Corneal Topography 3B

What are the different types of Topography maps?

Axial or Sagittal

Tangential or Instantaneous

Elevation

Difference

Aberration

Characteristics of Axial or Sagittal map?

Center of Radius of curvature is CENTRAL AXIS

Best to look at central cornea

Better to compare 2 different corneas

Assesses Optical State of Cornea

Characteristics of Tangential or Instantaneous Curve?

“True Curve data”

MORE DETAILED AND INDIVIDUALIZED to patient

****Most preferred map in contact lens FITTING along with elevation maps

Characteristics of Elevation Map?

Shows elevation of the eye at different points

Warm colors = more elevated

Cooler color = less elevated

Important for determining peripheral curvature of a lens

Characteristics of Difference Map?

Difference in curvature between EXAMS

Helpful with OrthK

Helpful to Monitor Changes in Cornea

Characteristics of Aberration Map?

Less commonly used

Need wave front aberrometer

Measures optical or refractive state of the eye

Uses Zernike Polynomials

1. Lower order: regular astigmatism, defocus (Can be corrected with glasses and soft contact lenses)

2. Higher order: trefoil, coma, spherical aberration (can be corrected with RGPs)

How are Irregularity Indices used?

Help detect corneal irregularity

Different on topographer vs tomographer

More extensive indices available on tomographer

What are common indices for topographers?

I-S or S-I (Inferior-Superior) Value: Superior and Inferior cornea

• Values higher than 1.5 D are suspicious for ectasia (thinning)

SAI (Surface Asymmetry Index): One eye to fellow eye

• Difference in more than 1 D considered at risk

• 0.10 to 0.42

SRI (Surface Regularity Index): Power of central 4.5 mm

• Normal corneas have low SRI values, usually 0 to 0.56

What are the irregularity indices for Topcon CA 800

AGC (Apical Gradient of Curvature): Curvature changes from center to peripheral

• Higher rates of change are more suspicious

AK (Apical Keratometry): Dioptric power at apex of cornea

• Steeper values are more suspicious

S-I Value

KPI (Keratoconus Probability Index)

• Combines previous values to determine probability of Keratoconus

What are the Irregularity Indices for Tomographers?

BAD (Belin-Ambrosio Display)

• Anterior elevation, posterior elevation, pachymetry

PPI (Pachymetric Progression Index)

• Change in corneal thickness over entire cornea

Why are Irregularity Indices used?

Help diagnose and monitor corneal disease

Good for general CL fitting and screening

***Different for Tomographer and Topographer

How do you interpret Topography Report?

G-General information

R- Reliability

A- Abnormal/normal

D- Defect

E- Evaluate

S- Subsequent Test

What are some keys to recognize patterns on a Topographer?

Look at color maps

What kind of map are you looking at?

Check Sim-K’s (normal curvature is 40-46.00 D)

What does this look like?

Spherical cornea

-Even coloring throughout central cornea

-Keratometry readings approximately the same in all quadrants

What is this look like?

With the Rule Astigmatism

What does this look like?

Against the Rule Astigmatism

What does this look like?

Oblique Astigmatism

**Angles usually at 120-150 or 30-60 degrees

What are the keys to look for if thinking about Irregular astigmatism?

May have “bowtie” look, but usually ASYMMETRICAL

May not have a particular pattern

Does not necessarily mean disease

Umbrella term

What are characteristics of Keratoconus?

Primary non-inflammatory Ectasia

• Cone-like shape INFERIORLY

• Progressive

• Starts in childhood

• Early diagnosis important for best prognosis

What does this look like?

Keratoconus

**Inferior steepening

Need more than topography findings to diagnose

What is Pellucid Marginal Degeneration?

Idiopathic thinning of peripheral cornea

• Rare

• Slowly progressive

• Usually Inferiorly

• Usually OLDER PATIENTS

What does this look like?

Pellucid Marginal Degeneration

***Crab Claws or Kissing Doves

Sagging appearance

**Need Pachymetry

What does this look like?

Corneal Transplant

• Variable appearance

• Usually irregular astigmatism

• Patient dependent

What does this look like?

Post-Refractive Surgery

***Myopic Correction

• Flatter in center with relative peripheral steepening

  • OrthoK can have similar topography

What does this look like?

Post-Refractive Surgery

***Hyperopic correction

• Steeper central cornea

• Flatter periphery

What is normal corneal shape?

Aspheric

Prolate: steeper in center, flatter in periphery

What measures rate of flattening?

e value

What does it mean if the e value is 0?

Spherical

What if the e value goes up?

Rate of flattening towards periphery increases

What is the average e-value for a cornea?

0.43

How do you apply topography readings clinically?

Standard vs custom fit contact lenses

• Custom contacts benefit from corneal topography

• Cornea to contact lens relationship is important

  • Prevents long-term complications

  • Improves vision and comfort