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