Corneal thickness and curvature in normal-tension glaucoma

Pharmaceutical Approaches to Normal Tension Glaucoma

Normal tension glaucoma (NTG) is defined as a subtype of primary open-angle glaucoma (POAG) in which the intraocular pressure (IOP) values are constantly within the statistically normal range without treatment and represents approximately the 30-40% of all glaucomatous cases. The pathophysiology of this condition is multifactorial and is still not completely well known. Several theories have been proposed to explain the onset and progression of this disease, which can be divided into IOP-dependent and IOP-independent factors, suggesting different therapeutic strategies. The current literature strongly supports the fundamental role of IOP in NTG. The gold standard treatment for NTG tends to be based on the lowering IOP even if "statistically normal". Numerous studies have shown, however, that the IOP reduction alone is not enough to slow down or stop the disease progression in all cases, suggesting that other IOP-independent risk factors may contribute to the NTG pathogenesis. In addition to IOP-lowering strategies, several different therapeutic approaches for NTG have been proposed, based on vaso-active, antioxidant, anti-inflammatory and/or neuroprotective substances. To date, unfortunately, there are no standardized or proven treatment alternatives for NTG when compared to traditional IOP reduction treatment regimes. The efficacy of the IOP-independent strategies in decreasing the risk or treating NTG still remains inconclusive. The aim of this review is to highlight strategies reported in the current literature to treat NTG. The paper also describes the challenges in finding appropriate and pertinent treatments for this potentially vision-threatening disease. Further comprehension of NTG pathophysiology can help clinicians determine when to use IOP-lowering treatments alone and when to consider additional or alternatively individualized therapies focused on particular risk factors, on a case-by-case basis.

Lab-on-a-Contact Lens: Recent Advances and Future Opportunities in Diagnostics and Therapeutics

The eye is one of the most complex organs in the human body, containing rich and critical physiological information (e.g., intraocular pressure, corneal temperature, and pH) as well as a library of metabolite biomarkers (e.g., glucose, proteins, and specific ions). Smart contact lenses (SCLs) can serve as a wearable intelligent ocular prosthetic device capable of noninvasive and continuous monitoring of various essential physical/biochemical parameters and drug loading/delivery for the treatment of ocular diseases. Advances in SCL technologies and the growing public interest in personalized health are accelerating SCL research more than ever before. Here, the current status and potential of SCL development through a comprehensive review from fabrication to applications to commercialization are discussed. First, the material, fabrication, and platform designs of the SCLs for the diagnostic and therapeutic applications are discussed. Then, the latest advances in diagnostic and therapeutic SCLs for clinical translation are reviewed. Later, the established techniques for wearable power transfer and wireless data transmission applied to current SCL devices are summarized. An outlook, future opportunities, and challenges for developing next-generation SCL devices are also provided. With the rise in interest of SCL development, this comprehensive and essential review can serve as a new paradigm for the SCL devices.

[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.

Relationship of axial length and corneal biomechanical properties with susceptibility to unilateral normal-tension glaucoma

PURPOSE

Corneal biomechanics, reflecting structural vulnerabilities of the eyeball, may participate in the pathogenesis of unilateral normal-tension glaucoma. This study investigated the pathophysiology of unilateral normal-tension glaucoma using Corvis ST (OCULUS Optikgeräte GmbH) and other ocular characteristics.

METHODS

Eighty-three patients with normal-tension glaucoma with unilateral visual field loss and structurally unaffected fellow eyes and 111 healthy controls were included in this prospective study. Dynamic corneal response parameters, intraocular pressure measured by rebound tonometry, central corneal thickness, and axial length were assessed on the same day. Measurements were compared between affected eyes, unaffected fellow eyes, and control eyes. Risk factors for normal-tension glaucoma and unilateral involvement were the main outcome measures.

RESULTS

A shorter first applanation time (adjusted odds ratio, 0.061; 95% confidence interval, 0.018-0.215) and a larger peak distance (adjusted odds ratio, 4.935; 95% confidence interval, 1.547-15.739) were significant risk factors for normal-tension glaucoma and were associated with greater glaucoma severity (both P < 0.001). Axial length (adjusted odds ratio, 29.015; 95% confidence interval, 4.452-189.083) was the predominant risk factor for unilateral involvement in patients with normal-tension glaucoma.

CONCLUSION

The eyes with normal-tension glaucoma were more compliant than healthy eyes. Axial elongation-associated optic nerve strain may play an important role in unilateral normal-tension glaucoma with similar corneal and scleral biomechanics in both eyes.

Effect of Topical Anesthetics on Intraocular Pressure and Pachymetry

Purpose

The determination of intraocular pressure (IOP) by noncontact tonometry (NCT) has been reported to be affected by central corneal thickness (CCT) and by the instillation of topical anesthetics.

Methods

In order to determine the influence of topical anesthetics on CCT and IOP measured by NCT, 80 eyes from 49 patients were examined before and after the instillation of topical anesthetics.

