Download Normal Thin Cornea CCT = 493 µm

ESCRS 2012 - Course IC-98 Modern Management of Irregular Cornea
Renato Ambrósio Jr, MD, PhD

Professor of Ophthalmology of Federal University of São Paulo and Pontific Catholic Federal
University of Rio de Janeiro

Scientific Coordinator of the Rio de Janeiro Corneal Tomography and Biomechanics Study Group

Director of Cornea and Refractive Surgery of Instituto de Olhos Renato Ambrósio, Visare Personal
Laser and Refracta-RIO
Correspondence to:
Renato Ambrósio Jr
Rua Visconde de Pirajá 595/808 – Ipanema
Rio de Janeiro, RJ – 22410-002
[email protected]
I - Understanding the Irregular Cornea.

Irregular Cornea can affect the visual outcomes of cataract surgery.
Preoperative evaluation is crucial to identify patients with irregular cornea, in
order to deal with their expectations but also to improve the visual results.
For this purpose it is necessary:


Clinical History

Previous Surgeries

Medications

Other Conditions
Understanding “Irregular Astigmatism”

Optical Properties (Wavefront
Analysis)


Opacity
Understanding “Architecture Stability”

Ectasia ?

Predisposition or Susceptibility to
Ectasia

Understanding the Ocular Surface &
Tear Film
II - The role of Corneal ToPography… Is it enough?

It is fundamental for tear film evaluation. Disturbance in the tear film layer is an
important cause of irregular astigmatism that can affect vision.

Computerized corneal topography provided more sensitivity to detect keratoconus
patterns in asymptomatic patients, but there are issues related to “false
keratoconus” diagnosis. These problems are due to irregular corneal surface and
not to true ectasia (Example 1).
Example 1: Is this Ectasia after PRK?

22 yo male, 6 months after PRK for -4D.

Progressive inferior steepening is documented in axial map.
BUT

No thinning is present in pachymetric map.Post-operative
haze was present in slit-lamp and Scheimpflug images.
This case represents the issues related to “false keratoconus” diagnosis with
corneal ToPpgraphy. These problems are due to irregular corneal surface and not
to true ectasia

The diagnosis of FFK is critical when screening refractive surgery candidates, because
such conditions are the most important risk factors for progressive “iatrogenic” ectasia
that may occur after LASIK and Surface Ablation.

About 1% of refractive candidates have ectasia detected during screening (keratoconus
and pellucid marginal degeneration). The majority of such cases may present with
normal BSCVA and unremarkable slit lamp biomicroscopy (Examples 2 and 3).
Example 2: Forme-Fruste or Early (or better “mild”) Pellucid Marginal
Corneal Degeneration (PMCD)

57 years old male, presented as a candidate for LASIK

Stable refraction for 8 years; no family history of keratoconus

UCVA was 20/400 in OD and counting fingers in OS

Manifest refraction in O: −8.50 +2.50 × 013, giving 20/20;
and in OS: −9.00 +2.00 × 173, giving 20/20.

Ultrasonic corneal pachymetry measurements: 550 and 541
micron in OD and OS respectively.

Regional inferior peripheral thickness were 518 and 522
micron in OD and OS respectively.

Cornea examination by slit lamp demonstrated superficial
punctuate keratitis in both eyes but no evidence of corneal
thinning, iron lines, or protrusion.

The patient was advised not to undergo LASIK or PRK and to
return for a new exam within 6 months. If stability is
documented, Custom Surface Ablation may be advocated with
a detailed informed consent.
(Ambrósio R Jr, Wilson SE. Cornea. 2002 Jan;21(1):114-7.)
 Corneal topography revealed inferior steepening with the
pattern of a “lazy C” or lobster claw shape and an area of
central corneal flattening. Age presentation and localized
inferior thinning are favorable for the diagnosis of early
pellucid corneal marginal degeneration. It has been
suggested the term forme-fruste pellucid for describing such
cases.
However, this may be debatable if this is a variant of keratoconus. We
believe that the complete differentiation between keratoconus and PMCD
may be done only with elevation 3D cornel tomography and a
comprehensive pachymetric evaluation over the entire cornea.

Computerized corneal topography provided more sensitivity to detect keratoconus
patterns in asymptomatic patients.

