Corneal hysteresis and axial length among Chinese secondary school children: the Xichang Pediatric Refractive Error Study (X-PRES) report no. 4

Comparison of factors that influence the measurement of corneal hysteresis in vivo and in vitro

PURPOSE

The purpose of this study was to compare measurements of corneal hysteresis (CH) obtained in vivo, with similar measurements from excised human eyes and from excised human corneas mounted in an artificial anterior chamber.

METHODS

Corneal hysteresis was measured using an ocular response analyser (Reichert Ophthalmic Instruments) from three groups: 53 healthy normal corneas of fifty-three patients, six excised eyes and 17 excised corneas.

RESULTS

In vivo, it was found that CH was independent of gender, age and mean spherical equivalent, but has a significant inverse relationship with intraocular pressure (IOP(cc)) (r = 0.53; p < 0.0001). However, there was no correlation between CH and IOP(G) (r = 0.10; p = 461). The same inverse relationship with IOP(cc) was recorded in intact, excised eyes (r = 0.74; p < 0.0001), with no significant differences between the behaviour each individual eye. Excised corneas also showed an inverse relationship between CH and trans-corneal pressure (r = 0.72; p < 0.0001), but the measured values of CH were lower than those recorded in vivo and from intact globes. In both excised eyes and excised corneas, we found a significant correlation between CH and central corneal thickness [r = 0.86; p < 0.0001 and r = 0.611; p < 0.0005 (respectively)].

CONCLUSION

The in vitro results indicate that every normal human eye at physiological hydration shows an identical dependence of CH on IOP(cc) , the same dependence as is observed in vivo. This therefore would appear to be an intrinsic response of the tissue to a change in IOP. However, it is possible that the lower values of CH recorded from excised corneas reflect the influence of the artificial chamber replacing the eye globe, so in vivo values of CH may be influenced to some extent by the presence of the other components of the eye.

Association between corneal biomechanical properties and myopia in Chinese subjects

PURPOSE

To examine the relationship between corneal biomechanical properties and the degree of myopia.

METHODS

Chinese subjects (n=172, age: 11-65 years) were divided into diagnostic groups with non-myopia (spherical equivalence (SE)>-0.50 D), low (-3.00 ≤ SE ≤ -0.50 D), moderate (-6.00 ≤ SE < -3.00 D), and high myopia (SE<-6.00 D). Only the right eye of each subject was analyzed. Central corneal thickness (CCT) was measured by optical coherence tomography. An ocular response analyzer was used to measure corneal hysteresis (CH), corneal resistance factor (CRF), intraocular pressure (IOP), and corneal compensated IOP (IOPcc). Refraction was measured by both automated and subjective refractometry and expressed as SE.

RESULTS

CH was significantly lower in high myopia compared with both low and non-myopia (P ≤ 0.002). CCT was 1.5 times more correlated to CH variation compared with refraction. Similarly, CRF was four times more dependent on CCT than refraction. CH (P<0.001) or CRF (P=0.005) was positively correlated to refraction. Both IOP and IOPcc were negatively correlated to refraction (P<0.001), respectively.

CONCLUSIONS

CH decreases only in high myopia. Refraction is positively correlated to both CH and CRF but negatively correlated to both IOP and IOPcc. These results indicate that the mechanical strength in anterior segment of the eye is compromised in high myopia. In addition, high myopia may increase the risk of glaucoma.

Central corneal thickness in children and adolescents with pediatric glaucoma and eye disorders at risk of developing glaucoma

BACKGROUND

To investigate central corneal thickness (CCT) in children with glaucoma and at risk for glaucoma.

METHODS

The study included 139 children with glaucoma: 66 at risk for glaucoma (ie, aphakia, aniridia, or uveitis) and 66 normal children. CCT was measured by ultrasound pachymetry and intraocular pressure (IOP) by applanation. Analysis of variance was used to compare CCT between groups. Correlation analysis assessed associations between CCT and ocular factors including spherical equivalent, cup-to-disc ratio, glaucoma medications, and number of intraocular surgeries.

RESULTS

CCT was significantly higher for 141 eyes with glaucoma (mean: 0.598 mm, P < .001) and 76 eyes at risk for glaucoma (mean: 0.604 mm, P = .001) than for 66 normal eyes (mean: 0.558 mm). No significant difference was observed between at-risk (P = .989) and glaucoma eyes. Eyes with aphakia (0.653 mm) and aniridia (0.639 mm) had the thickest CCT values. Thinnest CCT was found in anterior segment dysgenesis and uveitis (mean: 0.541 mm). A significant positive correlation between CCT and spherical equivalent was found for glaucoma (r = 0.413; P < .001) and at-risk (r = 0.412; P < .0003) eyes, and between CCT and intraocular surgery for at-risk eyes (P = .0066). A significant negative correlation was found between CCT and cup-to-disc ratio for glaucoma eyes (r = -0.223; P = .01).

