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

Question 1

READ THE DISPENSING BOOKLET

Answer 1

Search “Anisometropic”

Question 2

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

Answer 2

• Why?
  
  • In highly anisometropic prescriptions, significant horizontal prism imbalance can occur.
  • Without compensation (e.g., learned head turn), this imbalance can cause discomfort.
  • Even with similar right and left powers, a noticeable head turn may result if monocular PDs differ significantly due to yoked prism.
• Heights
  
  • Vertical prism can be induced for the same reasons as horizontal prism imbalance.
• Pantoscopic Angle (PA)
  
  • If measuring heights, follow the dispenser’s rule:
    
    • Every 2° of tilt induces a 1mm higher optical center (OC).
    • Reduce OC height by 1mm for every 2° of tilt.

Question 3

REMEMBER: SHOV

Answer 3

• (same horizontal opposite vertical - these 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 4

Fusional Reserves on Average:

Answer 4

• Vertical = 1D differential prismatic effect binoculalrly
• Horizontal tolerance = up to 10D out and 4D in for distance, up to 7D out and 7D in for near
  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

Question 5

General solutions to Anisometropia:

Answer 5

• 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 6

Unilateral cataract surgery

Answer 6

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 7

Anisometropia

Answer 7

o Difference of 2D+ in power between both eyes
o For each dioptre difference in power between lenses, there will be 1Δ differential prism induced
o Symptoms - headaches, blurred vision, diplopia etc.
o Some anisometropic subjects, however, are able to adapt to the differential prism and exhibit no symptoms. Some will just suppress, especially at higher levels. Others, at lower levels, may have good fusional reserves and tolerate the differential prism

Question 8

Single vision differential prismatic effect:

Answer 8

o Differential Prismatic effect occurs in single vision lenses when the patient looks down to read.
o Vertical diplopia may be induced due to differing amounts of vertical prism when looking away from OC.
o Your patient will complain of seeing 2 images positioned above one another.
o You could ask patient to look down and compensating prism can be placed. If response better then they would benefit from prism.

Question 9

Solutions

Answer 9

Two Pairs of Glasses
- Same prescription but differently centered
- One pair for distance vision (DV), the other for near vision (NV)
- Impractical solution

Slab-Off
- Removes base-down prism
- Applied to the most negative eye (induces the most base-down prism)
- Slab-off line should align with the lower limbus

Head Position
- Advise patient to drop head for NV, allowing eyes to look through distance OCs
- Not ideal for prolonged reading

Dropping Glasses Down Nose for Reading
- Not recommended
- Initially causes differential prism when worn normally
- When dropped down, patient looks through a different point on the lens, which may help induce prism compensation
Bifocal Differential Prismatic Effect
- Occurs when looking through the near vision portion (NVP)
- Must be eliminated or reduced to prevent diplopia and ensure tolerance
- Calculation at NVP (typically 10mm below and 2mm in from distance OC)
- Formula: ( P = cF ) (Prism = decentration (cm) × power (D))