest concavity time, pachymetry slope, the biomechanically corrected intraocular pressure and pachymetry. In a threefold cross-validation, the receiver operating characteristic (ROC) curve confirmed an area under the curve (AUC) of 0.814 with a sensitivity of 76% and a specificity of 77% using a logit cut-off value of a DBGF = 0.5.

CONCLUSION

The DBGF shows to be sensitive and specific to discriminate healthy from NPG eyes. Since diagnosis of NPG is often challenging, the DBGF may help with the differential diagnosis of NPG in daily clinical practice. Therefore, it might be considered as a new possible screening method for NPG.

Dependence of dynamic contour and Goldmann applanation tonometries on peripheral corneal thickness

AIM

To determine the effects of peripheral corneal thickness (PCT) on dynamic contour tonometry(DCT) and Goldmann applanation tonometry (GAT).

METHODS

A cross-sectional study. We created a software which calculates the corneal contour (CC) as a function of the radius from the corneal apex to each pixel of the contour. The software generates a central circumference with a radius of 1 mm and the remainder of the cornea is segmented in 5 rings concentric with corneal apex being its diameter not constant around the corneal circumference as a consequence of the irregular CC but keeping constant the diameter of each ring in each direction of the contour. PCT was determined as the mean thickness of the most eccentric ring. Locally weighted scatterplot smoothing (LOWESS) regression was used to determine the pattern of the relationship between PCT and both DCT and GAT respectively. Thereafter, two multivariable linear regression models were constructed. In each of them, the dependant variable was intraocular pressure (IOP) as determined using GAT and DCT respectively. In both of the models the predictive variable was PCT though LOWESS regression pattern was used to model the relationship between the dependant variables and the predictor one. Age and sex were also introduced control variables along with their first-degree interactions with PCT. Main outcome measures include amount of IOP variation explained through regression models (R²) and regression coefficients (B).

RESULTS

Subjects included 109 eyes of 109 healthy individuals. LOWESS regression suggested that a 2^nd-degree polynomial would be suitable to model the relationship between both DCT and GAT with PCT. Hence PCT was introduced in both models as a linear and quadratic term. Neither age nor sex nor interactions were statistically significant in both models. For GAT model, R² was 17.14% (F=9.02; P=0.0002), PCT linear term B was -1.163 (95% CI: -1.163, -0.617). PCT quadratic term B was 0.00081 (95% CI: 0.00043, 0.00118). For DCT model R² was 14.28% (F=9.29; P=0.0002), PCT linear term B was -0.712 (95% CI: -1.052, -0.372), PCT quadratic term was B=0.0005 (95% CI: 0.0003, 0.0007).

CONCLUSION

DCT and GAT measurements are conditioned by PCT though this effect, rather than linear, follows a 2^nd-degree polynomial pattern.

[Current aspects on the management of normal tension glaucoma]

In a considerable proportion of glaucoma patients (25-50 %) the intraocular pressure (IOP) is not elevated higher than 22 mmHg at first diagnosis and during subsequent follow-up controls. Although the IOP level remains in the low range < 22 mmHg, progression of glaucoma can still occur. A multitude of different factors are assumed to be involved in glaucoma progression, such as very low nocturnal diastolic blood pressure values, a low mean ocular perfusion pressure, extensive fluctuations in perfusion (e.g. in cases of vascular dysregulation), an increased vulnerability of the optic nerve support structures, an increased translaminar pressure gradient and various underlying systemic diseases. The most important evidence-based aspect of treatment in normal tension glaucoma is pharmaceutical or surgical reduction of the IOP by 30 % or more in comparison to the initial pressure level. Vascular and neuroprotective concepts of treatment for normal tension glaucoma have been strongly advocated and the object of experimental and clinical studies. As yet a clear clinical benefit has not been proven by large prospective randomized studies.

Corneal biomechanical properties in different ocular conditions and new measurement techniques

Several refractive and therapeutic treatments as well as several ocular or systemic diseases might induce changes in the mechanical resistance of the cornea. Furthermore, intraocular pressure measurement, one of the most used clinical tools, is also highly dependent on this characteristic. Corneal biomechanical properties can be measured now in the clinical setting with different instruments. In the present work, we review the potential role of the biomechanical properties of the cornea in different fields of ophthalmology and visual science in light of the definitions of the fundamental properties of matter and the results obtained from the different instruments available. The body of literature published so far provides an insight into how the corneal mechanical properties change in different sight-threatening ocular conditions and after different surgical procedures. The future in this field is very promising with several new technologies being applied to the analysis of the corneal biomechanical properties.