Results

Average age was 55.3 years (SD 18.4, range 18 to 93). Twenty-eight patients were female and 21 were male. Average basal IOP was 16.1 mmHg (SD 5.2, range 8 to 35.3). IOP was 14.8 mmHg (SD 4.6, 7.4 to 32.4) (p=0.0005, Student t test for paired data) 5 minutes after topical anesthetics instillation. CCT averaged 541 μm (SD 32, range 482 to 604) before topical anesthetic drops and 541 μm (SD 32, 490 to 607, p=0.89, Student t test for paired data) 5 minutes after topical anesthetics instillation.

Conclusions

The study confirms that the instillation of topical anesthetics causes a reduction in IOP. These changes do not seem to be associated with changes in CCT.

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.

Central corneal thickness in primary angle closure and open angle glaucoma

Purpose:

To quantitatively analyze central corneal thickness (CCT) in patients with primary angle closure glaucoma (PACG) and primary open-angle glaucoma (POAG), and to evaluate its correlation with severity of glaucoma.

Methods:

In this retrospective study, records of patients with previously diagnosed POAG or PACG at a tertiary glaucoma service were reviewed. CCT was measured by ultrasound pachymetry. Mean deviation (MD) on visual field (VF) testing was recorded for glaucoma severity determination. CCT and age- and sex-adjusted CCT were compared among the study groups using Student's t-test and analysis of covariance (ANCOVA), respectively. Univariate and multivariate regression models were used for correlation of age, MD and CCT.

Results:

A total of 115 patients with PACG, 215 with POAG, and 100 normal controls were included with mean age of 64.1 ± 10.4, 59.9 ± 10.5, and 62.04 ± 10.80 years, respectively. CCT was thicker in PACG eyes (545.5 ± 46.1 μm) as compared to POAG eyes (531.7 ± 37.3 μm) and controls (531.0 ± 38.3 μm) even after age and gender adjustment (ANCOVA, P = 0.05). CCT was found to decrease with increasing age only in the POAG group (β = -0.57, P = 0.01). Disease severity (MD of VF) was significantly and inversely correlated with CCT in both POAG and PACG eyes (β = 1.89, P = 0.02; and β = 1.38, P = 0.04, respectively) after age and sex correction.

Conclusion:

PACG eyes had thicker CCT as compared to POAG and normal healthy eyes in Iranian subjects. Severity of the disease was inversely correlated with CCT in eyes with both POAG and PACG.

Corneal biomechanical properties: precision and influence on tonometry

PURPOSE

To assess the precision and reproducibility of the corneal biomechanical parameters, and their relationships with the intraocular pressure (IOP) measured with the Goldmann tonometer and a noncontact tonometer.

METHODS

Readings for biomechanical properties and for IOP measured with the Goldmann and noncontact tonometers, were taken on one randomly selected eye of 106 normal subjects, on each one of two measurement sessions. Measurements with the ocular response analyzer (ORA) and the noncontact tonometer were randomized, followed by the measurement of central corneal thickness and with the Goldmann tonometer.

RESULTS

Repeatability coefficients for CCT, corneal hysteresis (CH) and corneal resistance factor (CRF) in Session 1 were ± 0.01 μm, ± 3.05 mmHg and ± 2.62 mmHg, respectively. The mean CCT, CH, CRF, Goldmann and noncontact tonometry did not vary significantly between sessions. Reproducibility coefficients for CCT, CH and CRF were ± 0.02 μm, ± 2.19 mmHg and ± 1.97 mmHg, respectively. Univariate regression analysis showed that CCT, CH and CRF significantly (P<0.0001) correlated with the IOP measured with the Goldmann and noncontact tonometers (and with the differences between tonometers) in Session 1. There were no significant correlations with the differences between tonometers in Session 2. Multivariate analysis revealed a minimal effect of CCT on Goldmann measurements but a significant effect on those of the noncontact tonometer.

CONCLUSIONS

Measurement of the biomechanical properties of the cornea, using the ORA, are repeatable and reproducible, affect Goldmann tonometry less than noncontact tonometry, and have a minimal influence on the difference in measured intraocular pressure between tonometers.

Intraocular pressure reduction with topical medications and progression of normal-tension glaucoma: a 12-year mean follow-up study

PURPOSE

To investigate whether the amount of intraocular pressure (IOP) reduction with topical medications is associated with the progression of normal-tension glaucoma (NTG) and to identify risk factors for NTG progression.

METHODS

The medical records of 121 eyes of 121 NTG patients, who were treated with topical medications for more than 7 years, were reviewed. NTG progression was defined by either structural (optic disc or retinal nerve fibre layer) or functional (visual field) deterioration. Patients were divided into tertile groups according to the percentage IOP reduction from baseline, and the cumulative probability of NTG progression between upper and lower tertile group was compared using Kaplan-Meier survival analysis. Multivariate analysis with Cox's proportional hazard model was performed to identify the hazard ratio (HR) of clinical factors for NTG progression.

RESULTS

The average follow-up period was 12.2 years, and 56 of 121 eyes (46.3%) showed the NTG progression. Kaplan-Meier analysis revealed that upper tertile group (percentage IOP reduction >22.1%) showed a greater cumulative probability of non-progression than lower tertile group (percentage IOP reduction < 13.3%; p = 0.012). Multivariate Cox's proportional hazard model indicated that percentage reduction of IOP (HR = 0.964; p = 0.007) and the occurrence of disc haemorrhage (HR = 2.410; p = 0.008) were significantly associated with NTG progression.

