A quick revision on Corneal topography

Corneal topographer is a useful tool for our clinical investigations on patient's with corneal problem. Knowing about its principle, function and interpretation is important.

1. Anis Suzanna Binti
Mohamad
Pegawai Optometri U41
Hospital Sultanah
Bahiyah
A quick revision
CORNEAL
TOPOGRAPHY

2. 1) The principles
2) Indications
Overview
3) Examination
4) Corneal topo guidelines
5) Interpreting cornel topo
6) Advantage &
disadvantages

3. PRINCIPLE
Placido Disk Technology
Cone-of-focus
Allow for cornea anterior
segment imaging

4. Healing Trend/Stars
Display
Optional
Software
Module
MasterFit Contact Lens
Module
Pathfinder corneal
Analysis
Advanced Refractive
Diagnostic
Simulated Ablation

5. INDICATIONS
Corneal
conditions
Refractive
surgery
Contact lens
fitting

6. Keratoconus
Pellucid Marginal Degeneration
(PMD)
Terrien's Marginal Degeneration
(TMD)
High Astigmatism
Ring Implant
Corneal scars
Corneal
conditions

7. Refractive
surgery
Screening
Wavefront Analysis
Post refractive surgery complication
Follow-up -condition is
stabilizing/getting worse

8. Contact lens
fitting
Suitable lenses:-
RGP
KC Lenses
Orthokeratology

9. Step 1
Enter
patient's
details
Step 2
Select
parameter set
for scanning
Step 3
Place
patient's
head in
position
Examination

10. Enter patient's
details
Choose patients's
category (if any)
Corneal disease
Contact lens fitting
Refractive surgery
Cataract
This is only for classification. It's optional.
E.g:-

11. Select parameter set
for scanning
Parameter set will determine the type
of maps to be printed
Parameters:-
Pathology
cataracts
Contact lens fitting
Refractive surgery
Screening
Places patient's head in
position (chin rest,
forehead rest)
Patient's head should be positioned at 45
degree angle-follow the arrows direction
Ask patient to fixate onto orange light
align white crosshair with the Placido rings
on the screen & adjust so that the rings are
in focus
image must be captured within 100 secs
If failed, press any key to continue

12. Corneal
topography
guidelines

13. Eye image with the
rings focused too far
out
Cone-of-focus is not equidistant
from neighbouring rings.
Ring outside of cone is spaced
further than ring inside the cone.
Center ring and surrounding rings
are blurry and not focused
correctly.

14. Eye image with
the rings focused
too closely
Cone-of-focus is not
equidistant from neighboring
rings.
Ring outside of cone is
spaced closer then ring inside
of cone.
Inner rings are blurry and
unfocused.

15. Eye image focused
too far to the left
The inner ring on the right side of the
image has almost no spacing between
it and the cone.
The inner ring on the left side of the
image has a greater spacing between
it and the cone, indicating that the
image is focused too far to the left.

16. Eye image
focused too far
to the right
The inner ring on the left side of the
image has almost no spacing between
it and the cone.
The inner ring on the right side of the
image has a greater spacing between
it and the cone, indicating that the
image is focused too far to the right.

17. Properly focused
image
The cone-of-focus is spaced
equidistant from neighboring
rings, indicating the correct
working distance from the target.
The center ring is clearly focused
and aligned with no uneven or
broken areas.

18. DISPLAYING CORNEAL
DATA AS MAPS
Humphrey corneal topography maps come
in four basic types:-
Axial map
Tangential map
Curvature maps
Refractive power
maps
Elevation maps
Irregularity maps

19. What corneal topo
can view?
Ring Image
Keratometry
Mean curvature
Corneal wavefront
Image simulation
HVID
Pupil size
(scotopic/Photopic)

20. Statistical Indices
in Humphrey
Corneal
Topography
Corneal Irregularity
Measurement (CIM)
Shape Factor (SF)
Mean Toric Keratometry
(TKM)

