Corneal Topography

Question 1

What is corneal topography used for?

Answer 1

• Measuring cornea, measuring thickness, shape & trying to generate numbers from measurements & colour coded maps
• Diagnosis of corneal abnormality & disease
• Pre-operative assessment for refractive surgery – laser/lasik
• CL fitting
• Postoperative assessment of corneal transplants
  Commonly used in CL practices & laser clinics – less common in optometric practice due to costs

Question 2

What are the three main advantages of a corneal topography scan?

Answer 2

• Non-contact
• V quick
• V comfortable for px

Question 3

Briefly describe refractive surgery?

Answer 3

• Cornea reshaped (often thinned), typically by laser, to correct refractive error  usually for myopia & astigmatism (& hyperopia to some extent)
• Corneal topography carried out on every px prior to surgery
• Identifies high-risk pxs (who should not have surgery)
• Helps detect mild forms of pre-existing corneal ectasia (pre-clinical keratoconus):
  o Forward bulging of cornea -> anyone with this should ideally not undergo refractive surgery -> cornea already slightly thinner & at risk of breaking post-surgery
• Identification of thin corneas
• 1.3% refractive candidates not suitable for surgery

Question 4

Describe CLs & Ortho-K? What are placedo rings?

Answer 4

• Requires detailed knowledge of peripheral corneal shape
• Topography provides peripheral Ks & asphericity readings
• These data are needed to determine a 1st trial lens & order lens (semi-custom made)
  Placedo rings -> used to illuminate cornea, capture those reflections, then do corneal analysis with attached computer

Question 5

Describe the outputs maps of corneal topographer?

Answer 5

• Topography provides geographic maps of various output measures:
  o Corneal radius of curvature
  o Corneal power
  o Corneal elevation
  o Corneal thickness – important for pressure measurement
• Difference to traditional methods of corneal assessment: maps, rather than single-point measurements.
  o Topography gives globe overview of corneal measurement – measurements across surface & not just single point measurement
   K readings typically only centre of cornea
• Diagnostic info on ‘entire’ cornea, rather than single spot

Question 6

How do you interpret a corneal topography map?

Answer 6

• Cool colours correspond to flat curves (K) & elevation values below the reference sphere (blue or violet)
• Mild colours correspond to medium curvature (K) & elevation values equal to reference sphere (green or yellow) – normal cornea w/ normal limits
• Warm colours correspond to high curvature (steeper K) & elevation values above reference sphere

Question 7

Describe the corneal radius map of the Oculus Keratography corneal topographer?

Answer 7

• Relatively simple topographer
• Global scale
• Reflections from placedo rings -> incomplete lines in lower half could be shadow of nose, in upper half could be obscured by lashes or lids
• Colour-coded output -> change in colour from orange to green indicates flattening of cornea

Question 8

Describe the four standards map of the Pentacam corneal topographer?

Answer 8

• Get maps – usually 4 but can get more
• Pxs like looking at colour coded images when having things explained -> improves px-practitioner relationship
• Corneal thickness -> thinner at centre, thickened towards periphery in all meridians

Question 9

Describe the colour-coded keratometry map of the Wave Analyser corneal topographer?

Answer 9

• Each device comes with scale telling you which colour means what
• Look for asymmetry – indicates part of cornea is steeper than other part

Question 10

Describe the output indices of corneal topographer?

Answer 10

• Calculated form global corneal data
• Allows quantification of level of irregularity of cornea
• Useful in determining whether cornea should be considered as outside normal range -> is it a cornea where should not fit CL or need particular type of CL?
• Instrument specific, not usually interchangeable between instruments
• Various keratoconus indices have been developed

Question 11

What is keratoconus?

Answer 11

Progressive condition – tends to affect younger people – cornea starts to bulge forward & becomes steeper & irregular -> tends to progress over number of years
Advanced: ↓acuity, high astigmatism & thinner cornea
Px to px w/ keratoconus may have widely different corneal powers & thickness

Question 12

What are the keratoconus indices from corneal topographer?

Answer 12

• Enhanced Ectasia Display:
  o Combines 9 parameters to come up with 1 number -> means can easily look at number & tell if normal vs abnormal (healthy vs keratoconus corneas)
• Keratoconus Severity Index:
  o <15% considered normal, 15-30% suspected keratoconus, >30% subclinical keratoconus
  o Combining various indicators for normality/keratoconus in a single index
  –> More than just 2 keratoconus indices

Question 13

Describe the reliability of measurement of corneal topography scan?

Answer 13

• Repeatability & reproducibility usually v good
• Objective measurements (minimal user input) - ↓operator related errors
• Differences in repeated measurements small, often 2%
• Measurements w/ different instruments are mostly NOT INTERCHANGEABLE

Question 14

What are the factors that influence corneal topography scan?

Answer 14

• Poor tear film
• Irregular ocular surface
• Blinking
• Considerable lid coverage of cornea
• Unsteady eye/ head position
• Uncooperative px behaviour

Question 15

What are the things to bear in mind with a corneal topograph scan?

Answer 15

• Look for asymmetry
• If in any doubt about lashes/lids affecting measurement -> repeat
• Corneal topography used to assess corneal shape changes after CL wear to monitor safety
  o Important for Ortho-K as they wear CLs overnight
• Pentacam – keratoconus detection map
  HIGHER K READING – FLATTER CORNEA
  LOWER K READING – STEEPER CORNEA
  NORMAL CORNEA – CORNEAL POWER <46D
  Px w/ KERATOCONUS – CORNEAL POWER >46D
  Px to px w/ keratoconus may have widely different corneal powers & thickness
  1/k = D