|Year : 2013 | Volume
| Issue : 4 | Page : 249-252
Evaluation of corneal biomechanics using ocular response analyzer for normal and primary open angle glaucoma eyes
Mohamed A El-Malah
Department of Ophthalmology, Faculty of Medicine, Al-Azhar University, Cairo, Egypt
|Date of Submission||20-Jul-2013|
|Date of Acceptance||12-Nov-2013|
|Date of Web Publication||28-Apr-2014|
Mohamed A El-Malah
MD, PhD, 252 St. Terat El Gabl, Hadayek El Zayton, Al-Azhar University, Cairo
Source of Support: None, Conflict of Interest: None
The aim of the study was to evaluate the differences in intraocular pressure measurements using the applanation Goldmann method (IOPg), in intraocular pressure after compensation of the cornea (IOPcc), in corneal hysteresis (CH), and in corneal resistance factor (CRF) between healthy individuals and patients diagnosed as primary open angle glaucoma (POAG) using ocular response analyzer (ORA).
Patients and methods
Ninety-four eyes of 94 individuals, 45 male individuals and 49 female individuals, were classified into two equal groups: the POAG (47 eyes) group and the normal (47 eyes) control group. Their mean age was 54 ± 7.1 years and 55 ± 7.8 years, respectively. All measures for the four selected parameters (IOPg, IOPcc, CH, and CRF) were performed using an ORA machine for all eyes. Statistical analysis was made using the Student t-test to search for any significant difference between the two groups.
All parameters including IOPg, IOPcc, CH, and CRF were measured and their mean values were calculated. In normal eyes, the mean of IOPg was 15.8 ± 1.3 mmHg, that of IOPcc was 14.7 ± 0.2 mmHg, that of CH was 11.6 ± 1.2 mmHg, and that of CRF was 12.4 ± 1.8 mmHg, whereas in eyes presented with POAG, the mean of IOPg was 23.6 ± 2.6 mmHg, that of IOPcc was 26.8 ± 2.4 mmHg, that of CH was 8.4 ± 2.3 mmHg, and that of CRF was 9.4 ± 0.6 mmHg; there was a significant difference between the two groups.
In this study, the selected parameters such as IOPg and IOPcc had higher readings and CH and CRF had lower readings in glaucomatous eyes and vice versa in normal eyes. The low readings of intraocular pressure in POAG eyes were most probably because of low readings of CH and CRF. Studies with larger sample size are needed to support the results and more accurately evaluate the ORA machine and its role in early detection of glaucoma patients.
Keywords: Central corneal thickness, corneal hysteresis, corneal resistance factor, ocular response analyzer, primary open angle glaucoma
|How to cite this article:|
El-Malah MA. Evaluation of corneal biomechanics using ocular response analyzer for normal and primary open angle glaucoma eyes. J Egypt Ophthalmol Soc 2013;106:249-52
|How to cite this URL:|
El-Malah MA. Evaluation of corneal biomechanics using ocular response analyzer for normal and primary open angle glaucoma eyes. J Egypt Ophthalmol Soc [serial online] 2013 [cited 2020 Sep 24];106:249-52. Available from: http://www.jeos.eg.net/text.asp?2013/106/4/249/131594
| Introduction|| |
Elevated intraocular pressure (IOP) is one of the most important factors for diagnosis and monitoring of glaucoma in general . IOP is currently the only modifiable risk factor for glaucoma, a disease that is the second leading cause of blindness worldwide ,. As IOP reduction is the mainstay of treatment, accurate IOP assessment is important in monitoring the efficacy of therapy and for assessing the risk of glaucomatous progression ,. In addition, reduction of IOP in eyes with ocular hypertension has been proven to reduce the rate of conversion to glaucoma ,.
Goldmann applanation tonometer, which is the most frequently used instrument to measure IOP, operates on the basis of the Imbert-Fick law. This law assumes that the cornea is an infinitely thin, perfectly flexible membrane; however, this assumption is not true. The force required to applanate the cornea depends not only on IOP, but also on corneal rigidity, thickness, curvature, hydration, and viscoelastic properties [8-11]. Studies by Ehlers et al.  in the 1970s revealed that this variation in central corneal thickness (CCT) had an effect on applanation-measured IOP. Many other studies have documented that there is an increase in measured IOP with increasing CCT, and that all commonly used types of tonometer are affected ,,,.
