Ocular Response Analyzer in Children Dr. Kaushik Hegde

CORNEA SESSION - III 

Ocular Response Analyzer in Children 

Dr. Kaushik Hegde, Dr. Rohit Shetty, Dr. Ajoy Vincent, Dr. Harsha K. 

(Presenting Author: Dr. Kaushik Hedge) 

Corneal biomechanics and its relevance in the 

diseases of the cornea1,2 keratorefractive 

surgeries3,4 and measurement of Intra ocular 

pressure (IOP) 4,5 is an area of intense and ongoing 

research. Various techniques have been 

235 

described to measure this parameter some of 

them being the dynamic corneal imaging 6 , high 

speed optical tomography 7 and more recently the 

ocular response analyser 3 (ORA; Reichert 

Ophthalmic Instruments, Depew, New York,

236 AIOC 2010 PROCEEDINGS 

USA). 

The principle of the ORA is based on non-contact 

tonometry (a patented bidirectional applanation 

event). 1 In addition to the IOP, it gives the 

measure of biomechanical properties of the 

cornea referred to as corneal hysteresis. 

Hysteresis refers to the energy lost during the 

stress-strain cycle. As with most biological 

materials, collagen is a viscoelastic material and 

therefore the cornea exhibits hysteresis. 

The effect of central corneal thickness (CCT) on 

IOP measurement is well known8,9 Corneal 

hysteresis may more effectively describe the 

contribution of corneal resistance to IOP 

measurements than CCT alone. 1 Preliminary 

studies have reported reduced hysteresis in 

keratoconus, fuch’s dystrophy3 , primary open 

angle glaucoma and normal tension glaucoma1. 

Biomechanical properties of the cornea in 

children10,11 or its contribution to the 

measurement of IOP12 are being better 

established. 

The primary aim of this study is to measure 

corneal hysteresis in children using ORA, with 

the intention to provide a reference to establish 

the normative values and ranges of the corneal 

biomechanical parameters. The secondary 

objective is to study the correlation between the 

various biomechanical parameters in refractive 

error subgroups, and discuss the scope of its 

utility as a clinical tool. 

Materials and Methods 

Study design: Prospective, non-interventional, 

cross sectional case series. 

Subjects and examinations: Informed consent 

was taken from the accompanying parent or 

guardian, and institutional ethical review board 

clearance was obtained. All children(less than 16 

years) underwent a complete eye examination. 

Any child with a history of ocular infection, 

ocular allergy, ocular surgery and other causes 

known to affect the corneal hysteresis, as well as 

children unable to cooperate with the procedure 

were excluded from the study. The child was 

comfortably seated and asked to fixate on the 

green light in the instrument. ORA recorded 

corneal hysteresis (CH), corneal resistance factor 

(CRF) and two intraocular pressure 

measurements: intraocular pressure corneal 

compensated (IOPcc), which is reportedly 

independent of corneal thickness3,4 and 

intraocular pressure Goldmann-correlated 

(IOPg), which is influenced by corneal thickness. 

A minimum of 4 readings were taken per eye and 

the best of the 4 readings was selected for 

analysis. CCT was measured using the IOPac 

Hand-Held Pachymeter (Standard, Model: 16011, 

Heidelberg Engineering, Inc., Germany). The 

probe was gently placed on the cornea in the mid 

pupillary plane in a perpendicular orientation, a 

minimum of 20 readings were taken and 

averaged out to get the mean CCT. The children 

were dilated with cyclopentolate (0.5%, twice at 

an interval of 30 minutes) and refracted (NIDEK; 

AR-600, Japan). The spherical equivalent was 

noted which was verified by objective refraction. 

For the purpose of analysis the children were 

categorized into 3 subgroups based on the 

refractive error (spherical equivalent) 13: >-0.50D 

(myopic), +2.00D to -0.50D (emmetropic), 

>+2.00D (hyperopic). 

