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Devi AV
Journal of Current GlaucomaSathi
Practice,
January-April 2009;3(1):24-27
The Ocular Response Analyzer
Sathi Devi AV
Glaucoma Services, Narayana Nethralaya, Rajajinagar, Bangalore, Karnataka, India
INTRODUCTION
The Ocular Response Analyzer (ORA; Reichert Inc, Depew,
NY) is a novel instrument for measuring the intraocular pressure
(IOP) of the eye (Fig. 1). It is also the only instrument capable of
measuring the biomechanical properties of the cornea. Corneal
biomechanical properties influence intraocular pressure
measurement, undergo alterations in corneal pathology and
following corneal refractive surgery. Corneal Hysteresis (CH),
which is the result of viscous damping in the corneal tissue, is
a new indicator of corneal biomechanical properties.
during the measurement process. These deformation changes
are monitored by the electro-optical detection system .Two
independent pressure values are derived from the inward and
outward applanation events. These two pressure values are
not the same. Due to the dynamic nature of the air pulse, viscous
damping (energy absorption) in the cornea causes delays in
the inward and outward applanation events, resulting in two
different pressure values (Fig. 2).The difference between these
two pressure values is Corneal Hysteresis (CH). The average of
these two applanation events provides a repeatable, Goldmanncorrelated IOP measurement (IOPg).
PRINCIPLE
The ORA utilizes a dynamic, bidirectional applanation
process for measuring IOP. A rapid air impulse is used to apply
force to the cornea. The deformation of the cornea is monitored
using an advanced electro-optical system. The precisely
metered, collimated air pulse causes the cornea to move inwards
causing applanation, similar to conventional noncontact
tonometers. However, in the ORA, the air impulse continues to
deform the cornea past applanation into slight concavity. Then,
the air pump shuts off and, as the pressure decreases, the cornea
begins to return to its normal configuration. During this process,
it once again passes through an applanated state. The entire
process takes only 20 milliseconds, a time sufficiently short to
ensure that ocular pulse effects or eye position does not change
Measurement signals—Red: Raw applanation signal
Blue: Filtered applanation signal
Green: Air pressure curve
Fig. 2: Ocular response analyzer measurement signals
METHOD OF OPERATION
Fig. 1: The ocular response analyzer
The patient is seated in front of the machine and asked to fixate
on a green light. The instrument has an innovative, automatic
alignment system that eliminates operator subjectivity and
provides precise, repeatable measurements. The system
positions an air tube to a precise position relative to the apex of
the cornea. The air pulse then applies pressure to the cornea.
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The Ocular Response Analyzer
Corneal deformation is recorded and measurement signals are
obtained. The measurement signals consist of a green symmetric
curve, which corresponds to the air pulse pressure, and a red
asymmetric curve, which indicates the raw signal corresponding
to the applanation of the cornea. The blue curve is a filtered
version of the red curve, designed to identify the optimum point
of applanation in less than ideal signals. The red curve has 2
principal peaks. The applanation pressure is determined by
drawing a line down from the peak of each applanation spike to
the intersection of the green pressure curve. These points, P1
and P2 are graphically indicated as blue squares on the green
curve. P1 is the pressure at the first applanation event as the
cornea moves inward under the increasing force of air pulse
(inward applanation). P2 is the pressure corresponding to the
second applanation event as the cornea returns to its normal
curvature under the decreasing force of the air pulse (outward
applanation). P2 is always lower than P1 due to Corneal
Hysteresis.
The applanation signal curves may vary significantly in
appearance from measurement to measurement. Ideally, the
height of the applanation signals (spikes) will be above the
green curve. Both spikes should have a clearly defined and
relatively well-centered high point (peak). Such signals would
generate highly reliable measurement results. Repeated
measurements on the same eye should produce similar looking
signals.
Measurement Parameters
The ORA provides 4 different measurement parameters:
IOPg: A Goldmann-correlated IOP value.
CH: (P1-P2) A measure of viscous damping in the cornea.
IOPcc (Corneal-Compensated Intraocular Pressure): An
intraocular pressure measurement that is less affected by corneal
properties.
CRF (Corneal Resistance Factor): An indicator of the overall
“resistance” of the cornea.
