• Users Online: 83
  • Print this page
  • Email this page


 
 Table of Contents  
ORIGINAL ARTICLE
Year : 2021  |  Volume : 33  |  Issue : 2  |  Page : 118-123

Tonometry by ocular response analyzer in keratoconic and warpage eyes in comparison with normal eyes


1 Eye Research Center, Department of Ophthalmology, Farabi Eye Hospital, Tehran University of Medical Sciences, Tehran, Iran
2 Eye Research Center, Department of Ophthalmology, Farabi Eye Hospital, Tehran University of Medical Sciences, Tehran; Department of Ophthalmology, Tabriz University of Medical Sciences, Nikookari Eye Hospital, Tabriz, Iran
3 Department of Health Services Management, School of Health Management and Information Sciences, Iran University of Medical Sciences, Tehran, Iran

Date of Submission12-Apr-2020
Date of Decision17-Oct-2020
Date of Acceptance06-Feb-2021
Date of Web Publication05-Jul-2021

Correspondence Address:
Narges Hassanpoor
Eye Research Center, Farabi Eye Hospital, Qazvin Square, Tehran
Iran
Login to access the Email id

Source of Support: None, Conflict of Interest: None


DOI: 10.4103/JOCO.JOCO_147_20

Rights and Permissions
  Abstract 


Purpose: To compare intraocular pressure (IOP) values measured by ocular response analyzer (ORA) in contact lens-induced corneal warpage, normal, and keratoconic eyes.
Methods: In a prospective, observational case–control study, 94 eyes of 47 warpage-suspected cases and 46 eyes of 23 keratoconic patients were enrolled. Warpage-suspected cases were followed until a definite diagnosis was made (warpage, nonwarpage normal, or keratoconus). ORA tonometry and corneal biomechanics testing were performed for all cases in each visit. We had 2–3 measured corneal-compensated IOP (IOPcc) and Goldmann-correlated IOP (IOPg) for each patient (based on group) with at least 2-week interval.
Results: After following up of warpage-suspected patients, finally 44 eyes of 22 patients had confirmed soft contact lens-related corneal warpage. Forty-six eyes of 23 people were finally diagnosed as nonwarpage normal eyes. Forty-six eyes of 23 known keratoconus patients were also included for comparison. The demographic and refractive data were not different between the warpage and nonwarpage normal groups but were different in the keratoconus group. Both IOPcc and IOPg were statistically different with the highest value in the warpage group followed by normal and keratoconus groups; the same trend was observed in central corneal thickness (CCT). The mean of IOPg was 14.94 ± 2.65, 13.7 ± 2.33, and 10.86 ± 3 and IOPcc was 15.73 ± 2.4, 15.28 ± 2.43, and 14.08 ± 2.55 in the warpage, normal, and keratoconus groups, respectively. IOPg and IOPcc in the warpage group (based on baseline diagnosis) did not regress to become closer to IOP of normal eyes after discontinuation of contact lens in their follow-up visits (P value for IOPg and IOPcc trends in the warpage group was 0.07 and 0.09 controlling for CCT, respectively). Both IOPcc and IOPg were significantly lower in keratoconic eyes in comparison with normal eyes. After correction for the confounding effect of CCT, a lower IOPcc in keratoconus versus warpage remained significant (P = 0.02).
Conclusion: Both IOPcc and IOPg were statistically different with the highest value in the warpage group followed by normal and keratoconus groups, just like their CCT. After correction for the confounding effect of CCT, there was no statistically significant difference between the three groups in their measured IOPcc and IOPg except for IOPcc in keratoconus versus warpage (P = 0.02).

