Central corneal thickness and corneal hysteresis associated with glaucoma damage

Long axial length as risk factor for normal tension glaucoma

BACKGROUND

The high prevalence of normal tension glaucoma (NTG) in the Japanese requires special screening tests other than measurements of only the intraocular pressure (IOP). This study was carried out to determine whether there is a significant association between the axial length of the eye and the presence of NTG.

METHODS

We reviewed the medical records of all patients who were scheduled to undergo cataract surgery alone or combined with glaucoma surgery at the same time. There were 87 patients with NTG, 137 with POAG, and 978 non-glaucomatous control cases. The axial length, IOP, curvature of the anterior corneal surface, age, and gender were determined at the time of the operation. If both eyes had surgery, data from only the right eyes were analyzed. An association of these parameters with NTG and POAG was analyzed by logistic regression analysis. The three groups were analyzed for differences in the axial length using the Kruskal-Wallis test followed by the Mann-Whitney U test.

RESULTS

The axial length was significantly associated with NTG (odds = 1.24, P = 0.002) and POAG (odds = 1.28, P = 0.001). The incidence of either POAG or NTG was significantly higher in patients with axial lengths >or=25.0 mm (odds = 2.29, P < 0.001, Fisher's exact test). The age at the time of cataract surgery was weakly but significantly correlated negatively with the axial length (r = -0.24, P < 0.001, Pearson's correlation coefficient test). Men had significantly longer axial lengths than women.

CONCLUSIONS

Long axial lengths can be considered a risk factor for NTG and POAG, and patients with long axial lengths need to be carefully examined for glaucoma.

Corneal biomechanical metrics in eyes with refraction of -19.00 to +9.00 D in healthy Brazilian patients

PURPOSE

To evaluate corneal biomechanical metrics with tomographic parameters (given by the Oculus Pentacam) and refractive data in a population of healthy Brazilian patients.

METHODS

Observational, cross-sectional study of 150 consecutive patients (53 men and 97 women; 260 eyes). Age, gender, central keratometric readings (central K), central corneal thickness (CCT), anterior chamber depth (ACD), spherical equivalent refraction, corneal hysteresis, and corneal resistance factor (CRF) were assessed and analyzed.

RESULTS

Mean patient age was 46.5+/-21.04 years, average central K was 43.59+/-1.54 diopters (D), CCT was 545.05+/-35.41 microm, ACD was 2.96+/-0.52 mm, spherical equivalent refraction was -1.16+/-3.48 D, corneal hysteresis was 10.17+/-1.82, and CRF was 10.14+/-1.8 (range: 5.45 to 15.1). Mean CRF and corneal hysteresis were distinct among gender: CRF 10.326 in women and 9.810 in men (P=.0266); corneal hysteresis 10.421 in women and 9.727 in men (P=.0031). A negative correlation was found between both CRF and corneal hysteresis with age (r=-0.1255, P=.0434; and r=-0.2445, P=.0001, respectively). No association was found between CRF and average central K (r=0.0633, P=.3086), ACD (r=-0.0474, P=.4498), or spherical equivalent refraction (r=0.1028, P=.1061). Corneal hysteresis was not associated with age and average central K (r=0.0572, P=.3573), ACD (r=0.0060, P=.9236), or spherical equivalent refraction (r=0.0975, P=.1253). Corneal resistance factor and corneal hysteresis were positively associated with CCT (r=0.5760, P=0; and r=0.4655, P=0, respectively).

CONCLUSIONS

Corneal biomechanical metrics of healthy Brazilian patients were associated with CCT, gender, and age. Corneal steepness, ACD, and spherical equivalent refraction did not affect comeal hysteresis and CRF values in the studied population.

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.

Effects of aging on corneal biomechanical properties and their impact on 24-hour measurement of intraocular pressure

PURPOSE

To study the effects of aging on corneal biomechanical properties and their impact on 24-hour measurement of intraocular pressure (IOP).

DESIGN

Experimental study.

METHODS

Fifteen older volunteers with healthy eyes (age range, 50 to 80 years) were housed for one day in a sleep laboratory with a 16-hour diurnal or wake period and an eight-hour nocturnal or sleep period. Every two hours, sitting corneal hysteresis, corneal resistance factor, and IOP were measured. Central corneal thickness (CCT) was measured using an ultrasound pachymeter. Data were compared with previous observations in 15 healthy younger volunteers (age range, 20 to 25 years).

