The glaucoma suspect: differentiation of the future glaucomatous eye from the non-glaucomatous suspect eye. 1. Ultrasonic measurements and eye-wall stress

Association between corneal biomechanical properties and glaucoma severity

PURPOSE

To investigate the association between corneal biomechanical parameters using the Ocular Response Analyzer (ORA) and glaucoma severity.

DESIGN

Observational cross-sectional study.

METHODS

Two hundred ninety-nine eyes of 191 patients with confirmed or suspect glaucoma were recruited at the University of California, San Diego. Corneal hysteresis (CH) and corneal resistance factor (CRF) were obtained from all participants. Standard automated perimetry was done using the 24-2 Swedish Interactive Threshold Algorithm. Retinal nerve fiber layer (RNFL) thickness measurements were obtained using GDx ECC and spectral-domain optical coherence tomography (SD-OCT). The association between ORA parameters and disease severity was evaluated using univariable and multivariable regression models.

RESULTS

CH and CRF were both positively associated with mean defect (MD) (R(2) = 0.03; P < .01 and R(2) = 0.10; P < .01, respectively). In multivariable analysis, the association between CRF and MD remained significant while CH to MD did not (P < .01 and P = .77). In the GDx ECC subgroup (204 eyes), there was a weak association between CH and CRF and average RNFL thickness (R(2) = 0.07; P < .01 and R(2) = 0.05; P < .01, respectively), which was not observed in the SD-OCT subgroup (146 eyes) (R(2) = 0.01; P = .30 and R(2) = 0.01; P = .21). After adjusting for central corneal thickness, age, and axial length, the relationship of CH and CRF to RNFL thickness no longer reached statistical significance.

CONCLUSIONS

The current study found only a weak relationship between corneal biomechanical parameters and measures of structural and functional damage in glaucoma.

Human corneal thickness and its impact on intraocular pressure measures: a review and meta-analysis approach

We determined the "normal" central corneal thickness (CCT) value in human corneas based on reported literature values for within-study average CCT values, and used this as a reference to assess the reported impact of physiological variables (especially age and diurnal effects), contact lens wear, pharmaceuticals, ocular disease, and ophthalmic surgery on CCT. With the expected CCT and its variance defined, it should be possible to determine the potential impact of differences in CCT in intraocular pressure (IOP) assessments, especially by applanation tonometry, using a meta-analysis approach. Some 600 sets of CCT data were identified from the worldwide literature over the period of 1968 through mid-1999, of which 134 included IOP measures as well. The within-study average CCT values and reported variance (SD) was noted along with the number of eyes and any special characteristics, including probable ethnic origin of the study subjects. Various sets of data were subjected to statistical analyses. From 300 data sets from eyes designated as normal, the group-averaged CCT was 0.534 mm. From 230 data sets where interindividual variance was reported, the group-averaged CCT was 0.536 mm (median 0.536 mm; average SD of 0. 031 mm, average coefficient of variation = 5.8%). Overall, studies using slit-lamp-based pachometry have reported marginally lower CCT values (average 0.530 mm, average SD 0.029 mm) compared to ultrasound-based studies (average 0.544, average SD 0.034 mm), which perhaps reflects the type of individual studied (non-surgical vs. pre-surgical patients) rather than the technique itself. A slight chronological increase in reported average CCT values (approximately 0.006 mm/decade) was evident, but a substantial chronological increase was evident for ultrasound pachometry studies (approximately 0.015 mm/decade). Within the meta-analysis-generated average and variance, age had no obvious impact on CCT measures for *whites, although an age-related decline in CCT is evident for non-whites. Any diurnal effects are likely concealed within the expected variance in CCT. Contact lens wear and pharmaceuticals generally produced changes in CCT that were well within the expected variance in CCT. Of the ocular diseases, only those associated with collagen disorders (including keratoconus) or endothelial-based corneal dystrophies (e.g., Fuchs) were likely to result in decreases or increases, respectively, of CCT beyond the normal variance. Routine contact lens wear and diseases such as diabetes seem unlikely to produce changes in CCT of a magnitude that would justify pachometry as a monitoring method beyond routine slit-lamp evaluation. Increases in CCT beyond the expected variance were reported after a range of intraocular surgeries (cataract operations, penetrating keratoplasty), whereas photorefractive surgery produces a measurable decrease in CCT. A meta-analysis of possible association between CCT and IOP measures of 133 data sets, regardless of the type of eyes assessed, revealed a statistically significant correlation; a 10% difference in CCT would result in a 3. 4 +/- 0.9 mm Hg difference in IOP (P </= 0.001, r = 0.419). The observed phenomenon was much smaller for eyes designated as healthy (1.1 +/- 0.6 mm Hg for a 10% difference in CCT, P = 0.023, r = 0. 331). For eyes with chronic diseases, the change was 2.5 +/- 1.1 mm Hg for a 10% difference in CCT (P = 0.005, r = 0.450), whereas a substantial but highly variable association was seen for eyes with acute onset disease (approximately 10.0 +/- 3.1 mm Hg for a 10% difference in CCT, P = 0.004, r = 0.623). Based on the meta-analysis, normal CCT in white adults would be expected to be within +/-11.6% (+/-2 SD) of 0.535 mm, i.e., 0.473-0.597 mm (95% CI, 0.474-0.596). The impact of CCT on applanation tonometry of healthy eyes is unlikely to achieve clinical significance, but for corneas of eyes with chronic disease, pachometry should be performed if the tonometry reveals IOP readi

