Corneoscleral GP CLs & Toric Over-Refraction

Question 1

Describe corneoscleral GP CLs?

Answer 1

Larger diameter GP lenses
Soft CLs are by nature corneoscleral lenses – they are designed to fit on the sclera
* These are not a scleral lens
* Provide similar vision correction of corneal GP
* Comfort of Soft CL – comfort comes from the large diameter
o Can therefore wear them part time as don’t really need to be adapted to them (unlike corneal GPs)
* Low cost same as corneal GPs
* Easy handling – compared to soft lenses
* Quality of vision is very good especially with astigmatic lenses
* Popular with pxs

Question 2

What is the difference between corneal GP, scleral lens and corneoscleral lenses?

Answer 2

Corneal GP - fits on cornea - touching eye only on sclera

Scleral lens - lens touching eye only on sclera - completely vaults the eye - gap between lens and cornea

Corneoscleral lens- align CL on zone of cornea, lens has to fit cornea reasonably well. Area of lens that goes over sclera - this diameter gives the comfort and stability of lenses. Corneaoscleral lenses are more akin to corneal lens than scleral lens. 2 zones in these lenses that can be used to fit the lens. OCT allowed for design, especially out in periphery, to be refined. These lenses are reasonably thick – thickness gives the lens stability (makes easier to handle)

Question 3

What happens when you make a GP and corneoscleral steeper or flatter?

Answer 3

• Steeping GP adds plus power into the lens (positive tear lens) – have to add minus power to lens to compensate
• Flattening GP adds minus power into lens (minus tear lens) – have to add positive power to compensate
• Steeping the base curve of a corneoscleral lens makes the lens deeper -> can push more pressure on the edge of lens
• Flattening the base curve of a corneoscleral lens makes the lens flatter -> lifts the edge of lens off
  Edge zone can be adjusted – can be moved up or down (without changing base curve) – can fit either plus or minus edge lift

Question 4

Describe the optimum fit of corneoscleral lens?

Answer 4

• Central alignment
• Edge lift:
  o Min – tear pump with NaFl (want to see NaFl under lens (but not too much))
  o Max – comfort (ask px – score out of 10 (7 or more means px is likely happy with it))
• Limbal clearance
• Immediate comfort – comfort is same as soft CL – don’t really feel lens in eye at all when have it in
  When fit a toric lens onto a toric eye then should result in a spherical looking eye (will not get the same toric pattern as in corneal GPs)
  Best comfort comes when there is a little bit of edge lift – allows for tear exchange
  If edge is too tight it causes discomfort on eye
• Unlike corneal GP, NO anaesthetic for fitting – use comfort to assess the fit of this lens
• NaFl in eye after lens
• NaFl under lens shows either adequate or large edge lift
• Comfort <7/10 edge too large or v sensitive px

Question 5

Describe how to insert corneoscleral CL?

Answer 5

• Put drops into CL – multi-purpose solution or wetting drops
• Get px to tip head forward slightly
• Ask them to look down – e.g. look at the skirting on the floor across the room
• Hold lids apart & place lens on cornea
• When lens is on eye – tends to go on quickly & stay on eye (not like soft lens where have to hold it on)

Question 6

Describe how to remove a corneoscleral lens?

Answer 6

• Can use sucker:
  o Hold lids apart – v gently place sucker onto CL
• OR tiddylwink method:
  o Squeeze the upper & lower lids together – dislodging & then catching the edge of the CL

Question 7

Describe the adverse conditions with corneoscleral CLs?

Answer 7

• Greasing – from tears, naturally gets attracted to dry patch on CL – grease then repels water component of tears
• Drying out of lens surface – Large area of ‘plastic’ like a SCL (plastic does not wet as well as eye surface)
• Dry eye lipid deposition
• Part time wear – compared to corneal GPs – if pxs have dry eyes it does not stop them wearing these lenses, they can wear them for shorter periods of time – they can use them for sports/activities – even if they cannot wear them on a full time basis as eyes are getting drier
• Tear reservoir debris
  o More common in keratoconic fits with steep apex
  o With keratoconics, often fit a steep central base curves – so have more of gap in front of apex
  o Metabolites from cornea can build up (can see this under soft SiHys too (muscin balls)) – mucin from tears can build up over middle
  o Can cause pxs vision to reduce – debris can be held over visual axis & some pxs may need to refresh their lenses throughout the day if they are prone to this
  o Solution: Flatten BOZR  so debris naturally gets pushed out towards edge of lens
• Solution reaction
  o Tear reservoir traps solutions
  o Less tear pump than corneal GPs
  o Use preservative free (PF) drops (solution or saline)for insertion
  o Image: diffuse SPK covering most of cornea

Question 8

Will manufacturers guidelines change for corneoscleral lenses?

Answer 8

Every manufacturers zones will be slightly different so look up their fitting guides
Fitting principles will be same across all manufacturers

Question 9

Describe how to do toric over refraction?

Answer 9

Toric lens if difference in Ks is ≥0.35mm (same for corneoscleral or corneal lenses)
First, assess how lens is sitting on eye – w/o knowing how the lens has rotated then cannot really do over-refraction
Note axis marks on CL – rotrationally stable – align with Ks (+/- 20 degrees)
IF have strange over refraction - rotationally unstable – oblique to Ks – alter BOZR to improve fit
Generally speaking corneal lenses will align with the Ks of the eye
Perform Over-Refraction:
* Put on trial frame
* -ve cyl aligned with axis marks
Example:
* 8.00 x 7.40:CSLT/ -4.00Flat -7.00Steep
* Rotationally stable, aligned with Ks
* Over-Rx +0.50/-1.00 x 170
* Add the +0.50 sphere in both meridians
* 8.00 x 7.40:CSLT / -3.50Flat -6.50Steep
* Add -1.00 at axis 170 (power @80 (power perpendicular to axis), 80 is the Steep CL axis)
* 8.00 x 7.40: CSLT/ -3.50Flat -7.50Steep