Keywords: Central corneal thickness, Contact lens, Goldmann applanation tonometry, Intraocular pressure, Keratoconus


How to cite this article:
Alipour F, Hassanpoor N, Letafatnejad M, Beheshtnejad AH, Mohammadi SF. Tonometry by ocular response analyzer in keratoconic and warpage eyes in comparison with normal eyes. J Curr Ophthalmol 2021;33:118-23

How to cite this URL:
Alipour F, Hassanpoor N, Letafatnejad M, Beheshtnejad AH, Mohammadi SF. Tonometry by ocular response analyzer in keratoconic and warpage eyes in comparison with normal eyes. J Curr Ophthalmol [serial online] 2021 [cited 2021 Oct 18];33:118-23. Available from: http://www.jcurrophthalmol.org/text.asp?2021/33/2/118/320693




  Introduction Top


Contact lens wear prevalence in different surveys has shown to be from 2% to 9% in different populations, and it seems to be increasing for both cosmetic and correcting purposes. Significant contact lens-induced corneal warpage incidence is estimated to be up to 12%, but it highly depends on contact lens material and design.[1],[2],[3] On the other hand, it has been shown that corneal biomechanics can be affected by corneal warpage.[4]

Corneal warpage is suspected by its typical corneal topography appearance which is bilateral and symmetrical superior flattening and dramatic inferior steepening in the absence of keratoconus clinical features such as characteristic “scissor reflex” and corneal slit-lamp changes. Corneal warpage diagnosis is confirmed after resolution of topographic abnormalities by discontinuation of contact lens wearing.[3],[4]

We previously suggested that specific corneal biomechanical indices can help predict the development of corneal warpage.[4] Considering the high prevalence of contact lens use among the general population, especially refractive surgery candidates and highly reported corneal biomechanical changes due to corneal warpage in this population, it seems necessary to work on interpretation of various methods of measuring corneal biomechanics in these patients. It is also important to know the potential limitations of intraocular pressure (IOP) measurement by ocular response analyzer (ORA) in these patients, especially due to higher prevalence of glaucoma in myopic patients who are the majority of soft contact lens wearers.

We worked previously on corneal hysteresis (CH) and corneal resistance factor (CRF) changes in contact lens-induced corneal warpage.[4] In the present report, we aimed to investigate changes of other corneal biomechanical indices measured by ORA in warpage syndrome: corneal-compensated IOP (IOPcc) and Goldmann-correlated IOP (IOPg). We also aimed to investigate the potential biases of measuring IOP by ORA in corneal warpage compared to normal eyes and IOP measurement trends after resolving warpage.

ORA was introduced in 2004 by Reichert Ophthalmic Instruments, Inc. (Depew, NY, USA). In addition to central corneal thickness (CCT) and corneal biomechanical indices such as CH and CRF, this instrument can also estimate IOP.[5],[6] Refractive surgeons have used the ORA to get information about cornea's viscoelastic properties. They have used CH and CRF to diagnose corneal degenerative diseases such as keratoconus, contact lens-induced corneal warpage, and Fuchs' corneal dystrophy.[7],[8],[9] Some clinicians also have used them to predict the risk of postrefractive surgery ectasia or to describe postpenetrating keratoplasty corneal changes.[10],[11] Furthermore, recent studies have shown that ORA can have an important role in increasing our knowledge in glaucoma pathophysiology.[12],[13],[14],[15],[16] Wells et al. have shown that in glaucoma patients, lower CH (but not CRF) has been associated with higher optic nerve head surface deformation during artificial IOP spikes. These phenomena have not been shown in control normal eyes.[17] It also has been shown that glaucoma patients with acquired optic nerve head pit have lower CH when compared to other primary open-angle glaucoma patients. CH, which is measured by ORA, could be a sign of susceptibility of the whole eye to deformative changes.[18]

ORA yielded higher IOP measurements in comparison with Goldmann applanation tonometry (GAT), which is known as the gold standard for IOP measurement.[16],[19] ORA also provides two IOP values: IOPcc and IOPg. IOPg is an estimation of Goldmann tonometry and is calculated by the average of the inward (P1) and outward (P2) applanation pressures. It has been found that the second applanation (outward) occurs at a lower pressure than the first applanation (inward). IOPcc correlates with both IOP and corneal biomechanical features.[19] To the best of our knowledge, there are few studies to evaluate ORA-based IOP values in prediction of corneal warpage and keratoconus.