CONCLUSIONS

The amount of IOP reduction using topical medications was related to NTG progression, and lower percentage reduction in IOP was a consistent risk factor for progression.

Effect of latanoprost on central corneal thickness in unilateral normal-tension glaucoma

PURPOSE

To evaluate the effect of latanoprost on central corneal thickness (CCT) in patients with unilateral normal-tension glaucoma (NTG).

METHODS

A total of 38 patients, with unilateral NTG, who were being treated in our glaucoma clinic and were receiving 0.005% latanoprost monotherapy over 24 months, were included in this study. The data were collected, retrospectively, with reviewing of the charts. The mean CCT and the CCT reduction from the baseline were assessed before the treatment, and at 3, 6, 9, 12, and 24 months after the initiation of the treatment. An unaffected eye was also evaluated for the control group. All the measurements were performed with a commercially available ultrasound pachymeter.

RESULTS

There were no significant differences between the eyes for the baseline intraocular pressure and CCT. The mean CCT trend decreased, except at 9 months after the treatment in the latanoprost group (affected eye), but only statistically significant after 24 months of treatment by using paired-sample t-test [544.6±38.4 vs. 540.3±37.8 μm (P=0.013)]. There was no statistically significant changes of CCT between the groups with repeated measures analysis of variance (P=0.635).

CONCLUSION

Topical therapy with latanoprost may decrease the CCT, over a period of 24 months, in patients with NTG. These data should be taken into consideration in a long-standing latanoprost treatment of NTG.

Corneal biomechanical properties in normal-tension glaucoma

PURPOSE

To investigate the intraocular pressure (IOP) and corneal biomechanical properties of normal and normal-tension glaucoma (NTG) eyes.

METHODS

This study included 83 normal and 83 NTG eyes. We measured corneal-compensated IOP (IOPcc), Goldmann-correlated IOP (IOPg), corneal resistance factor (CRF), corneal hysteresis (CH) and central corneal thickness (CCT) three times each for normal and NTG eyes using an Ocular Response Analyzer (ORA).

RESULTS

No significant difference in CCT was seen between normal eyes (541.4 ± 26.8 μm) and NTG eyes (535.4 ± 24.9 μm; p = 0.16). IOPcc was significantly higher in NTG eyes (16.1 ± 2.6 mmHg) than in normal eyes (15.1 ± 2.9 mmHg; p = 0.01), while IOPg was significantly lower in NTG eyes (14.1 ± 2.7 mmHg) than in normal eyes (15.1 ± 3.0 mmHg; p = 0.04). CRF and CH were significantly lower in NTG eyes (CRF, 8.9 ± 1.5 mmHg; CH, 9.2 ± 1.3 mmHg) than in normal eyes (CRF, 10.6 ± 1.4 mmHg; CH, 10.8 ± 1.3 mmHg; p < 0.0001 each).

CONCLUSION

IOPcc was significantly higher in NTG eyes than in normal eyes. The ORA may be useful for distinguishing between the IOPcc of NTG eyes with normal IOP and that of normal eyes. In addition, the ORA enables CRF and CH to be measured in vivo, and weakness of the lamina cribrosa may be clinically inferred from the fact that CRF and CH were reduced in NTG eyes in our study. Low CRF and CH may be clues to the pathology of NTG.

Robust automatic segmentation of corneal layer boundaries in SDOCT images using graph theory and dynamic programming

Segmentation of anatomical structures in corneal images is crucial for the diagnosis and study of anterior segment diseases. However, manual segmentation is a time-consuming and subjective process. This paper presents an automatic approach for segmenting corneal layer boundaries in Spectral Domain Optical Coherence Tomography images using graph theory and dynamic programming. Our approach is robust to the low-SNR and different artifact types that can appear in clinical corneal images. We show that our method segments three corneal layer boundaries in normal adult eyes more accurately compared to an expert grader than a second grader-even in the presence of significant imaging outliers.

Corneal thickness measurements in normal-tension glaucoma workups: is it worth the effort?

PURPOSE

To correlate central corneal thickness (CCT) and intraocular pressure (IOP) with disease severity in normal-tension glaucoma (NTG) patients.

METHODS

We conducted a retrospective review of all patients diagnosed with NTG in our institution between 2002 and 2006. NTG was diagnosed according to the glaucomatous visual fields loss, glaucomatous optic disc cupping, and an IOP <22 mm Hg on diurnal curve measurements. Mean CCT and IOP values before and after treatment were also evaluated. Patients were divided into 3 groups according to advanced glaucoma intervention score (mild, moderate, and severe visual field defects).

RESULTS

A total of 33 females and 35 males with bilateral NTG were enrolled. The mean follow-up was 4.6 years. CCT was inversely correlated with glaucoma severity. CCT was normal in both eyes in mild disease, thin in the right eye (RE) and normal in the left eye (LE) in moderate disease, and low in both eyes in severe disease. Initial bilateral mean maximal IOP was similar at all disease stages and became lower after treatment in parallel to disease severity: 13.44, 12.22, and 11.63 mm Hg in the RE and 13.29, 12.60, and 12.32 mm Hg in the LE, respectively. There was no statistical difference in disease severity between the RE and LE.

