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Irregular astigmatism:definition, classification, topographic and
clinical presentation
Ming X. Wang, MD, PhD
Clinical Associate Professor of Ophthalmology of University of
TennesseeDirector, Wang Vision Institute
Nashville, TN, USA
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Coworkers
� Tracey Swartz, O.D., MS;
� Shawna Hill, O.D.;� Helen Boerman, O.D.;
� Lav Panchal, M.D.;
� Yangzi Jiang, M.D.,O.D.;
� No financial interest.
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Irregular astigmatism
� Definition and classification;� Statistical indices and KC risk indices;� Clinical causes;� Other visually significant corneal changes;� Topo-WF correlates of irregular astigmatism;� Treatment.
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Definition and classification
Irregular astigmatism, defined as astigmatism where the principle meridians are not 90 degrees apart and associated with loss of vision, represents one of the most serious and frequent complications of corneal refractive surgery.
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Regular vs irregular astigmatism� Regular astigmatism is correctable using a cylindrical
spectacle lens;� Irregular astigmatism occurs when the orientation of the
principal meridians changes from one point to another across the pupil, or when the amount of astigmatism changes from one point to another;
� The further distinction of irregular astigmatism includes regularly or irregularly irregular astigmatism and relates to the presence of pattern recognition on computerized topography. Irregularly irregular astigmatism is rough or uneven, and shows no recognizable pattern on topography.
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Classification • Irregular astigmatism with defined pattern (Macroirregular, or regularly irregular astigmatism): There is a steep or flat area of at least 2 mm of diameter, which is the primary cause of the astigmatism.
• Irregular astigmatism with undefined pattern (Microirregular, or irregularly irregular astigmatism): Multiple irregularities; big and small, steep and flat, and profile maps are almost impossible to calculate.
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Regular astigmatism
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Irregular astigmatism: Astigmatism in which different parts of the same meridian have different degrees of curvature
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Irregular astigmatism
� Definition and classification;� Statistical indices and KC risk indices;� Clinical causes;� Other visually significant corneal changes;� Topo-WF correlates of irregular astigmatism;� Treatment.
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Statistical indices and KC risk indices
� SimK, SAI, SRI;
� SF, CIM, MTK, � KC risk index panel.
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Sim K, SAI, SRI.� Simulated keratometry (SimK) provides the power and axis of the
steepest and flattest meridian similar to values provided by thekeratometer. They are calculated from rings 7 to 9 corresponding to the position on the cornea at which keratometry measurements are obtained.
� The surface asymmetry index (SAI) is a centrally weighted summation of differences in corneal power between corresponding points 180°apart on 128 equally spaced meridians. The SAI approaches zero for a perfectly radially symmetrical surface and increases as the corneal shape becomes more asymmetrical.
� The surface regularity index (SRI) is calculated from a summation of local power fluctuations along 256 equally spaced meridians. The SRI rises with increasing irregular astigmatism and approaches zero for a smoothcorneal surface.
� Numerous other indices have been developed such as potential visual acuity, average corneal power, coefficient of variation of corneal power, and algorithms for the detection of keratoconus.
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SF
� Shape Factor (SF) is the measurement of corneal asphericity. A negative SF usually indicates a post-refractive surgery eye with the center flatter than the periphery (oblate).
� Normal 0.13 to 0.35 � Borderline 0.02 to 0.12 � Abnormal -1.0 to 0.01 .
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CIM
� Corneal Irregularity Measurement (CIM) is an index which represents the irregularityof the corneal surface. Higher the value of CIM predicts more irregularity
� Normal 0.03µm to 0.68µm � Borderline 0.69µm to 1.0µm � Abnormal 1.1µm to 5.0µm
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CIM
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MTK� Mean Toric Keratometry (MTK) indices use
elevation data to compare the toric reference to the actual cornea. The mean apical curvature value helps select the best toric fit using a sphero-cylinder design. This provides the most accurate toric representation of a patient's cornea.
� Normal 43.10 diopters to 45.90 diopters� Borderline 41.80 diopters to 43.00 diopters and
46.00 diopters to 47.20 diopters� Abnormal 36.00 diopters to 41.70 diopters and
47.3 diopters to 60.0 diopters
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Combining CIM, SF and MTK, to determine KC risk
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CL Distortion
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Subclinical keratoconus
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Irregular astigmatism
� Definition and classification;� Statistical indices and KC risk indices;� Clinical causes;� Other corneal visually significant
changes;� Topo-WF of irregular astigmatism;� Treatment.
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Clinical causes of irregular astigmatism� Irregular but stable cornea (e.g., irregular scar surface):
cornea is irregular because of LOCAL geography;� Irregular but unstable cornea (biomechanical
decompensation): cornea is irregular because of GLOBAL corneal weakness;
� Clinical causes:� CL warpage/overwear� FFKC/KC/Pellucid� Chalazion� Pterygium� Trauma� Limbal/corneal dystrophies� Keratorefractive surgery (LASIK, PRK, CK, ectasia)� Others
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CL warpage
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Our most common irregularity
without defined pattern post
LASIK is dry eye (47% of HOA in
on study)
Irregular astigmasim(dry eyes)
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Irregular astigmatism(dry eyes con’t)
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KC
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Elevation and curvature flipped with KC
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KC
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KC
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Pellucid curvature and elevation maps
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Corneal scarring
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Extreme GPC or other lid disorders may affect corneal shape
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HPV CIN causing changes
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Peripheral melt/Mooren’s
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Pterygium
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CK (left) vs. LASIK (right) for presbyopia (optical zone issues)
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Decentered treatment
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The power of 3 – 3-D stereo cornealtopographer
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3-Stereo Cameras
Stereo Views of 8mm
Sphere
3-D stereo corneal topography:
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Initial Images (3-camera, checker board)
@@@
@@
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S/P myopic LASIK with ridge
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S/p LASIK (irregularity map)
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Irregular astigmatism
� Definition and classification;� Statistical indices and KC risk indices;� Clinical causes;� Other visually significant corneal changes;� Topo-WF correlates of irregular astigmatism;� Treatment.
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Other visual significantly corneal changes
� Extremely flat (or steep) corneas;� Small optical zones;� Posterior corneal changes (anterior
and posterior corneal surface become decoupled after anterior ablative procedure).
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Flat cornea s/p LASIK (visual qualityissues)
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Posterior Surface changes after LASIK
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Posterior changes with normal anterior corneal surface (visual quality issue)
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S/P H-LASIK, small optical zone (night visionproblems)
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Irregular astigmatism
� Definition and classification;� Statistical indices and KC risk indices;� Clinical causes;� Other corneal visually significant changes;� Topo-WF correlates of irregular astigmatism;� Treatment.
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Corneal topographers
� Placido disc: Humphrey, EyeSys, Tomey Optikon, Topcon, Magellan, Orbscan, AstraMax 3-D;
� Scanning/projecting slit: Orbscan, AstraMax 3-D;
� 3-D stereo: AstraMax 3-D;� Schimpflug rotating camera:
Pentacam; Precisio;� Ultrasound: Artemis.
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3-D stereo checkboard AstraMax
3-Stereo Cameras
Stereo Views of 8mm
Sphere
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Magellan
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Pentacam
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Corneal topo and combined systems
� Placido disc: Humphrey, EyeSys, Tomey Optikon, Topcon, Magellan, Orbscan, AstraMax 3-D;
� Scanning/projecting slit: Orbscan, AstraMax 3-D;
� 3-D stereo: AstraMax 3-D;� Schimpflug rotating camera:
Pentacam;� Ultrasound: Artemis;
� Topo-WF combined systems.
New Book: “Irregular Astigmatism”(SLACK)
New comprehensive textbook
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Topo-WF correlates of irregular astigmatism
ComaDecenteredablations
Increase in coma and trefoil
Irregular astigmatism
Increase in spherical aberration
Loss of prolatecornea, small optical zone
WAVEFRONT ABERROMETRY
TOPOGRAPHY
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Decentered ablation (topo)
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Decentered ablation (WF: coma)
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Small optic zone s/p H-L (topo)
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Small optical zone s/p H-L (WF: spherical aberration)
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Irregular astigmatism (topo): night glare and decreased BSCVA
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Irregular astigmatism (topo, 3-D)
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Irregular astigmatism (WF: trefoil, SA, coma)
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Irregular astigmatism
� Definition and classification;� Statistical indices and KC risk indices;� Clinical causes;� Other visually significant corneal changes;� Topo-WF correlates of irregular astigmatism;� Treatment.