Statistical analysis: Statistical Analysis was 

performed using Stastical Package for Social 

Sciences (version 10.5, SPSS Inc). Group 

differences for continuous variables were tested 

using the unpaired student t tests and one-way 

analysis of variance (ANOVA) for normally 

distributed data. Results are presented as mean 

± standard deviation [SD]. All statistical tests 

were two-tailed, and P value less than 0.05 was 

considered to indicate statistical significance. 

Correlations between sub-groups were analyzed 

using the Pearson correlation coefficient. 

Results 

A total of 150 eyes of 76 patients with 42 male 

(55.3%), 34 female (44.7%), with a mean age of 

10.21 ± 3.2 years (range of 4 – 16 years) were 

examined. 

The mean CH was 10.71 ± 1.44 mm of Hg, mean 

CRF was 10.80 ± 1.94 mm of Hg, and the mean ± 

SD of the various parameters for the entire group 

is shown in table 1. 

None of the parameters showed any significant 

difference with age or sex with the student t test 

(table 2). 

IOPcc showed positive correlation with IOPg (r 

= 0.88), CRF(r = 0.4), CCT(r = 0.28) and negative 

correlation with CH(r = -0.25) and SE(r = -0.23). 

IOPg showed a positive correlation with CRF (r = 

0.7), CH (r = 0.2) and CCT(r = 0.5). CRF showed


Table-1: Mean and Range of the various 

parameters of the entire sample 

Variables Mean ±Std. Range 

N= 150 Deviation (SD) 

IOPcc ( mm of Hg) 15.83 ± 3.30 6.7 - 24.0 

IOPg ( mm of Hg) 15.70 ± 3.79 8.5 - 26.8 

CH ( mm of Hg) 10.71 ± 1.44 6.6 - 14.3 

CRF ( mm of Hg) 10.80 ± 1.94 5.6 - 18.4 

CCT(µm) 

Spherical 

549.93 ± 33.97 474- 650 

equivalent (DS) 0.50 ± 3.40 -13.50 - 10.75 

mm of Hg: millimeter of mercury, 

DS: Diopter sphere. 

µm: micro meter, 

Table-2: Correlation of the various biomechanical 

parameters with age and sex 

(student t test, unpaired). 

Biomechanical Age Sex 

parameters (P value) (P value) 

IOPcc 0.43 0.17 

IOPg 0.45 0.11 

CH 0.97 0.68 

CRF 0.50 0.40 

CCT 0.29 0.83 

Spherical Equivalent 0.10 0.91 

a positive correlation with both CH (r = 0.64), and 

CCT(r = 0.64). CH showed a positive correlation 

with CCT(r = 0.5). 

The distributions of the parameters in the 3 

subgroup population, consisting of myopic, 

emmetropic and hypermetropic children are 

shown in table 3. 

In one way ANOVA a significant difference was 

seen between the 3 subgroups among various 

parameters (table 4) except CH (P = 0.05). Post 

hoc test showed significant difference between 

emmetropic and hypermetropic groups for CRF 

(P = 0.02), CH (P = 0.01) and CCT (P = .007) and 

between myopic and hypermetropic groups for 

IOPcc (P = 0.006) and IOPg (P= 0.04) (table 5). 

Graphs 1 and 2 represent the CH (r2 = 0.022) and 

Table-3: Biomechanical parameters in Myopia, Emmetropia and Hypermetropia (±SD) 

Subgroups (DS) IOPcc IOPg CH CRF CCT 

(mm of Hg) (mm of Hg) (mm of Hg) (mm of Hg) (µm) 

> -0.5 (n = 42) 16.90 ± 3.28 16.95 ± 4.25 10.72 ± 1.27 11.11 ± 2.03 554.02 ±35.46 

-0.5 - 1.99 (n = 77) 15.67 ± 3.09 15.25 ± 3.74 10.49 ± 1.27 10.41 ± 1.77 542.81 ± 33.58 

≥+2.00 (n = 31) 14.75 ± 3.48 15.12 ± 2.86 11.23 ± 1.87 11.33 ± 2.05 562.10 ± 29.14 

Total (n=150) 15.83 ± 3.30 15.70 ± 3.79 10.71 ± 1.43 10.80 ± 1.93 549.93 ± 33.97 

237 

Figure-1: Shows a scatter plot of the corneal hysteresis 

(CH) in mm of Hg along the y-axis and the refractive 

error (spherical equivalent in DS) along the x- axis. 