CONCLUSIONS

CCT correlated with disease severity: the more advanced the disease, the thinner the cornea. Initial maximal IOP did not predict disease severity, but it was lower in the more severe cases after treatment, possibly representing a more aggressive treatment protocol.

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.

Associations with intraocular pressure in Latinos: the Los Angeles Latino Eye Study

PURPOSE

To evaluate the association of biologic factors with intraocular pressure (IOP) in a Latino population.

DESIGN

Population-based cross-sectional study.

METHODS

Latinos 40 years and older (n = 5,958) from the Los Angeles Latino Eye Study without a history of ocular hypotensive treatment underwent an interviewer-administered questionnaire and a complete ocular and clinical examination. IOP was obtained by applanation tonometry and was based on the mean of three measurements. Multivariable regression models were used to evaluate the independent association of biological factors with IOP.

RESULTS

Higher systolic blood pressure, higher central corneal thickness, and diabetes mellitus were the major factors associated with elevated IOP. Other positively correlated variables included age, female gender, higher diastolic blood pressure, larger body mass index, darker colored irides, and nuclear sclerosis. Axial length and family history of glaucoma had no association with IOP.

CONCLUSIONS

Several systemic and ocular characteristics are associated with elevated IOP in Latinos. By identifying and recognizing these risk factors, we can define subgroups of the population that may be most at risk of having elevated IOP.

Comparison of dynamic contour tonometry and goldmann applanation tonometry in deep lamellar and penetrating keratoplasties

PURPOSE

To compare the measurements of intraocular pressure (IOP) with dynamic contour tonometry (DCT) and Goldmann applanation tonometry (GAT) in eyes with corneal graft and to evaluate the influence of corneal thickness (CCT), corneal curvature (CC), and astigmatism on these methods.

DESIGN

Prospective, observational cross-sectional study.

METHODS

Eighteen eyes of 18 patients after penetrating keratoplasty (PKP) and 14 eyes of 14 patients after deep lamellar keratoplasty (DLKP) underwent IOP evaluation with DCT and GAT, and measurements of CCT, CC, and astigmatism. Bland-Altman plots were used to evaluate the agreement between tonometers. Multivariate regression analysis was used to evaluate the influence of ocular structural factors and running suture on IOP measurements obtained with both tonometers.

RESULTS

IOP values obtained by DCT and GAT were strongly correlated in all eyes (r = .91; P < .001). DCT values measured 2.5 +/- 1.7 mm Hg higher than GAT readings (P < .001). A reduction of the mean IOP difference between DCT and GAT with an increase in IOP values (P < .001) was found. Regression analysis showed no effect of CCT, CC, astigmatism, and running suture on both DCT and GAT readings, either in DLKP or in PKP eyes.

CONCLUSIONS

We found a good overall correlation between both tonometers but the agreement between instruments differs in high or low IOP ranges. The wide and varying 95% limits of agreement between DCT and GAT indicates that DCT provides IOP measurements on deep lamellar and penetrating keratoplasties which can be used in the clinical practice.

The treatment of normal-tension glaucoma

Normal-tension glaucoma (NTG) is generally defined as visual field loss and optic nerve defects consistent with glaucoma and an intraocular pressure (IOP) that does not exceed 21 mmHg (Allingham, R.R., Damji, K., Freedman, S., Moroi, S., Shafranov, G., Shields, M.B. (2005). In: Pine J. and Murphy J. (Eds.), Shields' Textbook of Glaucoma, 5th edn., Lippincott Williams & Wilkins, Philadelphia, PA, pp. 197-207, Chapter 11). If a patient has an atypical presentation (unilateral disease, decreased central visual acuity or visual field loss not consistent with optic disk appearance) then the clinician should rule out medical or neurologic etiologies. IOP-dependent and IOP-independent mechanisms play a role in NTG nerve damage. The exact mechanisms of IOP-independent damage are not currently known. Research has shown that vascular etiologies, such as vascular insufficiency and vasospasm, may be possible mechanisms for IOP-independent damage. The mainstay of glaucoma treatment remains robust IOP reduction. The chief goal of ongoing glaucoma research is to more clearly identify IOP-independent mechanisms of damage and to find neuroprotective treatment strategies to prevent retinal ganglion cell death and consequent visual loss.

Corneal curvature and central corneal thickness in eyes with pseudoexfoliation syndrome

BACKGROUND

Previous studies have reported different central corneal thickness (CCT) values in eyes with pseudoexfoliation syndrome (PXS). There has been no report about corneal curvature (CC) in PXS eyes. The aim of this study was to investigate the CCT together with the CC in PXS eyes with and without glaucoma.

METHODS

The CC (simulated keratometry 1 [SimK1], SimK2, and mean K) and CCT were prospectively studied by rotating a Scheimpflug camera in 72 PXS eyes and comparing them with 65 normal eyes.

RESULTS

In all PXS eyes, the mean K was significantly steeper than in control eyes (44.1 [SD 1.5] diopters (D) vs. 43.2 [SD 2.0] D, p = 0.04). When PXS eyes were subcategorized, the mean K was significantly steeper in both normotensive PXS eyes (44.6 [SD 1.6] D) and pseudoexfoliation glaucoma (PEXG) eyes (44.0 [SD 1.2] D) than normal eyes (43.2 [SD 2.0] D) (p = 0.001 and 0.02, respectively). In all PXS eyes, the mean CCT (546.4 [SD 39.6] microm) was not significantly different than the control eyes (542.9 [SD 32.2] microm) (p = 0.56). When PXS eyes were subcategorized, however, the CCT was significantly thinner in normotensive PXS eyes and significantly thicker in PEXG eyes.

