Understand the Cornea
Understand the Pressure
Corneal Biomechanics, Accurate IOP, and CCT
in one Simple Instrument
One Device, Five Parameters
•IOPG - Goldmann Correlated IOP
•IOPCC - Corneal Compensated IOP
•CH - Corneal Hysteresis
•CRF - Corneal Resistance Factor
•CCT - Central Corneal Thickness
ORA Technology Background
Measuring “Pressure”
Goldmann Tonometry Principles
The Goldmann Tonometer has long been
considered the gold standard for
measuring pressure. It is based upon the
Imbert-Fick Law (W = P x A) where:
- W is the force to applanate
- P is Intra Ocular Pressure (IOP)
- A is the area applanated
Measuring “Pressure”
Goldmann Tonometry Assumptions
-
Surface is dry
Volume is perfectly spherical
Surface is infinitely thin and perfectly flexible
Tear-film effect and corneal thickness effect cancel each other out
Recognizing that corneal effects and surface tension are factors which
influence the measurement; Goldmann selected a tonometer tip size of
3.06mm which he believed would nullify these effects based on a constant
central corneal thickness of 525 microns
Measuring “Pressure”
Goldmann Tonometry Flaws
- Experimentation done on cadaver eyes
- Not representative of live corneas
- Variation in corneal thickness is significantly greater than assumed
- Variations in corneal biomechanical properties unaccounted for
Accordingly, Goldmann tonometry cannot compensate for differences in
corneal thickness, corneal elasticity, and many other parameters that
influence tonometer readings.
This applies to all other Goldmann-correlated tonometers!
Non- Contact Tonometers
- Invented by Dr. Bernie Grolman in the 1960’s (American Optical)
- To enable OD’s in the USA to perform tonometry
- Introduced in 1971
- Uses rapid air pulse technology
- Easy to use
- Strong Goldmann correlation
- Objective: no operator bias
- No anesthetic required
- No risk of cross-contamination
Modern NCT - AT555
NCT Traditional Method of Operation
Traditional NCT vs. GAT
“In conclusion, the current study shows that the
XPERT vs GAT Sdiff (1.5 mmHg) is comparable to
single GAT instrument repeatability, and far superior
to that of two GAT instrument repeatability/reliability.”
Ocular Response Analyzer
Method of Operation
Static vs. Dynamic Measurement
Goldmann tonometers make ‘static’ measurements. That is they derive
IOP from the force measured during a steady state applanation of the
cornea.
The Ocular Response Analyzer makes a ‘dynamic’ measurement,
monitoring the movement of the cornea in response to a rapid air
impulse.
The ‘dynamic’ nature of the ORA measurement makes possible the
capture of other useful data about the eye.
Visco-Elastic System
An Automotive “Strut” Assembly
- Coil Spring: Static Resistance (Elasticity). strain
(deformation) is directly proportional to stress (applied force),
independent of the length of time or the rate at which the force
is applied.
- Shock Absorber: Viscous Resistance (Damping). The
resistance to an applied force depends primarily on the speed
at which the force is applied.
Method of Operation
Applanation Signal Plot
Definitions
Hysteresis
The phenomenon was identified, and the term coined, by
Sir James Alfred Ewing in 1890.
Hysteresis is a property of physical systems that do not
instantly follow the forces applied to them, but react
slowly, or do not return completely to their original state.
Corneal Hysteresis
The difference in the inward and outward pressure values
obtained during the dynamic bi-directional applanation
process employed in the Ocular Response Analyzer, as a
result of viscous damping in the cornea.
Corneal Hysteresis:
A New Ocular Parameter
Right/Left Eye Hysteresis
18.00
2
R = 0.6625
16.00
14.00
12.00
10.00
8.00
6.00
4.00
2.00
0.00
0.00
5.00
10.00
15.00
20.00
Hysteresis vs. Corneal Radius
12.0
10.0
Hysteresis - mmHg
R = 0.01
8.0
6.0
4.0
2.0
0.0
7
7.2
7.4
7.6
7.8
Average Corneal Radius - mm
8
8.2
8.4
Hysteresis vs. Corneal Astigmatism
12.0
Hysteresis - mmHg
10.0
R = 0.26
8.0
6.0
4.0
2.0
0.0
0
0.5
1
1.5
2
Corneal Astigmatism - D
2.5
3
CCT vs. CH - 184 normal eyes
Data courtesy Mitsugu Shimmyo, MD
IOPG vs CH - 339 Normal Eyes
Data courtesy New England College of Optometry
In/Out Applanation Regressions
32 eyes - 3 pressure levels (ODM induced)
Conclusion: Hysteresis stays constant over a wide range of
pressures for the same eyes
ORA and Corneal Specialties
Corneal Biomechanics:
A New Area of Clinical Interest
Ocular Response Analyzer is the only
instrument capable of measuring the
biomechanical properties of the cornea.
