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Keywords:

  • African;
  • central corneal thickness;
  • corneal hysteresis;
  • glaucoma;
  • intraocular pressure;
  • ocular response analyzer

Abstract.

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Materials and Methods
  5. Corneal biomechanics and ORA measurements
  6. Results
  7. Discussion
  8. Conclusion
  9. Conflict of interest
  10. References

Purpose:  To compare corneal hysteresis (CH) and corneal resistance factor (CRF) measured with the Ocular Response Analyzer® tonometer (ORA) between (i) African normals and treated primary open-angle glaucoma (POAG) patients and (ii) between normals and treated POAG Caucasians. To analyse the correlation of CH and CRF with visual field (VF) defects in the two groups.

Methods:  This comparative study included 59 African (29 (POAG), 30 normals) and 55 Caucasians (30 POAG and 25 normals) subjects. Goldmann applanation tonometry (GAT) and ORA measurements were performed in a randomized sequence. Visual field was tested with the Swedish interactive threshold algorithms standard strategy of the Humphrey perimeter. Hoddap classification was used to estimate the severity of VF defects.

Results:  Primary open-angle glaucoma Africans were younger than POAG Caucasians (p < 0.001). Goldmann applanation tonometry and central corneal thickness (CCT) did not differ significantly between the four subgroups. African normals had lower CH than Caucasian controls (p < 0.001). CH was 9.2 ± 1.1 and 8.3 ± 1.7 mmHg respectively in POAG Caucasians and Africans (p < 0.001). African controls had higher ORA corneal-compensated intraocular pressure (IOPcc) than Caucasian controls (p < 0.001). Primary open-angle glaucoma Africans had higher IOPcc values than Caucasian POAGs (p < 0.001). CH and IOPcc were associated with race (p < 0.001) but not with CCT. Based on mean deviation values (MD), POAG Africans had more severe VF defects. CH was correlated with MD (r = 0.442; p = 0.031) and severity of VF defects only in POAG Africans (= −0.464; p = 0.013).

Conclusions:  African normal subjects and POAG patients had an altered CH, which is associated with a significant underestimation of GAT IOP. This may potentially contribute to the earlier development and greater severity of glaucoma damage in Africans compared with Caucasians at diagnosis.


Introduction

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Materials and Methods
  5. Corneal biomechanics and ORA measurements
  6. Results
  7. Discussion
  8. Conclusion
  9. Conflict of interest
  10. References

Considering a large reported heterogeneity within and between African populations, primary open-angle glaucoma (POAG) has been found to have different characteristics in individuals of African descent than in subjects of North American and European descent. In addition to being more prevalent, having an earlier age of onset and being associated with a higher intraocular pressure (IOP), POAG has been found to be more rapidly progressive (thus leading to blindness twice as often as in the Caucasian patient) and to be diagnosed later in Africans compared with Caucasians (AGIS Investigators 2001; Murdoch et al. 2001; Denis 2004; Friedman et al. 2006; Cook 2009). Thinner corneas in Africans compared with Caucasians have also been potentially linked to a higher risk of glaucoma development thus a greater susceptibility to glaucoma damage in this ethnic group (Fansi et al. 2009; Sample et al. 2009).

Both central corneal thickness and viscoelastic properties of the cornea have been shown to influence the IOP measurements obtained by Goldmann applanation tonometry (GAT) (Liu & Roberts 2005; Medeiros & Weinreb 2006).

Corneal hysteresis (CH) is another biomechanical property related to the viscoelastic dampening of the cornea and is currently measured with the Ocular Response Analyzer® (ORA). This parameter had been found to be reduced in glaucomatous eyes compared with healthy eyes (Congdon et al. 2006; Mangouritsas et al. 2009). It has furthermore been suggested that this parameter is a risk factor for glaucoma progression, is independent of corneal thickness and is associated with optic disc surface compliance during acute IOP elevation (Wells et al. 2008).

Differences in CH between Africans and Caucasians could hypothetically reflect differences in susceptibility to the disease in the two ethnic groups. In a recent study, which compared the corneal biomechanical properties in 37 black and 82 white healthy subjects using the ORA, Leite et al. found that black normal subjects tended to have lower measurements of CH compared with whites. However, as this finding was largely explained by differences in corneal thickness, the authors could exclude any independent effect of CH in explaining the differences in glaucoma susceptibility between the two ethnic groups (Leite et al. 2010). No comparison of the corneal biomechanical properties measured with the ORA in African and Caucasian POAG patients has been made so far.

