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 JrRua Visconde de Pirajá 595/808 –

IpanemaRio de Janeiro, RJ – 22410-002Dr. renatoambrosio@gmail.com

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 genetic-driven 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)

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)

Normal Thin Cornea

CCT = 493 µm

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

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.

4

3

2

1

ORA SignalORA Signal

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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