Clinical data has shown that the Corneal Hysteresis measurement is
useful in identifying corneal pathologies and may be valuable in
identifying potential LASIK candidates who are at risk of developing
ectasia. In consequence, the instrument is attracting interest from
corneal specialists and refractive surgeons.
Corneal Biomechanics and Refractive Surgery
“Refractive surgery is not an exact science. Achieving the cornea’s
ultimate shape depends on our ability to predict the biomechanical
response to surgery.”
Cynthia Roberts, Ph.D. Associate Professor of Ophthalmology and Biomechanical Engineering, OSU
“The promise of wavefront-guided laser ablation will not be fully
realized until researchers gain a more complete understanding of
corneal biomechanics.”
John Marshall, Ph.D. “Father of the Excimer Laser”
“Wavefront by itself is a great tool but we still need to understand
corneal biomechanics to reap the whole benefit.”
David Williams, Ph.D. Direct of The Center For Visual Science, University of Rochester
Classifying Corneal Pathologies
Data courtesy Shah, Brandt, Pepose, Castellano
Classifying Corneal Pathologies
To investigate the biomechanical characteristics of eyes with:
- Fuchs’ Corneal Dystrophy (n=14)
- Post-Penetrating Keratoplasty (18±10 months postop, n=32)
- Corneal Ectasia (n=46)
- Advanced Keratoconus (CCT < 490 µm, n=15)
- Pellucid Marginalis (n=4)
- Early or Forme Fruste Keratoconus (CCT > 490 µm, n=27)
- Compared to 3 pachymetry matched control groups
Group 1: > 580 µm (n=31)
Group 2: between 510 and 580 µm (n=66)
Group 3: < 510 µm (n=17)
To compare IOP measurements using 3 testing techniques
GAT;
NCT with ORA;
Data courtesy Jay Pepose, MD - ASCRS 2006
PDCT
Classifying Corneal Pathologies
Control Group Differences
Controls
N
CCT µm
GAT mmHg
ORA-g mmHg
ORA-cc mmHg
PDCT mmHg
CH mmHg
CRF mmHg
OPA mmHg
Group 1
17
603.7 ± 20.0
15.3 ± 2.3
17.6 ± 3.7
15.5 ± 3.5
17.8 ± 2.3
11.5 ± 1.8
11.8 ± 1.8
2.3 ± 1.1
Group 2
66
543.9 ± 18.3
14.5 ± 3.2
15.3 ± 3.1
15.5 ± 3.2
17.5 ± 3.3
9.7 ± 1.4
9.5 ± 1.3
2.3 ± 0.8
Group 3
31
487.9 ± 20.0
12.8 ± 2.7
13.2 ± 3.5
15.0 ± 3.0
16.6 ± 2.7
8.4
± 1.2
7.8
± 1.5
2.2
± 0.9
= p<0.05 comparing Group 1 or 3 to Group 2, with Student’s t-test
Data courtesy Jay Pepose, MD - ASCRS 2006
Classifying Corneal Pathologies
Biomechanical Metrics in 3 Control Groups
12
mm Hg
10
8
6
4
2
0
CH
Gro up 1(mean 603.8 µm)
Data courtesy Jay Pepose, MD - ASCRS 2006
CRF
Gro up 2 (mean 543.9 µm)
OP
Gro up 3 (mean 487.9 µm)
Classifying Corneal Pathologies
N
Fuchs’
Group
1
14
17
KCN/
PKP
Group
2
advanced
PMD/
FFKCN
32
66
15
46
KCN
Group
3
31
CCT 585.3 ± 603.7 ± 533.0 ± 543.9 ± 400.9 ± 462.6 ± 487.9 ±
µm
52.5
20.0
72.2
77.4
20.0
47.1
18.3
± 9.7
± 7.0
± 11.5 ± 9.2
± 8.1
± 8.4
±
2.0
1.8
1.4
1.7
1.2
1.7
1.4
± 9.5
± 5.6
8.0
± 11.8 ± 9.2
± 7.0
± 7.8
±
CRF
2.0
1.8
1.5
1.8
1.5
2.1
1.3
± 2.0
2.3
± 2.3
± 2.6 ± 2.3
± 2.2
± 2.2
±
OPA
1.0
1.1
0.9
0.7
0.9
1.2
0.8
CH
8.0
= p<0.05 comparing study group to its respective control group, with Student’s t-test
Data courtesy Jay Pepose, MD - ASCRS 2006