The purpose of this study was to analyse and compare the corneal biomechanical properties in a healthy normal and a treated POAG population of Africans and Caucasians by using the ORA. It was also to investigate the relationship between the corneal biomechanical properties and the other ocular parameters, in particular the severity of the visual field (VF) defects in the two ethnic glaucoma groups.

Materials and Methods

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Materials and Methods
  5. Corneal biomechanics and ORA measurements
  6. Results
  7. Discussion
  8. Conclusion
  9. Conflict of interest
  10. References

Design

This was an observational single-centre comparative clinical study of Caucasian and African normal healthy subjects and medically treated patients with POAG. The participants were recruited from the Glaucoma Unit of the University Department of Ophthalmology, St Luc University Hospital, Brussels, over a 9-month period from June 2009 to March 2010. Normal subjects were recruited from the general consultation and/or accompanying persons. The study was discussed with each patient before his or her examination, and an informed consent was obtained from all participants according to the tenets of the Helsinki Declaration. Institutional review board approval was not required for the study because of the ORA’s widespread international use as a tonometer for a number of years.

Methods

Each subject underwent a detailed ophthalmologic examination including recording of medical history, glaucoma diagnosis, antiglaucoma medications, measurement of best-corrected visual acuity (BCVA), slitlamp biomicroscopy, GAT (Haag-Streit AG, Koning, Switzerland), gonioscopy, dilated fundus examination (66D or 90D lens) with stereoscopic optic disc photographs and standard automated perimetry (SAP) with Humphrey perimeter using the 30-2; 24-2 (and 10-2 algorithms in advanced defects) Swedish interactive threshold algorithms (SITA standard) (IRL; Carl Zeiss Meditec Inc, Dublin, CA, USA). The mean deviation (MD) (dB) and the pattern standard deviation (PSD) (dB) values were analysed in POAG patients. Hoddap classification was used to estimate the severity of the VF defects (Hodapp et al. 1993), and they were subsequently classified into early, moderate and advanced severity.

The eyes of normal controls should have BCVA of at least 0.8, a normal healthy aspect of the optic disc and retinal nerve fibre layer in fundus examination, normal GAT (no history of elevated IOP and open angle at gonioscopy). Moreover, to minimize misdiagnoses between a normal healthy and a glaucomatous optic disc with early damage in Africans, as has been previously suggested by Girkin et al. (2010), African controls should also have automated perimetry with the 24-2 SITA fast algorithm within the normal limits. This criterion was not considered for Caucasian normal subjects.

Patients with POAG were defined as those having open normal-appearing angles, typical glaucomatous optic disc changes and VF defects. After a careful evaluation of the optic disc size at the slit lamp using a three-mirror Goldmann contact lens, diagnosis of glaucomatous optic nerve head damage was based on the following criteria: the presence of a narrowing of the neuroretinal rim, localized and/or diffuse defects in the retinal nerve fibre layer and the possible detection of optic disc haemorrhages. An agreement between two of the authors (MDM and SP) was needed to confirm the existence of glaucoma changes.

All patients with glaucoma had undergone a minimum of two VF tests in the 6 months preceding the ORA measurements. Only reliable examinations were considered, i.e. with loss fixations of ≤ 20%, false positive answers of ≤ 10% and false negative answers of ≤ 10% except for very advanced defects. The diagnosis of an abnormal VF was based on the coexistence of a minimum of two of the following criteria: negative MD, PSD > 2 dB with p value < 1%, ‘abnormal’ and/or ‘borderline’ Glaucoma Hemifield Test, a minimum of three clustered points following the course of the retinal nerve fibre layer with significantly depressed sensitivity and with a significance of p < 1% for at least one of them in the probability maps (European Glaucoma Society, Terminology and Guidelines 2008).

Patients were excluded from this study if they had any ametropia > 6D of spherical equivalent and/or astigmatism ≥ 5D, any type of corneal disease or any history of corneal refractive surgery. Also, if they had had phakoextraction or filtering surgery within the 3 months preceding the ORA measurements and/or if they suffered from secondary causes of high IOP or coexisting intraocular disease other than glaucoma, they were excluded from the study.

Protocol

1. The subjects underwent testing with both the Reichert© (Ophthalmic Instruments, Buffalo, NY, USA) ORA and GAT in a non-masked fashion.

Two consecutive GAT and four consecutive measurements with the software version 2.02 of the ORA were performed in each eye on all subjects in a non-masked randomized sequence during the same visit scheduled between 9 AM and 1 PM. Three GAT readings were performed when the difference between the first two IOP measurements was > 2 mmHg. Mean IOP obtained for each eye was used for the analysis for GAT.