RESULTS

Variations in 24-hour corneal hysteresis and corneal resistance factor were not significant in the older subjects, but there were time-dependent variations in CCT and IOP. The nocturnal CCT was thicker than the diurnal CCT, but the IOP difference between the diurnal and nocturnal periods was not significant. Cosine-fits of CCT and IOP showed synchronized 24-hour rhythms. The phase timing of CCT rhythm appeared significantly earlier than the phase timing of IOP rhythm. Compared with younger subjects, older subjects had a lower mean 24-hour corneal hysteresis and corneal resistance factor, but not a lower CCT. Phase timings of 24-hour rhythms of CCT and IOP were significantly delayed by aging.

CONCLUSIONS

Aging may lower corneal hysteresis and corneal resistance factor, but neither parameter shows a significant 24-hour variation. Aging may not change CCT significantly, but can shift its 24-hour rhythm. The 24-hour IOP pattern in this group of older subjects is not an artifact resulting from a variation in corneal hysteresis, corneal resistance factor, or CCT.

Relationship between central corneal thickness and hypotony-related complications after glaucoma surgery

BACKGROUND AND OBJECTIVE

To examine the hypothesis that patients with increased central corneal thickness may have an overestimation of Goldmann applanation tension and a predisposition to hypotony-related complications.

PATIENTS AND METHODS

A case-control analysis of patients with an intraocular pressure of 7 mm Hg or less on two consecutive postoperative visits following glaucoma filtration or drainage implant surgery was performed. Forward stepwise multiple logistic regression was used to determine the model that best predicted hypotony-related complications defined as choroidal effusion or hypotony maculopathy.

RESULTS

Forty-three eyes (17 with hypotony-related complications and 26 controls) of 43 patients were enrolled. Eyes with pseudophakia (P = .006) and lower postoperative intraocular pressure (P = .013) were significantly more likely to develop hypotony-related complications. Mean central corneal thickness was similar in eyes with hypotony-related complications (519 +/- 32 microm) and controls (525 +/- 37 microm) and was not a significant predictor of hypotony-related complications in the multivariate model (P = .90).

CONCLUSION

Increased central corneal thickness does not represent a risk factor for hypotony-related complications following glaucoma surgery.

Corneal biomechanical properties in primary open angle glaucoma and normal tension glaucoma

PURPOSE

To determine and compare the corneal biomechanical properties between eyes with primary open angle glaucoma (POAG) and eyes with normal tension glaucoma (NTG).

PATIENTS AND METHODS

Prospective cross-sectional study. Consecutive eligible POAG and NTG patients attending the Glaucoma Clinic had assessment of their corneal biomechanical properties-corneal hysteresis (CH) and corneal resistance factor (CRF)-using the Ocular Response Analyzer by an observer masked to the diagnosis. Exclusion criteria included previous intraocular surgery, corneal pathology, inflammatory connective tissue disease, and refraction of 5-dimensional or over. If both eyes were eligible, then the right eye was used for analysis. The main outcome measures were corneal hysteresis and CRF measurements. Data analysis was performed using the t test and general linear model.

RESULTS

Eighty-one patients (80 whites) were analyzed. Forty had NTG, whereas 41 had POAG. Thirty-five were females. There was a statistically significant difference in mean CH (NTG 9.6+/-1.3 mm Hg; POAG 9.0+/-1.4 mm Hg; P=0.01), but not in mean CRF (NTG 9.9+/-1.4; POAG 10.8+/-1.7; P=0.06). The highest recorded Goldmann applanation intraocular pressure (IOP) was statistically significantly associated with lower CH (P=0.01) and higher CRF (P=0.02).

CONCLUSIONS

There was a small but statistically significant difference in the mean CH between POAG and NTG (CH was higher in NTG). The highest recorded Goldmann applanation IOP was also statistically significantly correlated with lower CH and higher CRF, suggesting that alterations to the corneal biomechanical properties may occur as a result of chronic raised IOP in POAG.

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

PURPOSE

To evaluate the association between corneal hysteresis and axial length/refractive error among rural Chinese secondary school children.

DESIGN

Cross-sectional cohort study.

METHODS

Refractive error (cycloplegic auto-refraction with subjective refinement), central corneal thickness (CCT) and axial length (ultrasonic measurement), intraocular pressure (IOP), and corneal hysteresis (Reichert Ocular Response Analyzer) were measured on a rural school-based cohort of children.

