Orthokeratology Flashcards
What layer does OrthoK work on?
Epithelium (~50 microns thick)
How does OrthoK work?
causes fluid to move from the center to the periphery; central epithelial cells are flattened and midperipheral cells are elongated
What does reverse geometry mean?
first peripheral curve is steeper than the base curve
What is the benefit of reverse geometry in OrthoK lenses?
Helps with centration, comfort and creating a predictable/ stable change in the cornea
How long does it take for full treatment to happen?
1-2 weeks depending on Rx
About how much of the prescription is corrected each night?
50%
Why do we overcorrect in OrthoK?
Long term expected daily regression is 0.25-0.75D; we want the patient to be close to emmetropic by the end of the day
How long does it take for reversal of OrthoK?
up to 1 month for full reversal; repeat topography to confirm back to baseline
How long to sleep in OrthoK lenses?
6-8 hours; higher myopia has more sleep required while low myopes may be able to wear lenses every other night
What is the Munnerlyn Formula
Depth = (optic zone diameter 2x refractive error)/3
use SE for astigmatism
may need to choose smaller OZD to get a smaller treatment zone
Do higher or lower prescriptions require a smaller treatment zone?
higher prescription
Why might a patient using OrthoK experience nighttime glare?
pupil size larger than treatment zone (beneficial for myopia control)
When might someone using OrthoK experience a slight decrease in visual acuity?
low contrast, low illumination (pupil size increased)
What Dk is used in OrthoK?
High Dk
Higher Dk= less wettable
No abrasive cleaners d/t plasma or other surface treatments
Which lens is the first therapeutic lens approved by FDA for overnight OrthoK for temporary reduction of myopia?
Paragon CRT
What are the refractive error limits of Paragon CRT?
up to -6.00 D of myopia, with or without -1.75 D of astigmatism
flat K between 40- 46 D
What are the 3 adjustable curves/ zones of a paragon CRT lens?
Return Zone depth, Landing Zone Angle, Base Curve
Which area of a Paragon lens is the key factor in centration and central applanation?
RZD
fit issue: change RZD
Desired applanation: 3-4 mm
What does the LZA do?
Provide appropriate edge lift
adjusted in 1 degree steps
larger (-) #: closer to cornea, less edge lift
Is the BC steeper or flatter than the corneal apex for OrthoK?
flatter
What should OrthoK over-refraction be to ensure proper over-correction?
plano to +0.50D
Paragon CRT lenses have a power of +0.50 D to neutralize overcorrection
What materials are Paragon lenses availabe in?
HDS100 (Dk=100); Menicon Z (Dk= 163)
tints: green, blut, red, yellow
What is the lens modification if there are large bubbles in the RZD of an OrthoK lens?
decrease RZD (25 micron steps)
bubbles form d/t excessive central clearance or no central applanation
How do you fix inferior positioning for an OrthoK lens?
Decrease sag depth by decreasing LZA by 1 degree
If doesn’t work, go back to original lens, decrease RZD by 1 step
What parameter is changed to correct Rx?
BC
RZD changes the fit
If over-refraction of an OrthoK lens yields minus power, the BC is too flat/steep
steep
flatten in 0.1mm steps for every 0.50 D until desired ROL
If ROL for OrthoK is plus power, the BC is too flat/ steep
flat
steepen in 0.10mm for every 0.50D until desired ROL
When treating a higher myope (>5.00), is it best to prioritize centration or treatment?
Centration
Munnerlyn’s formula
High myopes: smaller tx zone
What is the follow up schedule for OrthoK?
- first-morning
- one-week
- two-week
- one-month
don’t change parameters until 2-week mark
If patient is bothered by glare/ haloes at night, what can be prescribed?
Alphagan (brimonidine 0.2%)
miotic drop used off-label