Statistical analysis indicates a weak positive correlation 

of CH with the refractive error, indicating possibly that 

as the refractive error moves towards hyperopia the CH 

also increases. 

Figure-2: Shows a scatter plot of the corneal resistance 

factor (CRF) in mm of Hg along the y-axis and 

the refractive error (spherical equivalent in DS) along 

the x- axis. Statistical analysis indicates a weak positive 

correlation of CH with the refractive error, indicating 

possibly that as the refractive error moves towards 

hyperopia the CRF also increases. 

CRF (r2 = 0.002) plotted against the refractive 

error. As the refractive error moves towards 

hyperopia, the biomechanical parameters also 

increase, indicating a weak positive correlation.


Table-4: Correlation of the various biomechanical 

parameters within the subgroups (student t 

test, unpaired) 

Biomechanical Significance 

parameter (P value) 

IOPcc 0.02 

IOPg 0.04 

CH 0.05 

CRF 0.03 

CCT 0.02 

Table-5: Comparison of corneal biomechanical 

parameters among refractive error subgroups 

Para- Sub groups Significance 

meters (P value) 

IOPcc Myopia with Emmetropia .05 

Myopia with Hypermetropia .006 

Emmetropia with Hypermetropia .18 

IOPg Myopia with Emmetropia .02 



CH Myopia with Emmetropia .39 



CRF Myopia with Emmetropia .06 



CCT Myopia with Emmetropia .08 



Where: Myopia (spherical equivalent > - 0.5 DS) 

Emmetropia (spherical equivalent between - 0.5 DS and 

+ 2.00 DS) 

Hypermetropia (spherical equivalent > + 2.00 DS) 

Table-6: Comparison of mean CH with other 

reported studies 

Variable Kirwan Song Indian 

(mean) et al10 et al11 children 

(our data) 

Age(years) 

CH with SD 

11.2(0.8) 14.7(3.2) 10.3(3.2) 

(mm of Hg) 12.3(1.3) 10.7(1.5) 10.70(1.3) 

Discussion 

Corneal hysteresis is thought to represent the 

cumulative effects of corneal thickness, 

hydration, rigidity, and other as yet 

undetermined factors14. The mean CH and CRF 

were 10.71 ± 1.45 mm of Hg and 10.80 ± 1.90 mm 

of Hg respectively. In this study there was no 

statistically significant relationship with the 

student t test for both CH and CRF with age (P = 

0.97), (P = 0.5) or sex (P = 0.68), (P = 0.4). Earlier 

reported literature does mention of an inverse 

relationship between age and corneal 

biomechanics. 15,16,17 The corneal thickness in 

children reaches adult dimensions by 5-9 years of 

age. 18 Physiological changes do continue 

throughout their life. In this study the CH was 

similar to that reported on adults1, the authors 

could not find an inverse relationship with age 

possibly due to the fact that the range of age 

groups among whom this study was conducted 

was too small to establish a significant change. 

The results of our children were similar to those 

found in Asian Chinese children11 (Table 6), but 

were lesser than what was reported on Caucasian 

children. 10 

As reported by others in adults14, CRF showed a 

positive correlation with both CH (r = 0.67), and 

CCT(r = 0.64) 19, so also did CH with CCT(r = 

0.54) 19. This suggests that the corneal thickness 

is an important factor in determining 

biomechanical properties of the eye. 