RESULTS

Among 1,233 examined children, the mean age was 14.7 +/- 0.8 years and 699 (56.7%) were girls. The mean spherical equivalent (n = 1,232) was -2.2 +/- 1.6 diopters (D), axial length (n = 643) was 23.7 +/- 1.1 mm, corneal hysteresis (n = 1,153) was 10.7 +/- 1.6 mm Hg, IOP (n = 1,153) was 17.0 +/- 3.4 mm Hg, and CCT (n = 1,226) was 553 +/- 33 microns. In linear regression models, longer axial length was significantly (P < .001 for both) associated with lower corneal hysteresis and higher IOP. Hysteresis in this population was significantly (P < .001) lower than has previously been reported for normal White children (n = 42, 12.3 +/- 1.3 mm Hg), when adjusting for age and gender. This difference did not appear to depend on differences in axial length between the populations, as it persists when only Chinese children with normal uncorrected vision are included.

CONCLUSIONS

Prospective studies will be needed to determine if low hysteresis places eyes at risk for axial elongation secondary or if primary elongation results in lower hysteresis.

Correlations between corneal hysteresis, intraocular pressure, and corneal central pachymetry

PURPOSE

To analyze the correlation between corneal hysteresis (CH) measured with the Ocular Response Analyzer (ORA, Reichert) and ultrasonic corneal central thickness (CCT US) and intraocular pressure measured with Goldmann applanation tonometry (IOP GA).

SETTING

Bordeaux 2 University, Ophthalmology Department, Bordeaux, France.

METHODS

This study comprised 498 eyes of 258 patients. Corneal hysteresis, corneal resistance factor (CRF), and IOP corneal-compensated (IOPcc) were provided by the ORA device; CCT US and IOP GA were also measured in each eye. The study population was divided into 5 groups: normal (n = 122), glaucoma (n = 159), keratoconus (n = 88), laser in situ keratomileusis (LASIK) (n = 78), and photorefractive keratectomy (n = 39). The Pearson correlation was used for statistical analysis.

RESULTS

Corneal hysteresis was not strongly correlated with IOP or CCT US. The mean CH in the LASIK (8.87 mm Hg) and keratoconus (8.34 mm Hg) groups was lower than in the glaucoma (9.48 mm Hg) and normal (10.26 mm Hg) groups. The lower the CH, the lower its correlation with IOPcc and IOP GA. A CH higher than the CRF was significantly associated with the keratoconus and post-LASIK groups.

CONCLUSIONS

Corneal hysteresis, a new corneal parameter, had a moderate dependence on IOP and CCT US. Weaker corneas could be screened with ORA parameters, and low CH could be considered a risk factor for underestimation of IOP. The CCT US should continue to be considered a useful parameter.

Technical note: a comparison of central and peripheral intraocular pressure using rebound tonometry

PURPOSE

To compare central and peripheral intraocular pressure (IOP) readings obtained with rebound tonometry.

METHODS

Intraocular pressure was measured on the right eye of 153 patients (65 males, 88 females), aged from 21 to 85 years (mean +/- S.D., 55.5 +/- 15.2 years) with the ICare rebound tonometer at centre, and 2 mm from the limbus (in the nasal and temporal regions along the 0-180 degrees corneal meridian).

RESULTS

Intraocular pressure values obtained with the ICare were 14.9 +/- 2.8; 14.1 +/- 2.5 and 14.5 +/- 2.7 mmHg at centre, nasal and temporal corneal locations, respectively. On average, nasal and temporal IOP readings were 0.75 and 0.37 mmHg lower than the central reading (p < 0.05 and p > 0.05, respectively). A highly significant correlation was found between central and peripheral measurements in nasal (r(2) = 0.905; p < 0.001) and temporal (r(2) = 0.879; p < 0.001) regions along the horizontal meridian. Almost 80% of patients presented nasal IOP values within +/-1 mmHg of the central value.

CONCLUSIONS

Intraocular pressure values measured with the ICare rebound tonometer on the nasal corneal region is slightly lower on average and highly correlated with IOP values recorded at corneal centre. Both nasal and temporal readings are in good agreement with central IOP, and could be used to obtain a reliable estimate of rebound IOP in corneas where central readings cannot be taken.