Our first aim in this study was to see if IOPcc and IOPg (like other corneal biomechanical indices: CH and CRF) could be used to differentiate corneal warpage from keratoconus or not,[4] though understanding the potential error in IOP measurement by ORA induced by warpage is also interesting.

Clinical utility of ORA in glaucoma management is limited by lack of quality data.[4],[5],[6],[7],[8],[9],[10],[11],[12] This transient period has been passed with every new technology such as automated perimetry, optic nerve head OCT imaging, and even GAT. We need studies that help us figure out the ability of ORA in IOP measurement at different clinical situations. In this study, we also tried to compare ORA-measured IOP values in keratoconus and soft contact lens-induced corneal warpage in comparison with normal control eyes. We thought that there might be two hypothetical scenarios for measured IOP change in the warpage group. First, the IOP might be really affected – due to potential effects of long-term contact lens wear such as hypoxia. It has been shown that hypoxia can cause lower IOPs due to decreased oxygenation of ciliary body epithelium. Even lower systemic oxygen saturation levels were associated with lower IOPs as altitude increases in a study by Xie et al.[13],[14] Second, we were just suspicious about the measurement reliability of ORA-based IOP due to corneal biomechanical changes in these patients. A combination of these two hypotheses can also be considered.


  Methods Top


The protocol of the study was approved by the Ethics Committee of Tehran University of Medical Sciences and adhered to the tenets of the Declaration of Helsinki, and informed consent was obtained from all participants. This study has been done in the Refractive Surgery Clinic of Farabi Eye Hospital. Ninety-four eyes of 47 people who were suspected of soft contact lens-related corneal warpage based on their corneal topographic pattern were enrolled in the study in a case group. Forty-six eyes of 23 known keratoconic patients who had never worn contact lenses were included in the control group of the study. Control patients were selected from the Cornea Clinic of Farabi Eye Hospital.

Patients with any history of corneal scarring, corneal diseases, previous corneal surgery, diabetes, and connective tissue disorders were excluded from the study. Our exclusion and inclusion criteria, instrument details, and method of measurement were explained in detail in our previous published work with the same data.[4]

We measured all three groups at baseline and after 2–4 weeks. In the corneal warpage suspect group, patients were asked not to wear their contact lenses for 2–4 weeks and come back to the clinic for reexamination. Refraction, corneal imaging (Orbscan II or Pentacam), and ORA measurements (CH, CRF, IOPcc, and IOPg) were repeated. At this stage, based on the changes in the topographic pattern, the patients were diagnosed as follows: (1) confirmed contact lens-induced corneal warpage – complete resolution, (2) possible contact lens-induced corneal warpage – incomplete resolution, (3) keratoconus, and (4) normal patients with stable irregular topographic pattern.

The criteria for stabilization were defined as (1) manifest refraction changes within 0.50 diopter (D) and (2) keratometry changes within 0.50 D.[4]

Those with suspicious incomplete warpage resolution were requested to wait for 2–4 more weeks, and all the previous examinations were repeated at this follow-up visit. In this stage, patients with normal topography were also defined as confirmed warpage. To reduce variability due to diurnal variations in the IOP, corneal thickness, and CH, all evaluations were performed between 11:00 and 14:00 p.m. At least three acceptable ORA measurements were recorded for each patient, and the mean value was considered for data entry. We had at least 2 measured IOPcc and IOPg for each patient with at least 2-week interval. In the suspected corneal warpage group, some patients had three measurements.

The final categories were based on the consensus of three cornea subspecialists (A.H.B., F.A., and S.F.M.), reviewing all the records as: (1) confirmed soft contact lens-induced corneal warpage, (2) nonwarpage normal, and (3) keratoconus.

Descriptive statistics were used to evaluate the distribution of the data. The normality of the data was tested with Kolmogorov–Smirnov/Shapiro–Wilk test. We used the paired t-test to assess the changes within the groups (corneal warpage). As for the difference in the baseline pachymetry, analysis of covariance with adjustment for corneal thickness was used to evaluate the difference between the groups. Considering the possible correlation of the results in two eyes, we applied the generalized estimating equation (GEE) analysis. P < 0.05 was considered statistically significant. All statistical analyses were performed with SPSS software (IBM SPSS Statistics for Windows, Version 22.0., Armonk, NY, USA: IBM Corp.).