INTERPRETATION

It appears that PEXG eyes have a significantly steeper CC and higher CCT than normal eyes. This could be of clinical significance because overestimation of true intraocular pressure may then occur in these eyes.

Central corneal thickness among Aboriginal people attending eye clinics in remote South Australia

PURPOSE

To determine the central corneal thickness (CCT) and its demographic associations among Aboriginal people attending eye clinics in remote South Australia.

METHODS

A clinic-based cross-sectional study was conducted involving opportunistic sampling of patients. Eligible participants underwent measurement of CCT by ultrasound pachymetry. The results were compared with a group of Caucasian control patients.

RESULTS

All patients (189) who were invited to participate in the study had their CCT measured. The mean age was 44.8 +/- 14.5 years, and women comprised 57.7% of the sample. The control group consisted of 115 Caucasian participants. The mean age was 47.1 +/- 14.8 years, and women accounted for 55.7% of the sample. Mean CCT for Aboriginal participants was 514.9 +/- 30.5 microm in the right eye and 515.6 +/- 30.5 microm in the left eye (t = 1.1, P = 0.3). Mean right CCT for Caucasian participants was 544.6 +/- 31.9 microm and mean left CCT in this group was 547.1 +/- 32.2 microm (t = 4.6, P < 0.001). There was a significant difference between the right (t = 8.4, P < 0.001) and left (t = 8.8, P < 0.001) CCT of Aboriginal and Caucasian participants.

CONCLUSIONS

The CCT among Aboriginal patients attending eye clinic in remote South Australia was significantly thinner than that of a Caucasian control group. Thinner corneas among this group of Aboriginal patients may indicate a need to adjust intraocular pressure according to CCT and to be more vigilant for glaucoma.

Central corneal thickness in pediatric glaucoma

PURPOSE

To compare the central corneal thickness (CCT) of children with and without glaucoma and to estimate the effect CCT may have on the intraocular pressure (IOP) assessment of children.

PATIENTS AND METHODS

CCT and IOP measurements were performed in an unmasked fashion in 34 consecutive children with glaucoma and 28 consecutive children without glaucoma. Patients with corneal edema or extensive corneal scarring were excluded. IOPs in the pediatric glaucoma group were corrected for CCT using a linear algorithm; adjustments of 3 mm Hg or more were considered clinically significant.

RESULTS

Mean CCT for eyes with glaucoma was 610.2+/-121.9 microm; for the control eyes, mean CCT was 555.6+/-38.4 microm (P < .001). Mean CCT was 543.3+/-66.9 microm for eyes with primary congenital glaucoma, 591.9+/-23.1 pm for those with Sturge-Weber syndrome, 662.7+/-68.7 microm for those with aphakic glaucoma, 754.5 +/-92.6 microm for those with aniridia, and 820.6+/-133.7 microm for those with microcornea (P < .001). Applying a correction formula for IOP with CCT adjustment, the adjusted IOP was overestimated by > or =3 mm Hg in 14 (41.2%) eyes and underestimated by > or = 3 mm Hg in 5 (14.7%) eyes.

CONCLUSIONS

The mean CCT was higher for the pediatric glaucoma group than for the control group. However, the CCTs varied significantly depending on the specific diagnoses. CCT was estimated to have a clinically significant effect on IOP measurements in more than half of the patients with pediatric glaucoma. Pachymetry results should be considered in the management of these patients.

Central Corneal Thickness in the European Glaucoma Prevention Study

PURPOSE

To measure central corneal thickness (CCT) within the participants of the European Glaucoma Prevention Study (EGPS). This study was designed to test if lowering intraocular pressure (IOP) by means of dorzolamide is able to prevent or delay conversion from ocular hypertension to glaucoma.

DESIGN

Randomized, double-masked, controlled, observational clinical trial.

PARTICIPANTS

Eight hundred fifty-four of 1077 ocular hypertensive participants within the EGPS were investigated. Four hundred twenty-nine patients were treated with dorzolamide and 425 patients received placebo.

INTERVENTION

Treatment with dorzolamide or placebo (the vehicle of dorzolamide) in 1 or both eyes.

MAIN OUTCOME MEASURES

Central corneal thickness as measured by ultrasound pachymetry (DGH-500 Pachette; DGH Technologies, Exton, PA). The CCT measurements were obtained in the morning before measuring IOP. Five measurements were taken from each eye of each patient within 5 minutes of application of anesthetic eye drops.

RESULTS

Mean CCT was 572.6+/-37.4 microm (range, 458.5-695.6 microm). The CCT was higher in younger patients, male patients, and diabetic patients. Mean CCTs for the 429 patients receiving dorzolamide were 574.2+/-38.48 microm (range, 458.5-695.6 microm) and 571.0+/-36.21 microm (469.7-690.1 microm) for the 425 patients receiving placebo (P = 0.205). Central corneal thickness did not correlate with refraction, baseline IOP, or systemic hypertension.