Lower corneal hysteresis in glaucoma patients with acquired pit of the optic nerve (APON)

OBJECTIVE

Acquired pit-like changes of the optic nerve head (APON) are characteristic of glaucomatous damage and may be a sign of a localized susceptibility of the optic nerve. Thus, it is possible that biomechanical properties of the ocular tissues may play a pressure-independent role in the pathogenesis of glaucoma. Corneal hysteresis (CH) appears to provide information of the biomechanical properties of the ocular hull tissues. The purpose of this study was to compare CH of patients with primary open angle glaucoma (POAG) with and without APON.

METHODS

A prospective case control study was done. POAG patients with and without APON were measured using the Ocular Response Analyzer by masked investigators. Patients in both groups were matched for sex, age, corneal thickness, and type of glaucoma according to maximal IOP (NTG or POAG). Statistical analysis was done using ANOVA.

RESULTS

Corneal hysteresis of 16 glaucomatous eyes with APON and 32 controls (glaucoma without APON) was measured. The mean (+/-SD) CH in the APON group was 8.89 (+/-1.53) and 10.2 (+/-1.05) in the control group. The difference is statistically significant (p=0.005).

CONCLUSIONS

Corneal hysteresis in POAG patients with APON was significantly lower than in patients that did not have such structural changes of the optic disc. These findings may reflect pressure-independent mechanisms involved in the pathogenesis of such glaucomatous optic nerve changes.

Intraocular pressure and central corneal thickness

From the results of the Ocular Hypertension Treatment Study emerged the conclusion that ocular hypertensive subjects with thinner central corneal thickness (CCT) are at increased risk of developing glaucoma. Although possible underlying biases that could have led to this conclusion are still under investigation, there is an increasing interest in the scientific community to understand the potential mechanisms of this increased risk profile. It has been proposed that interindividual differences in CCT might be purely responsible for inaccuracies of the tonometric readings with potential underestimation of the true IOP in subjects with thinner CCT although it is becoming progressively clearer that the true IOP is unpredictable with linear correction formulas for CCT, and it is likely that other material properties of the cornea contribute, together with CCT, to the tonometric artifact. Recently, it has become possible to measure the biomechanical properties of the cornea in vivo and it has been suggested that differences in corneal biomechanics may be the expression of interindividual structural differences of the ocular tissues (including lamina cribrosa), with potential consequences on the interindividual susceptibility to the glaucomatous damage under the same IOP level. A possible underlying biological risk related to thinner CCTs, independent of the influence on tonometric reading, has been proposed and largely studied after the results of the OHTS were published. Besides the understanding of the mechanism underlying the role of CCT as a risk factor for the development of glaucoma, it is important to understand how the information about CCT should be integrated in the clinical management of both ocular hypertension (OHT) and glaucoma and whether other ocular properties should be measured to better understand the individual risk profile.

Predictors of Long-term Progression in the Early Manifest Glaucoma Trial

PURPOSE

To determine progression factors at the end of the Early Manifest Glaucoma Trial (EMGT) based on all EMGT patients and evaluate separately patients with higher and lower baseline intraocular pressure (IOP; median split).

DESIGN

Cohort of clinical trial participants.

PARTICIPANTS

Patients with early open-angle glaucoma randomized to argon laser trabeculoplasty plus betaxolol (n = 129) or no immediate treatment (n = 126), examined every 3 months for up to 11 years.

METHODS

Cox proportional hazard analyses, expressed by hazard ratios (HRs) and 95% confidence intervals (CIs).

MAIN OUTCOME MEASURE

Time to progression, defined by perimetric and photographic disc criteria.

