Lecture 17 Orthokeratology Flashcards

1
Q

What is orthokeratology?

A

*A technique for correcting refractive errors in vision by changing the shape of the cornea with the temporary use of progressively flatter hard contact lenses
*A form of refractive correction

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2
Q

What happens when a ortho-k lens is fitted?

A

*We put a flatter than standard lens RGP cl on a cornea
*Worn overnight
*Once is removed, you recheck the cornea
*The longer you wear the lens, the more reshaping.
*Re moulds soft corneal tissue overnight.
*Takes 2-3 weeks for full correction.

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3
Q

What are other terms attributed to orthokeratology?

A

*Corneal bending
*Remodelling of the anterior corneal layers
*Hydraulic theory
*Central corneal epithelial thinning
*Mid peripheral thickening of the stroma and epithelium

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4
Q

Explain the NaFl pattern seen in orthokeratology.

A

*There is positive compression centrally on the cornea
*Causes negative tension in the periphery of the cornea
*Forces cells out into reverse curve. This is shown by band of fluorescein. This is where cells get pushed to.
*There should be NO touch with cornea, always central clearance.
*We use positive compression and negative tension to reshape cornea. This happens beneath the lens.

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5
Q

Why can it looks like there is touch between lens and cornea in NaFl pattern?

A

*Fluorescein doesn’t fluoresce at less than 30 microns.
*Ideal central clearance is 10-13 microns.
*Not enough fluorescein beneath centre of lens to fluoresce.

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6
Q

What changes do you do to the structure of cornea after wearing ortho-k lenses?

A

Central thinning of epithelial cells

Thickening of cells and stroma in periphery

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7
Q

What equipment must you have to fit ortho-k lenses?

A

corneal topographer

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8
Q

What amount can ortho-k lenses correct?

A

The higher the correction, the smaller the treatment zone.

  • If px has a large pupil and treatment zone is small over pupil, you can cause visual disturbance especially in low lighting.
  • myope correction is max 5.00D and 2.50D WTR astg.
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9
Q

What patients won’t be suitable for orthokeratology?

A

*Occupation- e.g., shift workers, hard to get consistent 8 hours wear.
*Compliance
*Hygiene- not disposable, will have them for at least 6 months
*Motivation

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10
Q

What are the features of topography?

A

*High degree of accuracy and repeatability
*Statistical analysis of repeated readings of apical radius and eccentricity or elevation
*Axial, tangential, and refractive power and curvature maps
*A difference or subtractive map function. Not all do this.
*Pupil recognition from centration pov.
*A large area of corneal coverage with minimal interpolation. We get 9-14 mm of anterior surface of cornea. Minimum of 10 mm needs to be measured.

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11
Q

What does the tangential graph show?

What does the axial/refractive graph show?

A

-localised changes
-best indicator of corneal shape and centration after ortho k

-monitoring change and showing central island post ortho k

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12
Q

What does the tangential subtractive map show?
What does the refractive subtractive map show?
What does the axial subtractive map show?

A
  • centration

the size of treatment zone and change in refractive power

change in axial corneal surface power

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13
Q

What is required to do topography on a px?

A

*Px needs to keep eyes wide open
*Tilt head, if necessary, as eyelashes/nose can obstruct good image
*Normal blink rate required
*Good tear quality (can use artificial tears if necessary)
*Take a minimum of four images to compare
*Delete poor images
*Keep 3-4 good images as base line

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14
Q

What indicates a poor image?

A

-missing ares, px not opening eyes wide enough

-wavy mires, unstable tear film

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15
Q

What are some of the types of ortho-k lenses?

A

*Paragon CRT: fitting set lens. based on eccentricity and other values from topographer
*Eye dream by no 7: software-based lens
*Euclid, Aiko, emerald
*Menicon Z night using easy fit software

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16
Q

What does the optimum fluorescein pattern look for an ortho-k fitting?

A

clearance: 0.4mm
central zone: 3-5mm
reverse zone: 0.5-1mm

17
Q

What is the advantages of topography in ortho-k fittings?