The ORA also has a built in 20 MHz ultrasound pachymeter
that measures CCT - Central corneal thickness.
Corneal hysteresis is a function of the corneal viscous
resistance (damping/energy absorption) properties.1 Corneal
hysteresis is the difference between pressures P1and P2, a
numerical value denoting viscoelastic corneal tissue response
to a dynamic deformation. A greater difference generates a
higher CH, suggestive of a stiffer cornea. CH is influenced by
the rate of force application and is likely linked to the stromal
collagen nature and state of hydration.2 Normal values of CH
and CRF measured in a recent study were 10.8 mm Hg +1.5 (SD)
and 11.0 mm Hg ± 1.6 mm Hg respectively.3 Corneal resistance
factor and IOPcc are calculated using a linear combination of P1
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and P2.4 IOPcc measurement is designed to reduce the effect of
corneal thickness and properties on the IOP measurement
process, wherein the IOP has been adjusted utilizing the
information provided by the corneal hysteresis measurement.
CRF is derived from the formula P1 – kP2, where k is a constant,
derived from the relationship between changes in P1 and P2,
with change in IOP. CRF is more heavily weighted by corneal
elasticity (static resistance).
ORA IN GLAUCOMA
The influence of central corneal thickness (CCT) on intraocular
pressure (IOP) measurements using Goldmann applanation
tonometry (GAT) has been well-recognized.5-7 Intraocular
pressure is overestimated in thick corneas or underestimated in
thin corneas. However, it is likely that factors other than CCT,
including corneal hydration, connective tissue composition,
and bioelasticity, contribute to the response of the corneoscleral shell, to the force applied during the measurement of
IOP.8 The ORA provides IOP measurements taking into
consideration these biomechanical properties of the cornea
(Figs 3 to 5). IOPcc and IOPg have shown good correlation
with GAT IOP measurements.1 GAT IOP measurements are
significantly associated with CCT, whereas IOPcc measurements
are not associated with any of the variables - CCT, corneal
curvature, axial length, and age.9,10 The difference between GAT
and IOPcc measurements is significantly influenced by corneal
thickness.8 Patients with thicker corneas tend to have higher
GAT IOP measurements compared with IOPcc, whereas in
patients with thin corneas, GAT IOP measurements tend to be
lower than IOPcc. Variations of the elasticity of the cornea within
a range predicted to occur in a normal population, was found to
result in an error of IOP measurement as high as 17 mm Hg.6
Also, the influence of CCT on applanation tonometry readings
was found to depend on the modulus of elasticity of the cornea.
In a study by Congdon et al, lower corneal hysteresis value,
but not CCT, was associated with visual field progression.8 In
the study by Touboul et al, CH values were found to be lower in
glaucomatous eyes than in normal eyes.1 CRF was higher than
CH values in normal and glaucomatous eyes.1 True IOP was
underestimated by Goldmann applanation tonometry in
underdampened corneas and should be an interesting factor in
glaucoma management.1
ORA IN CORNEAL PATHOLOGY
Clinical data has shown that the Corneal Hysteresis
measurement is useful in identifying corneal pathologies and
may be valuable in identifying potential LASIK (laser assisted
in situ keratomileusis) candidates who are at risk of developing
ectasia (Figs 6 to 8). In patients with Fuchs’ dystrophy, the
cornea is thicker, but they are less rigid (lower CH)11 and
Sathi Devi AV
generally show lower Goldmann readings. Eyes with
keratoconus also have low CH values.1,11 CH was higher than
CRF in eyes with keratoconus.1 Low CH and CRF were wellcorrelated with the weakest corneas and should be helpful in
keratoconus screening as a new risk factor.1 In a study by Ortiz
et al, it was found that higher the keratoconus grade, the lower
the corneal hysteresis and corneal resistance factor values.12
ORA IN REFRACTIVE SURGERY
Refractive surgery currently uses corneal thickness as a basic
qualification and planning parameter.10,11 However, corneal
hysteresis may be more useful as a qualification factor for LASIK
and related corneal refractive surgery procedures because
different subjects with the same corneal thickness may display
significantly different corneal mechanical properties 11
(Figs 6 and 7). A significant decrease in the IOP and
biomechanical properties is found in eyes following LASIK
surgery. In the study by Ortiz et al, corneal hysteresis and
corneal resistance factor decreased significantly, one month
post LASIK.12 The creation of the corneal flap, and corneal
thinning caused by ablation weaken the cornea. A good
correlation was observed between the corrected refractive defect
and the change in biomechanical properties. Both IOPcc and
IOPg showed a decrease after LASIK surgery. However, the
decrease in IOPcc was much lower than the decrease in IOPg,12
suggesting that the IOPcc is a more accurate indicator of the
true IOP.