ORA measurements with almost symmetrical height applanation peaks on the ORA waveform and with a wavelength score (WS) value ranging from 3.7 to 6.1 and higher were retained (Vantomme, M. et al. Oral presentation, EVER 2010, abstract 2354). Integrated into the latest version 2.02 of the ORA, the WS reflects the quality of the signals and the reliability of the measurements.

When GAT was measured first, a break of 15 min (Carbonaro et al. 2008) was respected between GAT and ORA measurements to eliminate the possible effects of topical oxybuprocaine and applanation tonometry on the CH value.

2. Central corneal thickness (CCT) was then measured with ultrasonic pachymetry (Quantel Medical Pocket II, Clermont-Ferrand, France).

Corneal biomechanics and ORA measurements

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Materials and Methods
  5. Corneal biomechanics and ORA measurements
  6. Results
  7. Discussion
  8. Conclusion
  9. Conflict of interest
  10. References

The ORA determines the corneal biomechanical properties using an applied force-displacement relationship. The device uses an air pulse to flatten the corneal surface thereby causing the cornea to shift inwards, thus passing from a flat to a concave state and then back to its initial convex shape.

The times required for inwards and outwards flattening are measured by monitoring a 3-mm diameter circular area in the central part of the cornea for approximately 20 ms and the air pressures at the two applanation points are calculated. An infrared electro-optical system monitors this process and records the inwards (P1) and outwards applanation (P2) movements. Corneal hysteresis is simply the mathematical difference between the two applanation pressures P1–P2 and is presumed to be the result of viscoelastic damping in the cornea (Luce 2005; Medeiros & Weinreb 2006; Shah et al. 2007).

Four ORA in- and out-applanation curves are displayed on the screen together with four numerical parameters expressed in mmHg and with the WS in front of each curve (Fig. 1).

image

Figure 1.  Ocular Response Analyzer in- and out-applanation curves with the corresponding values of corneal-compensated IOP, Goldmann-correlated IOP, corneal hysteresis and corneal resistance factor and of the waveform score.

Download figure to PowerPoint

  • 1
     Corneal-compensated IOP (IOPcc) that minimizes corneal influence and is obtained from the difference between the two applanation pressures using the formula kP1 times P2, where the constant k has a value of 0.43, which was derived from clinical studies that have been performed in eyes that had laser treatment in situ for keratomileusis (Luce 2005).
  • 2
     Goldmann-correlated IOP (IOPg) that corresponds to the average of P1 and P2. For simplification purposes, this parameter has not been analysed per se nor discussed.
  • 3
     Corneal hysteresis represents the numerical difference P1–P2 in the two applanation processes and is an indicator of viscoelastic dampening of the cornea.
  • 4
     The corneal resistance factor (CRF) is derived from CH and is P1 – (0.7P2). This formula has been proven to yield the maximum correlation between CCT and ORA measurements and also as having a greater element of elasticity because of its pressure component. Corneal resistance factor corresponds to the elastic resistance to deformation.

Statistical analysis

One eye per subject was selected at random for analysis. In subjects with unilateral blindness, the other eye was considered for analysis. Chi-square test and analysis of variance were used for comparison between groups with LSD test for 2 by 2 comparisons. The Pearson and Spearman coefficient was applied to assess the correlation between the different parameters. The correlation between numerical parameters was assessed by the Pearson coefficient or by the Spearman rank coefficient when an ordinal variable was involved. All statistical tests were two tailed. Multivariate linear regression was used to study the simultaneous influence of various clinical variables on CH and corneal-compensated IOP (IOPcc). A p value of < 0.05 was considered statistically significant. Statistical analyses were performed using spss software version 15.0 (SPSS Inc., Chicago, IL., USA).

Results

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Materials and Methods
  5. Corneal biomechanics and ORA measurements
  6. Results
  7. Discussion
  8. Conclusion
  9. Conflict of interest
  10. References

Fifty-nine African (30 normal controls, 29 POAG,) and 55 Caucasian (25 normal controls, 30 POAG) subjects were recruited. Forty-four of fifty-nine (75%) African subjects were natives of Congo, and the remaining 15/59 (25%) of the African subjects were natives of the Ivory Coast, Burkina Faso, Cameroon and Nigeria.

Primary open-angle glaucoma patients within each group were receiving topical antiglaucoma medications.

