Corneal topography and Pachymetry Flashcards
What is the central radius of curvature of the anterior cornea (K and D)?
7.8mm
43.50D
What is the central radius of curvature of the posterior cornea?
6.7mm
What does keratometry assume?
Spherical surface
Where does keratometry measure?
Two points either side of central 3mm of cornea
What are the benefits of keratometry?
Can directly relate to spec rx
Useful for CL fitting
What are the indications for corneal topography?
CL fitting/AC (complex cases)
Corneal injury/disease (diagnosis and monitoring)
Surgery (mostly for refractive surgery)
What is an advantage of corneal topography?
Non-invasive
Few contra-indications
What can affect the reliability of corneal topography?
Dry eye
Tear film quality
Poor fixation
Poor focus
Lid/lash artefacts
What are the applications of keratometry?
Refraction for a normal cornea
CL fitting in simple cases
Routine diagnosis and follow-up in corneal disease
Simple cataract surgery cases (planning, IOL calculation, suture removal)
Routine follow up for corneal graft and refractive surgery
What are the applications of topography?
Complex CL fitting
Orthokeratology
Detect warpage
Monitoring and subclinical detection of corneal disease
Complex cataract surgery cases (investigation of poor outcome)
Corneal graft surgery (assess regularity, suture removal and CL fitting)
Refractive surgery (preop screening, planning, document surgery, investigation of poor outcome, discuss with pxs)
What are the types of topographer?
Placido disc
Tomography (Orbscan and Pentacam)
How else can you perform topography?
Ultrasound
Anterior OCT
How does the placido disc work?
Pattern reflected from anterior cornea
What are the disadvantages of the placido disc?
Only assesses anterior corneal curvature due to relying on reflection
Compromised tear film can affect the result
How does the orbscan work?
Tomography - cross-sectional photos
Combines placido disc and vertical slit scanning images
How does the pentacam work?
Tomography - cross-sectional photos
Scheimpflug photography gives cross-section images in 360 degrees
What can be seen using tomography techniques?
3D maps
Anterior and posterior surfaces
Iris
Lens
AC depth and angle measurement
What are the types of scaling in data maps?
Absolute - fixed step sizes, any data outside range is given same colour
Relative - step size adjusted to fit the overall range
What are the advantages of absolute scales?
Standardised
Easy comparison
Large power range
Good for screening
Good for gross pathology
What are the disadvantages of absolute scales?
Large step sizes
Low resolution
What are the advantages of relative scales?
Smaller step sizes
High resolution
Subtle features visible
Good for detail
What are the disadvantages of relative scales?
Non-standardised
Difficult to compare
Narrower power range
What is axial/sagittal corneal curvature measurement?
Equivalent measurement to keratometry but measures across the whole cornea, measuring radius of curvature at each point on the cornea to the optical axis
What assumption does axial/sagittal corneal curvature measurement make and what bias does it cause?
Assumes surface is spherical
Causes spherical bias
What is axial/sagittal corneal curvature measurement good for and not so good for?
Good for measuring regular astigmatism
Not good at detecting localised disease
What is tangential/true corneal curvature measurement?
Measures each point independently, not assuming the surface is spherical (doesn’t intersect the optical axis) so less spherical bias exists
What is tangential/true corneal curvature measurement good for?
CL fitting
Keratoconic scans
What is a corneal power map?
Curvature is presented in dioptric power
Common in US/Canada
What does a corneal power map assume?
Assumes refractive index of media is 1.33
Doesn’t take into account corneal thickness
What is an elevation map?
Shows raised and lower areas of the cornea by comparison to a sphere with a fixed RoC (7.99mm for anterior, 6.59mm for posterior)
How does regular astigmatism present on a topography map?
Symmetrical bow tie
Visual axis at centre of bow tie
Bow tie will be lined up with one of the principal meridians
Meridians cross at the centre of the bow tie
How does irregular astigmatism present on a topography map?
Asymmetrical bow tie
Visual axis is not at the centre of the bow tie
Meridians don’t cross at the centre of the bow tie
What is with-the-rule astigmatism?
Higher positive power on vertical axis
Minus cyl axis horizontal
More common in children and young adults
What is against-the-rule astigmatism?
Higher positive power on horizontal axis
Minus cyl axis vertical
More common in older adults
What is Javal’s rule?
Spectacle astig= 1.25x(corneal cyl in D)+(-0.50x90)
How can keratoconus be identified on a topography map?
Cone - thinnest area
Earliest signs on posterior corneal surface
Combined increased curvature, thinning and elevation at same location (cone)
How can pellucid marginal degeneration be identified on a topography map?
Kissing bird appearance
Thinning inferiorly (4-8 o’clock)
Why is pachymetry needed?
Assess glaucoma risk
To account for IOP measurement errors
Corneal transplant and post-op evaluation
Minimum thickness for refractive surgery
Corneal disease assoc with oedema
Corneal diesease assoc with thinning (min thickness for cross-linking)
What is the average CCT with SD and range?
540+/-35
range: 470-610
What are the indications for ultrasound pachymetry?
Corneal disease
Corneal injury
Glaucoma
IOP measurement
Refractive surgery
What are the contra-indications for ultrasound pachymetry?
Infection (due to contact)
Compromised cornea
