Limitations of current topographies, clinical problems, and new development

1. Limitations of current topographies, clinical problems, and new development Ming Wang, M.D.,Ph.D. Director, Vanderbilt Laser Sight Center http://www.net-serv.com/drmingwang

2. Corneal topography – an indispensable integral part of any refractive surgical treatment

3. Limitations of wavefront analysis • Wavefront is a total visual axis Z-axis “head-on” shot (including both cornea and lens); • No information regarding the LOCATION of aberration (cornea? lens?); • No information outside entrance pupil; • Accommodation dependent, rather than fixed spatial physical dimension-dependent as in corneal topography.

4. Corneal topography will always be needed • To determine corneal contribution to refractive error; • To gather information outside entrance pupil; • Needed for planning of any cornea-based refractive treatment.

5. Limitations of current corneal topographies • Lack of stereoposis, relying on one camera/one shot, giving rise to over-dependency on ONE set of data and low degree of data degeneracy • “Free-standing” point measurement therefore not possible as it depends on nearby point relationship for mathematical construction. Can thus cause accumulation of errors; • Not possible to discard bad points due to optical image artifact as one only has one image shot for each corneal point.

6. Limitations of current topographiesProne to errors of measurement arising from optical artifact due to one angle/one shot and low data degeneracy • Errors of measurement from abnormal light scattering: dry eye surfaces; • Errors from abnormal light reflection: surface scar; • Errors from abnormal light transmission: corneal edema, corneal stromal opacities.

7. Clinical cases in which existing topo systems break downCase 1 • S/p PTK, with KED; • Topo showed erroneous reading of the posterior surface and pachymetry, while anterior elevation and anterior curvature are OK:

9. Case 1 conclusion • SLE showed KED at 6:00 and 9:00 o’clock; • Blockage of light REFLECTION from corneal endothelium causes erroneous reading of posterior and pachymetry data (both depends on posterior data).

10. Clinical cases in which existing topo systems break downCase 2 • S/p LASIK with epithelial ingrowth; • Topo showed abnormal “thinning” in the area of epithelial ingrowth:

12. Case 2 con’t • The “thinned” portion at 12:00 o’clock was artifactual, due to the BLOCKAGE of light transmission by epithelial ingrowth, affecting DATA COLLECTION.

13. Case 2 conclusion • Conclusion: Erroneous topo reading can be caused by • Prevention of light REFLECTION back from endothelium, as in KED; • Blockage of light TRANSMISSION, as in epithelial ingrowth or scar; • SCARTERING of light by irregular surface, as in DES.

14. Clinical cases in which existing topo systems break downCase 3 • A patient who has no KC hx and normal vision shows “FFKC” in topography:

16. Case 3 con’t • But there are no other signs or symptoms of FFKC. Topography repeated: normal!

18. Case 3 conclusion • What is going on? • The repeated topo was done after instillation of artificial tears • Conclusion: • Dry surface can lead to erroneous topo reading due to light SCATTERING.

19. Clinical cases in which existing topo systems break downCase 4 • Reis-Buckler dystrophy; • Clinical photos:

20. OD

21. OS

22. Case 4 con’t • Topo shows 280um “thinned” area centrally, despite of nomral ultrasound pachy reading of 550um:

25. Case 4 conclusion • Cornea indeed 280um thin? • Ultrasound pachy: 550um. • Conclusion: • Corneal opacity, such as dystrophy or scar, can block light TRANSMISSION and affect topo DATA COLLECTION.

26. Clinical cases in which existing topo systems break downCase 5 • 56 y/o woman with corneal scar os; • Ultrasound os showed 450um centrally, and 520um inferiorly; • Topo showed:

29. Case 5 con’t • Topo showed inferior corneal thickness of only 312um; • While ultrasound showed 520um; • The reason for the difference?

30. Case 5 conclusion • The difference is due to artifactual reading of topo pachymetry, in the setting of corneal scar; • Corneal scar causes abnormal light REFLECTION and block light TRANSMISSION; • Existing topographers are sensitively dependent on optical information, data are subject to surface reflection/transmission abnormalities.

31. Clinical cases in which existing topo systems break downCase 6 • 43 y/o came for LASIK screen; • All w/u normal, except topo showed:

33. Case 6 con’t • Topo curvature map showed central steepening; • No signs or symptoms of KC; • Dx?

34. Case 6 con’t • Repeat topo:

36. Case 6 conclusion • Repeat topo is normal. It was done after copious lubrication; • Conclusion: Dry eyes can cause artifactual topo maps in existing topographers due to irregular surface SCATTERING and reflection.

37. Clinical cases in which existing topo systems break downCase 7 • Repeated topo in a post LASIK pt showed an “void” area os:

39. Case 7 con’t • Explanation for the lack of data points in the temporal area of the cornea os? • Let’s look at clinical photo:

41. Case 7 conclusion • Clinical photo shows an area of temporal corneal scar, blocking light transmission,; • Existing topo systems can show “no data” points, when light transmission/reflection is abnormal.

42. Clinical cases in which existing topo systems break downCase 8 • LASIK screen, normal exam except DES, but topo showed dramatic thinned cornea of 268um centrally:

44. Case 8 con’t • Repeat topo with more tears:

46. Case 8 con’t • Repeat topo was very different from the initial one; • Etiology? • What to do?

47. Case 8 con’t • Copious lubrication and closure of eyes for 30 minutes, in this severe DES eye; • Repeat topo again:

49. Case 8 conclusion • Conclusion: Artifactual topo reading can occur due to severe DES(causing abnormal light REFLECTION).

50. Clinical cases in which existing topo systems break downCase 9 • 2nd opinion, s/p LASIK, with epi ingrowth os; • Topo shows: