Specialty testing in CL Fitting Flashcards
In a ____________ cornea, there is gradual flattening from center to periphery
prolate
When do we see an oblate cornea?
post surgery (flatter centrally, steeper peripherally)
What is eccentricity?
amount of flattening
What is the E value for a prolate cornea? An Oblate Cornea?
Prolate: between 0-1; Oblate <0
What is the average E value?
0.50 (range 0.3-0.7)
What is Asphericity (Q)?
The rate of change of curvature from apex to periphery
What type of cornea has a (-) Q?
prolate
What is the average Q?
-0.26
What type of keratometry is the Oculus Pentacam?
Projection/ slit scanning
What is the axial map used for?
showing overall shape, following progression, power change (assumes center of curvature is the center of cornea)
What is the tangential map used for?
local curvature; keratoconus, orthoK centration
What is the elevation map used for?
interpreting NaFl patterns in corneal GPs
What are the 5 parameters given in the Belin/ Ambrosio Enhanced Ectasia Display?
- Df: changes in anterior elevation from standard to enhanced reference surface
- Db: changes in posterior elevation
- Dt: corneal thickness at the thinnest point
- Da: thinnest point displacement
- Dp: pachymetric progression
How is Final D calculated in the BAD?
considers all 5 parameters and performs a linear regression against a standard database of normal and KC corneas
What is the index of vertical asymmetry?
Mean difference between superior and inferior corneal curvature by assessing the level of curvature symmetry with respect to the horizontal meridian as the axis of reflection
Is the IVA or BAD better at predicting KCN?
BAD
What measurement is KCN staging based on?
posterior curvature and thickness measurements based on the thinnest point, not the apical measurement
Which instrument has the best repeatability for min/ max K readings?
IOLMaster
What is the gold standard for CCT?
Ultrasound pachymetry
When is CCT helpful?
keratorefractive procedures; glaucoma
When is ECC helpful?
early stages of dystrophic endothelial disease
When considering corneal grafts, is a higher or lower ECC associated with a higher chance of rejection?
lower ECC
What happens to the cornea when the Na/K ATP pumps become less effective?
stromal swelling
How does endothelial cell density change throughout life?
decreases (children: 3,000-4,000 cells/ mm 2; 80 year olds: 2300 cells/ mm2)
what does a lower CD value mean?
endothelium depleting faster than normal
What is the average adult axial length?
24 mm
What change in axial length do we see in children with progressive myopia?
1D of myopia = ~0.50 mm AXL
What are the uses for axial length measurements?
cataract surgery; myopia control progression
How does the lens compensate for the initial increase in AXL?
thins until it reaches physiological limit, then myopia begins
What age is the peak incidence of myopia?
8-10 years old
When does myopia usually stabilize?
by 16 years old
What are the goals for myopia control?
keep myopia < 6.00 D and axial length <26 mm
What are uses of Anterior Seg OCT?
glaucoma, retina, cornea; scleral lens fitting