Classifying Corneal Pathologies
Thin Cornea with no ectasia
CH=11.2
CRF=10.8
Data courtesy Renato Ambrosio, MD - ASCRS 2006
Thin Cornea with Keratoconus
CH=8.1
CRF=7.9
Classifying Corneal Pathologies
CCT = 605 um; CH = 8.4mmHg; CRF = 8.0mmHg
3+ Corneal Gutata
(Fuchs’Dystrophy)
NORMAL
CCT = 597 um; CH = 11.9mmHg; CRF = 11.4mmHg
Data courtesy Renato Ambrosio, MD - ASCRS 2006
Pre / Post Lasik
This patients pre-lasik CH is lower than the population
average post-lasik CH. This patient may be a candidate
for ectasia!
Data courtesy Dr. David Castellano, MD / Dr. Jay Pepose, MD
Normal vs. Keratoconic Signals
KERATOCONUS
NORMAL
Data courtesy Mr. Sunil Shah, MD
Normal vs. Fuchs’ Signals
FUCHS’
NORMAL
Data courtesy Dr. James Brandt, MD
Pre and Post Lasik Signals
POST-LASIK
PRE-LASIK
Data courtesy Dr. David Castellano, MD
Signals are “Corneal Signature”
NORMAL
FUCHS’
KERATOCONUS
POST LASIK
Predicting Ectasia Risk
Data courtesy Peter Hersh
ORA and Glaucoma
Landmark Studies
Many recent studies have concluded, for the first time, that controlling
IOP in glaucoma patients and suspects stops or slows the progression
of the disease. These studies include:
- OHTS - Ocular Hypertension Treatment Study
- AGIS - Advanced Glaucoma Intervention Study
- CNTGS - Collaborative Normal-Tension Glaucoma Study
- CIGTS - Collaborative Initial Glaucoma Treatment Study
Many of these studies have also investigated the role of the cornea in the
diagnosis and management of glaucoma.
The cornea and glaucoma
• Some studies have investigated
Corneal thickness as a contaminating
factor in measuring IOP
• Others have investigated Corneal
thickness as an independent indicator
of glaucoma risk - Could a thin
cornea be a surrogate for eyes
susceptible to glaucoma damage?
Central Corneal Thickness
Recently a great deal of attention has been focused on the relationship
between central corneal thickness (CCT) and Goldmann-obtained IOP
values. Studies have found that corneal thickness influences the
accuracy of IOP measurements.
- Thicker corneas, on average, tend to overstate GAT IOP values
- Thinner corneas, on average, tend to understate GAT IOP values
HOWEVER, this is only true ON AVERAGE for large populations
- The IOP/CCT relationship is actually quite weak and varies from study
to study, making correcting IOP based on CCT impractical
The problem with CCT
184 Normal Eyes
Data courtesy New England Collage of Optometry
The problem with CCT
•
•
•
•
•
Two corneas, both 0.65 mm
One is clear
The other is edematous
The first reads high (compared to manometry), the second low
Thickness can’t be the whole answer
• Other corneal factors besides thickness determine response of
corneo-scleral shell to force
– Hydration
– Connective tissue composition
– Bio-elasticity
Data courtesy Harry Quigley, Wilmer Eye Institute
The problem with CCT
“We should not assume that corneal thickness is the parameter of
greatest interest in monitoring glaucoma or in determining what
features of the eye are important in optic nerve damage”.