The advent of progressive “iatrogenic” ectasia after LASIK and Surface Ablation
despite of normal topography and without other identifiable risk factors lead to the
understanding of the need for more sensitive diagnosis.
Example 3: Asymmetric Forme-fruste Keratoconus (not unilateral)
with normal curvature maps in the contra-lateral eye

23 years old male, presented as a candidate for LASIK

No contact lens history; no family history of keratoconus

Mild allergy; Positive for eye rubbing

UCVA was 20/200 in OD and 20/80 in OS and BSCVA to
20/20 in OD and 20/15 in OS

MRx: -2.75 -1.25 x 27 – OD and -1.00 -0.50 x 126 – OS

Corneal Hysteresis (CH) and Corneal Resistance Factor (CRF)
were 8.4 and 9.1 mmHg and 6.1 and 7.2 mmHg in OD and
OS respectively with a low amplitude ORA signal in OU.

CCT (US): 519 and 531 micron in OD and OS

Slit Lamp is Normal in OU
 Placido’s axial curvature map revealing keratoconus pattern in
OD and a normal pattern OS. Considering the patient is
asymptomatic unless for myopic astigmatism, but with normal
BSCVA, the right eye would be considered as a forme-fruste
or mild keratoconus. Interestingly, the left eye has a
remarkably normal topography. Cases like the left eye
represent the best model to test if the enhanced screening tests
are sensitive to detect any abnormality (see enhanced test
results).

There are also cases with topographic signs of keratoconus, such as
inferior steepening which are stable with no progression over time.
This may represent 0.5% of normal population (Example 3).
Example 3: Asymmetric bow tie, stable for over 10 years

33 years old male; UCVA 20/15 OU

MRx: +0.25 = -0.25 x 21 - OD and plano OS

Corneal Hysteresis (CH) and Corneal Resistance Factor (CRF)
were 11.8 and 10.6 mmHg and 11.2 and 10.1 mmHg in OD
and OS respectively with a normal ORA signal in OU.

CCT (US) is 502 and 505 micron in OD and OS

Slit Lamp is Normal in OU

Placido’s Topography is remarkably similar to the Pentacam’s
Sagittal Anterior Map in OU with inferior steepening and
asymmetric bow tie.

Conclusion: Normal thin cornea (this case illustrates enhanced
specificity).
 Axial curvature maps with IS /ABT, a keratoconic suspect
pattern OD.
 USVA is 20/15 and there is also documented topographic
stability over 10 years.
III - The Concept of Ectasia Susceptibility

Corneal thinning is a hallmark of these ectatic diseases. The area of maximal
thinning, relative to the location of maximal corneal protrusion differentiates
keratoconus, pellucid marginal degeneration, and keratoconus.

Ectasia is a process of biomechanical failure, which is the biological equivalent of a
well-known composite science process described in biomechanical engineering by
Puk and Knops: interfiber fracture.

Significant evidence supports that thinning does occur prior to steepening.

A genetic predisposition, combined with behavioral (eye rubbing) and
environmental stress factors influence the biomechanical susceptibility to develop
ectasia.

Thereby, a concept of a balance between corneal resistance (individual geneticdriven corneal biomechanical and biochemical properties) and stress factors
(individual phenotype) would lead to a net result which we refer as ECTASIA
SUSCEPTIBILITY.

More sensitive analyses reveal a continuum of findings from normal cornea towards
ectatic disease, even in its earliest presentations. For example, this is well
documented that some family members of keratoconus patients have mild
topography abnormalities.

Studies from asymmetric keratoconus and examples indicate that novel tests based
on corneal tomography and biomechanical measurements are sensitive to detect
abnormalities in the contra-lateral eyes with normal topography (Salomão, ASCRS
2008).

We believe that any cornea may undergo ectasia if enough stress is applied to
overpass its resistance limit, leading to biomechanical failure.

For example, some corneas may undergo spontaneous ectasia (keratoconus) even
without eye rubbing history. Others cases may undergo ectasia if there is enough
stress, such as eye rubbing and corneal surgery to overcome corneal resistance limit.
IV - Evolution in Corneal Imaging: from ToPography to ToMography.

Along with anterior curvature data, Corneal ToMography (CTm) provides detailed
architecture information so that elevation maps from the front and back surfaces
are calculated, along with the pachymetric map.

Much attention has been devoted to the posterior corneal elevation map. The BFS
(best fit sphere) for the 8 mm corneal area is the most accepted parameters for
referencing the elevation map.

Anterior and Posterior Enhanced Elevation: standard BFS “subtracted” from the
enhanced BFS (best fits to peripheral cornea excluding 4mm in diameter centered
on the thinnest).
Figure 2: Belin’s Concept for Enhanced Elevation. Peripheral fit highlights the cone
area.
Figure 3: Normal Enhanced Elevation in a thin cornea.