CONCLUSION

Central corneal thickness measurements within the EGPS were greater than those reported in other studies of normal eyes without ocular hypertension. Larger CCT measurements correlated with male gender, younger age, and diabetes.

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

PURPOSE

To determine whether there is a temporal relationship between the diurnal variation of central corneal thickness (CCT) and intraocular pressure (IOP) by Goldmann applanation tonometry in young normal human participants in vivo.

METHODS

Twenty-five eyes of 25 young healthy normal participants were examined in a prospective observational cross-sectional study. IOP, CCT and corneal curvature were measured using standard clinical techniques over a 24-hour period, and the temporal interrelationships between these parameters were examined.

RESULTS

The overnight change in IOP measured by Goldmann tonometry was 3.1+/-2.4 mm Hg (P<0.001), CCT was 20.1+/-10.9 mum (P=0.016), with no statistical change in central corneal curvature (0.05 mm, P=0.477, paired t test with Bonferroni correction). Both IOP and CCT were highest on awakening at 7:00 then dropped rapidly to baseline levels by 9:00 (linear mixed models), and these two parameters were highly correlated (r=0.978, P<0.001). After 9:00, there was no correlation between these parameters (r=-0.453, P=0.260).

CONCLUSIONS

The results of this study have highlighted a potential link between the diurnal variation of CCT and the accuracy of Goldmann tonometry estimates of IOP during the first 2 hours after awakening. Clinicians should be wary of using Goldmann tonometry to estimate IOP until the overnight increase in CCT has resolved.

Simultaneous measurement of eye stiffness and contact area for living human eyes

Goldmann applanation tonometry is commonly used for measuring IOP (IntraOcular Pressure) to diagnose glaucoma. However, the measured IOP by the applanation tonometry is valid only under the assumption that all the subjects have the same structural eye stiffness. Abnormal eye stiffness makes abnormal corneal deformation and thus the current applanation tonometer misestimates the IOP. This study challenges to measure the eye stiffness in vivo with a non-invasive approach for detecting the abnormal deformation. The deformation of the cornea and the contact area between the probe and the cornea are simultaneously captured by cameras during the experiment. Experimental results show that some subjects have different relationship among the force, the displacement and the contact area even with same IOP. The proposed eye stiffness measurement can help detecting the abnormal deformation and the eyes with misestimated IOP.

Central corneal thickness in congenital glaucoma

PURPOSE

The aim of this study was to compare central corneal thickness between eyes with congenital glaucoma and normal fellow eyes in unilateral glaucoma or less affected fellow eyes in bilateral glaucoma.

METHODS

Eyes of consecutive phakic children with congenital glaucoma and previous glaucoma surgery were examined under chloral hydrate. Complete ophthalmologic examination, central corneal thickness (CCT), axial length, and corneal diameter measurements were performed. Patients were included in the study if presented with intraocular pressure (IOP) less than 21 mm Hg and no biomicroscopic signs of corneal edema.

RESULTS

Nine patients were included in the study. The mean CCT in the more affected eye/glaucomatous eye was 522.3 +/- 65.2 microm and in the less affected eye/healthy eye was 579.7 +/- 44.5 microm. This difference was statistically significant (P = 0.0013).

CONCLUSION

CCT was significantly thinner in glaucomatous eyes than in normal fellow eyes in phakic children with congenital glaucoma. This finding may be another confounding factor when measuring IOP in these patients.

Dynamic contour tonometry: principle and use

Interindividual variability of central corneal thickness has been found to be a source of error for conventional Goldmann applanation tonometry. The dynamic contour tonometer represents a potentially new technology for non-invasive and direct intraocular pressure (IOP) measurement, and has been proposed to accurately measure the true IOP irrespective of the corneal thickness. It is based on the principle that when the tip of the device exactly matches the contour of the cornea, the pressure measured by a transducer placed on its tip is an accurate indicator of the true IOP. This device is also capable of measuring the ocular pulse amplitude, a variable that has controversial significance in the diagnosis and management of glaucoma. Even though this technique seems to be very promising, further studies are required to conclusively determine the effectiveness of the dynamic contour tonometer in patients having an abnormal or irregular corneal contour.

Comparison between central corneal thickness measurements by ultrasound pachymetry and optical coherence tomography

PURPOSE

Measurement of central corneal thickness (CCT) plays an important role in both diagnostic and therapeutic assessment of ocular diseases. Although ultrasound pachymetry (U-PACH) is regarded as the golden standard for measurement of CCT, optical coherence tomography (OCT) may offer advantages as it can locate the central cornea with precision with no corneal touch. Nevertheless, the agreement of OCT with U-PACH has not yet been gauged by Bland-Altman analysis. This study compares CCT measurement by OCT with that by U-PACH.

METHODS

Healthy subjects without ocular abnormality (except refractive errors less than or equal to -6.0 D), contact lens wear or ocular surgery were recruited. CCT was measured in one eye of normal subjects using OCT and U-PACH. Results were compared using correlation and Bland-Altman plots.

