Topography

Question 1

Absolute scale

Answer 1

Colour coded dioptric values - large intervals - detail lost
Useful for screening
Can compare between eyes
Compares with computerised reference eye

Question 2

Relative scale

Answer 2

Colour coded dioptric values - smaller intervals - more detail
Cant compare between eyes

Question 3

Axial (saggital) map

Answer 3

Measures corneal curvature @ axial direction to centre
Global view of curvature
Usually used with absolute scale

Question 4

Tangential map

Answer 4

Measures curvature @90deg to points on particular ring
Doesn’t assume eye is spherical
Doesn’t assume apex is steepest part
Doesn’t assume where centre of radius is

Question 5

Elevation map

Answer 5

Height difference in curvature from reference eye
Red = above ref = steepest meridian
Blue - below ref = flattest meridian
Most useful for predicting nafl pattern in RGPs

Question 6

Refractive map

Answer 6

Changes in rx across corneal surface
Corneal contribution to eyes power
Magnitude of cyl in astigmatic eye
Useful for estimating LASIK values/ selecting IOL

Question 7

Difference map

Answer 7

Before/after 
Often used in OK fittings 
Monitor changes 
Before/after lens fit, surgery
Only one eye comparisons
Red = steepened
Blue = flattened

Question 8

Aberration map

Answer 8

Mathematic representation of deviations of real wave front from ideal one
Specs correct low order aberrations
Tear film can mask corneal irregularities - improving higher order aberrations

Question 9

Keratoconus diagnosis

Answer 9

Srax >15deg = abnormal
>21deg = KC indication

ABR >1 = KC