“Physiology is more important than anatomy”
- Harry Quigley, Director of Glaucoma Service, Wilmer Eye Institute
“Adjusting IOP by means of a fixed CCT algorithm is almost
certainly wrong in the majority of our patients and is attempting to
instill a degree of precision, into a relatively flawed instrument (the
Goldmann tonometer), that simply is not there”
- James Brandt, Director of Glaucoma Services, UC Davis
CH distribution - Normals & Glaucoma
Data courtesy New England College of Optometry and Mitsugu Shimmyo, MD
Corneal Properties and Glaucoma Risk
Additional Parameters:
P1 and P2 provide independent information
about the eye
Background
Background
Data courtesy Dr. David Castellano, MD / Dr. Jay Pepose, MD
Background
Data courtesy Dr. David Castellano, MD / Dr. Jay Pepose, MD
Gaining additional Useful Information
•Clinical data analysis demonstrated that p1 and p2 respond
independently to various factors (CCT, LASIK, IOP reduction, etc)
•Therefore, an “optimum combination” of the two independent
parameters may yield the best IOP and Corneal Parameter, resulting in:
•Reduced or eliminated ORA IOP change after LASIK
•Reduced or eliminated Corneal Parameter change after pressure reduction
•Increased correlation of Corneal Parameter and CCT
•Reduced or eliminated correlation of ORA IOP and CCT
•Reduced or eliminated correlation of ORA IOP and Corneal Parameter
•Reduced (slightly) correlation of ORA IOP and GAT
•Higher correlation of Corneal Parameter with GAT than CCT with GAT
•Reduced or eliminated anomalous low IOP for keratoconus, fuch’s patients
IOPcc
Corneal Compensated IOP
Define & Describe IOPCC
Corneal-Compensated Intraocular Pressure
- An Intraocular Pressure measurement that is less affected by corneal
properties than other methods of tonometery, such as Goldmann
(GAT). IOPCC has essentially zero correlation with CCT in normal
eyes and stays relatively constant post-LASIK.
- Developed using clinical data and a proprietary algorithm.
Method for finding “invariant” pressure
• Use linear combination of P1 & P2 - avoids potential coupling
of IOP & CH
• Vary ratio of P1 & P2 to minimize difference of pre-post
LASIK IOP
•Upon achieving desired post-LASIK results, verify that:
•Correlation with Goldmann is still strong
•Correlation of IOP with CCT in various data sets is minimal
•Correlation of IOP with CH in various data sets is minimal
•Optimum formula: IOPcc = P2 - (0.43*P1)
IOPG vs. CCT - 184 normal eyes
Data courtesy New England Collage of Tonometry
IOPCC vs CCT 184 Normals
•
Data courtesy New England Collage of Optometry
IOPcc vs. GAT and DCT IOP
Thin, Average, and Thick Cornea Groups
18
16
14
mm Hg
12
10
8
6
4
2
0
GAT
ORA-g
Gro up 1(mean 603.8 µm)
Data courtesy Jay Pepose, MD - ASCRS 2006
ORA-cc
Gro up 2 (mean 543.9 µm)
PDCT
Gro up 3 (mean 487.9 µm)
28 eyes Pre/Post LASIK IOPCC
26% IOP drop
Data courtesy Dr. David Castellano, MD / Dr. Jay Pepose, MD
3% IOP drop
24 “NTG” eyes
IOPCC is higher than traditional IOP in “NTG” subjects
Data courtesy Mitsugu Shimmyo, MD
Is IOPcc Better than GAT?
•IOPcc correlates strongly with GAT on the average
•HOWEVER, IOPcc has the following advantages over GAT
•Not affected by CCT
•Not affected by corneal biomechanical properties (rigidity)
•As such, it is more accurate in KC, Fuchs’, OHT, NTG eyes
•In addition, it has less measured IOP reduction post-LASIK
•No operator bias
Is IOPcc Better than GAT?
CRF
Corneal Resistance Factor
Define & Describe CRF
Corneal Resistance Factor
An indicator of the overall “resistance” of the cornea, including
both the viscous and elastic properties. It is significantly
correlated with Central Corneal Thickness (CCT) and GAT, as
one might expect, but not with IOPCC.