RESULTS

Fifty subjects were recruited. Mean +/- SD CCT measured by OCT was 565 +/- 33 microm. This was highly correlated (Pearson's coefficient = 0.934) with the mean thickness measured by U-PACH (543 +/- 33 microm). The coefficients of variation were good and comparable at 7.9% for U-PACH and 3.5% for OCT. Compared with U-PACH, OCT consistently overestimated the CCT by a mean of 23 microm as shown on Bland-Altman plot.

CONCLUSION

CCT measured by OCT and U-PACH is highly correlated. With appropriate adjustment factor, OCT agrees well with U-PACH and is a reliable alternative for CCT measurement.

The Effects of Eye Drops on Corneal Thickness in Adult Myopia

PURPOSE

To study the effects of Complex Tropicamide (0.5% Tropicamide and 0.5% phenylephrine HCI) and Saline solution on corneal thickness in adult myopic patients with the Orbscan II system.

SETTING

Department of Ophthalmology, Yantai Yuhuangding Hospital.

DESIGN

Prospective, nonrandomized, clinical trial.

METHODS

The thinnest pachymetry of the cornea was obtained before and 1.5 hours after administration of three drops of Complex Tropicamide to the left eyes of 58 patients (58 eyes) and Saline solution to their 31 right eyes, respectively. The corneal thickness of the other 27 right eyes before and 1.5 hours after eyelid closure without exposure to eye drops was used as the control group.

RESULTS

The thinnest pachymetry of the cornea was significantly higher after exposure to eye drops in the Tropicamide group (23.36 +/- 15.01 microm; t = -11.855, P < 0.001). Similar findings were also noted in the Saline group (7.13 +/- 8.11 microm, t = -4.894, P < 0.001). The difference between the two groups was also significant (t = 6.737, P < 0.001). There were no statistically changes in corneal thickness in control group. The drops tested have no effect on the location of the thinnest corneal site and its distance form the visual axis.

CONCLUSION

Eye drops including Saline solutions may have significant effects on the corneal thickness in myopia, and this may have implications for corneal refractive surgery.

Corneal thickness and visual field damage in glaucoma patients

PURPOSE

To verify whether there was a significant correlation between central corneal thickness (CCT) and visual field damage in patients with primary open angle glaucoma (POAG).

METHODS

A total of 99 eyes with POAG were consecutively recruited. Patients were classified as glaucomatous based on visual field and optic nerve head damage. All underwent applanation tonometry, Humphrey perimetry, and measurement of CCT with ultrasonic pachymetry. Based on CCT value, the sample was split at the mode in two groups (group 1<535 microm, n=49; group 2>or=535 microm, n=50).

RESULTS

Entire cohort: mean CCT 554 microm+/-45.03; mean deviation (MD) -6.68 dB+/-7.32; pattern standard deviation (PSD) 5.33+/-3.75; intraocular pressure (IOP) 17.91+/-4.16 mmHg with treatment. Group 1: CCT was 504.8 microm+/-30.8; MD -9.01 dB+/-8.72; PSD 6.38+/-3.99; IOP 18.02 mmHg+/-4.66. Group 2: mean CCT 574.6 microm+/-35.03; MD -4.39 dB+/-4.70; PSD 4.25+/-3.19; IOP 17.79 mmHg+/-3.57. A significant difference was found between the two groups for both MD and PSD. Linear regression analysis showed a significant correlation between CCT and PSD (P<0.001).

CONCLUSIONS

Our data show that patients with a thinner cornea had a worse MD and PSD. As a thinner CCT causes an underestimation of the true IOP, there may be a delay in the diagnosis of POAG or an inadequate estimate of the clinical course despite apparently desirable IOP applanation readings.

Relationship Between Central Corneal Thickness and Localized Retinal Nerve Fiber Layer Defect in Normal-tension Glaucoma

PURPOSE

To determine whether central corneal thickness (CCT) is related to the extent of localized retinal nerve fiber layer (RNFL) defect at the initial examination of normal-tension glaucoma (NTG) patients.

PATIENTS AND METHODS

Seventy-five eyes of 75 NTG patients showing localized RNFL defects on RNFL photographs and corresponding visual field defects at the initial visit to a glaucoma specialist were selected for this study. All participants completed refraction, Goldmann applanation tonometry, CCT measurement, stereoscopic disc photography, RNFL photography, and automated perimetry. Each patient's age, spherical equivalent, intraocular pressure, CCT, approximation of the RNFL defect to the fovea (angle alpha), circumferential width of the RNFL defects (angle beta), horizontal and vertical cup-to-disc ratios, and mean deviation of visual field were analyzed.

RESULTS

In univariate and multivariate analyses, lower CCT was significantly associated with increased horizontal and vertical cup-to-disc ratios, decreased angle alpha, and increased angle beta. For a decrease of 10 microm of CCT, horizontal and vertical cup-to-disc ratios increased by 0.020, angle alpha decreased by 1.58 degrees, and angle beta increased by 1.71 degrees, respectively.

CONCLUSION

CCT is a significant factor in predicting the extent of localized RNFL defect at the initial examination of NTG patients.

Central corneal thickness and normal tension glaucoma: A cross-sectional study

BACKGROUND

Recently published evidence has identified thinner central corneal thickness (CCT) as a strong predictive factor for the conversion from ocular hypertension (OHT) to primary open-angle glaucoma (POAG). The association between CCT and development of normal-tension glaucoma (NTG), however, is less clear. Accordingly, we designed this cross-sectional study to further explore the relationship between CCT and NTG.