Recently, ocular response analyzer (ORA) is a new, noninvasive device that analyzes corneal biomechanical properties simply and rapidly, especially corneal hysteresis (CH), corneal resistance factor (CRF), intraocular pressure by Goldmann (IOPg), and intraocular pressure after compensation of the cornea (IOPcc) ,. ORA measures two corneal biomechanical parameters, CH and CRF, and provides a measure of IOP that is corrected for these parameters. CH represents 'viscous damping' in the corneal tissues; in addition, it is a direct measure of the corneal biomechanical properties, and therefore may more completely describe the contribution of corneal resistance to IOP measurements than does CCT alone ,,.
In this study, we compared IOPg, IOPcc, CH, and CRF in normal eyes and eyes affected with primary open angle glaucoma (POAG). The purpose of the study was to evaluate the changes in ORA parameters and its value for glaucoma diagnosis and follow-up.
| Patients and methods|| |
A total of 94 eyes of 94 individuals were included in this study, 45 male individuals (47.9%) and 49 female individuals (52.1%). They were classified into two groups: 47 eyes (50%) of normal individuals (the normal or control group) without any ophthalmic diseases and 47 eyes (50%) of individuals presented with POAG.
The study was conducted in Al-Azhar University Hospitals, Bab-El-Sheriyia Hospital, and a private hospital (International Eye Hospital) Eloyoun Eldawly Hospital) in Cairo, the capital of Egypt. Informed consent was obtained from all participants after explanation of the ORA examination as an easy, simple, noninvasive, and quick procedure.
All patients included in both study groups were above 18 years of age, with exclusion of juvenile glaucoma. The mean age in the POAG group was 54 ± 7.1 years (range 34-73 years) and in the control group was 55 ± 7.8 years (range 30-72 years), with nonstatistically significant difference.
All participants included in both groups underwent a complete eye examination, including a review of medical history, fundus examination, slit-lamp assessment, IOP, and gonioscopy examinations. For the POAG group, visual field examination was added.
Three ORA measurements were taken and unreliable measurements were excluded. Accepted measurements should be with the typical signals and a good wave score not less than 6.5.
Ocular response analyzer
The machine produces a precisely metered air pulse to the cornea to assess the corneal biomechanical properties using ORA (Reichert Ophthalmic Instruments Inc., Depew, New York, USA). CH results from the dynamic nature of the air pulse and viscous damping inherent in the cornea. It was measured as the difference between the inward (P1) and outward (P2) applanation pressures ,,,. CRF is an indicator of the overall resistance of the cornea, which seems to be related to CCT-determined and Goldmann applanation tonometry-determined IOP but not to corneal-compensated IOP  [Figure 1].
A good quality reading of the ORA printout was defined as one with symmetrical height of fore-in (P1, in applanation) and fore-out (P2, out applanation) waveform peaks and a waveform score of at least 6.5 in a software-generated scale of 0-10 ,,. Mostly, it looks like reliability indices in the visual field printout. If the wave score is less than 6.5, the test should be repeated.
For statistical analysis, data were collected using commercial software (Excel 11.1) (http://www.microsoft.com) and the Student t-test was performed to search for any significant difference between the two groups.
| Results|| |
After data analysis, it was found that there was no significant difference among the age of the two groups, as the mean age in the control group was 55 ± 7.8 years (range 30-72 years) and in the POAG group was 54 ± 7.1 years (range 34-73 years). In addition, the mean values of four selected parameters were calculated; there was a significant difference between the two groups as shown in [Table 1].
|Table 1: Demographic data of all eyes in the control and primary open angle glaucoma groups|
Click here to view
As shown in [Table 1], in the control group, the reading of IOPg was 15.8 ± 1.3 mmHg, that of IOPcc was 14.7 ± 0.2 mmHg, that of CH was 11.6 ± 1.2 mmHg, and that of CRF was 12.4 ± 1.8 mmHg. It was noticed that the IOPcc reading was usually lower than the IOPg reading, with good CH and CRF readings.
