CL - Corneal topography - Week -3 Flashcards

1
Q

Name 5 limitations of keratometry

A

assumes refractive index is 1.3375
assumes cornea is spherical
only measures central 3mm of cornea
inaccurate readings if any epithelial anomalies
limited capacity to measure irregular corneal surfaces (e.g. keratoconus)

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2
Q

What baseline data can you measure with corneal topography? (6)

A

Detect corneal pathology
Establish corneal parameters (e.g. HVID, pupil size)
Contact lenses fit
Astigmatism
Screening tool
Tear film (NITBUT)

remember as DECAST (i made this up)

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3
Q

How can corneal topography be used in follow up care? (4)

A

monitor pathology
corneal stability
corneal shape changes
orthoK

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4
Q

List the 5 types of topography maps

A

axial
tangential
elevation/spherical offset
subtractive/difference
refractive power

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5
Q

What does the axial map measure?

A

refraction/corneal power; closely mimics K values

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6
Q

What does the axial map assume?

A

centre of radius of curvature is on central axis

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7
Q

Does the axial map highlight minor variations in curvature?

A

No. Only global view of cornea

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8
Q

What type of astigmatism is the most challenging to fit contact lenses for?

A

limbal-to-limbal astigmatism

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9
Q

What type of astigmatism is the easiest to fit contact lenses for?

A

central/apical astigmatism

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10
Q

Define peak elevation index (PEI)

A

distance from centre of cornea (apex) to the steepest part of the cornea

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11
Q

What are the average PEIs for Keratoconus and PMD?

A

KC: 1.95mm
PMD: 3.5mm

So PMD has the larger PEI

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12
Q

Define shape factor (P)

A

asphericity of the cornea

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13
Q

What are the average shape factors for Keratoconus and PMD?

A

KC: highly positive (>0.50)
PMD: usually negative (<0.15)

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14
Q

What does the tangential map measure?

A

shows the location of any corneal irregularity

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15
Q

Does the tangential map highlight minor variations in curvature?

A

Yes. Gives local radii of curvature, independent of central axis

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16
Q

How might tangential maps be useful for keratoconus and orthoK patients? Explain

A

KC: good for locating the apex of the cone
OrthoK: evaluating shape of the peripheral cornea (b/c tangential maps provide the most accurate peripheral data)

17
Q

Which topography map should be used to map out the structural parameters of the cone for contact lens fitting? Axial or Tangential?

A

Tangential

18
Q

What does the elevation/spherical offset map measure? How does it do this?

A

Measures the high and low areas on the cornea. Does this by subtracting a normal reference spherical surface (shape of avg. cornea) from the actual cornea to find how much it differs

19
Q

What do the colours represent on an elevation map?

A

Blue/green = cornea below reference sphere
Red/yellow = cornea above reference sphere

20
Q

What do subtractive (difference) maps measure?

A

shape change in a cornea comparing before and after to see how it differs

21
Q

When might a subtractive map be used? Provide 2 examples

A

Observing the change from:
An orthoK lens
A laser surgery for myopia correction (which looks identical to the map of an orthoK lens)

22
Q

What does a refractive power map measure?

A

correlates corneal shape to vision quality

23
Q

What is a refractive power map based on?

A

snell’s law of refraction, which is the best estimate of corneal power

24
Q

List the 3 topography display parameters

A

Standard power
K scale
Normalized (useful for KC)

25
Q

What is the downside to the normalized scale?

A

can over emphasize subtle irregularities and make them look larger than they are

26
Q

What can be done to ensure the accuracy of baseline topography maps? (8)

A

Weekly calibration
Multiple captures
Non-preserved lubricants (improve TF stability)
Avoid soft CL wear/tonometry
Minimize eyelash interference
Miniimize patient/practitioner movement
Ensure absence of tear film debris
Compare R vs L

27
Q

What is corneal eccentricity a measure of?

A

the rate of corneal flattening from centre to periphery

28
Q

What are the normal and abnormal values for corneal eccentricity?

A

normal <0.65
abnormal >0.65

29
Q

What is the normal range for:
A: a steep K value
B: An apical power value (steepest part of cornea)

A

A: < 47D
B: < 47D

30
Q

What does the inferior superior (IS) index measure? and what is the normal range?

A

mean power difference between inferior and superior corneal hemispheres: normal < 1.40D

31
Q

What does the surface asymmetry index (SAI) measure? and what is the normal range?

A

Detects radial asymmetry. Difference in superior vs inferior corneal powers at oblique perpendicular axis.
Normal < 1.0D

32
Q

What does the surface regularity index (SRI) measure? and what is the normal range?

A

Assess the smoothness of central cornea. Measures local fluctuations in corneal power over the pupil
Normal < 0.56D

33
Q

How many microns of tear film clearance in topography based rgp fitting is needed for fluorescein to be observed/seen?

A

Generally 20-25 microns. When tear film clearance is lower than this (but not 0), we have a zone of apparent applanation

34
Q

How much central tear film clearance do we typically aim for?

A

about 30-40um. Looks like a gentle glow of fluorescein

35
Q

How much tear film clearance do we typically aim for at the edge of the lens? Why?

A

About 70-90 (so a brighter glow of fluorescein). Want more clearance to enable tear exchange.