PATIENTS AND METHODS

All patients with a clinical diagnosis of NTG and NTG suspect (NTGS) who were seen from September 2002 through May 2003 at the Albuquerque VA Medical Center eye clinic were identified retrospectively. After eligible subjects were categorized into no, mild, moderate, and advanced visual field loss groups, analysis of variance (ANOVA) and regression analyses were used to determine group differences for several IOP variables, several systemic variables, and CCT. Additional analyses were completed after eligible subjects were recategorized into thin, intermediate, and thick CCT groups.

RESULTS

Eighty-four eyes in 84 NTGS subjects and 56 eyes in 56 NTG subjects were studied. Mean CCT was significantly thicker in the no field loss group (NTGS) when compared with all 3 groups with glaucomatous visual field loss (NTG). In multivariate regression analysis, the association between CCT and the presence of NTG-related visual field loss was robust and independent. Conversely, no relationship was found between CCT and severity of NTG-related visual field loss.

CONCLUSIONS

In eyes characterized by statistically normal intraocular pressure (IOP) measurements as measured by Goldmann applanation tonometry, we found a significant relationship between CCT and the presence, but not severity, of glaucomatous visual field loss. A prospective study is required to further explore and confirm these relationships.

Circadian variations in central corneal thickness and intraocular pressure in patients with glaucoma

AIM

To analyse 24 hour variations in intraocular pressure (IOP) and central corneal thickness (CCT) in a group of glaucomatous patients.

METHODS

30 patients with primary open angle glaucoma were hospitalised and underwent circadian evaluations (at 8 pm, midnight, 4 am, 8 am, noon, and 4 pm) of supine and sitting IOP, respectively, measured using a Perkins and a Goldmann tonometer, and CCT measured using an ultrasonic pachymeter (the mean value of three measurements within 5 mum). All patients were treated with timolol 0.5% twice daily and latanoprost 0.005% once daily.

RESULTS

Mean supine IOP was 15.3 (SD 3.7) mm Hg (range 10-25), with circadian fluctuations of 7.3 (3.3) mm Hg. Mean sitting IOP was 15.1 (3.9) mm Hg (range 8-26), with circadian fluctuations of 5.4 (3.1) mm Hg. Mean CCT was 534 (39) microm (range 443-637 microm) with circadian fluctuations of 16.5 (6.2) microm (range 6-31 microm). Both the within patient and within time point fluctuations in CCT were statistically significant (p<0.0001, ANOVA).

CONCLUSIONS

The authors found considerable fluctuations in 24 hour IOP. The circadian fluctuations in CCT were small and, although statistically significant, did not seem to interfere with the circadian IOP assessment.

Eye stiffness measurement by probe contact method

The internal eye pressure is an important index for judging whether an eye suffers from glaucoma or not. The conventional eye pressure measurement is valid only under the condition that all subjects have the same structural eye stiffness. This paper challenges the practice of measuring the stiffness of a human eye by pressing the cornea with a contact probe. The displacement of the eye is captured by a camera with high resolution. Experimental results suggest that the measured eye stiffness nicely matches with the theoretical estimation. Based on the experimental results, the difference between the eye stiffness measured by the contact method and the non-contact method is discussed.

New thoughts on normal tension glaucoma

Normal tension glaucoma (NTG) is considered a primary optic neuropathy within the glaucoma family. Whereas NTG shares many of the same pathologic characteristics as primary open angle glaucoma (POAG), there are several differences such as pattern of visual field defect and incidence of disc hemorrhaging. The most notable difference is that NTG does not manifest a recordable intraocular pressure (IOP) greater than 21 mmHg. Debate has existed as to whether NTG is a pressure-sensitive neuropathy and if pressure reduction affects the course of the disease. Likely, there are aspects in the pathophysiology of NTG that are both pressure sensitive and insensitive, and the etiology of NTG has never been adequately explained.

Dynamic Contour Tonometry

PURPOSE

With tonometers currently in use intraocular pressure is indirectly determined by measuring a physical quantity related to a specified deformation of the cornea. We present a new principle of direct, continuous, and transcorneal intraocular pressure measurement, describe its theoretical foundation, and evaluate its application on the basis of an in vitro model.

METHODS

On a living human eye an optimized pressure-sensitive contact surface was determined by performing pressure measurements with differently shaped tonometer heads. Based on these results and on the theoretical model, a Dynamic Contour Tonometer was constructed and validated on eye bank bulbi against a manometric reference pressure.

RESULTS

A concave contact surface with a radius of curvature of 10.5 mm creates a distribution of forces between the central contour matching area of the tip and the cornea that equals the forces generated by the internal pressure of the eye. A sensor integrated into the surface having the same contour measures the intraocular pressure closely to the manometric reference pressure in human cadaver eyes. The accuracy of the tonometer appears to be unaffected by variations in corneal properties.

CONCLUSION

Dynamic Contour Tonometry eliminates most of the systematic errors arising from individual changes of corneal properties that adversely influence all types of applanation tonometers. The advantage of measuring the true pressure in combination with the capability of registering dynamic pressure fluctuations discloses new tonometric opportunities to diagnose and classify different types of glaucoma.