CONCLUSION

This is the largest series of CCT in pediatric glaucoma and related disorders. The data suggest caution in application of standard formulas for IOP-to-CCT correction when evaluating children with glaucoma because their mean CCT values extend far beyond values reported for normal eyes.

Keratoconus and normal-tension glaucoma: a study of the possible association with abnormal biomechanical properties as measured by corneal hysteresis

PURPOSE

To test the hypothesis that patients with keratoconus and pellucid who have glaucoma or are glaucoma suspects have lower corneal hysteresis (CH) and/or corneal resistance factor (CRF) measurements compared with controls.

METHODS

A prospective study at a tertiary eye center of patients with keratoconus and pellucid, with glaucoma or suspect glaucoma and age-matched keratoconus and pellucid controls, was performed. After informed consent was obtained, corneal topography, ocular response analyzer (ORA; Reicher, Buffalo, NY), pachymetry, intraocular pressure, A scan measurements, Humphrey visual fields (VFs), and disk photographs were done. Analyses compared cases with controls on primary (CH and CRF) and secondary variables. Disk photographs and VFs were rated in a masked fashion.

RESULTS

The mean CH [8.2 (SD = 1.6) and 8.3 (SD = 1.5)] and CRF [7.3 (SD = 2.0) and 6.9 (SD = 2.1)] were low and did not differ significantly between 20 study (29 eyes) and 40 control patients (61 eyes), respectively. CH had a negative significant correlation with maximum corneal curvature by topography (P < 0.002) and positive significant correlation with central corneal thickness (P < 0.003). The mean cup to disk ratio was larger (0.54, SD = 0.20) among cases than in controls (0.38, SD = 0.20), P = 0.003. VFs were suspicious for glaucoma more often among the study eyes (11 of 29, 33.9%) than controls (8 of 60, 13.3%), P = 0.019.

CONCLUSIONS

CH was low in study and control patients and was correlated with severity of keratoconus/pellucid but not with glaucoma/glaucoma suspect or control status. Evidence of glaucoma was more common in study eyes than in controls but was present in both.

Ocular response analyzer measurements in keratoconus with normal central corneal thickness compared with matched normal control eyes

PURPOSE

To compare corneal hysteresis (CH) and corneal resistance factor (CRF) in eyes with keratoconus with a central corneal thickness (CCT) ≥ 520 μm with CH and CRF in matched controls, and to estimate the sensitivity and specificity of these parameters for discriminating between the two groups.

METHODS

This prospective, comparative case series comprised 19 eyes of 19 patients with keratoconus with CCT ≥ 520 μm and 19 eyes of 19 healthy sex-, age-, and CCT-matched patients who underwent a complete clinical eye examination, corneal topography, tomography, and biomechanical evaluation. The receiver operating characteristic (ROC) curve was used to identify cutoff points that maximized the sensitivity and specificity for discriminating between groups.

RESULTS

Central corneal thickness was 543.1 ± 13.9 μm (range: 520 to 568 μm) in the keratoconus group and 545 ± 12.5 μm (range: 527 to 575 μm) in the control group (P=.6017). Corneal hysteresis was 9.22 ± 1.44 mmHg (range: 6.2 to 11.35 mmHg) in the keratoconus group and 10.58 ± 1.91 mmHg (range: 7.34 to 13.53 mmHg) in the control group (P=.0075). Corneal resistance factor was 8.62 ± 1.52 mmHg (range: 5.60 to 11.20 mmHg) in the keratoconus group and 10.30 ± 1.92 mmHg (range: 6.95 to 14.12 mmHg) in the control group (P=.0049). The ROC curve analyses showed a poor overall predictive accuracy of CH (cutoff, 9.90 mmHg; sensitivity, 78.9%; specificity, 63.2%; test accuracy, 71.05%) and CRF (cutoff, 8.90 mmHg; sensitivity, 68.4%; specificity, 78.9%; test accuracy, 73.65%) for detecting keratoconus in the eyes studied.

CONCLUSIONS

Corneal hysteresis and CRF were statistically lower in the keratoconus group compared with the control group. Given the large overlap, both CH and CRF had low sensitivity and specificity for discriminating between groups.