Fig. 4: NTG—IOPcc greater than IOPg, low CH and CRF
low amplitude peaks
INTERPRETING ORA SIGNALS
The morphological signal that is produced by the Ocular
Response Analyzer is a unique “signature” for the eye being
measured. Following are few typical examples.
Fig. 5: Blind eye—Signals are low amplitude, noisy and lumpy
IOPcc and IOPg are elevated. CH is very low, CRF is elevated
Fig. 3: Stable POAG on medications—CH and CRF in
normal range IOP well-controlled. Signal is smooth
Fig. 6: Pre LASIK—IOPcc and IOPg close and in normal range
CH and CRF close and in normal range
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The Ocular Response Analyzer
invaluable in appropriate, long-term management in these
conditions.
REFERENCES
Fig. 7: Post LASIK—(same patient as in Figure 6) IOPcc higher than
IOPg. CH, CRF and CCT low. Low amplitude signals with thin sharp
peaks, noisy signals
1. Touboul D, et al. Correlations between corneal hysteresis, intraocular
pressure, and corneal central Pachymetry. J Cataract Refract Surg
2008;34:616-22.
2. Rouse EJ, et al. IOVS 2007; 48: ARVO E-Abstract 1247.
3. Ortiz D, Pinero D, Shabayek MH, et al. Corneal biomechanical
properties in normal, post-laser in situ keratomileusis, and
keratoconic eyes. J Cataract and Refract Surg 2007;33:1371-75.
4. Luce DA. IOVS 2006;47:ARVO E-Abstract 2266.
5. Whitacre MM, Stein RA, Hassanein K. The effect of corneal
thickness on applanation tonometry. Am J Ophthalmol
1993;115:59-596.
6. Liu J, Roberts CJ. Influence of corneal biomechanical properties
on intraocular pressure measurement: quantitative analysis.
J Cataract Refract Surg 2005;31:146-55.
7. Whitacre MM, Stein R. Sources of error with use of Goldmanntype tonometers. Surv Ophthalmol 1993;38:1-30.
8. Congdon NG, et al. Central Corneal Thickness and Corneal
Hysteresis Associated With Glaucoma Damage. Am J Ophthalmol
2006;141:868-75.
9. Medeiros FA, Weinreb RN. Evaluation of the Influence of Corneal
Biomechanical Properties on Intraocular Pressure Measurements
Using the Ocular Response Analyzer. J Glaucoma 2006;15:364-70
10. Pepose JS, et al. How Should We Measure IOP Following LASIK?
Cataract and Refractive Surgery Today 2005;2-4.
11. Luce DA. Determining in vivo biomechanical properties of the
cornea with an ocular response analyzer. J Cataract Refract Surg
2005;31:156-62.
12. Ortiz D, et al. Corneal biomechanical properties in normal, postlaser in situ keratomileusis, and keratoconic eyes. J Cataract Refract
Surg 2007;33:1371-75.
Fig. 8: Keratoconus—IOPcc higher than IOPg, low CH and CRF
noisy, low amplitude signals, less repeatable values
CONCLUSION
The assessment of corneal biomechanical properties with the
ORA is useful in evaluating the influence of corneal properties
on IOP measurements. These new, objective methods of IOP
measurement appear to be less dependent on corneal rigidity
than Goldmann applanation tonometry. The clinical applications
of this novel device range from corneal pathology diagnosis,
pre-ectasia screening and post LASIK IOP measurement to
glaucoma screening, diagnosis and treatment efficacy
monitoring. The information obtained using this instrument is
Sathi Devi AV
([email protected])
“I learned this, at least, by my experiment: that if one advances confidently in the direction of his dreams and endeavors
to live the life he has imagined, he will meet with success unexpected in common hours.”
—Thoreau
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