Corneal thickness and curvature in normal-tension glaucoma

PURPOSE

To determine whether normal-tension glaucoma, defined as a condition in which glaucomatous optic nerve and visual field changes exist without documentation of intraocular pressure greater than 21 mm Hg or other apparent cause for these changes, is overdiagnosed in patients with decreased central corneal thickness and curvature.

METHODS

Twenty-one patients with normal-tension glaucoma were compared with 25 patients with primary open-angle glaucoma and 27 age-matched healthy subjects. Corneal thickness was determined by ultrasonic pachymetry. Corneal curvature was determined using a keratometer. Eyes with corneal pathology or previous intraocular surgery were excluded.

RESULTS

Mean corneal thickness +/- SD in 21 eyes of 21 patients with normal-tension glaucoma was 0.521 +/- 0.037 mm, significantly (P = .0028) lower than in 25 eyes of 25 patients with primary open-angle glaucoma (0.556 +/- 0.035 mm) and 27 eyes of 27 healthy subjects (0.555 +/- 0.034). Mean corneal curvature in the three groups was not appreciably different: 43.90 +/- 1.81 diopters, 43.66 +/- 1.68 diopters, and 44.36 +/- 1.13 diopters in the patients with normal-tension glaucoma and primary open-angle glaucoma and the healthy subjects, respectively.

CONCLUSIONS

Corneal thickness is significantly reduced in patients with normal-tension glaucoma compared with patients with primary open-angle glaucoma (P = .0028) and normal subjects (P = .0037). This may lead to underestimation of intraocular pressure and misdiagnosis in some of these patients. Corneal curvature was similar in patients with normal-tension glaucoma and primary open-angle glaucoma and in healthy subjects.

Ocular hypertension and central corneal thickness

PURPOSE

To determine whether ocular hypertension (OHT) is over diagnosed in patients with increased corneal thickness.

METHODS

Thirty-six patients with OHT were compared with 29 control subjects and 31 patients with glaucoma. Corneal thickness was determined by ultrasonic pachymetry and measured at a single point, 1.5 mm temporal to the corneal light reflex.

RESULTS

The corneal thickness (mean +/- standard deviation) in patients with OHT was 0.610 +/- 0.033 mm. This was significantly greater than in patients with glaucoma (0.557 +/- 0.039 mm) and control subjects (0.567 +/- 0.036 mm).

CONCLUSION

It is well documented that increased corneal thickness leads to artificially high estimations of intraocular pressure. This study confirms that a significant number of patients with OHT have a normal intraocular pressure if corneal thickness is taken into account. Corneal pachymetry is clinically helpful in estimating intraocular pressure, determining the risk of visual loss, and establishing a target pressure.

The effect of corneal thickness on applanation tonometry

Simultaneous manometry and Perkins tonometry were performed at 10.0, 20.0, and 30.0 mm Hg on 15 eyes on which intraocular procedures were performed. There was a statistically significant relationship between corneal thickness and the error of Perkins tonometry. Thin corneas produced underestimations of the intraocular pressure by as much as 4.9 mm Hg, whereas thick corneas produced overestimations by as much as 6.8 mm Hg. Measuring the corneal thickness is necessary to interpret properly the results of Goldmann applanation tonometry, particularly in eyes with thin corneas.

The glaucoma suspect: differentiation of the future glaucoma eye from the non-glaucomatous suspect eye. 2. Visual field decay

Visual-field areas to a I2e stimulus were measured planimetrically using an X-Y digitizer and a computer program. Sampling of normal subjects and patients suspected of having glaucoma was done at two points in time. Calculations of eye-wall stress were done using ultrasonic data and intra-ocular pressure (IOP) measurements from patient records. For those suspected of having glaucoma who developed chronic open-angle glaucoma (COAG), the time of transition was the second point in time. The visual field area was regressed against patient age at the two points in time. No difference in the regression slopes was found for the normal subjects and unchanged patients. The patients who did develop glaucoma were significantly different. The mean annual rate of visual-field change (rate of decay) was calculated and found to be 28.5 mm2/year for the normals, 153.5 mm2/year for the suspects, and 376.4 mm2/year for those patients who developed glaucoma. The rate of visual-field decay only correlated with patient age (P = 0.03) and eye-wall stress (P less than 0.01) in the patients who developed glaucoma.