2. Corneal Topography Flashcards
What are the 6 limitations of keratometry?
- Assumes refractive index = 1.3375
- Assumes cornea is spherical
- Estimates the avg curvature of the central 3mm in two principal meridians
- Peripheral cornea is not measured
- Inaccurate readings with minor surface epithelial anomalies
- Limited capacity to measure and monitor irregular corneal surfaces and/ or decentred corneal apices (incl. KCN)
How many points across the cornea does a corneal topographer measures?
How much cornea does it measure?
7000-70000 points across the cornea
Cornea area = 7.0-11.00mm
What are the 6 different analysis does corneal topography do?
- Axial map
- Tangential mal
- Elevation map
- Refractive map
- Difference map
- Irregularity indices
For what 6 purposes does corneal topography obtain baseline data for?
- Corneal pathology
- Corneal parameters
- Contact lenses
- Astigmatism - qualitative & quantitative
- Screening tool
- Tear film - NITBUT
For what 4 purpose does corneal topography provided as a follow-up care?
- Monitor pathology
- Corneal stabilty - post-surgery
- Corneal shape changes
- Orthokeratology - optimise lens fit/ refractive outcome, record of course of treatment
Axial map
* relates to ...
* closely mimics ...
* assumes centre of the ... is on the central axis
* ... overview
* Ignores ...
Axial map
* relates to refraction/ corneal power
* closely mimics K values
* assumes centre of the radius of curvature is on the central axis
* Global overview
* Ignores minor variation
Tangential map
* location of ...
* instantaneous/ true ...
* sensitive to noise as detects ...
Tangential map
* location of corneal irregularities
* instantaneous/ true radius of curvature
* sensitive to noise as detects small variations
Elevation map
* ... height of the cornea above or below a reference sphere
* shows the location of any ...
* gives the local ...
* uses a reference e.g. ...
* high points and low points of ...
* can be used to simulate ... and ...
Elevation map
* sagittal height of the cornea above or below a reference sphere
* shows the location of any corneal irregularity
* gives the local radius of curvature
* uses a reference e.g. Best Fit Sphere
* high points and low points of elevation
* can be used to simulate CL fitting and NaFl pattern
What is Peak Elevation Index (PEI)?
PEI = distance from centre of cornea (apex) to the steepest part of the cornea
KCN = 1.95mm
PMD = 3.5mm
What is Shape Factor (P)?
P = asphericity of the cornea
Normal ~0.15-0.30
KCN highly positive (>0.50)
PMD usually negative/ low (<0.15)
How is a Subtractive (Difference) Map constructed?
Post-treatment axial map - Pretreatment axial map
Give change that happen over time after treatment
What do the colours on a Subtractive Map represent?
Red = steepening
Blue = corneal power reduction
Green = no change
What is the Refractive Power Map?
Correlates corneal shape to vision quality.
This is based upon Snell’s law of refraction.
Best estimate of corneal power.
Compare Standard Power, K Scale and Normalised Scaled.
- Standard Power = project the full spectrum of power the topography assumes its normal
- K Scale = incraeses the lowest and highest values, which decreases sensitivity
- Nomalised Scale = particular to px’s parameters, inidividualised
How can you prevent ring jam? What are they actually?
Use non-preserved lubricant drops to improve px’s tear film stability.
Tear filim break up
What are the 8 things that can be done to ensure topography is reliable and accurate?
- Weekly calibration of topographer
- At least 4 captures and select the best
- Lubricant drops
- Avoid SCL wear/ tonometry
- Minimise eyelash interference
- Minimise patient/ practitioner movement
- Ensure absence of tear film debris
- Compare R & L
What are the 7 topographical indices?
- Normalised dioptric range
- Mean corneal eccentricity
- Steep keratometry value
- Apical power value
- Inferior-Superior (IS) index
- Surface Asymmetry index (SAI)
- Surface Regularity index (SRI)
Normalised Dioptric Range
* Normative range: ...
* Normative scale on ...
* Higher range in ...
Normalised Dioptric Range
* Normative range: <10D
* Normative scale on axial power map
* Higher range in corneal ectasia
Corneal eccentricity is based upon the mathematical description of an ellipse. It measures the rate of corneal flattening from centre to periphery.
* Normal range: 0.4-0.6
* Normal: <0.65
* Abnormal: >0.65
How would the corneal eccentricity value (e) change when it is a:
* Prolate ellipse
* Sphere
* Oblate ellipse
- Prolate ellipse: normal & higher e
- Sphere: e value = 0
- Oblate ellipse: = lower or negative e
Apical power value (APV) represents the ..., which is therefore the ... value on the ... scale (...).
* Normative range: ...
Apical power value (APV) represents the steepest point of the entire cornea, which is therefore the highest value on the normalised scale (Kmax).
* Normative range: <47.2D
Inferior-Superior (IS) index measures inferior-superior asymmetry, which is the mean power difference between inferior and superior corneal hemispheres.
* 5 superior and inferior dioptric values
* 3mm from corneal apex
* 30-degree intervals
* over central 6mm(area)
* Normative range <1.60D
Surface Asymmetry Index (SAI) measures ... asymmetry, which is the difference in superior vs inferior ... at ... axes.
* Centrally weighted summation of difference in ... between points ... degrees apart
* Measures ... equally spaced meridians
* Over central ...mm (area)
* Normative range ...
Surface Asymmetry Index (SAI) measures radial asymmetry, which is the difference in superior vs inferior corneal powers at oblique perpendicular axes.
* Centrally weighted summation of difference in corneal power between points 180 degrees apart
* Measures 128 equally spaced meridians
* Over central 4.5mm (area)
* Normative range <1.00D
Surface regularity infex (SRI) assesses the ... of the ... cornea and also local fluctuation in .... SRI is calculated from summating ... across ... equally spaced semi-meridians on the ... central mire. SRI is measured over the ....
* Normative range ...
* High values indicate ...
Surface regularity infex (SRI) assesses the smoothness of the central cornea and also local fluctuation in corneal power. SRI is calculated from summating local power fluctuations across 256 equally spaced semi-meridians on the 10 central mire. SRI is measured over the pupil.
* Normative range <0.56D
* High values indicate corneal ectasia
Corneal topographers can simulate ...with RGP fitting. These patterns represent ....
Corneal topographers can simulate fluorescein patterns with RGP fitting. These patterns represent tear film clearance.
Fluorescein assessment
* ...μm tear film clearance = threshold for NaFl
* ...μm central clearance = threshold for reduction in VA
* ...μm edge clearance = ideal
* ...μm central clearance = threshold for bubble formation beneath the lens
Fluorescein assessment
* 20μm tear film clearance = threshold for NaFl
* 30-40μm central clearance = threshold for reduction in VA
* 70-90μm edge clearance = ideal
* >90μm central clearance = threshold for bubble formation beneath the lens