[Intraocular pressure measurement in children]

Intraocular pressure (IOP) measurement in children is often difficult to perform because younger children are non-compliant and resisting the examination. Normal IOP in children is not well established yet because in the studies reporting about IOP, the instruments used and clinical conditions have varied. Non contact tonometer often overestimates IOP in blinking children and is not always reliable. But all the others measurement techniques use contact (GAT, Perkins, Tono-Pen, ORA, RBT), and are not always suitable and easy-touse. Under general anaesthesia, mean IOP measured with Perkins applanation tonometer is under 8 mmHg before age of 3 months and under 12 mmHg between ages of 6 and 9 months. After, IOP shows an increasing trend with age of 1 mmHg per year up to 12 years. Some studies have proposed as normal pediatric IOP: To=0.71 x age (years) +10, up to age 10. Then, IOP tends to approach adult levels by 12 years of age. However pediatric glaucoma is rare: congenital glaucoma, before age of 3 years, autosomal dominant juvenile glaucoma, with family history of glaucoma and elevated IOP, or secondary glaucoma with special context. Thus, hypertony has to be confirmed by another measurement technique, correlated to central corneal thickness, and clinical examination (optic nerve head and visual field).

The influence of refractive errors on IOP measurement by rebound tonometry (ICare) and Goldmann applanation tonometry

PURPOSE

To evaluate the effect of refractive errors and central corneal thickness (CCT) on the measurement of intraocular pressure (IOP) by ICare rebound tonometer (RT), and its agreement with measurements by Goldmann applanation tonometer (GAT).

PATIENTS AND METHODS

Two observers measured the IOP by using RT and GAT in four groups of healthy volunteers with emmetropic (n = 78), hyperopic (n = 83), myopic (n = 87) and astigmatic (n = 79) eyes. Refraction was assessed by an autorefractometer. CCT was assessed by ultrasound pachymetry.

RESULTS

In all groups, no significant interobserver difference was seen in IOP values detected by both tonometers (Wilcoxon signed-rank test not significant). In all groups, IOP values were higher as measured by RT than by GAT (paired t-test p = 0.000): mean RT-GAT difference was higher in myopic eyes (+1.6 +/- 1.8 mmHg), and it was less than 1 mmHg in the other groups. RT-GAT difference was correlated to the refraction (p < 0.001), and it was greater when an higher IOP was detected by RT (significant correlation between RT-GAT difference and IOP by RT, p < 0.001). Compared with GAT values, the IOP readings by RT were greater than 2 mmHg in respectively 17.9% (emmetropic), 13.3% (hyperopic), 34.5% (myopic) and 7.6% (astigmatic) of the eyes. With both tonometers, in all groups the IOP values were correlated with CCT (p < 0.05), but the discrepancy between RT and GAT values was not related to CCT.

CONCLUSIONS

In all groups of subjects, higher IOP values were detected by RT; the IOP readings exceed the GAT values usually in a range of less than 1 mmHg, except when RT detects IOP >18 mmHg and generally in myopic eyes; RT-GAT discrepancy is related to the refractive error, but not to CCT.

Intraocular pressure spikes in keratectasia, axial myopia, and glaucoma

PURPOSE

To review a range of activities associated with intraocular pressure (IOP) spikes. To examine the possible significance of IOP spikes in conditions such as keratectasia, axial myopia, and glaucoma.

METHODS

Hypotheses concerning mechanisms for adverse responses to IOP spikes were examined.

RESULTS

Apart from the possibility that IOP spikes might cause susceptible corneal, posterior scleral, or optic nerve head tissue to yield to associated distending forces, there is the possibility that these tissues will be also be damaged by increased hydrostatic pressure.

CONCLUSIONS

In-office tonometry does not indicate the degree to which ocular tissues are exposed to IOP spikes. For eyes that are exposed to IOP spikes of longer duration, that occur frequently and which result in a larger IOP increment, the risk of an adverse response may be greater. Changes in ocular tissues because of increased hydrostatic pressure may include morphological cellular changes and alterations to enzyme function. Eye rubbing may be the most significant mechanism for creating IOP spikes because of the large IOP increments that may be involved, as well as the possibility that abnormal rubbing can become a chronic habit. As appears to be the case in keratoconus, asymmetric exposure to IOP spikes may help to explain some asymmetric presentations of post-laser-assisted in situ keratomileusis, glaucoma, or myopia. Ideally methods for the objective assessment of patient risk for adverse responses to IOP spikes will continue to be developed. A self-administered questionnaire may help identify patients who are significantly exposed to IOP spikes. Family history may indicate an increased risk of diseases for which IOP spikes may have significant implications. Patient counseling regarding the possibility that IOP spiking activities may contribute to the development and/or progression of conditions such as keratectasia, axial myopia, and glaucoma may be indicated.