4.1.1 Identifies anomalies in a prescription and implements the appropriate course of action.

Question 1

What to record:

Answer 1

Mono pds taken. Same lens thickness/design as prev to make easier for px to adapt. Px not complained of diplopia when looking through reference point that isn’t OC before. Adv if any problems/diplopia/magnification effect, to return to consider slab off prism in RE or separate pairs

Question 2

Mono pds is a MUST. Assessor will ask why certain measurements are taken.

Answer 2

• Why? → If the prescription is highly anisometropic, however, a significant horizontal prism imbalance would be induced (at least without some degree of compensatory head turn learned empirically by the wearer). Further, even when the right and left powers are similar, a noticeable head turn may be induced if the monocular PDs differ considerably due to the presence of significant yoked prism.
• Heights → For same reason as above but for vertical prism
• PA → (if taking heights) - dispenser’s rule, every 2 degrees of tilt induces 1mm higher OC so reduce by 1mm for every 2 degrees

Question 3

What type of lens is preferred?

Answer 3

Ideally aspheric in one lens & spherical in other but not a must for the comp

Question 4

Evidence of talking about aniseikonia, adaptation, any solutions

Answer 4

• If patient returns complaining of diplopia e.g. for distance glasses, it may be that they’re looking through different part of lens to use for near so suggest separate spex or at worst, slab off or biprism lens for SVD if single frame wanted
• Must know if anisometropia is longstanding or new

Question 5

Rules of thumb

Answer 5

• REMEMBER: SHOV (same horizontal opposite vertical) - will Add together
  
  • up and down = add together (ie 1 up R 3 down L = 1+3 = 4)
  • down and down = subtract (i.e. 1 down R 4 down L = 4-1 = 3 down in L)
    out and out = add together (ie 1 out R and 3 out L = 1+3 = 4)
    in and in = add together (ie 1 in R and 3 in L = 1+3 = 4)
    in and out = subtract (ie 1 in R and 3 out L = 3-1 = 2 Base Out L)

Question 6

Fusional Reserves on Average:

Answer 6

Vertical —> tolerance issue if 1 dioptre or more! - depends also on the length of the near vision task, the individual’s binocular vision status, and their habitual refractive correction. If for short periods of work, then patient may be asymptomatic but if prolonged then probably an issue
Horizontal tolerances: up to 10Dioptres out and 4 dioptres in for distance, up to 7Dioptres out and 7dioptres in for near.

Question 7

General solutions to Anisometropia:

Answer 7

• 2 pairs of spex - different centrations
• Head position - for NV, drop head to look through distance OCs but uncomfortable
• Drop spex down nose - look through distance OCs
• Franklin split - ridge felt on both sides, looks awful
• Slab off - removing base down in more -ve eye, line coincides with lower limbus
  
  • Minimum slab-off 2Δ down
• Bonded prism segment
• Prism controlled - segment on a prism segment blank is depressed from the back surface of the distance portion, thus allowing prism in any direction to be worked on the segment
• Grossly decentred D segs - for H prism
• Different R seg sizes - larger to most positive but looks awful
• Contact lenses

Question 8

Unilateral cataract surgery

Answer 8

Unilateral cataract surgery is a big cause for anisometropia as patient will be waiting for other eye to be done or patient lives in area where cataract surgery is unable to be done due to lack of NHS funding

Question 9

Why do we need to determine the patients tasks?

Answer 9

• Determine patient visual needs before assessing if anisometropia will affect them i.e. length of near vision task, BV status & habitual refractive correction
  
  • Even vertical prism may not be problematic if near vision isn’t used for prolonged periods/fusional reserves are good to adapt to prism
• If px has vertical prism differential > 1, then separate SVD + SVN can be ordered with OCs aligned for each otherwise if single pair wanted, then slab-off/biprism SV lenses where base down taken off more minus lens (as it induces more base down), to balance differential prism in near portion only.

Question 10

Unequal R segs?

Answer 10

• Unequal R segs - larger diameter = larger base down
  
  • These segs are not that suitable for myopes but more for hyperopes. Slab off is far better
  • Unequal R segs can only go so far eliminating differential prism. May need slab off from higher levels of prism. Slab-off line positioned level with the segment top to improve cosmesis.
  • Franklin split bifocals offer another alternative solution, where the bifocal consists of two separate lenses which are cut in two and glazed together (see Figure 4). This solution provides independent positioning of the optical centres of the distance and near portions, and thus allows separate vertical and horizontal prism control for distance and near

Question 11

When is aniseikonia a problem?

Answer 11

• When is aniseikonia a problem? a difference of around 5% or greater may be problematic
  
  • Then stereoscopic fusion will be difficult and can result in poor binocular vision as a result of aniseikonia
  • Spectacle mag:
• Iseikonic lenses - leave Rx the same but only change the magnification
• Even if spectacle mag balanced, does not mean perceived mag will be balanced

Question 12

Know slab on Vs slab off

Answer 12

• Slab off is with D segs
• Slab off is for negative - taking away base down
• Slab on - positive Rx that don’t want unequal R segs

• Slab off can be done on varifocals but not done as much