  Results Top


As it is shown in the study design algorithm of our previous published work,[4] in the case group after follow-up of patients, 44 eyes of 22 patients were found to have confirmed contact lens-related corneal warpage based on three anterior segment expert clinicians' opinion. Forty-six eyes of 23 patients were diagnosed as nonwarpage normal eyes, and two other patients had warpage in only one eye and a nonwarpage normal stable pattern in the other eye. Forty-six eyes of 23 known keratoconus patients were included for comparison as the control group.

The demographic and ophthalmology examination data are shown in [Table 1]. Case and keratoconic control eyes were statistically different in age, sex, astigmatism (P < 0.001), and CCT (P = 0.004). Age, CCT, and female-to-male ratio was significantly lower in keratoconic patients in comparison with normal and confirmed warpage group patients, but there was no significant difference between the normal and warpage groups in age, sex, and CCT according to the post hoc analysis. P values are adjusted for age to avoid bias. Myopic astigmatism average was significantly higher in keratoconic eyes: −3.42 ± 3.14 (mean ± standard deviation [SD]) versus −0.63 and −0.5 in the normal and confirmed warpage groups, respectively. However, there was no statistically significant difference in spherical refractive error, spherical equivalent, and best corrected visual acuity between keratoconic control eyes and normal and confirmed corneal warpage groups (P = 0.521, 0.847, and 0.775, respectively). The mean values are shown in [Table 1].
Table 1: Demographic and ophthalmology examination data based on final diagnosis

Click here to view


Baseline and final IOP values by ORA are shown in [Table 2] and [Table 3], respectively. The Bonferroni method was used for multiple comparisons. The significant values are bolded in [Table 2] and [Table 3].
Table 2: Intraocular pressure values by ocular response analyzer measurements according to the baseline diagnosis

Click here to view
Table 3: Intraocular pressure values by ocular response analyzer measurements according to the final diagnosis

Click here to view


The mean IOPcc and IOPg in the warpage suspect group were 15.66 ± 2.44 (mean ± SD) and 14.39 ± 2.54 (mean ± SD), respectively. The mean IOPcc and IOPg in the keratoconus group were 14.08 ± 2.55 (mean ± SD) and 10.86 ± 3 (mean ± SD), respectively. Both IOPcc and IOPg were significantly lower in the keratoconic group than the warpage suspect group at baseline (P = 0.006 and P < 0.001, respectively) [Table 2].

As it is shown in [Table 3], both IOPcc and IOPg were statistically different, with the highest value in the confirmed warpage group followed by normal and keratoconus groups, just like their CCT. IOPg was 14.94 ± 2.65, 13.7 ± 2.33, and 10.86 ± 3 and IOPcc was 15.73 ± 2.4, 15.28 ± 2.43, and 14.08 ± 2.55 in the confirmed warpage, normal, and keratoconus groups, respectively. Therefore, we decided to control CCT effect on IOPcc and IOPg measurements [Table 4]. After CCT control, there was no statistically significant difference between the three groups in their measured IOPcc and IOPg except for IOPcc in keratoconus versus confirmed warpage (P = 0.02).
Table 4: Intraocular pressure P values by ocular response analyzer measurements according to the final diagnosis adjusted for central corneal thickness

Click here to view


We also tried to check IOPcc and IOPg trends in warpage diagnosed eyes to see if there is any statistically significant change by improving warpage and corneal deformities after discontinuing contact lenses. We had at least 2 measured IOPcc and IOPg for each patient with at least 2-week interval. In the corneal warpage suspect group, some patients had three measurements. As shown in [Table 5], there was no statistically significant trend in IOP by improving warpage. IOPg and IOPcc in the warpage suspect group (based on baseline diagnosis) did not regress to become closer to IOP of normal eyes in their follow-up visits (P value for IOPg and IOPcc trends in the warpage group was 0.07 and 0.09 with CCT control, respectively).
Table 5: Intraocular pressure trends in the warpage group based on final diagnosis

Click here to view



  Discussion Top


Analysis of the demographic data showed no significant differences in age, sex, and refractive error (sphere, spherical equivalent, and astigmatism) between patients who suffered from warpage and normal participants without warpage. However, the participants in the keratoconus group were younger with a higher male-to-female ratio, thinner corneas, and higher myopic astigmatism [Table 1].

Although this finding could be predicted because of our selection method which was to include documented keratoconus patients who never used contact lenses in our control group, it might have resulted in some biases in our findings.[20],[21],[22] For example, it has been shown that corneal stiffness parameters such as CCT and age can affect ORA measurements. These two parameters were different in our case and control groups that might be a source of bias.[23] To avoid this bias, we tried to control CCT and age effect on IOPcc and IOPg measurements. Controlling age effect did not make any change; however, there was no statistically significant difference between the three groups in their measured IOPcc and IOPg after controlling for CCT, except for IOPcc in keratoconus versus confirmed warpage.

In our study, both IOPcc and IOPg in keratoconic eyes were lower than the normal and confirmed warpage groups. Even with GAT, measured IOP in keratoconic eyes is lower than normal.[24] This might be due to lower CCT and CH in keratoconic patients.

Özcura et al. have shown that dynamic contour tonometry seems to be the most accurate instrument for IOP measurements in keratoconus because it is least affected by CCT and corneal radius of curvature.[25] In this study, we can tell inferentially that ORA-measured IOPs are affected by CCT. The lower the CCT, the lower the ORA-measured IOP values (correlation coefficient of IOPg in the keratoconus group was 0.87, P = 0.02). This may be a red flag that shows us ORA cannot be the instrument of choice for measuring IOP in keratoconus; however, this hypothesis should be evaluated by other studies and could not be confirmed by the methodology of this study.

Finally, we think that manometric studies and long-term longitudinal prospective studies are needed to show ORA-measured IOP correlation with glaucoma development to optimize the clinical utility of ORA in glaucoma management.

Our other aim in this study was to see if IOPcc and IOPg could be clinically useful modalities to detect corneal warpage. Are they useful to differentiate warpage from keratoconus before refractive surgery? Based on our study, IOPg could be a potential factor that can help us differentiate them (P < 0.001). However, IOPcc did not show this capability (P = 0.060). Clinicians may also be able to use this difference in IOPg as a factor added to other corneal biomechanical features for predicting the development of corneal warpage in those who wish to use contact lenses. Further longitudinal and prospective research studies in this regard are needed.

Interestingly, despite the fact that our first theory was “those who develop warpage are possibly more similar to keratoconus patients”, we found that participants with documented corneal warpage were more similar to the normal (nonwarpage) group than the keratoconus group in IOPcc and IOPg [Table 3].

The idea of using IOPg for differentiating contact lens-induced corneal warpage from keratoconus is a novel idea. Although we have shown that patients with corneal warpage have higher IOP, the reverse is not necessarily true, and future longitudinal studies with different designs are needed to investigate their relationship. More importantly, in this study, we showed the potential false measurement of ORA-based IOP in warpage (possible overestimation of IOP) and keratoconic eyes (possible underestimation of IOP) that could be an important applicable point in a clinic; however, it needs to be proven in future longitudinal studies.

On the other hand, as shown in [Table 5], with improvement of corneal warpage, IOP measurements did not change significantly. As mentioned previously, we suggested two different potential mechanisms that ORA-based measured IOP could be affected in corneal warpage patients. First, the IOP might be truly affected due to potential effects of long-term contact lens wear such as hypoxia. Second, the measurement validity of ORA-based IOP due to corneal “biomechanic” changes in these patients might be under question. The fact that we did not see returning to normal group IOP values after contact lens discontinuation might be either due to primary difference in the biomechanics properties of those participants who are susceptible to warpage or due to “permanent” effects of the contact lens on the cornea. These theories should be further evaluated in future studies.