Table 1 shows the characteristics of the studied population. There was no significant discrepancy in the gender distribution between the different subgroups. The mean ages of normal control Caucasians and Africans were 58.4 ± 14.7 years and 43.9 ± 11.4 respectively (p < 0.001). Primary open-angle glaucoma Africans were also younger than POAG Caucasians (70.6 ± 9.2 years versus 53.8 ± 12.7 years; p < 0.001). Mean BCVA ± SD was not statistically different between the two ethnic groups; neither for far nor near vision (p > 0.05).

Table 1.   Mean ± SD of clinical and ocular variables for the African and Caucasian normal and POAG subgroups.
ParameterCaucasiansAfricansp-value*
Normal (n = 25 eyes)POAG (n = 30 eyes)Normal (n = 30 eyes) POAG (n = 29 eyes)
  1. CCT, central corneal thickness; GAT, Goldmann applanation tonometry; MD, mean deviation; POAG, primary open-angle glaucoma; PSD, pattern standard deviation; VF, visual field; NS, not significant.

  2. *p-value refers to the calculation of a statistically significant difference between normal, POAG Caucasians, normal, POAG Africans for each demographic considered parameter.

Age (years)58.4 ± 14.770.6 ± 9.243.9 ± 11.453.8 ± 12.7< 0.001
Sex
 Male1219159NS
 Female13111520
BCVA (far vision) 0.87 ± 0.17 0.84 ± 0.26NS
GAT (mmHg)16.8 ± 2.716.4 ± 3.715.8 ± 3.018.0 ± 5.0NS
CCT (μm)554 ± 19544 ± 37537 ± 35537 ± 47NS
Visual field
 MD (dB) −6.4 ± 7.0 −12.2 ± 13.40.044
 PSD (dB) 6.7 ± 4.6 5.7 ± 4.6NS
Severity of VF defects
 Early 18 14NS
 Moderate 4 0
 Severe 8 15

Goldmann applanation tonometry measurements were not significantly different between the different subgroups. Mean CCT was slightly thinner in Africans (537 ± 35 μm in normals and 537 ± 47 μm in POAG patients respectively) compared with Caucasians (554 ± 19 μm in normals and 544 ± 37 μm in POAG patients respectively), but there was no significant difference between the two ethnic groups.

With reference to the MD values, POAG Africans tended to have more severe VF defects (MD = −6.4 ± 7.0 dB in POAG Caucasians versus 12.2 ± 13.4 dB in POAG Africans; p = 0.044), whereas mean PSD values were not statistically different between the two subgroups.

Ocular Response Analyzer measurements are shown in Table 2.

Table 2.   ORA parameters (mean ± SD) and their statistical significance, with the mean waveform score (WS of the ORA measurements in each subgroup.
ParameterCaucasiansAfricansp-value
Normal (n = 25 eyes)POAG (n = 30 eyes)Normal (n = 30 eyes)POAG (n = 29 eyes)
  1. CH, corneal hysteresis; CRF, corneal resistance factor; IOPcc, corneal-compensated IOP; ORA, Ocular Response Analyzer; POAG, primary open-angle glaucoma; WS, wavelength score; IOPg, Goldmann-correlated IOP; NS, not significant.

WS7.0 ± 1.46.4 ± 1.85.5 ± 1.65.3 ± 1.90.002
CH (mmHg)10.8 ± 1.69.2 ± 1.19.2 ± 1.58.3 ± 1.7< 0.001
CRF (mmHg)10.7 ± 1.59.7 ± 2.19.8 ± 2.09.6 ± 2.1NS
IOPcc (mmHg)16.0 ± 3.218.0 ± 4.118.4 ± 3.020.6 ± 5.70.001
IOPg (mmHg)15.6 ± 3.316.4 ± 5.016.7 ± 3.718.4 ± 5.8NS

Mean waveform score (WS ± SD) was significantly lower in POAG Africans than in POAG Caucasians.

Among the four measured parameters with the ORA, only CH and IOPcc were significantly different between the subgroups. Table 3 summarizes the results of 2 by 2 comparison analyses of age, CH and IOPcc between subgroups. Again, normal Africans were significantly younger than normal Caucasians, and POAG Africans were also significantly younger than POAG Caucasians. African controls had significantly lower CH than Caucasian controls (9.2 ± 1.5 versus 10.8 ± 1.6 mmHg) (p<0.001). Primary open-angle glaucoma Africans also had lower mean CH values when compared with Caucasian POAG (8.3 ± 1.7 and 9.2 ± 1.1 mmHg respectively) (p < 0.001).

