CL - Corneal topography - Week -3 Flashcards
Name 5 limitations of keratometry
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)
What baseline data can you measure with corneal topography? (6)
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)
How can corneal topography be used in follow up care? (4)
monitor pathology
corneal stability
corneal shape changes
orthoK
List the 5 types of topography maps
axial
tangential
elevation/spherical offset
subtractive/difference
refractive power
What does the axial map measure?
refraction/corneal power; closely mimics K values
What does the axial map assume?
centre of radius of curvature is on central axis
Does the axial map highlight minor variations in curvature?
No. Only global view of cornea
What type of astigmatism is the most challenging to fit contact lenses for?
limbal-to-limbal astigmatism
What type of astigmatism is the easiest to fit contact lenses for?
central/apical astigmatism
Define peak elevation index (PEI)
distance from centre of cornea (apex) to the steepest part of the cornea
What are the average PEIs for Keratoconus and PMD?
KC: 1.95mm
PMD: 3.5mm
So PMD has the larger PEI
Define shape factor (P)
asphericity of the cornea
What are the average shape factors for Keratoconus and PMD?
KC: highly positive (>0.50)
PMD: usually negative (<0.15)
What does the tangential map measure?
shows the location of any corneal irregularity
Does the tangential map highlight minor variations in curvature?
Yes. Gives local radii of curvature, independent of central axis
How might tangential maps be useful for keratoconus and orthoK patients? Explain
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)
Which topography map should be used to map out the structural parameters of the cone for contact lens fitting? Axial or Tangential?
Tangential
What does the elevation/spherical offset map measure? How does it do this?
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
What do the colours represent on an elevation map?
Blue/green = cornea below reference sphere
Red/yellow = cornea above reference sphere
What do subtractive (difference) maps measure?
shape change in a cornea comparing before and after to see how it differs
When might a subtractive map be used? Provide 2 examples
Observing the change from:
An orthoK lens
A laser surgery for myopia correction (which looks identical to the map of an orthoK lens)
What does a refractive power map measure?
correlates corneal shape to vision quality
What is a refractive power map based on?
snell’s law of refraction, which is the best estimate of corneal power
List the 3 topography display parameters
Standard power
K scale
Normalized (useful for KC)
What is the downside to the normalized scale?
can over emphasize subtle irregularities and make them look larger than they are
What can be done to ensure the accuracy of baseline topography maps? (8)
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
What is corneal eccentricity a measure of?
the rate of corneal flattening from centre to periphery
What are the normal and abnormal values for corneal eccentricity?
normal <0.65
abnormal >0.65
What is the normal range for:
A: a steep K value
B: An apical power value (steepest part of cornea)
A: < 47D
B: < 47D
What does the inferior superior (IS) index measure? and what is the normal range?
mean power difference between inferior and superior corneal hemispheres: normal < 1.40D
What does the surface asymmetry index (SAI) measure? and what is the normal range?
Detects radial asymmetry. Difference in superior vs inferior corneal powers at oblique perpendicular axis.
Normal < 1.0D
What does the surface regularity index (SRI) measure? and what is the normal range?
Assess the smoothness of central cornea. Measures local fluctuations in corneal power over the pupil
Normal < 0.56D
How many microns of tear film clearance in topography based rgp fitting is needed for fluorescein to be observed/seen?
Generally 20-25 microns. When tear film clearance is lower than this (but not 0), we have a zone of apparent applanation
How much central tear film clearance do we typically aim for?
about 30-40um. Looks like a gentle glow of fluorescein
How much tear film clearance do we typically aim for at the edge of the lens? Why?
About 70-90 (so a brighter glow of fluorescein). Want more clearance to enable tear exchange.