RESULTS

Overall progression was 67% when follow-up ended (median, 8 years). Treatment approximately halved progression risk (HR, 0.53; 95% CI, 0.39-0.72); results were similar for patients with higher and lower baseline IOP (HRs, 0.41 and 0.55). Baseline progression factors (HRs, 1.51-2.12; P<0.01) were higher IOP, exfoliation, bilateral disease, and older age, as previously reported. New baseline predictors were lower ocular systolic perfusion pressure in all patients (< or =160 mmHg; HR, 1.42; 95% CI, 1.04-1.94), cardiovascular disease history (HR, 2.75; 95% CI, 1.44-5.26) in patients with higher baseline IOP, and lower systolic blood pressure (BP) (< or =125 mmHg; HR, 0.46; 95% CI, 0.21-1.02) in patients with lower baseline IOP. Postbaseline progression factors were IOP levels at follow-up, with 12% to 13% average increase per millimeter of mercury in all patients (HRs, 1.12-1.13 per mmHg higher) and similar results in patients with higher and lower baseline IOP (HRs, 1.15 and 1.13 per mmHg higher). Disc hemorrhages (HR, 1.02; 95% CI, 1.01-1.03 per percent higher frequency) also predicted progression. Thinner central corneal thickness (CCT) (HR, 1.25; 95% CI, 1.01-1.55 per 40 microm lower) was a new significant factor, a result observed in patients with higher baseline IOP (HR, 1.42; 95% CI, 1.05-1.92 per 40 microm lower) but not lower baseline IOP, with significant IOP-CCT interaction.

CONCLUSIONS

Treatment and follow-up IOP continued to have a marked influence on progression, regardless of baseline IOP. Other significant factors were age, bilaterality, exfoliation, and disc hemorrhages, as previously determined. Lower systolic perfusion pressure, lower systolic BP, and cardiovascular disease history emerged as new predictors, suggesting a vascular role in glaucoma progression. Another new factor was thinner CCT, with results possibly indicating a preferential CCT effect with higher IOP.

Comparison of IOP Measurements Between ORA and GAT in Normal Chinese

PURPOSE

To compare intraocular pressure (IOP) obtained from the ocular response analyzer (ORA) and Goldmann applanation tonometer (GAT) on a group of normal Chinese.

METHODS

One hundred twenty-five normal subjects were recruited, with one eye randomly selected for this study. Each eye was measured first with the noncontact tonometer ORA, followed by the GAT and ultrasound pachometry, in a randomized order. Four readings were obtained from the ORA, and three measurements were taken with the GAT. The mean was used for analysis. The ORA provided a Goldmann-correlated IOP (IOPg) and a corneal-compensated IOP (IOPcc). Three central corneal thickness (CCT) values were measured using an ultrasound pachometer, and the mean was used for analysis.

RESULTS

IOP obtained from the ORA was similar to that from the GAT (IOPg minus GAT: mean difference = 0.33 mm Hg, 95% limits of agreement = 4.55 to -4.44 mm Hg; IOPcc minus GAT: mean difference = 0.24 mm Hg, 95% limits of agreement = 4.83 to -5.07 mm Hg). CCT was positively associated with corneal hysteresis (CH) (r2 = 0.30, p < 0.01), corneal resistance factor (r2 = 0.38, p < 0.01), GAT (r2 = 0.09, p < 0.01) and IOPg (r2 = 0.16, p < 0.01). IOPcc was not associated with CCT (r2 = 0.01, p = 0.33).

CONCLUSIONS

Both IOPg and IOPcc have good agreement with GAT on normal subjects. The influence of CCT on IOPcc was insignificant.

Changes in corneal hysteresis after clear corneal cataract surgery

PURPOSE

To assess the changes in corneal hysteresis (CH) as measured by the Ocular Response Analyzer (ORA; Reichert Ophthalmic Instruments, Buffalo, New York, USA) to describe the influence of clear corneal cataract surgery on corneal viscoelastic properties and intraocular pressure (IOP) measured by noncontact tonometry (NCT) and Goldmann applanation tonometry (GAT).

DESIGN

Retrospective, interventional, comparative study.

METHODS

One hundred and one eyes of 101 consecutive patients who underwent routine clear corneal cataract surgery were evaluated. CH, NCT, and central corneal thickness (CCT) were measured by ORA before surgery and at postoperative day 1. A control group of 48 pseudophakic eyes (surgery >3 months previously) was included.

RESULTS

CCT increased from 556.82 +/- 32.5 microm before surgery to 580.26 +/- 45.5 microm after surgery (P < .001; control, 555.16 +/- 42.33 microm). Mean CH decreased from 10.35 +/- 2.5 mm Hg before surgery to 9.20 +/- 1.9 mm Hg after surgery (P < .001; control, 10.47 +/- 1.63 mm Hg). NCT values rose from 17.85 +/- 3.8 mm Hg before surgery to 20.10 +/- 6.3 mm Hg after surgery. GAT values were 14.85 +/- 2.8 mm Hg before surgery and 15.24 +/- 4.1 mm Hg after surgery (P = .52). There was no significant difference of CCT or CH between the preoperative values and the values of the control group (CCT, P = .986; CH, P = .166), in contrast to the difference between postoperative values and the values of the control group (CCT, P = .005; CH, P = .031).