Orbscan topography in primary open-angle glaucoma

PURPOSE

To compare anterior and posterior corneal curvatures between eyes with primary open-angle glaucoma (POAG) and healthy eyes.

METHODS

This is a prospective, cross-sectional, observer-masked study. A total of 138 white subjects (one eye per patient) were consecutively recruited; 69 eyes had POAG (study group), and the other 69 comprised a group of healthy control eyes matched for age and central corneal pachymetry with the study ones. Exclusion criteria included any corneal or ocular inflammatory disease, previous ocular surgery, or treatment with carbonic anhydrase inhibitors. The same masked observer performed Goldmann applanation tonometry, ultrasound pachymetry, and Orbscan II topography in all cases. Central corneal thickness, intraocular pressure, and anterior and posterior topographic elevation maps were analyzed and compared between both groups.

RESULTS

Patients with POAG had greater forward shifting of the posterior corneal surface than that in healthy control eyes (p < 0.01). Significant differences in anterior corneal elevation between controls and POAG eyes were also found (p < 0.01).

CONCLUSIONS

Primary open-angle glaucoma eyes have a higher elevation of the posterior corneal surface than that in central corneal thickness-matched nonglaucomatous eyes.

Thickness mapping of the cornea and epithelium using optical coherence tomography

PURPOSE

To measure corneal and epithelial thickness across four meridians using Optical Coherence Tomography (OCT) and to compare these measurements between normal non-lens wearers (NLW), rigid gas permeable (RGP) lens wearers, and RGP-wearing keratoconics (KC).

METHODS

Both eyes of 60 subjects were measured (20 NLW, nine female:11 male, 27.6 +/- 5.9 years; 20 RGP, 20 female, 23.9 +/- 7.6 years; and 20 KC, seven female:13 male, 32.4 +/- 8.1 years). A customized fixation target employing LEDs in eight directions of gaze was attached to the OCT and corneal images obtained. Raw OCT scans were analyzed to yield values for corneal and epithelial thickness and color-coded maps were compiled.

RESULTS

Central corneal thickness (CCT) was thinnest in KC (447 +/- 68 microm) and similar between RGP (518 +/- 32 microm; pKC < 0.001) and NLW (517 +/- 21 microm) (p(KC) < 0.001 NLW pRGP > 0.05). Peripheral corneal thickness in NLW was thickest in the superior temporal and thinnest in the inferior (I) regions (superior temporal(thickest) vs. I(thinnest) p < 0.001). Central epithelial thickness was thinnest in KC (44 +/- 7 microm), followed by RGP (50 +/- 4 microm), then NLW (54 +/- 2 microm) (pKC < 0.001 NLW p(RGP) < 0.05). Central epithelial thickness in the KC group was significantly thinner than in the RGP group (p < 0.001). In the NLW group, peripheral epithelial thickness was thicker (63 +/- 5 microm) than central (p < 0.001) and was thickest in the superior (S) region and thinnest in the inferior (I) region (S(thickest) vs. I(thinnest) p < 0.001). KC epithelium was thinnest in the inferior temporal meridian (42 +/- 5 microm).

CONCLUSIONS

Thickness of the normal cornea and epithelium was greatest in the superior region. In all groups, the inferior cornea and epithelium was thinnest, and to a greater extent in the KC group.

Correlations between corneal biomechanical properties measured with the ocular response analyzer and ICare rebound tonometry

PURPOSE

To investigate the biomechanical properties of the normal cornea, and correlate them with central and peripheral corneal thickness and age.

METHODS

Seventy-six right eyes of volunteers were measured with Ocular Response Analyzer (ORA), ICare rebound tonometry and an ultrasound pachymeter at corneal center and at 4 mm from corneal center in the nasal and temporal directions.

RESULTS

ICare readings were significantly correlated with central and peripheral corneal thickness and corneal biomechanical properties. Corneal resistance factor was the biomechanical parameter with the higher correlation with ICare intraocular pressure (IOP) values. ICare tonometry at center and Goldmann equivalent IOP obtained with ORA were significantly higher for thicker than thinner corneas (P<0.05). IOP compensated for corneal properties with the ORA was lower than the remaining IOP values measured in the study. Higher correlation was found between Goldmann equivalent IOP with ORA and ICare IOP values.

CONCLUSIONS

IOP values obtained with the rebound tonometer are higher in thicker corneas and are positively correlated with biomechanical corneal parameters, namely corneal resistance factor. Although corneal thickness plays a significant role in rebound tonometry, elastic and viscous properties of the cornea seem to play a significant role in the interaction of the tonometer probe with the ocular surface. However, the mechanism behind this process is presently unknown.