Table 3.   Two by 2 comparisons between subgroups.
  p-value
  1. CH, corneal hysteresis; IOPcc, corneal-compensated IOP; POAG; primary open-angle glaucoma; WS, wavelength score; NS, not significant.

AGENormal Africans < normal caucasians< 0.001
POAG Africans < POAG Caucasians< 0.001
CHNormal Africans < normal caucasians< 0.001
POAG Africans < POAG caucasians0.033
IOPccNormal Africans > normal caucasians0.041
POAG Africans > POAG caucasians0.018
WSNormal Africans < normal caucasiansNS [p = 0.068]
POAG Africans < POAG caucasians0.002

African controls had significantly higher mean IOPcc than Caucasian controls (18.4 ± 3.0 versus 16.0 ± 3.2 mmHg) (p = 0.041), and when comparing POAG Africans with Caucasian POAGs, they had significantly higher mean IOP cc values (20.6 ± 5.7 and 18.0 ± 4.1 mmHg respectively) (p = 0.018).

A correlation analysis between CH, CRF, IOPcc and other ocular parameters, especially GAT and CCT in Caucasian and African normal subjects, is shown in Table 4. No correlation was seen between CH, CRF, IOPcc and CCT neither in African nor Caucasian healthy controls. CH was not correlated with GAT measurements.

Table 4.   Correlation between corneal hysteresis (CH), corneal resistance factor (CRF) and other ocular parameters in Caucasian and African normal healthy subjects.
 GATCCTIOPccCHCRF
  1. CCT, central corneal thickness; GAT, Goldmann applanation tonometry; IOPcc, corneal-compensated IOP; NS, not significant.

Caucasians (n = 25 eyes)
 CHr = −0.321r = 0.125r = −0.328 r = 0.719
NSNSNS p < 0.001
 CRFr = 0.237r = 0.330r = 0.303  
NSNSNS  
 IOPccr = 0.750r = 0.271   
p = 0.000NS   
Africans (n = 30 eyes)
 CHr = 0.275r = −0.026r = −0.167 r = 0.821
NSNSNS p < 0.001
 CRFr = 0.569r = 0.076r = 0.365  
p = 0.001NSp = 0.047  
 IOPccr = 0.530r = 0.082   
p = 0.003NS   

Table 5 shows that CH, CRF and IOPcc were not correlated with CCT in African POAG patients, whereas it was only CH that was not correlated with CCT in Caucasian POAGs. In the two ethnic POAG groups, there was no correlation between CH and GAT measurements. A significant association was found between CRF and GAT in Africans as well as between CRF and CCT in POAG Caucasians.

Table 5.   Correlation between corneal hysteresis (CH), corneal resistance factor (CRF) and other ocular parameters in Caucasian and African POAG patients.
 GATCCTIOPccCHCRF
  1. CCT, central corneal thickness; GAT, Goldmann applanation tonometry; IOPcc, corneal-compensated IOP; POAG, primary open-angle glaucoma; NS, not significant.

Caucasian POAG (n = 30 eyes)
CHr = −0.271r = 0.282r = −0.082 r = 0.733
NSNSNS p < 0.001
CRFr = 0.772r = 0.619r = 0.734  
p = 0.000p = 0.000p = 0.000  
IOPccr = 0.870r = 0.625   
p = 0.000p = 0.000   
African POAG (n = 29 eyes)
CHr = −0.502r = 0.087r = −0.563 r = 0.441
p = 0.005NSp = 0.001 p = 0.017
CRFr = 0.429r = 0.337r = 0.434  
p = 0.020NSp = 0.019  
IOPccr = 0.901r = 0.272   
p = 0.000NS   

A multivariate regression analysis of the influence of clinical variables on CH and IOPcc that had shown to be significantly different in normals and Africans POAG compared with normals and POAG Caucasians in the previous analyses had thus been constructed. It was found that CH and IOPcc were significantly associated with ethnicity, diagnosis (i.e. normal versus POAG) and GAT measurements (Table 6).

Table 6.   Results of multivariate regression analysis of the influence of clinical variables on CH and IOPcc.
VariableCHIOPcc
  1. CCT, central corneal thickness; CH, corneal hysteresis; GAT, Goldmann applanation tonometry; IOPcc, corneal-compensated IOP; POAG, primary open-angle glaucoma; NS, not significant.

Racep < 0.001p < 0.001
Diagnosis (normal versus POAG)p = 0.009p = 0.017
AgeNSNS
SexNSNS
GATp < 0.001p < 0.001
CCTNSNS
CRFp < 0.001NS

Finally, contrary to POAG Caucasians, only CH was significantly correlated to the MD value (r = 0.442, p = 0.031), and the severity of the VF defects (r = −0.464, p = 0.013) in African POAG patients (Table 7).