CONCLUSIONS

At day 1 after clear corneal cataract surgery, CH is diminished, whereas CCT is increased significantly. Postoperative corneal edema leads to a change of corneal viscoelastic properties, resulting in a lower damping capacity of the cornea. It is supposed that GAT and NCT measurements are significantly different because of postoperative changes in viscoelastic properties of the cornea.

Corneal biomechanical properties in normal, post-laser in situ keratomileusis, and keratoconic eyes

PURPOSE

To compare the biomechanical properties of normal, post-laser in situ keratomileusis (LASIK), and keratoconic corneas evaluated by corneal hysteresis and the corneal resistance factor measured with the Reichert Ocular Response Analyzer (ORA).

SETTINGS

Instituto Oftalmológico de Alicante, Vissum, Alicante, Spain.

METHODS

Two hundred fifty eyes were divided into 3 groups: normal (control group), post-LASIK, and keratoconus. The corneal biomechanical properties were measured with the ORA, which uses a dynamic bidirectional applanation process. The main outcome measures were intraocular pressure, corneal hysteresis, and the corneal resistance factor.

RESULTS

The control group had 165 eyes; the LASIK group, 65 eyes; and the keratoconus group, 21 eyes. In the control group, the mean corneal hysteresis value was 10.8 mm Hg +/- 1.5 (SD) and the mean corneal resistance factor, 11.0 +/- 1.6 mm Hg. The corneal hysteresis value was lower in older eyes, and the difference between the youngest age group (9 to 14 years) and oldest age group (60 to 80 years) was statistically significant (P = .01, t test). One month after LASIK, corneal hysteresis and the corneal resistance factor decreased significantly, from 10.44 to 9.3 mm Hg and from 10.07 to 8.13 mm Hg, respectively. In the keratoconus group, the mean corneal hysteresis was 7.5 +/- 1.2 mm Hg and the mean corneal resistance factor, 6.2 +/- 1.9 mm Hg. There were statistically significant differences in both biomechanical parameters between keratoconic eyes and post-LASIK eyes (P<.001, t test).

CONCLUSIONS

The corneal hysteresis and corneal resistance factor values were significantly lower in keratoconic eyes than in post-LASIK eyes. Future work is needed to determine whether these differences are useful in detecting keratoconus when other diagnostic tests are equivocal.

Der Einfluss von kornealer Hysterese und kornealem Resistenzfaktor auf die Messung des intraokularen Drucks

BACKGROUND

The influence of central corneal thickness (CCT) on the measurement of intraocular pressure (IOP) has been discussed extensively in recent years. The problem, however, has not been solved so far. In addition to CCT there are probably further biomechanical properties that play a role in IOP measurement. We wanted to find out whether these properties are related to Goldmann applanation tonometry (GAT), noncontact tonometry (NCT), or CCT.

MATERIAL AND METHODS

Biomechanical properties of the cornea such as corneal hysteresis (CH) and corneal resistance factor (CRF) can be measured with the Ocular Response Analyzer (ORA, Reichert Ophthalmic Instruments, Depew, NY, USA). Furthermore, a corneal compensated IOP (IOPcc) is given. We examined 156 normal eyes of 80 patients who did not show corneal pathology nor glaucoma. In each eye GAT, NCT, and ORA data as well as CCT were measured. Data were statistically analyzed with respect to agreement and the influence of CH and CRF on IOP measurement.

RESULTS

In our patients the following average values were calculated: GAT 14.8+/-3.0 mmHg, NCT 16.4+/-3.9 mmHg, IOPcc 16.2+/-4.1 mmHg, CH 10.6+/-2.3 mmHg, CRF 10.9+/-2.4 mmHg, and CCT 557+/-36 microm. IOPcc was not related to CCT in normal eyes and the only IOP value related to CH (p<0.01). CRF, however, was related to GAT and NCT values (p<0.01).

DISCUSSION

In our group of normal eyes IOPcc, i.e., the value that is adjusted by measurement of viscoelastic properties of the cornea, in contrast to GAT and NCT does not depend on central corneal thickness. Corneal hysteresis and corneal resistance factor provide further information about biomechanical properties of the cornea beyond central corneal thickness.

