Corneal Topography & Tomography

corneal topography

• shows the landscape of the cornea

• tells us the rate of change of curve across the cornea & whether the eye is optically regular or irregular

Placido Disc Topography

• reflects a series of rings off the cornea

• measures how the corneal curvature affects the reflection of the rings back

• interprets corneal shape based on these reflections

yes

is the placido disc topography tear layer dependent?

large

a ____ cone Placido Disc Topgrapher is more patient friendly but captures a smaller amount of data (8mm)

small

a ____ cone Placido Disc Topographer is not as patient friendly as it must get very close to the eye but provides more data (10mm)

scales

• the range by which you are measuring the topography of the cornea

• can be in D or mmm for curvature maps

• in microns for elevation maps

steeper, higher

hotter colors on a scale mean _____ curvature, ______ elevation

flatter, lower

cooler colors on a scale mean ______ curvature, _______ elevation

normalized, absolute, adjustable

what are the 3 types of scales?

normalized

determines the scale based upon the lowest & highest powers/points on the map & uses them as the low & high ends of the scale

absolute

• type of scale that takes the full range of powers into account

• not sensitive

• apples to apples w/ all other maps

adjustable scale

• allows you to choose the high & low end of the scale to suit your needs

• can make the top & bottom values anything you want

• can allow you to compare apples to apples on a more detailed scale

axial, tangential, elevation

what are the 3 types of maps?

axial curvature

• map that gives a global view of the corneal curvature as a whole

• tendency to ignore minor variations in curvature

• distance along the normal from the corneal surface to the optic axis

• reference distance, not a true curvature

• running average-excludes extreme values

• error increases towards the periphery

• spherically biases

• may miss abrupt changes

• significant smoothing

• best for determining K values & understanding corneal refractive power

determining K values & understanding corneal refractive power

what are axial maps best for?

tangential curvature

• map that is similar to axial but uses different mathematics to create a more sensitive display

• instantaneous rate of curvature based map

• based on mathematical formula for local radius at a point along a curve

• more sensitive to abrupt changes

• less smoothing (more noise)

• includes extreme curvature values

• localizes exact position of pathology

• more detailed

• more accurate interpretation of the peripheral cornea

elevation

shows areas of elevation & depression relative to a “reference sphere” generated by the average height of the cornea

corneal tomography

• uses a slit beam & Sheimpflug camera to take several cross-sectional images of the anterior segment of the eye

• all images go through the central cornea

• coverage is nearly limbus to limbus

• 2D slices are used to create a 3D representation of the object

• true measurement, not based on reflections

no

is corneal tomography tear film dependent?

true elevation of anterior & posterior cornea, corneal thickness, corneal curvature

what information can corneal tomography tell you?

curvature, elevation

corneal topography measures ______ & extrapolates _______

elevation, curvature

corneal tomography measures _______ & extrapolates _______

anterior

corneal topography gives you _____ corneal information

anterior & posterior, pachymetry

corneal tomography gives you _________ corneal information, as well as ________

steep/flat

what do curvature maps tell you?

high/low points

what do elevation maps tell you?

tangential map w/ a normalized scale

what map & scale combo will show you the most detail about the cornea?

adjacent to

the area of steepest slope on a curvature map is typically ______ the highest point on an elevation map

flatter

the blue arrow represents _____ slope

steeper

the red arrow represents _______ slope

normalized

what type of scale is this?

absolute

what type of scale is this?

WTR astigmatism

what does this elevation map show?

diopters of curvature

what do Placido Disc topographers measure directly?

elevation of anterior & posterior cornea

what do slit-scanning or elevation devices directly measure? (ex: Orbscan, Pentacam, Visante OCT)

patient’s refractive status

what do axial maps allow you to correlate anterior surface shape to?

tangential

_______ maps allow for a clearer view of the size & shape of the cone in a keratoconus patient

tangential

_______ maps define the position of the treatment or effect of corneal reshaping & refractive surgery

positive, red

corneal elevation above the reference sphere is measured in _____ microns & appears as ____ on the elevation map

negative, blue

corneal elevation below the reference sphere is measured in _____ microns & appears as ____ on the elevation map

difference display

• shows 2 exams for the same eye & the difference b/t them

• helpful when monitoring corneal changes from one exam to the next

symmetric, hourglass

normal astigmatism is ____ & appears as an _____ shape either w/in the pupil margin or extending the entire length of the cornea

horizontal visible iris diameter (white to white measurement)

most topographers are capable of measuring the corneal diameter & label this as the __________

yes

do topographers automatically generate a pupil diameter measurement?

tangential

What type of corneal topographical map tends to accentuate subtle changes in shape?

adjustable

What type of scale is best to use to compare two maps from different dates when you want to look for signs of SUBTLE changes in the map?

B

Which of the following statements about Placido Disc Topography is FALSE?

a. Is tear layer dependent

b. Captures true elevation data about the cornea

c. Reflects a series of rings off the cornea

the location of the steepest rate of curvature change/slope

Look at the attached topography.  What does point C describe? 

D

Look at the attached topography.  Which point is the FLATTEST in terms of the slope of the cornea?

the highest spot of elevation on this cornea relative to the reference sphere

Look at the attached topography.  What does point C describe?

D

Look at the attached topography.  Which point is the in lowest in terms of elevation relative to the reference sphere?

C

Choose the CORRECT answer:

 

A. Corneal Topography is tear layer independent and measures curvature

B. Corneal Topography is tear layer dependent and measure true elevation

C. Corneal Tomography is tear layer independent and measures true elevation

D. Corneal Tomography is tear layer dependent and measures curvature

keratoconus

keratoconus

keratoconus

keratoconus

keratoconus (with plastic inlays)

keratoconus (with plastic inlays)

corneal ectasia

Salzman nodular degeneration

Salzman nodular degeneration

post radial keratotomy

post LASIK

ortho K

post radial keratotomy

post radial keratotomy

off-center ortho K

off-center ortho K

post PK

post PK

post PK

post PK