Table 7.   Correlation between corneal hysteresis (CH), corneal resistance factor (CRF) and MD, PSD and severity of VF defects in African and Caucasian POAG patients.
 MD (dB)PSD (dB)Severity of VF defects
  1. MD, mean deviation; POAG, primary open-angle glaucoma; PSD, pattern standard deviation; VF, visual field; NS, not significant.

Caucasian POAG (n = 30 eyes)
CHr = 0.325r = −0.066r = −0.091
NSNSNS
CRFr = 0.320r = −0.181r = −0.209
NSNSNS
African POAG (n = 29 eyes)
CHr = 0.442r = −0.145r = −0.464
p = 0.031NSp = 0.013
CRFr = 0.282r = −0.0164r = −0.218
NSNSNS

Discussion

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Materials and Methods
  5. Corneal biomechanics and ORA measurements
  6. Results
  7. Discussion
  8. Conclusion
  9. Conflict of interest
  10. References

In the present sample of African subjects who were mainly natives of Congo, Africans with POAG were younger than Caucasians with POAG and suffered from more severe glaucoma VF loss, which is in accordance with the findings of others (Murdoch et al. 2001; Fansi et al. 2009). Furthermore, African healthy subjects had significantly lower CH than Caucasian controls, and Africans with treated POAG had lower CH values when compared with POAG Caucasians. Worth noting is that the CRF was not significantly different between the two ethnic groups whether they were healthy controls or had glaucoma.

Importantly, GAT and CCT measurements were not significantly different between Africans and Caucasians in our studied sample. Unlike previously published studies that have shown that African people presented with thinner corneas than Caucasians (Sample et al. 2009), our series of individuals of African descent did not have significantly thinner corneas than Caucasians.

We found that African controls had significantly higher ORA IOPcc than Caucasian controls and that POAG Africans also had significantly higher IOPcc values than Caucasian patients with glaucoma did. In both Caucasian and African healthy controls, we did not observe any correlation between CH, CRF, IOPcc and CCT. We also noted that CH was not correlated with GAT measurements in either healthy ethnic group.

In the same way, no correlation was observed between CH, CRF, IOPcc and CCT in African POAG patients. CH was not correlated with GAT in either group whereas the CRF was correlated with this parameter.

Moreover, in multivariate regression analyses dealing with the influence of clinical variables on CH and IOPcc, we found that CH and IOPcc were significantly associated with race, diagnosis (normal versus glaucoma) and GAT, but not with corneal thickness. This may suggest that reduced CH could be an inherited trait in African subjects.

Considering the group of normal healthy African subjects only, our results were in disagreement with those that have been reported recently by Leite et al. (2010) found in normal healthy subjects of African Descent who were living in USA. Leite et al. had found that this population of normal subjects had effectively lower CH and lower CRF values when compared with normal whites but that these differences became insignificant when adjusted for differences in corneal thickness between the two ethnic groups. The authors concluded therefore that at least in normal subjects of African descent, differences in CH between blacks and whites could largely be attributed to differences in corneal thickness measurements between the two ethnic groups, rather than reflecting an additional structural difference between their corneas (Leite et al. 2010).

Although the CRF value is mathematically derived from CH, there was some discrepancy between CH and CRF in our data, suggesting and confirming that each parameter definitely reflects a different corneal biomechanical property.

In contrast to what we observed in POAG Caucasians, we also found that CH was correlated to the MD values and the severity of the VF defects in African patients with glaucoma.

Our findings could suggest that CH would be an independent risk factor for glaucoma in African subjects and could also have a independent effect in explaining differences in susceptibility to glaucoma damage between Africans and Caucasians (Gordon et al. 2002; Congdon et al. 2006). Broman et al. (2007) have found that in Caucasian patients with glaucoma, lower CH measurements were related to VF progression even after adjusting for CCT. It has been previously suggested that an eye with a low CH and a fortiori reduced corneal thickness would be more vulnerable thus having a predisposition to developing glaucoma. Changes in CH could be related to modifications of the corneal architecture; especially of the stroma and the arrangement of the collagen thin strips (Broman et al. 2007). Some authors have also argued that the biomechanical properties of the cornea could be correlated with those of the lamina cribrosa and could be a risk factor for the development of glaucoma (Albon et al. 2000; Kotecha 2007; Ang et al. 2008; Bochmann & Ang 2008). Furthermore, it has been demonstrated recently that CH, not central corneal thickness, is associated with an increased deformation of the optic nerve surface during transient elevations of IOP in patients with glaucoma (Wells et al. 2008).

