maps the corneal curvature

1. CORNEAL TOPOGRAPHY KURAISHA PARI.R

2. VIDEOKERATOGRAPHY
This is the method by which the surface curvature of the cornea is mapped
It provides description of the shape and power of the cornea
Gives quantitative measurement of the whole of cornea

3. CLINICAL APPLICATION
Detecting pathological conditions in cornea e.g.: keratoconus, PMD
Detecting and evaluating the severity of keratoconus.
Screening tool before refractive surgery.
Evaluation of irregular astigmatism especially after PK.
Evaluation of the effects of corneal refractive surgery.
Helps in planning of the suture removal and post operative fitting of
CL.

4. WORKING
Multiple light concentric rings are projected on the cornea.
The reflected image is captured on CCD camera.
Computer software analyse the thousands of data points captured
and displays the result in the form of topography map.

5. CHARACTERISTICS OF NORMAL CORNEA:
Flattens from the centre to periphery 2 – 4 D
Topographic pattern - unique to the individuals.
The 2 corneas of the 1 individual normally exhibits non superimposable
mirror image.
Exhibits relative smoothness and absence of significant astigmatism.
Normal cornea is usually prolate – steep in the periphery and flatten in the
centre.

6. TYPES OF CORNEAL TOPOGRAPHY
Round
Oval
Symmetric bowtie
Asymmetric bowtie
Irregular bowtie

7. INTERPRETING TOPOGRAPHY
MAPS
Warm colours - red, orange, yellow (denotes steeper portions of the
cornea).
Intermediate colour – green.
Cool colours – blue (denotes flatter portions).

8. QUANTITATIVE DESCRIPTIONS OF
TOPOGRAPHY
Sim K : obtained from the greatest or least power meridian in the
cornea, by averaging the measured values of 6 – 8th rings of the
meridian.
Sim K1 : It is the power & axis of the meridian with the highest power.
Sim K2 : It is the power of the meridian 90 deg from Sim K1.
Higher value indicates keratoconus, s/p PK or a normal steep eye.
Lower than normal values occur with myopic surgery and rare flat
cornea.

9. Cylinder: simulated keratometer cylinder is obtained from the difference of
sim K1 & sim K2 reading.
SAI (surface asymmetrical index) : measured as the difference in corneal
powers at every ring 180 deg apart over the entire corneal surface.
SAI is higher in keratoconus, PK, decentrated myopic refractive surgery,
trauma, CL warpage.
SRI (surface regularity index): measures local fluctuation in corneal powers.
Higher values found in cases of dry eyes, CL wear, trauma, PK.

10. Asphericity (Q): this is a measure of the rate of flattening of the cornea
from the apex to the periphery.
It allows the positive values to describe oblate surfaces and negative
values for prolate surfaces.
Normal Q – 0.26.
Predicated visual acuity (PVA) : provides a single values in units of snellen
acuity of the optical quality of the cornea surface within the 3mm zone
ranging from 2010 to 20200
SDP (standard deviation of corneal power) : this is calculated from the
distribution of all the corneal power present on corneal topography.

11. OSI (opposite sector index):
 Cornea is divided into 8 segments.
 Greatest difference of mean corneal power existing between 2 opposite sectors.
DSI (differential sector index):
 Subdivides the cornea into 8 segments.
 Maximum difference extends between the mean of 2 of the 8 segments.
KPI (keratoconus predictive index): is obtained from statistical analysis of
indices, this is a numerical estimator of keratoconus which spans from 0,
when there are no topographical characteristics relative to keratoconus
these indices reveal the presence.

12. AA (analysed area): this value expresses the percentage of corneal
surface covered by the instrument. The value is low in case od
advanced keratoconus.

13. Indices Normal Abnormal
SAI 0.42 0.50
SRI 1.01 1.97
DSI 2.96 3.51
OSI 1.65 2.09
SDP 1.17 1.33
IAI 0.44 0.49
KPI 0.23 0.30
AA 69.43 73.49

14. TYPES OF TOPOGRAPHIC MAPS
AXIAL MAPS :
Describes the overall shapes of the cornea.
Calculates the curvature rather than power.
Easy to understand.
Colours visually represented flatness & steepness.
Cannot measure periphery.

15. TANGENTIAL MAP:
More sensitive.
Calculates corneal curvature based on a tangent to the normal.
Best to identify corneal pathologies and localized defects.
REFRACTIVE POWER MAP:
Accounts for aberration while calculating power and radius.
Useful in assessing the visual performance or the end result of post refractive
surgeries.

16. ELEVATION MAP:
Measures the difference in height or elevation of the cornea in microns from
the reference surface.
IRREGULARITY MAP:
Similar to the elevation map but uses the best fit toric surface reference.

17. KERATOSCOPIC ERRORS
Errors due to misalignment of central mires.
Patients misalignment errors due to poor fixation.
Tear abnormality.
Misinterpretation error occurs due to the analysis of only the absolute
or relative map.