Document 6586763

Corneal Topography
Aim
The aim of corneal topography is to obtain
detailed, accurate data about the corneal
contour and display it in a clinically useful
format.
Topography display
Measurements
Height
A more sensitive way of presenting height data is
to plot the difference in height from a sphere of
known size or from an idealized cornea shape.
Elevation maps
Astigmatism interprtation
Flat axis appearance
BFS
Flat axis
Steep corneal appearence
Steep axis
BFS
Curvature
It is a sensitive measure of variation in contour
across the corneal surface
Global (axial/sagittal):
Measurements are made
relative to visual axis.
Less accurate in the
corneal periphery and for
irregular surfaces.
Local ( instantaneous / tangential )
`
Curvature of each point
is calculated with
respect to its
neighbours
More accurate
Power
Refractive power is a slightly less accurate
measure of corneal contour than curvature
Display
Three dimensional wire mesh representation,
for detection of gross distortion.
Two dimensional colour coded contour
mapping:
Colours
Warmer colours
(red, orange, yellow)
Represent the steeper
areas.
Cooler colours
(green, blue)
Represent the flatter
areas.
Scales
Absolute/standardized scale
There is a fixed colour coding system. The same
colours always represent the same curvature of
powers.
Normalized/relative scale
Uses a Fixed number of colours (11) which are
automatically adjusted to fill the range of diopteric
values for that single map.
Adjustable scale
It enables the operator to select the step interval and
diopteric range of the contours so the topographic
information can be displayed to optimum effect to
emphasize a certain information, used to compare the
clinical situation.
Axes
Axis is the meridian of either greatest or least slope
Orthogonal axes
Represent the major and
minor meridians.they are
determined by averaging
the power from the rings
within the central 3mm
zone along every
meridian
Zonal axes
Are the steepest and flattest meridian within the
3 mm, 5 mm, and 7 mm zones
Instantaneous axes
Are the true major and minor axes.they are
continuos lines joining points of max.or min. power
on each ring axes
Statistical indices
Corneal asphericity Q
Classification of normal corneas
Round: 22.6%
Classification of normal corneas
Oval : 20.8%
Classification of normal corneas
Symmetric bow-tie: 17.5%
Classification of normal corneas
Asymmetric bow-tie: 32.1%
Classification of normal corneas
Irregular:7.1%
Artifacts of corneal topography
Alignments and focusing
Artifacts of corneal topography
Tear film irregularities
Pooling of tears in lower meniscus lead to
focal steepening
Artifacts of corneal topography
Tear film irregularities
Thinning of the tear film by drying lead to
localized flattening
keratoconus
Topographic features
VKS mires are distorted
They lie closest
together
in
the
inferiocentral region
where the cornea is
steepest and furthest
apart
superiorly
where the cornea is
flattest
Colour coded curvature map
Show an asymmetric bow-tie corresponding to
the exaggerated prolate shape and irregular
astigmatism of the corneal surface
Diagnostic features of keratoconus
Central corneal power greater than 47 D
A difference of 3 D or more in corneal power
comparing points 3 mm inferior to the center
to points 3 mm superior to the center
Asymmetry between central corneal
power of the fellow eyes in excess of 1 D
Keratoconus detection indices
Keratoconus predictability index(KPI)
KPI is derived by inputting eight quantitative
indices into an automated keratoconus detection
algorithm
Sensitivity 68%
specificity 99%
accuracy 90%
Keratoconus detection indices
Keratoconus predictability index(KPI)
Classification of keratoconus
Severity
Is related to the difference in power between the
steepest and flattest portions of the cornea. The
more sever cases are also associated with
increasing surface irregularity
Location
Most commonly inferiorly or inferiocentrally
Location
Rare - central
Location
Rare - superior
Shape
Most cones have an oval shape involving one or two
quadrants
Globus cone,cone is
extensive and involve
up to three quarter
Nipple cone,much more
localized and are
completely surrounded by
relatively flat (normal
cornea)