CL - Corneal topography - Week -3

Question 1

Name 5 limitations of keratometry

Answer 1

assumes refractive index is 1.3375
assumes cornea is spherical
only measures central 3mm of cornea
inaccurate readings if any epithelial anomalies
limited capacity to measure irregular corneal surfaces (e.g. keratoconus)

Question 2

What baseline data can you measure with corneal topography? (6)

Answer 2

Detect corneal pathology
Establish corneal parameters (e.g. HVID, pupil size)
Contact lenses fit
Astigmatism 
Screening tool
Tear film (NITBUT)

remember as DECAST (i made this up)

Question 3

How can corneal topography be used in follow up care? (4)

Answer 3

monitor pathology
corneal stability
corneal shape changes
orthoK

Question 4

List the 5 types of topography maps

Answer 4

axial
tangential
elevation/spherical offset
subtractive/difference
refractive power

Question 5

What does the axial map measure?

Answer 5

refraction/corneal power; closely mimics K values

Question 6

What does the axial map assume?

Answer 6

centre of radius of curvature is on central axis

Question 7

Does the axial map highlight minor variations in curvature?

Answer 7

No. Only global view of cornea

Question 8

What type of astigmatism is the most challenging to fit contact lenses for?

Answer 8

limbal-to-limbal astigmatism

Question 9

What type of astigmatism is the easiest to fit contact lenses for?

Answer 9

central/apical astigmatism

Question 10

Define peak elevation index (PEI)

Answer 10

distance from centre of cornea (apex) to the steepest part of the cornea

Question 11

What are the average PEIs for Keratoconus and PMD?

Answer 11

KC: 1.95mm
PMD: 3.5mm

So PMD has the larger PEI

Question 12

Define shape factor (P)

Answer 12

asphericity of the cornea

Question 13

What are the average shape factors for Keratoconus and PMD?

Answer 13

KC: highly positive (>0.50)
PMD: usually negative (<0.15)

Question 14

What does the tangential map measure?

Answer 14

shows the location of any corneal irregularity

Question 15

Does the tangential map highlight minor variations in curvature?

Answer 15

Yes. Gives local radii of curvature, independent of central axis

Question 16

How might tangential maps be useful for keratoconus and orthoK patients? Explain

Answer 16

KC: good for locating the apex of the cone
OrthoK: evaluating shape of the peripheral cornea (b/c tangential maps provide the most accurate peripheral data)

Question 17

Which topography map should be used to map out the structural parameters of the cone for contact lens fitting? Axial or Tangential?

Answer 17

Tangential

Question 18

What does the elevation/spherical offset map measure? How does it do this?

Answer 18

Measures the high and low areas on the cornea. Does this by subtracting a normal reference spherical surface (shape of avg. cornea) from the actual cornea to find how much it differs

Question 19

What do the colours represent on an elevation map?

Answer 19

Blue/green = cornea below reference sphere
Red/yellow = cornea above reference sphere

Question 20

What do subtractive (difference) maps measure?

Answer 20

shape change in a cornea comparing before and after to see how it differs

Question 21

When might a subtractive map be used? Provide 2 examples

Answer 21

Observing the change from:
An orthoK lens
A laser surgery for myopia correction (which looks identical to the map of an orthoK lens)

Question 22

What does a refractive power map measure?

Answer 22

correlates corneal shape to vision quality

Question 23

What is a refractive power map based on?

Answer 23

snell’s law of refraction, which is the best estimate of corneal power

Question 24

List the 3 topography display parameters

Answer 24

Standard power
K scale
Normalized (useful for KC)

Question 25

What is the downside to the normalized scale?

Answer 25

can over emphasize subtle irregularities and make them look larger than they are

Question 26

What can be done to ensure the accuracy of baseline topography maps? (8)

Answer 26

Weekly calibration
Multiple captures
Non-preserved lubricants (improve TF stability)
Avoid soft CL wear/tonometry
Minimize eyelash interference
Miniimize patient/practitioner movement
Ensure absence of tear film debris
Compare R vs L

Question 27

What is corneal eccentricity a measure of?

Answer 27

the rate of corneal flattening from centre to periphery

Question 28

What are the normal and abnormal values for corneal eccentricity?

Answer 28

normal <0.65

abnormal >0.65

Question 29

What is the normal range for:
A: a steep K value
B: An apical power value (steepest part of cornea)

Answer 29

A: < 47D
B: < 47D

Question 30

What does the inferior superior (IS) index measure? and what is the normal range?

Answer 30

mean power difference between inferior and superior corneal hemispheres: normal < 1.40D

Question 31

What does the surface asymmetry index (SAI) measure? and what is the normal range?

Answer 31

Detects radial asymmetry. Difference in superior vs inferior corneal powers at oblique perpendicular axis.
Normal < 1.0D

Question 32

What does the surface regularity index (SRI) measure? and what is the normal range?

Answer 32

Assess the smoothness of central cornea. Measures local fluctuations in corneal power over the pupil
Normal < 0.56D

Question 33

How many microns of tear film clearance in topography based rgp fitting is needed for fluorescein to be observed/seen?

Answer 33

Generally 20-25 microns. When tear film clearance is lower than this (but not 0), we have a zone of apparent applanation

Question 34

How much central tear film clearance do we typically aim for?

Answer 34

about 30-40um. Looks like a gentle glow of fluorescein

Question 35

How much tear film clearance do we typically aim for at the edge of the lens? Why?

Answer 35

About 70-90 (so a brighter glow of fluorescein). Want more clearance to enable tear exchange.