Method for finding “CRF”
corneal resistance factor
• Use linear combination of P1 & P2 - avoids potential coupling
of IOP & CH
• Vary ratio of P1 & P2 to:
•Maximize correlation of CH and CCT in various populations
•Minimize CH change after pressure reduction / increase
•Maximize correlation of CH and GAT
•Ensure CH remains significate indicator of corneal
conditions such as Keratoconus, fuch’s, etc
•Ensure significant CH change post-LASIK remains
•Optimum corneal parameter: CRF = P1-(0.7*P2)
Correlation of CRF and CCT
Correlation of CH and CRF vs. CCT
339 Normal Eyes
Correlation of CH & CRF vs. IOPG (“GAT”)
CRF - Normals, Keratoconus, Fuchs’
CRF is a better indicator of KC than CH
Data courtesy Shah, Brandt, Pepose, Castellano
CRF distribution - Normals & Glaucoma
Data courtesy New England College of Optometry and Mitsugu Shimmyo, MD
How do CH and CRF Differ
Correlation of CH, CRF & IOPg
Data courtesy Dr. Mitsugu Shimmyo, MD
IOPCC vs CRF 339 Normals
•
Are CH and CRF Related to
the “Modulus of Elasticity”?
NO!
Researchers have attempted to identify the young's modulus of the
cornea - but the reported values in the literature, vary by four orders
of magnitude!
The cornea is a system, not an isotropic material such as steel or
rubber. Attempting to identify the youngs modulus is a gross oversimplification of a complex subject.
Interpreting ORA
Measurement Results
Measurement Signal Components
Applanation Events
Raw Signal
Filtered Signal
Air Pulse
P1
P2
Identifying Normal Signals
“Rules of thumb”
High
Ave
Low
IOPg
IOPcc
CH
CRF
CCT
X
X
X
X
X
- Watch for:
- Clean, smooth signals
- Similar amplitude peaks
- Repeatable values
- Consistent measurements in both eyes
Identifying Normal Signals
IOPcc and IOPg are close and in normal range
CH and CRF are close and in normal range
Filtered peaks “line up” under raw peaks
Similar signal amplitude
Raw signal
has clean points
Raw signal is fairly smooth
Baseline signal is “flat” and nearly same amplitude on both sides
Identifying Normal Signals
Identifying Normal Signals
Identifying Normal Signals
Identifying Keratoconus
“Rules of thumb”
High
Ave
Low
IOPg
IOPcc
X
X
X
CH
CRF
CCT
X
X
X
- Watch for:
- Low amplitude peaks
- less repeatable signals than normal subject
- “noisy” signals
- Often present in one eye and not the other.
Identifying Keratoconus Signals
IOPcc Higher than IOPG
Low CH
Low CRF
Thin CCT
Low amplitude peaks
Sharp, thin peaks
P2 raw signal “bounce”
More “noisy” raw signal
Noisy signals cause less repeatable values
Identifying Keratoconus Signals
Identifying Keratoconus Signals
Questionable Keratoconus Signal
Measurement may yield unreliable results
Raw signal is too “lumpy”
Identifying Severe Keratoconus
“Rules of Thumb”
IOPg
High
Ave
Low
IOPcc
CH
CRF
???????????
- Look at the signal, the numbers may be unreliable
- Thin CCT
- Very low amplitude peaks - practically a flat line
- General signal shape is very repeatable
- Often present in one eye and not the other
CCT
Severe Keratoconus Signal
Measurement values will be unreliable
CH and CRF are unreliable
due to signal amplitude
Forget about the CH, no question this Keratocouns!!
Identifying Forme Fruste KC
“Rules of Thumb”
High
Ave
Low
IOPg
IOPcc
X
X
X
CH
CRF
CCT
X
X
X
- Watch for:
- Rule out past history of refractive surgery
- lower amplitude peaks
- Rapid P2 raw signal falloff with small “ricochet bounce”
- Suspicious topography
- “noisy” signals, but cleaner than advanced KC and more repeatable
- Family history, frequent eye-rubbing, trouble wearing contacts
“Sub-Clinical” Keratoconus Signal
Signal looks nearly normal but low CH and CRF
IOPcc Higher than IOPG
CH just below normal range
CRF just below normal range
Mild P2 raw signal “bounce”
Identifying Refractive Surgey
“Rules of Thumb”
High
Ave
Low
IOPg
IOPcc
X
X
X
CH
CRF
CCT
X
X
X
- Watch for:
- low amplitude peaks (cleaner in LASIK than PRK)
- “Sharp / thin” raw signals (especially in LASIK)
- Rapid P2 raw signal falloff with pronounced “ricochet bounce”
- less repeatable signals than normal subject (Especially in PRK)
- “noisy” signals (Especially in PRK)
Pre / Post-LASIK Signals
PRE-LASIK
IOPg, IOPcc close and in normal range
CH, CRF close and in normal range
Normal Signal
POST-LASIK
IOPcc higher than IOPg, closer to normal
CH, CRF low
CCT lower
Thin, sharp peaks
Reduced signal amplitude
Some “noise”
P2 “bounce”
Pre / Post-LASIK Signals
PRE-LASIK
POST-LASIK
Pre / Post-LASIK Signals
Example of less reliable, but still useful, signals
PRE-LASIK
CH, CRF may be higher in reality
P1, not ideal
POST-LASIK
IOP probably higher in reality
CH may be lower in reality
But the CRF is reduced!
Neither signal is “ideal” but the
post-lasik difference is still clear.
TAKE MULTIPLE READINGS!!
P1, not ideal
Pre / Post PRK Signals
PRE-PRK
2 Months POST-PRK
IOPg, IOPcc close and in normal range
CH, CRF close and in normal range
But CH, CRF stay low
Signal improves with time
Normal Signal
CH, CRF reduced
Note how well IOPcc works!
2 wks POST-PRK
PRK signals are noisy
Identifying Ectasia
“Rules of Thumb”
IOPg
High
Ave
Low
IOPcc
CH
CRF
X
X
CCT
X
X
X
- Watch for:
- Has had LASIK, PRK, other surface ablation procedure
- Very low amplitude, noisy, “messy” signals
- Signal quality does not improve over time
- Suspicious topography
- Often present in one eye and not the other
Identifying Ectasia Signals
IOPcc Higher than IOPG
Low CH
Low CRF
Thin CCT
Low amplitude peaks
Lots of noise
Sharp, thin peaks
P2 raw signal “bounce”
Identifying POAG
“Rules of Thumb”
High
Ave
Low
IOPg
X
X
IOPcc
X
X
CH
CRF
CCT
X
X
X
X
X
X
- Watch for:
- IOPcc higher than IOPg
- Noisy signals
- Family history, race, age, CDR, Diabetes status, Visual fields
results, optic nerve status
Identifying POAG Signals
Uncontrolled Subject, moderately high IOP
Signals are high amplitude, noisy
IOPg, IOPcc both elevated
Low CH
CRF higher than CH
Identifying POAG Signals
Subject on meds, but progressing
IOP is in normal range
but IOPcc Higher than IOPG
Low CH
Low CRF
Signal is smoother
than high IOP signals
Identifying POAG Signals
Subject on meds and stable
IOP is well controlled
CH, CRF in normal range
Signal is smooth
Identifying POAG Signals
Uncontrolled Subject - Blind
IOPg, IOPcc both elevated
Low CH
High CRF
Signals are low amplitude, lumpy, and noisy
Identifying OHT
“Rules of Thumb”
High
Ave
Low
IOPg
X
IOPcc
CH
X
CRF
X
CCT
X
X
- Watch for:
- IOPcc lower than IOPg
- Smooth signal
- Family history, race, age, CDR, Diabetes status, Visual fields
results, optic nerve status
Identifying OHT Signals
Subject is a “false positive”
IOPg much higher than IOPcc
CH, CRF are very high
Signal is smooth
Rules-of-thumb
Spotting NTG/LTG
IOPg
High
Ave
Low
X
X
IOPcc
X
X
CH
CRF
CCT
X
X
X
- Watch for:
- IOPcc higher than IOPg, but may still be in “normal” range
- Low amplitude signals, some noise
- Family history, race, age, CDR, Diabetes status, Visual fields
results, optic nerve status
Identifying NTG Signals
IOPcc Higher than IOPG
Low CH
Low CRF
Low amplitude peaks
Thin CCT
Rules-of-thumb
Spotting “unusual” eyes / corneas
- Atypical measurement signals that are:
- less repeatable than normal
- highly variable numeric measurement values
- Investigate previous ocular history for surgery, disease, trauma, etc
- What to do:
- take a series of measurement
- look for the “best signals” possible
- try to get two that look similar and yield similar results
- delete clearly “bad” signals and use average values of good ones
Unusual Signals
“Bizarre” signals are often very repeatable
Just the fact that they are “different” is
telling us something about the cornea / eye