Factors influencing central corneal thickness in a Turkish population

PURPOSE

The aim of this study is to find out the central corneal thickness (CCT) values for a Turkish patient group and to investigate the possible influences of age, sex, IOP, refractive status, keratometry readings, systemic disorders (hypertension, diabetes mellitus, hyperlipidemia, heart disease and asthma) on CCT values.

MATERIALS AND METHODS

Six hundred twenty five subjects (276 male (44%), 349 female (56%)) of ages 6 to 88 years were recruited. Subjects who had corneal diseases, purulent conjunctivitis or blepharitis were excluded. Refraction and keratometry readings were made by MRK-3100 premium auto-ref/keratometer, IOP was measured by Reichert AT-555 auto noncontact pneumotonometer.

RESULTS

Mean age was 44.1 +/- 16.6 years +/- SD for male subjects, 41.0 +/- 16.9 for females. Mean CCT +/- SD values for male was 552.2 +/- 35.9 microm, for female was 552.3 +/- 35.4 microm, respectively. There was no significant difference between right and left eye CCT values for both genders. Age and CCT was not correlated for the whole study group but there was a slight negative correlation in male subjects. IOP and CCT had moderately significant correlation for males and females. There was a slight significant correlation between Kh-Kv and CCT values for the whole group. In a multivariate regression model only Kv values seemed to affect CCT values.

DISCUSSION

There are studies showing the variation of CCT values among different nations and ethnicities. There is no agreement about the relationship between age, IOP, Kh-Kv, spherical equivalence of refractive error, systemic disorders, menopause and CCT. In our study CCT was correlated with Kh-Kv and IOP in correlation analysis but in multivariate regression analysis only Kv appeared to influence corneal thickness.

Effect of central corneal thickness on Goldmann applanation tonometry measures - a different result with different pachymeters

PURPOSE

To assess whether the use of Orbscan pachymetry, as an alternative to ultrasound pachymetry, had an effect on the IOP-CCT relationship for Goldmann tonometry in normal human eyes.

PATIENTS AND METHODS

The right eye of 50 healthy subjects, aged 19 to 82 years, were assessed by Orbscan (three scans), and then central corneal thickness (CCT) measured by ultrasound pachymetry (three readings) and Goldmann tonometry performed (three readings).

RESULTS

Goldmann tonometry indicated average IOP values from 9.7 to 20 mm Hg (group mean 14.3 +/- 2.5 mm Hg) that were highly highly correlated with ultrasound pachymetry (r = 0.608, p < 0.001). Orbscan central zone pachymetry (2-mm sample zone) yielded higher values than ultrasound, averaging 0.591 +/- 0.044 mm compared to 0.523 +/- 0.037 mm (p < 0.001), with the differences between the instruments being proportional to the average thickness (p < 0.001; r = 0.461). Notwithstanding, the Goldmann tonometry values were still highly correlated with the Orbscan central zone thickness data (r = 0.595, p < 0.001); but, due to the difference in the thickness data generated by the two pachymeters, the absolute slope of the IOP-CCT relationship was slightly less if Orbscan measures were used, with or without use of the acoustic factor.

CONCLUSION

Orbscan measures of the thickness of a central corneal zone can be used to assess the impact of central corneal thickness on Goldmann tonometry data.

Age differences in central and peripheral intraocular pressure using a rebound tonometer

AIM

To evaluate the influence of age on the measurements and relationships among central and peripheral intraocular pressure (IOP) readings taken with a rebound tonometer.

METHODS

The IOPs were measured using the ICare rebound tonometer on the right eyes of 217 patients (88 men and 129 women) aged 18-85 years (mean 45.9 (SD 19.8) years), at the centre and at 2 mm from the nasal and temporal limbus along the horizontal meridian. Three age groups were established: young (< or =30 years old; n = 75), middle aged (31-60 years old; n = 77) and old patients (>60 years old; n = 65).

RESULTS

A high correlation was found between the central and peripheral IOP readings, with the central readings being higher than the peripheral ones. Higher IOP values for the central location were found in the younger patients. Older patients had significantly lower temporal IOP readings than those for the remaining two groups (p<0.001), whereas no significant differences were found among groups when IOP was measured at the central and nasal locations. A significant decrease was observed in the nasal and temporal IOP readings as the age increased (p = 0.011 and 0.006, respectively).

CONCLUSION

Older patients had lower IOP values than the middle-aged and younger patients in the temporal peripheral location. A negative correlation was found between age and IOP by rebound tonometry in the corneal periphery but not in its centre.