Week 3 - Corneal topography and imaging

Question 1

what is the difference between Topography vs tomography?

Answer 1

• Topography: most commonly used term for corneal mapping
• Tomography: Performed by scanned slit, Scheimpflug cameras or optical coherence tomography

Question 2

Why is corneal topography and imaging important and relevant?

Answer 2

• Diagnosis of corneal abnormality and disease
• Contact lens fitting
• Pre-operative assessment refractive surgery
• Postoperative assessment of corneal transplants

Question 3

Is corneal topography commonly used?

Answer 3

• Depends, varies between settings and countries
• Instruments commonly available since 1990’s
• Contact lens practises and laser clinics have been using these instruments for 20+ years
• optometric practice: probably less common, prohibiting factor likely costs

Question 4

How is corneal topography relevant to refractive surgery?

Answer 4

• Corneal topography is carried out on every patient prior to surgery
• Identifies high risk px
• Helps detect mild forms of pre-existing corneal ectasia, e.g preclinical keratoconus
• Identification of thin corneas
• 1.3% refractive candidates not suitable for surgery

Question 5

what is Ortho-keratology, and how does it relate to contact lenses?

Answer 5

• Ortho-K requires detailed knowledge of peripheral corneal shape
• Topography provides peripheral Ks and asphericity readings
• These data are needed to determine a first trial lens and then order lenses

Question 6

What and How is Ortho-Keratology used for contact lenses?

Answer 6

• Ortho-K requires detailed knowledge of peripheral corneal shape
• Topography provides peripheral Ks and asphericity readings
• These data are needed to determine a first trial lens and the order lens (semi-custom made)

Question 7

What can the Topcon Corneal analyzer (CA-800) do, for contact lenses?

Answer 7

• Measures cornea
• Pupil measurements for application of contact lenses

• Fluorescence analysis for contact lens positioning
• Contact lens simulation
• Analysis of wavefront corneal aberrations
• Optional - Toric IOL calculation

Question 8

What are the CA-800 outputs?

Answer 8

• Corneal radius of curvature
• Corneal power
• Corneal elevation
• Corneal thickness
Difference to traditional methods of corneal assessment: maps, rather than single-point measurements.
- Diagnostic information on ‘entire’ cornea, rather than just a small spot

Question 9

How are the maps interpreted?

Answer 9

• Cool colors correspond to flat curves (K) and elevation values below reference sphere (blue or violet)
• Mild colors correspond to medium curvature (K) and elevation values to the reference sphere (green or yellow)
• Warm colors correspond to high curvature (steeper K) and elevation values above the reference sphere

Question 10

What do topography indices outputs do?

Answer 10

• Calculated from global corneal data
• Allows quantification of level of irregularity
• Useful in determining whether cornea should be considered as outside normal range
• Instrument specific, not usually interchangeable between instruments
• There are various types developed

Question 11

What 9 parameter indices that are on a Belin/Ambrosio Enhanced ectasia display?

Answer 11

Permits discrimination between healthy and keratoconic corneas:-
• anterior elevation at the minimum thickness point,
• posterior elevation at the minimum thickness point,
• change in anterior elevation,
• change in posterior elevation,
• corneal thickness at minimum thickness point,
• location of thinnest point,
• pachymetric progression,
• Ambrósio relational thickness and Kmax.

Question 12

What is the keratoconus severity index?

Answer 12

• KSI value <15 % is considered normal, values between 15 and 30 % as suspected keratoconus, and above this value is considered subclinical keratoconus

Question 13

What are the reliability of measurements?

Answer 13

• Repeatability/reproducibility usually very good
• Objective measurements (minimal user input), reducing operator-related error
• Differences in repeated measurements small, often <2%
• Measurements with different instruments are mostly not interchangeable

Question 14

What ate factors affecting measurements:-

Answer 14

• Often patient related:
- Poor tear film
- Irregular ocular surface
- Blinking
- Considerable lid coverage or cornea
- Unsteady eye/head position
- Uncooperative patient behaviour

Question 15

What are some methods/instruments of corneal topography?

Answer 15

• Orbscan
• Pentacam
• Oculus keratograph
• Galilei
• Topcon CA-800
• Wave Analyzer
• OCT (primarily designed for posterior assessment)