In this study, we did not have one normal group with no history of the contact lens use at the beginning of the study. We also did not compare our measured IOPs with GAT, which is the gold standard of IOP measurement. This study did not include rigid gas permeable (RGP) contact lenses.

Further studies also are required on the corneal structure and histology of these three groups, including one normal group with no history of contact lens use and/or another group with keratoconus diagnosis and contact lens wearing history, to prove our findings and find biological explanations for these differences. We also recommend similar studies for RGP and/or scleral contact lenses.

The other limitation of the study was a short follow-up period of 4–6 weeks in the corneal warpage group to assess IOP trends in them. We had borderline P values for IOP trend (P value for IOPg and IOPcc trends in the warpage group was 0.07 and 0.09, respectively). With a longer follow-up period, these P values could be significant.

ORA measures the corneal biomechanics and IOP in the central cornea while most striking warpage changes occur in eccentric locations. This can reduce the chance to detect the full magnitude of changes.

Acknowledgments

This study was supported by a grant from the Tehran University of Medical Sciences.

The authors acknowledge the help of Elham Ashrafi, PhD, and Mehdi Yaseri, PhD, in analysis of the data.

The authors appreciate the help and support of LASIK clinic staff of Farabi Eye Hospital for their help and support.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

1.
Omar M, Jassim FA, Sheikh F, Abbas AY, Munshi A. Prevalence of contact lens use and its complications among students of Gulf Medical University, Ajman. GMJ, ASM 2013;2:S92-8.  Back to cited text no. 1
    
2.
Lee YC, Lim CW, Saw SM, Koh D. The prevalence and pattern of contact lens use in a Singapore community. CLAO J 2000;26:21-5.  Back to cited text no. 2
    
3.
Schornack M. Hydrogel contact lens-induced corneal warpage. Cont Lens Anterior Eye 2003;26:153-9.  Back to cited text no. 3
    
4.
Alipour F, Letafatnejad M, Beheshtnejad AH, Mohammadi SF, Ghaffary SR, Hassanpoor N, et al. Corneal biomechanical findings in contact lens induced corneal warpage. J Ophthalmol 2016;2016:5603763.  Back to cited text no. 4
    
5.
Luce DA. Determining in vivo biomechanical properties of the cornea with an ocular response analyzer. J Cataract Refract Surg 2005;31:156-62.  Back to cited text no. 5
    
6.
Pepose JS, Feigenbaum SK, Qazi MA, Sanderson JP, Roberts CJ. Changes in corneal biomechanics and intraocular pressure following LASIK using static, dynamic, and noncontact tonometry. Am J Ophthalmol 2007;143:39-47.  Back to cited text no. 6
    
7.
Kerautret J, Colin J, Touboul D, Robrerts C. Biomechanical characteristics of the ectatic cornea. J Cataract Refract Surg 2008;34:510-3.  Back to cited text no. 7
    
8.
Kotecha A, Elsheikh A, Roberts CR, Zhu H, Garway-Heath DF. Corneal thickness and age-related biomechanical properties of the cornea measured with the ocular response analyzer. Invest Ophthalmol Vis Sci 2006;47:5337-47.  Back to cited text no. 8
    
9.
Pflugfelder SC, Liu Z, Feuer W, Verm A. Corneal thickness indices discriminate between keratoconus and contact lens-induced corneal thinning. Ophthalmology 2002;109:2336-41.  Back to cited text no. 9
    
10.
Ryan DS, Coe CD, Howard RS, Edwards JD, Bower KS. Corneal biomechanics following Epi-LASIK. J Refract Surg 2011;27:458-64.  Back to cited text no. 10
    
11.
Yenerel NM, Kucumen RB, Gorgun E. Changes in corneal biomechanics in patients with keratoconus after penetrating keratoplasty. Cornea 2010;29:1247-51.  Back to cited text no. 11
    