Central corneal thickness and corneal hysteresis during corneal swelling induced by contact lens wear with eye closure

PURPOSE

To determine if corneal hysteresis (CH) was associated with increased central corneal thickness (CCT) induced by wearing soft contact lenses during eye closure.

DESIGN

A prospective laboratory investigation.

METHODS

CCT was measured with a modified optical coherence tomography (OCT), and CH was measured with a Reichert Ocular Response Analyzer (ORA) [Reichert Ophthalmic Instruments, Depew, New York, USA]. The ORA also determined values for intraocular pressure (IOP), corneal compensated IOP (IOPcc), and corneal resistance factor (CRF). One randomly selected eye of 20 non-contact lens wearers (four males and 16 females, age 19.7 +/- 1.1 years) was patched during three hours of soft contact lens wear. Measurements were made before lens insertion, immediately upon removal, and every 20 minutes thereafter for 100 minutes.

RESULTS

Immediately after contact lens removal, CCT was increased by 13.1 +/- 2.2% (mean +/- SD) compared with baseline (post hoc, P = .001). After 100 minutes, it remained elevated by 2.4 +/- 1.6% (post hoc, P = .001). However, there were no significant differences of CH at any time after lens wear (analysis of variance [ANOVA], P = .9). Immediately after lens removal, there were significant increases in IOP (post hoc, P = .003) and corneal resistance factor (CRF) (post-hoc, P = .015), but not in IOPcc (post hoc, P = .07). After lens wear, there were significant but weak correlations between the percentage change of CCT (CCT%) and IOP (r = 0.32, P = .001) and IOPcc (r = 0.29, P = .001). However, there was no significant correlation between CCT% and CH (r = 0.07, P = .458).

CONCLUSION

CH as measured by ORA was not associated with corneal swelling induced by soft contact lens wear in this study group.

Corneal hysteresis following descemetorhexis with endokeratoplasty

The corneal biomechanical properties following descemetorhexis with endokeratoplasty (DXEK) and in normal subjects were studied in 100 eyes (12 DXEK and 88 age-matched normal subjects). Corneal hysteresis measurements were significantly lower in DXEK vs normal subjects. DXEK has a direct effect on corneal hysteresis and corneal biomechanical properties of the human cornea.

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.

How Might Corneal Elasticity Help Us Understand Diabetes and Intraocular Pressure?

PURPOSE

To determine how changes in corneal elasticity/stiffness might influence intraocular pressure (IOP) readings in diabetic patients.

METHODS

A systematic review of the literature was performed to understand the conflicting relationship between diabetes, elevated IOP, and glaucoma progression.

RESULTS

Diabetic patients have been found to have statistically significant higher IOP readings in some population-based studies compared to non-diabetics. Insulin resistance states, hyperglycemia and glycosylated hemoglobin have been correlated with higher IOP (1 mmHg) measurements in diabetic patients. In the Ocular Hypertension Treatment Study (OHTS), a self reported history of diabetes was found to be protective against the progression of primary open-angle glaucoma. Small differences in IOP measurements in diabetic patients may be due to corneal stiffening, as demonstrated by the protective effect of glucose-mediated collagen cross-linking against the manifestation and keratometric progression of keratoconus. Different collagen cross-linking agents may induce different degrees of corneal stiffening, which can result in differences in measured IOP.

CONCLUSIONS

Glucose-mediated corneal stiffening due to collagen cross-linking might be responsible for IOP overestimation in diabetic patients. Corneal stiffening might explain why diabetic eyes tend to have higher IOP readings in large population-based studies and why those with ocular hypertension have a reduced risk for glaucoma progression.

Corneal structure and biomechanics: impact on the diagnosis and management of glaucoma

PURPOSE OF REVIEW

Highlights recent studies relating to the impact of corneal structure and biomechanical properties on glaucoma evaluation and management.

RECENT FINDINGS

Central corneal thickness has been shown to play a role in the interpretation of intraocular pressure. Central corneal thickness has also been suggested as a glaucoma risk factor. The potential role of other corneal factors, such as stromal makeup, in the accurate measurement of intraocular pressure and the assessment of glaucoma risk remains to be determined.

SUMMARY

Improved understanding of central corneal thickness and corneal biomechanical properties may someday lead to a better understanding of glaucoma risk and its assessment.