Lecture 15 Topography

Question 1

What is the centre of the cornea called?

Answer 1

apical zone
4 mm in diameter

Question 2

What are the limitations of keratometry?

Answer 2

*Keratometry only measures 3mm in diameter so poor guide of corneal shape as actual corneal diameter is 12 mm
*We may need to know the peripheral shape in RGP fittings and after post refractive surgery
*Keratometers can’t measure corneal curvature accurately if surface being measured doesn’t have a constant radius of curvature or is not radially symmetrical

Question 3

Whar is keratoscopy?

Answer 3

Study of the light reflected from the anterior surface of cornea (360 degrees)
8-10 mm

Question 4

What is the aim of corneal topography?

Answer 4

To accurately describe the shape of the corneal surface in all meridians

Question 5

What is a placido disc?

Answer 5

a reflective device that assesses the quality of corneal topography. A series of concentric black and white rings on a flattened disc with a sight hole.

Question 6

How can you use placid rings to assess the shape of the cornea?
What is a con of this method?

Answer 6

*Disc is positioned in front of cornea and reflections are observed
*If reflected rings are closer to each other, then corneal curvature is steeper
*Widely separated rings indicate flatter curvature
*Rings become oval in astigmatism- they stretch over horizontal axis if WTR and vertical axis if ATR

Using this method, only gross corneal irregularities, and high astigmatism (3 D above) can be picked up

Question 7

What do the colours represent in topography?

Answer 7

cooler colours (blue): slow rates of change (flatter curvature)

hotter colours (hot): steeper curvature

green: mean/reference value

Question 8

What do elevation maps show?

Answer 8

*Useful for looking at regularity of astigmatism and location of it (keratoconus)
*Used for planning corneal surgeries

Question 9

What is the WTR astigmatism?

Answer 9

Steeper axis is vertical and flatter one is horizontal

Question 10

What is ATR astigmatism?

Answer 10

Steeper axis is horizontal and flatter axis is vertical

Question 11

What are the signs of keratoconus on topography maps?

Answer 11

-Steepened area in the inferior nasal area of cornea is found in keratoconus eyes
-Looks like an uneven bow tie
-Top part of bow ties disappears when it gets worse
-Will find a thinning of cornea in this area
-Cones looks larger in axial map as averaging values mean effect of this steepening becomes more visible