CRF (P = 0.03) and CCT (P = 0.02) showed 

significant difference between the refractive error 

subgroups. A post hoc test revealed CH (P = 0.02) 

and CRF (P = 0.03) to be significantly higher in 

hypermetropes than emmetropes. This could 

possibly be due to a significantly higher CCT (P 

= 0.007) in the hyperopes in our sample, although 

it is generally accepted that CCT is independent 

of the refractive error status. 20 Reported literature 

does indicate that CH does not vary in myopia, 

or its progression. 19,21 In this study there was no 

significant difference in the value of CH (P = 0.13) 

between the myopic and hyperopic groups, 

though a significant difference in the value of CH 

was seen between emmetropic and hyperopic 

groups (P = 0.02). None of the reported studies 

had hyperopic group as a control to identify any 

significant association between refractive error 

groups. 

In our study there was no correlation between 

IOPcc and IOPg with age (P = 0.4), (P = 0.5) or sex 

(P = 0.2), (P = 0.1). On analyzing the parameters 

across various subgroups, IOPcc and IOPg 

increased in the same proportion with refractive 

error. In each group, measured IOP values were 

strongly and positively correlated with each (r = 

0.9, r = 0.9, r = 0.8; among myopic, emmetropic


and hyperopic groups respectively). This is a 

little surprising as the IOPcc values are supposed 

to be independent of corneal parameters like 

CCT and corneal biomechanics, and one would 

ideally expect varied correlation of IOPcc with 

IOPg. As the IOPcc takes into consideration the 

viscoelastic nature of the cornea to evaluate the 

‘‘corneal-compensated IOP,’’ we can expect to 

find a significant difference between IOPcc and 

IOPg when tissue viscous damping is abnormal. 22 

The normal range of the biomechanical 

parameters would probably explain the positive 

correlation between the IOP values in our 

subgroups. 

IOPcc (P = 0.006) and IOPg (P = 0.04) was found 

to be more in myopic than hyperopic eyes (table 

5), although the CCT between the 2 groups in our 

study was not significant (P = 0.31). Since myopes 

are known to have a higher IOP23 than nonmyopes 

and myopia is a definite risk factor for 

the development of glaucoma23, 24, 25 the same 

pattern may exists in children too. 

CH is likely to be a useful additional 

measurement to assess the progression of 

diseases of the cornea, as it may change before 

topographic or clinical changes becomes 

apparent. 1,2 Since the CCT in Asian Indian 

population is known to be lesser than that found 

in other parts of the world, 26 and also keratoconus 

that is known to present itself at an earlier age 

1. David A. Luce. Determining in vivo biomechanical 

properties of the cornea with an ocular response 

analyzer. J Cataract Refract Surg 2005;31:156–62. 

2. Dolores Ortiz, David Pin˜ ero, Mohamed H. 

Shabayek. Corneal biomechanical properties in 

normal, post-laser in situ keratomileusis, and 

keratoconic eyes. J Cataract Refract Surg 

2007;33:1371–5 . 

3. Jun Liu, Cynthia J. Roberts. Influence of corneal 

biomechanical properties on intraocular pressure 

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4. Doughty MJ, Zaman ML. Human corneal thickness 

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review and meta-analysis approach. Surv 

Ophthalmol 2000;44:367–408. 

5. Jay S. Pepose, Susan K. Feigenbaum, Mujtaba A. 

Quazi, Jeffrey P. Sanderson, and Cynthia J. Roberts. 

Changes in Corneal Biomechanics and Intraocular 

Pressure Following LASIK using Static, Dynamic, 

and Noncontact Tonometry. Am J Ophthalmol 

2007;143:39–47. 

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among Indians, 27 planning refractive surgery in 

adults should incorporate CH and CRF 

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these factors in mind recording the 

biomechanical parameters and monitoring it over 

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Though the ORA is able to measure corneal 

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Ocular Response Analyzer in Children Dr. Kaushik Hegde

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