The roles of cornea and axial length in corneal hysteresis among emmetropes and high myopes: a pilot study.

PURPOSE: To observe the roles of cornea and axial length (AL) in corneal hysteresis (CH) among emmetropes and high myopes. MATERIALS AND METHODS: The CH, corneal resistance factor (CRF) and corneal-compensated intraocular pressure (IOPcc) of 40 young emmetropes (spherical equivalent +/-0.50 D) and 40 age-matched high myopes (-6D or more) were measured by the Ocular Response Analyzer. AL was measured using a partial coherence interferometer. The mean keratometry reading (meanK) and corneal volume (CV) in 4 zones were measured using the Pentacam. Full stromal thickness (FST) and epithelial and Bowman's thicknesses (Epi + BT) were evaluated using confocal microscopy. Valid confocal microscopic results from the more emmetropic eyes of 30 emmetropes and the more myopic eyes of 27 high myopes were studied. RESULTS: High myopes exhibited significantly longer AL (27.34 ± 0.90 mm versus 23.71 ± 0.79 mm), lower CH (10.17 ± 1.38 mmHg versus 11.11 ± 1.25 mmHg), and higher IOPcc (16.50 ± 3.05 mmHg versus 13.91 ± 2.49 mmHg) compared with emmetropes. The 2 groups showed similar CRF, CVs, meanK, FST and Epi + BT. By combining the results of both groups, CH and CRF were significantly correlated with FST (r(2)= 0.368 and r(2 )= 0.408, respectively), CVs (r(2)= 0.251-0.391 and r(2)= 0.201-0.346, respectively), but not with Epi + BT or meanK. Furthermore, AL and IOPcc were significantly correlated with CH (r(2)= 0.112 and r(2)= 0.161, respectively) but not with CRF. The multivariate analysis indicated that FST (standardized coefficients, ß = 0.591) and CV (ß = 0.575) had stronger association than IOPcc and AL for CH. CONCLUSIONS: High myopes exhibited lower CH than emmetropes. Although CH had stronger correlation with FST than AL, it might contain the biomechanical properties of components other than the cornea.

Improving interobserver variation in corneal sublayer pachymetry using ConfoScan4 with z ring.

OBJECTIVES: To observe and improve the interobserver variation of corneal sublayer pachymetry using ConfoScan4 (CS4) z ring. METHODS: Right corneas of 34 normal subjects were scanned using CS4 (z ring) (Nidek Technologies, Padova, Italy). Corneal sublayer pachymetry was performed by two masked observers based on the written frame selection criteria from literature. The full stromal thickness (FST), epithelial and Bowman layer thickness (Epi+BT), endothelial thickness (EndoT), and total corneal thickness (CT) were obtained. Upon completion of the measurements (measurement 1), the frame selection criteria were reviewed between the observers by going through some corneal frames together. Eight subjects were excluded and corneal sublayer pachymetry was re-evaluated by the two observers using the revised criteria (measurement 2). RESULTS: Corneal thickness showed significant interobserver difference in measurement 1 (n=34), but no significant difference in measurement 2 (n=26). Endothelial thickness remained significant difference between observers throughout the study. Interobserver difference significantly reduced using the revised criteria in CT (1.93 ± 3.23 µm to 0.28 ± 1.75 µm) and EndoT (4.01 ± 4.82 µm to 0.86 ± 1.81µm). There was no significant difference between observers in Epi+BT and FST for the two measurements. Interobserver variation and the 95% confidence limits between observers for CT and corneal sublayers were reduced at least by half in measurement 2. CONCLUSIONS: The interobserver variation in corneal sublayer pachymetry could be improved by having the observers going through some corneal frames together rather than just following the written criteria. The use of CS4 (z ring) to measure CT, FST, and Epi+BT is suitable for monitoring corneal changes clinically.

Influence of corneal astigmatism, corneal curvature and meridional differences on corneal hysteresis and corneal resistance factor.

BACKGROUND: This study investigated the influence of corneal astigmatism, corneal curvature and meridional differences on corneal hysteresis (CH) and the corneal resistance factor (CRF) in a group of normal Chinese persons. METHODS: Ninety-five participants were recruited and data from the eye with higher corneal astigmatism were analysed. The anterior corneal curvature was measured by corneal topography. The Goldmann-correlated intraocular pressure (IOPg), corneal-compensated intraocular pressure (IOPcc), CH and CRF at different meridians (default horizontal position, 10°, 20° and 30° along the superotemporal and inferonasal meridians) were obtained from an ocular response analyser. The corneal powers at these specific meridians also were calculated. RESULTS: At the default position, the IOPg and CRF had weak correlations with corneal astigmatism, while the IOPcc and CH were not significantly correlated with corneal astigmatism. Both the IOPg and IOPcc were measured significantly higher at the default position. The CH and CRF were lower at the default position but the difference in the CRF from obliquity could not reach statistical significance. The CH was not significantly correlated with the corneal power at all meridians. The CRF correlated with the corneal power only at 30° superotemporal. CONCLUSION: Corneal astigmatism and head tilt did not have much effect on the measurement of CH and the CRF, both of which were lowest along the horizontal meridian. Clinically, the difference was small. The influence of corneal power on CH and the CRF was minimal.

Corneal volume measures for monitoring contact lens induced corneal swelling: a pilot study.

BACKGROUND: This study investigated the use of corneal volume to monitor the corneal swelling response induced by wearing high plus power contact lenses. METHODS: Twelve young non-contact lens wearers were recruited with one eye fitted with a soft contact lens (Polymacon material, 38.6 per cent water, Dk of 9 and 0.27 mm centre thickness) and the fellow eye served as control. The treated eye was patched for two hours leaving the control eye uncovered. Central corneal thickness (CCT) was measured with non-contact specular microscopy and corneal volumes (at three, five and 10 mm zones) were measured with a corneal topographer (Pentacam, Oculus Inc, Germany), before eye patching as well as immediately after and every 20 minutes for 100 minutes. RESULTS: Subjects had similar CCT and corneal volumes between the two eyes before treatment. The treatment eyes showed a mean corneal swelling of 8.1 per cent immediately after lens removal. Corneal volume was significantly increased at the three (mean swelling of 5.9 per cent), five (5.6 per cent) and 10 millimetre (3.3 per cent) zones. It took 20 minutes for the corneal volume at the 10 mm zone to return to baseline but required 60 minutes for the three and five millimetre zones to return to the baseline level. The central corneal thickness was still significantly thicker 80 minutes after patching. CONCLUSION: Corneal volume could be a useful parameter to monitor corneal change when an event affects the entire cornea. Corneal volume combined with central corneal thickness could give more comprehensive information to monitor central corneal swelling.