8 - corneal topography and tomography

what is the advantage of topographers vs keratometers?

topographer measures larger area of cornea (8-10 mm) → helpful in fitting GP

keratometer only measures at center 3-4 mm of cornea

What does a corneal topographer measure?

curvature of the cornea (NOT elevation)

how does a more curved radius of curvature look like? less curved?

more curved → smaller circle → steeper curve

less curved → larger circle → flatter curve

How do you convert diopters into radius in mm?

337.5/diopters = radius (mm)

what is the diopter power equivalent to?

refractive element

what does the OPD measure?

topography

autorefraction

auto-K

pupilometer

aberrometer

what is the advantage of a small cone on a topographer?

capable of collecting more data points

better accuracy

what is a disadvantage of a small cone on a topographer?

more difficult to capture image

example of small cone

medmont and pentacam

what is an advantage of a large cone on a topographer?

easier to manipulate and capture data

large cone example

OPD

does the Medmont topographer require repeated scans?

recommend 3 scans to ensure reliability (serial scans) → uses placido rings that are tear film dependent

does the OPD require repeated scans?

can be repeated - not usually

what is the OPD often used for?

entrance testing

often NOT used for BC selection

does the pentacam require repeated scans?

one is often enough (take another if unreliable or identified something suspicious) → tomographic so tear film independent

In corneal topography, what color indicates steepest curvature? flattest?

steepest: red

orange

yellow

green

flattest: blue

what map does not use diopters as a unit?

elevation map

What does the axial map show on corneal topography?

overall shape of cornea → avg of the data

curvature at different points of cornea compared to optical axis

What is a disadvantage to the axial map on corneal topography?

limited accuracy in peripheral cornea → central power is more accurate

what is the axial map good for?

choosing corneal GP and SCL BC

What does the tangential (instantaneous) map show in corneal topography?

localized corneal power/curvature

curve measured via tangent to the normal

what is the most sensitive map?

tangential map

what is the tangential (instantaneous) map good for?

diagnosing keratoconus

ortho-K F/U

what map is the most true representation of corneal shape?

elevation map

what is the elevation map most useful for?

determining the best lens for an irregular cornea

what is an advantage of the elevation map?

can predict the Fl pattern based on elevation

what is the difference between elevation map of topographer and tomographer?

topography: complicated algorithm used to indirectly calculate elevation relative to a best fit sphere

tomography: directly measures elevation relative to a best fit sphere

What does the refractive power map on a corneal topographer show?

provides an interpretation of the quality of vision through the pupil

when is the refractive power map on a corneal topographer used?

pre- and post-refractive surgery procedures

what kind of asphericity is an oblate cornea?

positive

what kind of asphericity is a spherical cornea?

0

what kind of asphericity is a prolate or parabolar hyperbola?

negative

what is the IS Index?

difference between the avg inferior and superior power

what is the Surface Asymmetry Index (SAI)?

a measure of the asymmetry of the eye

difference in power between corresponding points at the same chord 180° apart

what value of SAI indicate disease?

>1

what is the Surface Regularity Index (SRI)?

a measure of surface regularity → increases with increasing asymmetry

what value of SRI is considered abnormal?

>0.80

what are Scheimpflug imaging devices?

Pentacam (Oculus)

Galilei (Zeimer)

what does the Pentacam measure?

pachymetry

topography

corneal curvature (ant/post)

astigmatism determination

Scheimpflug imaging of lens

what are the advantages of the Pentacam?

gives maximum depth of focus and minimal image distortion for an obliquely tilted object

depth of focus of a device extends from cornea to posterior lens

what maps are used on Pentacam?

individual → instantaneous (tangential), axial, elevation (front and back)

4 map refractive (total)

how is the regular BFS (best fit sphere) calculated?

regular BFS - compares the cornea against a reference derived from the entire corneal surface. Because the central cone is included, the reference becomes steeper and more anterior, which makes ectatic areas appear less elevated than they truly are

ectasias are not flat/less elevated it's just less steep!

how is the enhanced BFS calculated?

enhanced BFS - compares the cornea against a reference sphere fit to the normal peripheral cornea, excluding the central ectatic (cone) area. Because the cone is excluded, the reference becomes flatter and more posterior, which makes the ectatic area appear more elevated and steeper relative to this baseline

what is the CTSP (corneal thickness spatial profile)?

22 imaginary circles in 0.4 mm increments taking avg thickness surrounding the thinnest point

what is the equation for percent of increase thickness (PIT)?

(mean corneal thickness in the ring - thinnest corneal thickness)/thinnest corneal thickness

(mean - thinnest) / thinnest

what is the D (differential) value?

changes from BFS vs enhanced BFS, grades the severity of an ectasia

what do the D values indicate?

white

what KISA value indicates keratoconus?

>100

what is progression defined as in Belin ABCD keratoconus staging?

change in 2 of the following:

anterior corneal surface

posterior corneal surface

thinnest patch

What does scheimpflug imaging allow us to measure?

1. anterior corneal curvature

2. posterior cornea curvature

3. pachymetry

4. crystalline lens densitometry

what test can help differentiate keratoconus VA decrease vs. non-optical pathology?

pinhole

what type of cross linking occurs in keratoconus?

epithelial off crosslinking