Enhanced Elevation Map (Three colors Format):
Anterior: Green < 6,
Yellow: 6 – 12, Red > 12 µm
Posterior: Green < 8, Yellow: 8 – 20, Red > 20 µm

Corneas with higher elevation around the TP (likely ectatic) have pronounced
differences between the standard and enhanced BFS (YELLOW and RED).

The thickness map provides detailed information regarding the thinnest point
(value and location in relation to the apex [0;0]) and pachymetric distribution
(Figure 2)
Normal Thin Cornea
CCT = 493 µm
Figure 4: CTSP and PTI Graphs for the thickness profiles. Example from a normal thin
cornea.

Corneal Thickness Spatial Profile (CTSP): average of the thickness values along
twenty-two imaginary circles centered on the thinnest point (TP).

Percentage Thickness Increase (PTI): percentage of increase of each of these circles
from the TP.

CTSP and PRI Graphs displays 95%CI limits of normals.

Thinned corneas (likely ectatic) have profiles out of the 95% CI - more abrupt
(going down) increase.
V - The importance of Scheimpflug Images - Understanding ToMography.

Pentacam system provides three-dimensional Scheimplflug images.

These images can complement our assessment (e.g. Pellucid Marginal
Degeneration; Haze after PRK; Recurrent Ectasia after PKP for Keratoconus)
Example 4: The importance of Scheimpflug Images
 Thick Flap related ectasia
 “True” Pellucid Marginal Degeneration
 Recurrent Ectasia after PKP for Keratoconus
VI - Corneal Biomechanics with the ORA (Ocular Response Analyzer, Reichert)

Corneal response to a collimetric air pulse is monitored by the infrared light
reflection (applanation => peak)

Detects two applanation events correlated with the air pulse pressure (INWARD p1 and OUTWARD - p2)poe



The delay of p2 caused by corneal viscous damping
[CH = p1 – p2] and [CRF = p1 - (K * p2)]
Normal Values: CH: 10.17 ± 1.82 mmHg (3.23 to 14.58)
CRF: 10.14 ± 1.8 mmHg (range 5.45 to 15.1) A B
ORA Signal
2
1
4
3
Figure 5: ORA Measurement and ORA Normal Signal

Ectasia leads to lower CH and CRF and altered signals

CH or CRF < 8.8mmHg is considered a relative contra indication for LASIK
based on normal population values

Advanced Bio-corneagram Analysis provides 38 waveform morphology
parameters.

Combination of these new parameters can provide new information regarding
corneal behavior, allowing a better biomechanical study.
VII - Corneal Biomechanics with the Corvis ST (Oculus, Germany)

Ultra High-Speed (UHS ST) Scheimpflug Technology: 4,330 frames/sec that
monitors 8mm horizontal Scheimpflug image in response to a symmetrically
metered air pulse with fixed peak pressure.

The metered collimated air pulse or puff has a symmetrical configuration and fixed
maximal internal pump pressure of 25 kPa. The bidirectional movement of the
cornea in response to the air puff is monitored. Measurement time is 30ms, with
140 frames acquired. Advanced algorithms for edge detection of the front and
back corneal contours are applied for every frame.

IOP is calculated based on the first applanation momentum.

Deformation amplitude is determined as the highest displacement of the apex in
the highest concavity momentum. Applanation length and corneal velocity are
recorded during ingoing and outgoing phases.

Such parameters provide clinical in vivo characterization of corneal biomechanical
properties, which are relevant for different applications in Ophthalmology.
Figure 6: Corvis ST and corneal Scheimpflug imaging in response to a
symmetrically metered air pulse with fixed peak pressure
II. Conclusions
A.
Curvature -based Corneal Topography is a more traditional and intuitive language for
Ophthalmologists and will always be critical since it reflects refractive power of the cornea
and optical regularity. It is (and will always be) a critical step for the evaluation of
refractive properties of the cornea and quality of the ocular surface tear film.
B.
However, curvature maps does not represent all the picture for screening candidates for
Refractive Surgery.
C.
Corneal Tomography is defined as a 3D representation of the corneal architecture, with
detailed and reliable data from the front and back surface of the cornea and a pachymetric
map.
D. Elevation subtraction maps is the preferred method for describing the front and back
surfaces of the cornea. However, this is a more complex way and less intuitive for the
general Ophthalmologist. In addition, there are many possible options to calculate the
reference surface for the subtraction with the corneal surface (front or back).
E.
Corneal Thickness Distribution enables the identification of ectasia and the differentiation
of a normal thin cornea from ectasia.
F.
Corneal Biomechanical measurements represent a complementary method for the
enhanced screening for ectasia.
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