21. Measure of the aspherivity of the
cornea and derivative of eccentricity.
Shape factor index - determine cornea
is more oval or elliptical in shape
0 shape factor = perfect spehere
+ve shape factor = keratoconus
-ve shape factor = post refractive
surgery
Shape Factor (SF)

22. Corneal
Irregularity
Measurement
(CIM)
Is a number to represent the
irregularity of the corneal surface
Determine the difference in 'height'
or elevation between the patient
cornea and perfect model toric
cornea
The higher value - the more
uncorrectable/uneven the surface is
optically
Unit: microns

23. Mean Toric
Keratometry
(TKM)
Is derived using elevation data from
a best toric reference surface as
compared to actual cornea
2 values are calculated at the apex
of the flattest meridian and their
mean is determined
The higher the TKM index becomes
the greater likehood that excessive
corneal toricity exists leads to
keratoconus

24. Interpreting
corneal
topography

25. Lousiana State University
colour-coded map:
Colour map gives qualititative
information
Important to check the colour scale to
see which colours correspeonds to
which dioptric powers, to determine
the dioptric interval between colour
changes and the entire range of
powers represented
1.9 Billion
Green
Intermediate
Hot colour
(red,orange,yellow)
Steep
Cool colour
(light/dark
blue)
Flat

26. Important
parameters to
look at...
Shape Factor (SF)1.
2. Corneal Irregularity
measurement (CIM)
3. Mean Toric
Keratometry (TKM)

28. Normal cornea
Flattens from center to
periphery : nasal >
temporal
Topographic pattern
unique to the individual
Relative smoothness
Absence of significant
irregular astig

29. Regular
astigmatism
The most common naturally
occurring deviation from
normal
Symmetrical bowtie pattern
Most often : with the rule - steep
meridian at 90°

30. Irregular
astigmatism
Component of the
astigmatism that cannot be
corrected with glasses
Commom cause : dry eyes,
scar, pterygium, trauma

31. Keratoconous
Most frequent ectatic disorder
Topo useful to make diagnosis when no
other clinical signs
3 corneal topo features
high central power : 47D
A difference of 3D or more between
points 3mm inf. to the center & 3mm
sup. to the center
Asymmetry , difference between
central corneal power of fellow eye >
1D

32. Penetrating
keratoplasty
Regular astigmatism
Irregular astigmatism

33. Refractive
surgery
Radial keratotomy
PRK
Lasik
Central flattening

34. Ergonomic :-
Comfortable light
itensity for
patient to fixate
their eyes
Unique chin rest
Easy image
capture
Auto-detect OD
& OS
Advantage 1
Pupillometry
Photopic
Scotopic
Advantage 2

35. Chin rest & head rest
Not friendly for
patients with
sunken forehead
Patient need to
move head
Patient need to
lean forward
Disadvantage 1
Without extrapolation function, missing area
means missing area. The only solution is to
repeat the scan with copious amount of
artificial tears. However, liberal use of artificial
tears can mask the actual appearance of the
keratoconus.
Large Placido Disk
Difficult for patients
with Roman nose,
Protruded brows &
Sunken eyeball
Disadvantage 2
Mapping function
Lack map extrapolation
capability present on newer
topographers (such as
pentacam)
Extrapolation function allows
machine to calculate and
predict the topography of any
area where sufficient placido
data could not be obtained.
Calculation is based on data in
surrounding area.
This creates incomplete
map appearance for
patients with:-
Dry eye
Corneal conditions
(such post PK)
Protruded nose or
eyebrows
Sunken eyeball
Disdvantage 3
The same eye scanned with topographer
with extrapolation function. A reliable
map is obtainable with a single scan.

36. GET IN TOUCH
FOR QUESTIONS AND FEEDBACK
013-9184407
[email protected]
Jabatan Oftalmologi, Hospital Sultanah
Bahiyah, KM 6, Jalan Langgar
05460 Alor Setar, Kedah.
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