A

Pre-treatment
*Topography sets a baseline
*Helps to select lens
*Screening of abnormal cornea, poor corneal shape, decentred apexes
*Allows to fit with more caution in people with tilted cornea, abnormal lid architecture, limbus to limbus astigmatism

Post treatment
*Determine centration
*Do we have a good treatment zone size
*Refractive change

18
Q

What topographical responses can you get?

A
  1. Bulls eye
  2. Smiley face
  3. frowny face
  4. true central island
  5. smiley face with false central island
  6. lateral displacement
  7. central divot
19
Q

What is the ideal topography pattern you expect to see after ortho-k?

A

Bull’s eye
A well-centered area of corneal flattening
A circle of mid-peripheral corneal steepening
Little or no peripheral corneal change

20
Q

What symptoms can a small treatment zone cause?

A

flare
poor vision

21
Q

What causes a smiley face topography pattern?

What are the consequences?

What is the treatment?

A

Causes by flat fitting lens

*Px will have poor vision or variable vision
*Negative over refraction and unwanted cyl or increase in astigmatism
*Increased WTR astigmatism
*Px will experience ghosting, glare and flare

*Need to increase lens sag, steepen the lens centrally or peripherally in landing zone, steepen RC/BOZR
*Need to increase RZA, LZA or BOZR
*Decrease cone angle

22
Q

What causes a frowny face topography pattern?

What are the consequences?

What is the treatment?

A

Caused by Flat fitting

*Causes some visual disturbances such as ghosting, glare and flare
*Negative over refraction

*Need to increase lens sag, steepen AC,RC/BOZR, RZD, LZA
*Need to decrease cone angle

23
Q

What causes a true central island topography pattern?

What are the consequences?

What is the treatment?

A

steep fitting lens

*Over refraction: no end point
*Poor BCVA
*Too much lens sag and eccentricity

*If central island is small (less than 1D), may self-resolve in a week
*If island doesn’t resolve, refit with reduced sag

24
Q

What are the risks/disadvantages of ortho-k?

A

*Requires more time to fit
*Topographer is essential
*Aftercare is more frequent to ensure efficacy and safety
*Noncompliance or poor compliance due to lens care or handling
*Full correction may not be achievable in all px
*Top up contact lens may be needed during the treatment process. 1/3 of px prescription may be corrected after first night. Need top up lenses during first initial weeks.

25
Q

What are the benefits of orthokeratology?

A

*Vision correction
-Successful overnight wear results in good vision that can be maintained throughout the day post lens removal
-Average reduction of myopia of -4.00 D gives the highest rate of success
-Completely reversible: if lens wear is stopped, cornea can revert to its pre-treatment state. Depends on how well fitted lenses were and length of time of wear.

*Lifestyle
-Convenience for those who do sports, some occupations and those who don’t want to wear conventional contact lenses or spectacles during the day.
-Complications associated with dryness, dust, allergies with conventional lens types avoided as only wearing them overnight.

26
Q

What can corneal staining indicate about the fitting?

How can you prevent lens binding?

A

-sub optimal lens fit (not enough clearance)
-solution sensitivity
-will expect some superficial staining in all px to begin with

-px should wait 15 minute to make sure tears are circulating rather than taking them out first thing.

27
Q

What types of staining can you get and what is the cause?

A

central staining- flat fit

inferior conjunctival staining- steep fit

superior conjunctival staining- flat fit

dimple veil- steep fit (too much clearance) or optimum fit but trapped tears

lens binding- poor fit

28
Q

How does myopic progression work?

A

*We need to stop axial elongation
*Myopes have longer axial length
*We believe there is a trigger for axial elongation
*Putting a negative lens Infront of the cornea, we cause hyperopic defocus in peripheral retina
*Eye stretches in response to this defocus. Increases axial elongation.
*We need to create myopic defocus in peripheral retina to stop this trigger for axial elongation

29
Q

How does orthokeratology slow myopic progression?

A

*When using an ortho k lens, we change the shape of the cornea. This creates myopic defocus on peripheral retina.
*Due to increased thickness in peripheral cells, we are effectively creating an ADD on the cornea