2. Corneal Topography

Question 1

What are the 6 limitations of keratometry?

Answer 1

• Assumes refractive index = 1.3375
• Assumes cornea is spherical
• Estimates the avg curvature of the central 3mm in two principal meridians
• Peripheral cornea is not measured
• Inaccurate readings with minor surface epithelial anomalies
• Limited capacity to measure and monitor irregular corneal surfaces and/ or decentred corneal apices (incl. KCN)

Question 2

How many points across the cornea does a corneal topographer measures?
How much cornea does it measure?

Answer 2

7000-70000 points across the cornea
Cornea area = 7.0-11.00mm

Question 3

What are the 6 different analysis does corneal topography do?

Answer 3

• Axial map
• Tangential mal
• Elevation map
• Refractive map
• Difference map
• Irregularity indices

Question 4

For what 6 purposes does corneal topography obtain baseline data for?

Answer 4

• Corneal pathology
• Corneal parameters
• Contact lenses
• Astigmatism - qualitative & quantitative
• Screening tool
• Tear film - NITBUT

Question 5

For what 4 purpose does corneal topography provided as a follow-up care?

Answer 5

• Monitor pathology
• Corneal stabilty - post-surgery
• Corneal shape changes
• Orthokeratology - optimise lens fit/ refractive outcome, record of course of treatment

Question 6

Axial map
* relates to ...
* closely mimics ...
* assumes centre of the ... is on the central axis
* ... overview
* Ignores ...

Answer 6

Axial map
* relates to refraction/ corneal power
* closely mimics K values
* assumes centre of the radius of curvature is on the central axis
* Global overview
* Ignores minor variation

Question 7

Tangential map
* location of ...
* instantaneous/ true ...
* sensitive to noise as detects ...

Answer 7

Tangential map
* location of corneal irregularities
* instantaneous/ true radius of curvature
* sensitive to noise as detects small variations

Question 8

Elevation map
* ... height of the cornea above or below a reference sphere
* shows the location of any ...
* gives the local ...
* uses a reference e.g. ...
* high points and low points of ...
* can be used to simulate ... and ...

Answer 8

Elevation map
* sagittal height of the cornea above or below a reference sphere
* shows the location of any corneal irregularity
* gives the local radius of curvature
* uses a reference e.g. Best Fit Sphere
* high points and low points of elevation
* can be used to simulate CL fitting and NaFl pattern

Question 9

What is Peak Elevation Index (PEI)?

Answer 9

PEI = distance from centre of cornea (apex) to the steepest part of the cornea
KCN = 1.95mm
PMD = 3.5mm

Question 10

What is Shape Factor (P)?

Answer 10

P = asphericity of the cornea
Normal ~0.15-0.30
KCN highly positive (>0.50)
PMD usually negative/ low (<0.15)

Question 11

How is a Subtractive (Difference) Map constructed?

Answer 11

Post-treatment axial map - Pretreatment axial map
Give change that happen over time after treatment

Question 12

What do the colours on a Subtractive Map represent?

Answer 12

Red = steepening
Blue = corneal power reduction
Green = no change

Question 13

What is the Refractive Power Map?

Answer 13

Correlates corneal shape to vision quality.
This is based upon Snell’s law of refraction.
Best estimate of corneal power.

Question 14

Compare Standard Power, K Scale and Normalised Scaled.

Answer 14

• Standard Power = project the full spectrum of power the topography assumes its normal
• K Scale = incraeses the lowest and highest values, which decreases sensitivity
• Nomalised Scale = particular to px’s parameters, inidividualised

Question 15

How can you prevent ring jam? What are they actually?

Answer 15

Use non-preserved lubricant drops to improve px’s tear film stability.
Tear filim break up

Question 16

What are the 8 things that can be done to ensure topography is reliable and accurate?

Answer 16

• Weekly calibration of topographer
• At least 4 captures and select the best
• Lubricant drops
• Avoid SCL wear/ tonometry
• Minimise eyelash interference
• Minimise patient/ practitioner movement
• Ensure absence of tear film debris
• Compare R & L

Question 17

What are the 7 topographical indices?

Answer 17

• Normalised dioptric range
• Mean corneal eccentricity
• Steep keratometry value
• Apical power value
• Inferior-Superior (IS) index
• Surface Asymmetry index (SAI)
• Surface Regularity index (SRI)

Question 18

Normalised Dioptric Range
* Normative range: ...
* Normative scale on ...
* Higher range in ...

Answer 18

Normalised Dioptric Range
* Normative range: <10D
* Normative scale on axial power map
* Higher range in corneal ectasia

Answer 19

Corneal eccentricity is based upon the mathematical description of an ellipse. It measures the rate of corneal flattening from centre to periphery.
* Normal range: 0.4-0.6
* Normal: <0.65
* Abnormal: >0.65

Question 20

How would the corneal eccentricity value (e) change when it is a:
* Prolate ellipse
* Sphere
* Oblate ellipse

Answer 20

• Prolate ellipse: normal & higher e
• Sphere: e value = 0
• Oblate ellipse: = lower or negative e

Question 21

Apical power value (APV) represents the ..., which is therefore the ... value on the ... scale (...).
* Normative range: ...

Answer 21

Apical power value (APV) represents the steepest point of the entire cornea, which is therefore the highest value on the normalised scale (Kmax).
* Normative range: <47.2D

Answer 22

Inferior-Superior (IS) index measures inferior-superior asymmetry, which is the mean power difference between inferior and superior corneal hemispheres.
* 5 superior and inferior dioptric values
* 3mm from corneal apex
* 30-degree intervals
* over central 6mm(area)
* Normative range <1.60D

Question 23

Surface Asymmetry Index (SAI) measures ... asymmetry, which is the difference in superior vs inferior ... at ... axes.
* Centrally weighted summation of difference in ... between points ... degrees apart
* Measures ... equally spaced meridians
* Over central ...mm (area)
* Normative range ...

Answer 23

Surface Asymmetry Index (SAI) measures radial asymmetry, which is the difference in superior vs inferior corneal powers at oblique perpendicular axes.
* Centrally weighted summation of difference in corneal power between points 180 degrees apart
* Measures 128 equally spaced meridians
* Over central 4.5mm (area)
* Normative range <1.00D

Question 24

Surface regularity infex (SRI) assesses the ... of the ... cornea and also local fluctuation in .... SRI is calculated from summating ... across ... equally spaced semi-meridians on the ... central mire. SRI is measured over the ....
* Normative range ...
* High values indicate ...

Answer 24

Surface regularity infex (SRI) assesses the smoothness of the central cornea and also local fluctuation in corneal power. SRI is calculated from summating local power fluctuations across 256 equally spaced semi-meridians on the 10 central mire. SRI is measured over the pupil.
* Normative range <0.56D
* High values indicate corneal ectasia

Question 26

Corneal topographers can simulate ...with RGP fitting. These patterns represent ....

Answer 26

Corneal topographers can simulate fluorescein patterns with RGP fitting. These patterns represent tear film clearance.

Question 27

Fluorescein assessment
* ...μm tear film clearance = threshold for NaFl
* ...μm central clearance = threshold for reduction in VA
* ...μm edge clearance = ideal
* ...μm central clearance = threshold for bubble formation beneath the lens

Answer 27

Fluorescein assessment
* 20μm tear film clearance = threshold for NaFl
* 30-40μm central clearance = threshold for reduction in VA
* 70-90μm edge clearance = ideal
* >90μm central clearance = threshold for bubble formation beneath the lens