[Table 1] also shows that, in the POAG group, the reading of IOPg was 23.6 ± 2.6 mmHg, that of IOPcc was 26.8 ± 2.4 mmHg, that of CH was 8.4 ± 2.3 mmHg, and that of CRF was 9.4 ± 0.6 mmHg. In this group, the IOPcc reading was much higher than the IOPg reading, whereas CH and CRF readings were low in comparison with the control group, despite their values being at the lower end of the normal range.
The most important factor that gains by ORA, is CH and CRF which are significantly low in the POAG group (CH, P < 0.051; CRF, P < 0.052) [Figure 2] and [Figure 3].
| Discussion|| |
After data analysis in this prospective study, it was found that CH and CRF readings were significantly lower in POAG eyes in comparison with normal eyes. In comparison with other previous studies, from pathological view and value of CH as a risk factor in glaucoma, Gordon et al.  concluded that the lower values of CH might be associated with a progressive visual field change and worsening of the glaucoma disease.
Abitol and colleagues, in a retrospective study, evaluated CH in 133 eyes, 75 healthy individuals and 58 patients, and concluded that CH reading in glaucomatous eyes was 8.77 ± 1.4 mmHg and in healthy eyes was 10.46 ± 1.6 mmHg. In this study, 94 eyes were evaluated and CH reading was found to be 8.4 ± 1.7 mmHg in POAG eyes and 11.6 ± 1.2 mmHg in the eyes of the healthy control group. The results were similar, with low CH values in glaucomatous eyes in both studies compared with that in normal eyes (normal value of CH=8.5-12.5 mmHg) .
The CH reading was also found to be lower in glaucomatous eyes than in normal eyes as documented by Martinez-de-la-Casa et al. . CH represents a dynamic resistance component of the cornea. More elastic or distensible ocular structures may be associated with the progression of glaucoma lesions and according to this hypothesis, the biomechanical status of the cornea, if low, may reflect weakness of the lamina cribrosa ,,,,.
With respect to CRF, Martinez-de-la-Casa and colleagues found in their study that the CRF value was 8.8 ± 2.1 mmHg in glaucomatous eyes and 11.5 ± 2.1 mmHg in healthy eyes. In this study, the CRF value was 9.4 ± 2.2 mmHg in POAG eyes and 12.4 ± 1.8 mmHg in normal eyes. The results are nearly similar, with low CRF value in glaucomatous eyes and high CRF value in normal eyes (normal values of CRF=8.5-12.5 mmHg) .
With respect to IOP, Moreno-Montances and colleagues found that the IOPg value was higher than the IOPcc value by at least 1.5 mmHg in normal eyes compared with the present study in which the IOPg value was higher than the IOPcc value by 1.1 mmHg. In glaucomatous eyes, they found that IOPg value was higher than the IOPcc value by at least 3 mmHg compared with the present study in which the IOPg value was higher than IOPcc value by 3.2 mmHg. Hence, both studies showed higher value of IOPcc than IOPg in glaucomatous eyes in a range of 3 mmHg compared with normal eyes in a range of 1.5 mmHg only . Anand et al.  also reported in their study that IOPg value is little higher than IOPcc value in normal healthy eyes in comparison with glaucomatous eyes in which IOPcc value is higher than IOPg value in a range of 3 mmHg as documented in this study.
| Conclusion|| |
IOP measurements were significantly different between the two groups of patients, the normal and POAG groups, especially the IOPcc. CH and CRF measurements were considered to be highly specific corneal factors for progression of glaucoma from the beginning at the time of diagnosis, and even for follow-up. These factors - IOPg, IOPcc, CH, and CRF - are nowadays more critical especially with the spread of refractive surgery, specifically LASIK. Other studies with large sample size and with inclusion of CCT to previous four assisted factors are needed for better evaluation and documentation.
| Acknowledgements|| |
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3]