Interpretation of corneal tomography

Transcription

1 Interpretation of corneal tomography Presented by Chameen Samarawickrama - Westmead Hospital - Liverpool Hospital - University of Sydney - University of New South Wales The University of Sydney Page 1

2 Financial disclosures Early Career Research Fellowship (Westmead Charitable Trust) The University of Sydney Page 2

3 Uses of corneal tomography Cataract Toric IOL insertion Lens densimetry Post-refractive IOL calculations Limbal relaxing incisions Cornea/refractive Ectasias Intra-corneal ring segments Corneal graft planning Ocular surface diseases Pterygia Scars Salzman nodules Glaucoma Anterior chamber depth Angle estimation The University of Sydney Page 3

4 Torics at Westmead Westmead Hospital Audit Aug Oct ophthalmologists 5 registrars 2 fellows 22 toric IOL Pre-op cyl: 1.61 ± 0.9 Post-op cyl: 1.23 ± 0.23 Pre-op cyl Post-op cyl Change The University of Sydney Page 4

5 Introduction of a toric pathway Regular astigmatism Even bow tie pattern Axis of astigmatism Match within 15 deg Power of astigmatism Match within 0.5D The University of Sydney Page 5

6 Power of appropriate utilization of tomography Feb April torics (25 followed pathway) By 1 month 26 of 29 had uncorrected vision of 6/12 or better 21 (78%) had SER within 0.5D of target Mean preop versus postop cylinder Refractive Cylinder (D) Mean ± SD Range Available data n = 34 Preoperative 2.77 ± to Month Postoperative 0.89 ± to 1.75 P- value The University of Sydney Page 6

7 Aims 1. Understand differences between tomography and topography 2. Reference plane and standardized settings 3. Systematically read tomography maps 4. Examples of common pathologies 5. Specific maps that are useful for diagnosis of ectasia The University of Sydney Page 7

8 1. Methods to assess cornea Red reflex Retinoscopy Keratometry Keratoscopy Topography* Tomography * Topography: 2-dimensional surface mapping Tomography: 3-dimensional modelling The University of Sydney Page 8

9 Topography 2-dimensional mapping of surface contours Accuracy and data acquisition affected by: Working distance Disc size Alignment with cornea Focusing of rings (increased difficulty with peripheral cornea) No central corneal data Susceptibility to error due to corneal irregularity Anterior surface data only Data from 60% of corneal surface (limitations with peripheral ectasias) The University of Sydney Page 9

10 Effect of misalignment Misalignment of corneal apex with reference axis Danger of misdiagnosis if only looking at curvature! The University of Sydney Page 10

11 Tomography is different 3-dimensional digital rotating Scheimpflug ELEVATION based system Rotating camera takes optical cross-sectional images Reconstructs complete anterior segment The University of Sydney Page 11

12 Scheimpflug principle Theodore Scheimpflug The University of Sydney Page 12

13 Scheimpflug principle The University of Sydney Page 13

14 Advantages of a Scheimpflug system Camera rotates around fixation point Minimises artifacts generated by small movements More accurate image registration Huge number of data points Up to 138,000 analyzable data points per map Accurate anterior and posterior surface data Cross-sectional data allows accurate pachymetry Reconstructions based on elevation data (not curvature) Not dependent of reflections Irregular surfaces Corneal opacities Post-op posterior surface Corneal apex The University of Sydney Page 14

15 Difference between topography and tomography The University of Sydney Page 15

16 2. Importance of reference plane Chimborazo Everest Mauna Kea Equador Nepal Hawaii A. 6,268m 8,848m 4,205m B. 4,118m 4,650m 10,200m C. 6,384km ~4,000km ~3,500km The University of Sydney Page 16

17 Reference plane for tomography The University of Sydney Page 17

18 Understanding the best fit sphere The University of Sydney Page 18

19 Standard settings for BFS Diameter of 8mm Not too flat, not too steep Most maps will contain only valid data Missing data is usually not an issue except in markedly abnormal corneas Standardized interpretation Belin and Ambrosio screening indicies are set to BFS Dia=8mm Float setting Allows software to move the reference BFS radius to best match the individual cornea Elevation scale Set to -75 microns to +75 microns Standardized colour scheme The University of Sydney Page 19

20 3. The 4 map refractive display The University of Sydney Page 20

21 A. Check quality of scan The University of Sydney Page 21

22 B. Look at the pictures Curvature Front elevation Thickness Back elevation The University of Sydney Page 22

23 Interpretation order The University of Sydney Page 23

24 Keratoconus The University of Sydney Page 24

25 Normal elevation values Back elevation Front elevation Normal <6µm <8µm Suspect 6-15µm 8-17µm Pathological >15µm* >17µm The University of Sydney Page 25

26 C. Detailed numbers - shape Anterior and posterior corneal surface data K1 and K2 simulated K s representing calculated power of surface Km mean K within 3mm zone Axis axis of astigmatism (note if set for flat or steep) Astig amount of astigmatism (difference between K1 and K2) The University of Sydney Page 26

27 Detailed numbers - thickness Pachymetry data Pachy apex = corneal apex Thinnest local may defer in KCN - Normal > 500 µm - Suspect µm - Pathology < 480 µm Kmax data Steepest K on anterior cornea KPD Anterior K true net K Influence of posterior corneal K The University of Sydney Page 27

28 4. With the rule astigmatism The University of Sydney Page 28

29 Regular vs irregular astigmatism Regular astigmatism Irregular astigmatism The University of Sydney Page 29

30 Keratoconus The University of Sydney Page 30

31 5. Belin Ambrosio Display (BAD) The University of Sydney Page 31

32 Understanding the BAD The University of Sydney Page 32

33 Compare 2 maps The University of Sydney Page 33

34 Myopic refractive surgery The University of Sydney Page 34

35 Post LASIK ectasia The University of Sydney Page 35

36 Summary Tomography is different to topography Newer generation of scanning technology Powerful tool to aid in diagnosis and management of many common conditions Best results are achieved with standardizing the settings and displays Using a systematic method of interpretation it is possible to consistently diagnose many corneal pathologies Pattern recognition! The University of Sydney Page 36