12.
Congdon NG, Broman AT, Bandeen-Roche K, Grover D, Quigley HA. Central corneal thickness and corneal hysteresis associated with glaucoma damage. Am J Ophthalmol 2006;141:868-75.  Back to cited text no. 12
    
13.
Wang J, Chagnon M, Bayoumi A, Wajszilber M, Descovich D, Lesk MR. Correlation between glaucoma damage, ocular biomechanical properties, and vasospasm. Fort Lauderdale, FL: Poster presented at: The Annual Meeting of the Association for Research in Vision and Ophthalmology; 2008.  Back to cited text no. 13
    
14.
Xie Y, Sun YX, Han Y, Yang DY, Yang YQ, Cao K, et al. Longitudinal observation of intraocular pressure variations with acute altitude changes. World J Clin Cases 2019;7:3226-36.  Back to cited text no. 14
    
15.
Ang GS, Bochmann F, Townend J, Azuara-Blanco A. Corneal biomechanical properties in primary open angle glaucoma and normal tension glaucoma. J Glaucoma 2008;17:259-62.  Back to cited text no. 15
    
16.
Sullivan-Mee M, Billingsley SC, Patel AD, Halverson KD, Alldredge BR, Qualls C. Ocular Response Analyzer in subjects with and without glaucoma. Optom Vis Sci 2008;85:463-70.  Back to cited text no. 16
    
17.
Wells AP, Garway-Heath DF, Poostchi A, Wong T, Chan KC, Sachdev N. Corneal biomechanics and optic nerve head compliance: Corneal hysteresis but not corneal thickness correlates with optic nerve surface deformation in glaucoma patients. Invest Ophthalmol Vis Sci 2008;49:3262–68.  Back to cited text no. 17
    
18.
Bochmann F, Ang GS, Azuara-Blanco A. Lower corneal hysteresis in glaucoma patients with acquired pit of the optic nerve (APON). Graefes Arch Clin Exp Ophthalmol 2008;246:735-8.  Back to cited text no. 18
    
19.
Kaushik S, Pandav SS. Ocular Response Analyzer. J Curr Glaucoma Pract 2012;6:17-9.  Back to cited text no. 19
    
20.
Wilson SE, Lin DT, Klyce SD, Reidy JJ, Insler MS. Topographic changes in contact lens-induced corneal warpage. Ophthalmology 1990;97:734-44.  Back to cited text no. 20
    
21.
Wang X, McCulley JP, Bowman RW, Cavanagh HD. Time to resolution of contact lens-induced corneal warpage prior to refractive surgery. CLAO J 2002;28:169-71.  Back to cited text no. 21
    
22.
Terai N, Raiskup F, Haustein M, Pillunat LE, Spoerl E. Identification of biomechanical properties of the cornea: The ocular response analyzer. Curr Eye Res 2012;37:553-62.  Back to cited text no. 22
    
23.
Elsheikh A, Joda A, Abass A, Garway-Heath D. Assessment of the Ocular Response Analyzer as an instrument for measurement of intraocular pressure and corneal biomechanics. Curr Eye Res 2015;40:1111-9.  Back to cited text no. 23
    
24.
Altinkaynak H, Kocasarac C, Dundar H, Sayin N, Kara N, Bozkurt E, et al. Which tonometry in eyes with keratoconus? Eye (Lond) 2016;30:431-7.  Back to cited text no. 24
    
25.
Özcura F, Yıldırım N, Tambova E, Şahin A. Evaluation of Goldmann applanation tonometry, rebound tonometry and dynamic contour tonometry in keratoconus. J Optom 2017;10:117-22.  Back to cited text no. 25
    



 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]



 

Top
 
 
  Search
 
Similar in PUBMED
   Search Pubmed for
   Search in Google Scholar for
 Related articles
Access Statistics
Email Alert *
Add to My List *
* Registration required (free)

 
  In this article
Abstract
Introduction
Methods
Results
Discussion
References
Article Tables

 Article Access Statistics
    Viewed950    
    Printed50    
    Emailed0    
    PDF Downloaded86    
    Comments [Add]    

Recommend this journal


[TAG2]
[TAG3]
[TAG4]