Most of the described variations in the optic nerve morphologic characteristics between healthy African and Caucasian descent individuals have been attributed to differences in the optic disc area. Moreover, differences in Heidelberg Retinal Tomograph cup depth, optical coherence tomograph (OCT) macular thickness and volume and OCT retinal nerve fibre layer thickness between Africans and Caucasians have recently been reported and are considered as being independent of the optic nerve area (Girkin et al. 2010). Accordingly, it is difficult to differentiate clearly between a normal healthy and a glaucomatous optic disc with early damage in Africans. To reduce the incidence of misdiagnosed eyes in our study, all normal healthy African participants had to have a 24-2 SITA fast SAP within the normal limits whereas the size of their optic discs was carefully evaluated. In spite of this requirement, it should be pointed out that a large percentage of healthy eyes of African descent were revealed to have a significantly worse performance than people of Caucasian descent on all tests of visual function and this needs to be born in mind (Racette et al. 2010).

We also found that POAG Africans had a significantly lower WS than POAG Caucasians during the ORA measurements. This finding and its cause(s) still need to be confirmed and clarified.

Potential effects of topical antiglaucoma medications on intraocular pressure (as well as on the above-mentioned biomechanical properties) are susceptible to complicate the evaluation of the ORA corneal biomechanics. We have recently experienced that CH, unlike CRF, was not altered by topical antiglaucoma medications regardless of the total duration with topical medicines (Detry-Morel et al. 2011). Until further confirmation, we only recruited intentionally the patients with glaucoma who were receiving topical antiglaucoma medications in our current investigation.

Our study has, however, a number of limitations. One such limitation is that the participants recruited for this study belonged to a selected group and as such not necessarily representative for a more general POAG population. Also, given the small sample size of the different subgroups analysed, this study may not be sufficiently scaled to detect statistical differences in corneal thickness measurements between Africans and Caucasians. In addition to the unavoidable bias in the recruitment of participants, our ORA measurements were only performed once. Their potential change over time could therefore not be assessed, and this fact is likely to interfere with the reliability of our findings. Moreover, corneal biomechanical properties were established based only on the ORA and should be confirmed by an additional method of evaluation of these properties together with a larger sample of African normal subjects and patients with glaucoma.

Although we observed a negative correlation between CH and MD values, as well as between CH and severity (i.e. early, moderate and severe) of VF defects in POAG Africans only, further studies are necessary to elucidate the exact relationship between CH and CRF and the well-known susceptibility of African subjects to glaucoma damage.

Conclusion

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Materials and Methods
  5. Corneal biomechanics and ORA measurements
  6. Results
  7. Discussion
  8. Conclusion
  9. Conflict of interest
  10. References

To summarize, the results of this study suggest that African normal subjects as well as African POAG patients could have an altered CH that is associated with a significant underestimation of the IOP measured with GAT. This may potentially contribute to the earlier development and greater severity of glaucoma damage in this ethnic group at diagnosis compared with Caucasians.