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Treatment options for irregular astigmatism
� RGP;� C-CAP for decentered treatment;� Wavefront-driven custom;� Topo-driven custom;� Intacs for KC;� Topo-wavefront combined approach (ACAP);� Others.
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RGP
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C-CAP for decentered treatment
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C-CAP: pre and post and difference (volume of corneal tissue removed) map
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Wavefront-driven custom: Irregular astigmatism (coma, trefoil)
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Wavefront-driven custom Rx for irregular astigmatism (coma and trefoil, volume of corneal tissue removed Rx plan)
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Wavefront-driven custom for irregular astigmatism: preop WF and corneal removal volume Rx plan
Preop WF Rx plan (corneal tissueto be removed)
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Topography-guided custom LASIK
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Topo-driven custom: preop topo (central island) and postop topo, comparing custom (OD) with non-custom (OS) (Stojanovic)
Pre-therapeutic Rx Pre-therapeutic Rx
Post (custom) Post (conventional)
Central islandcorrected
Central islandremains
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What to base to treat, topo or WF?� Topo-driven: correct where the problem is (s/p keratorefractive
surgery). Can create ideal cornea, but can have non-ideal overall WF, and hence may not has as good vision as WF-driven; Let the cornea bear what is rightfully its burden;
� WF-driven: Can create ideal overall WF, but can have irregular cornea. Vision can be good (but can be temporally, since cornea is corrected to compensate HOA of lens, and lens HOA changes more sensitively over time). Let the poor cornea bear the ENTIRE visual axis WF error correction burden (i.e., assuming location of aberration, i.e., lens vs cornea, does not matter).
� Topo-WF weighted approach: Alpin’s. But, how about the issue of non-simultaneity of optimization of lens and cornea?
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Location of aberration along visual axis DOES make a difference: Treating anterior corneal astigmatism on corneagives good result (lesser untreated cylinder)
End result: a nice circular image on retina, minimal residual uncorrected cylinder.
Circular incoming light.
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Location of aberration along visual axis DOES make a difference: treating non-anterior corneal cyl oncorneagives unsatisfactory result (more cyl left untreated)
Circular incoming light.
End result: a bizarre imperfect image on retina, due tomoreresidual uncorrected cylinder.
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VISX model:the Advanced Corneal Ablation planner (ACAP)
AblationAchieved CAP Treatment
Surface Target
AblationCalculated Treatment
Optical Target
Patient 2Patient 1
Pre-op Elevation map
AblationCalculated CAP Treatment
Surface Target
AblationAchieved CAP Treatment
Surface Target
AblationCalculated Treatment
Optical Target
Patient 2Patient 1
Pre-op Elevation map
AblationCalculated CAP Treatment
Surface Target
Combined topo-WF approach
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ACAP-topo-WF combined approach
� Integration of wavefront, topography, and VISX laser using WaveStar software;
� Software allows surgeons to build various models representing the “optical target”, defining the desired optical properties of the cornea after surgery based on combinations of wavefront and topographic information.
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ACAP-topo-WF combined approach
Ablation profileCornea
Target corneal shape
Ablation profile
Alternative target corneal shapes
(A)
(B)
Cornea
Target corneal shape
Ablation profile
Alternative target corneal shapes with same power change
(A)
(B)
Cornea
Target corneal shape
Ablation profile(A)
(B)
CorneaAblation profile(A)
(B)
Ablation profileCornea
Target corneal shape
Ablation profile
Alternative target corneal shapesAlternative target corneal shapes
(A)
(B)
(A)
(B)
Cornea
Target corneal shape
Ablation profile
Alternative target corneal shapes with same power changeAlternative target corneal shapes with same power change
(A)
(B)
(A)
(B)
Cornea
Target corneal shape
Ablation profile(A)
(B)
(A)
(B)
CorneaAblation profile(A)
(B)
Various models for treatment (corneal tissue removal volume) based on various weighted combinations of wavefront and topography data.
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Intacs for KC
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Intacs for KC
PRE-OP POST-OP
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The first case of femtosecond laser- Intacsfor recurrent KC on graft
BSCVA=20/200DiplopiaRGP intolerantNeeds repeat graft
BSCVA=20/20Happily wearing SPECTACLESNo diplopia
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Irregular astigmatism
� Definition and classification;� Statistical indices and KC risk indices;� Clinical causes;� Other visually significant corneal
changes;� Topo-WF correlates of irregular
astigmatism;� Treatment.