References

  1. Top of page
  2. Abstract.
  3. Introduction
  4. Materials and Methods
  5. Corneal biomechanics and ORA measurements
  6. Results
  7. Discussion
  8. Conclusion
  9. Conflict of interest
  10. References
  • AGIS Investigators. (2001): The Advanced Glaucoma Intervention Study (AGIS): 9 comparison of glaucoma outcomes in black and white patients within treatment groups. Am J Ophthalmol 132: 311320.
  • Albon J, Purrslow PP & Karwatowski WSS (2000): Age related compliance of the lamina cribrosa in human eyes. Br J Ophthalmol 84: 318323.
  • Ang GS, Bochmann F, Townend J, Azua Ra-Blanco A, Oochmann F, Townend J & Azua Ra-Blanco A (2008): Corneal biomechanical properties in primary open angle glaucoma and normal tension glaucoma. J Glaucoma 17: 259262.
  • Bochmann F & Ang GS (2008): Azua-Blanco A. Lower corneal hysteresis in glaucoma patients with acquired pit of the optic nerve (APON). Graefes Arch Clin Exp Ophthalmol, 246: 735738.
  • Broman AT, Congdon NG, Bandeen-Roche K & Quigley HA (2007): Influence of corneal structure, corneal responsiveness, and other ocular parameters on tonometric measurement of intraocular pressure. J Glaucoma 16: 581588.
  • Carbonaro F, Andrew T, Mackey DA, Spector TD & Hammond CJ (2008): The heritability of corneal hysteresis and ocular pulse amplitude. A twin study. Ophthalmology 115: 15451549.
  • Congdon NG, Broman AT, Bandeen-Roche K, Grover D & Quigley HA (2006): Central corneal thickness and corneal hysteresis associated with glaucoma damage. Am J Ophthalmol 141: 868875.
  • Cook C (2009): Glaucoma in Africa: size of the problem and possible solutions. J Glaucoma 18: 124128.
  • Denis P (2004): Le glaucome chez le mélanoderme. J Fr Ophtalmol 27: 708712.
  • Detry-Morel M, Jamart J & Pourjavan S (2011): Evaluation of the corneal biomechanical properties with the Reichert Ocular Response Analyzer (ORA). Eur J Ophthalmol 21: 138148.
  • European Glaucoma Society. (2008): Terminology and guidelines for glaucoma, 3rd edn. Savona, Italy: DOGMA, Srl, 1.4:82-88.
  • Fansi AA, Papamatheakis DG & Harasymowycz PJ (2009): Racial variability of glaucoma risk factors between African Caribbean sand Caucasians in a Canadian urban screening population. Can J Ophthalmol 44: 576581.
  • Friedman DS, Jampel HD, Munoz B & West SK (2006): The prevalence of open-angle glaucoma among black and whites 73 years and older: the Salisbury Eye Evaluation Glaucoma Study. Arch Ophthalmol 124: 16251630.
  • Girkin CA, Sample PA, Liebmann JM et al. (2010): African Descent and Glaucoma Evaluation Study (ADAGES): II. Ancestry differences in optic disc, retinal nerve fiber layer, and macular structure on healthy subjects. Arch Ophthalmol 128: 541550.
  • Gordon MO, Beiser JA, Brandt JD, Heuer DK, Higginbotham EJ & Johnson CA (2002): The Ocular Hypertension Treatment Study: baseline factors that predict the onset of primary open-angle glaucoma. Arch Ophthalmol 120: 714720.
  • Hodapp E, Parrish RK & Anderson DR (1993): Clinical decisions in glaucoma. St Louis: CV Mosby Company.
  • Kotecha A (2007): What biomechanical properties of the cornea are relevant for the clinician? Surv Ophthalmol 52: 109114.
  • Leite MT, Alencar LM, Gore C et al. (2010): Comparison of corneal biomechanical properties between healthy blacks and whites using the Ocular response Analyzer. Am J Ophthalmol 150: 163168.
  • Liu J & Roberts CJ (2005): Influence of corneal biomechanical properties on intraocular pressure measurement: quantitative analysis. J Cataract Refract Surg 31: 146155.
  • Luce DA (2005): Determining in vivo corneal biomechanical properties of the cornea with an ocular response analyzer. J Cataract Refract Surg 31: 156162.
  • Mangouritsas G, Morphis G, Mourtzoukos S et al. (2009): Association between corneal hysteresis and central corneal thickness in glaucomatous and non-glaucomatous eyes. Acta Ophthalmol 87: 901905.
  • Medeiros FA & Weinreb RN (2006): Evaluation of the influence of the corneal biomechanical properties on intraocular pressure measurements using the ocular response analyzer. J Glaucoma 15: 364370.
  • Murdoch IE, Cousens SN, Babalola OE, Yang YF, Abiose A & Jones BR (2001): Glaucoma prevalence may not be uniformly high in all « black » populations. Afr J Med Sci 30: 337339.
  • Racette L, Liebmann JM, Girkin CA et al. (2010): African Descent and Glaucoma Evaluation Study (ADAGES): III. Ancestry differences in visual function in healthy eyes. Arch Ophthalmol 128: 551559.
  • Sample PA, Girkin CA, Zangwill LM et al. (2009): The African Descent and Glaucoma Evaluation Study (ADAGES): design and baseline data. Arch Ophthalmol 127: 11361145.
  • Shah S, Laiquzzaman M, Bhojwanjr R, Mantry S & Cunliffe I (2007): Assessment of the biomechanical properties of the cornea with the ocular response analyzer in normal and keratoconic eyes. Invest Ophthalmol Vis Sci 48: 30263031.
  • Wells AP, Garway-Heath DF, Poostchi A et al. (2008): Corneal hysteresis but not corneal thickness correlates with optic nerve surface compliance in glaucoma patients. Invest Ophthalmol Vis Sci 49: 32623268.