4.1.1 Anomalies in a prescription

What issues can anisometropia create?

Issues start to occur when difference greater than 2.00DS

• when looking away from the OC (e.g looking down to read)

  • Blurred vision

  • Double vision

  • unable to tolerate reading for long periods due to greater than 1D of vertical differential prism effect

What is the near visual point?

• 10mm below and 2mm in from the OC

What happens at the near visual point?

For each dioptre difference between eyes, there is 1D of vertical differential prism effect at the near visual point

VDPE according to British standards?

• Px can tolerate 1D of VDPE

• tolerance much greater horizontally

  • 10 BO - 4 BI at dist

  • 7 BO - 7 BI at near

Actual Vertical prism reserves range between 2-4PD

Formula which links Prism, Centration and Power

P = C x F

• P = prism at NVP

• C = centration in cm (always 1cm)

• F = power along the vertical meridian

e.g of formula

RE: +4.25/-0.50x90 LE: +1.50/-0.50x90

Don’t include the cyls in this example as they are at an axis of 90, they are actually acting at 180, making them horizontal for this equation, we focus on the vertical meridian ONLY.

RE: 4.25 BU - LE: 1.50 BU

Total: 2.75 Base UP RE - It’s labelled as RE as it has the highest power of prism to begin with

What is Aniseikonia?

• For Px with anisometropia of greater than 2.00D, they may experience a difference in the size and shape of visual images and magnification

Sx include;

• Visual discomfort

• Visual distortion

• Difficulty with stereopsis/BV

• Dizziness, H/A, Nausea

Solutions for Dispensing an anisometropic Patient - single vision

• Aspheric lens to the most +ve powered eye

  • retinal image size is reduced as aspheric surface is 2.00D flatter = reduces difference between eyes

• Iseikonic lens to least +ve eye - aims to make retinal image size equal

  • increases thickness of lens (BC 8.00D) which increases the spec mag = reduces difference between both eyes

• Give Px separate pairs for different distances as OC will be set for that particular distance so won’t get any VDPE

• Can potentially compromise by partially prescribing to try and avoid sx

• Contact lenses

• Minimise centre thickness in +ve lenses

• Minimise edge thickness in -ve lenses

• Frame with smaller eye size

What formula does Iseikonic lenses link to

Spectacle magnification = Shape Factor x Power Factor

Iseikonic lenses adjust lens design parameters that are included in shape factor and power factor

What is shape factor?

• A ratio of surface curvatures that describe the shape of the lens and range from -2 to +2

What is power factor?

• Dioptric power of the lens considering the power and vertex distance

Why do CLs help with anisometropia?

• Reduces image size difference drastically

  • improves BV

Bi-focal dispensing for Anisometropic Px

• Slab off / bicentric grinding near-portion for most -ve eye

  • induces base up prism neutralising the base down prism of the main lens

  • most -ve eye induces the most base down

• Unequal round segments - only successful in round segs

  • Larger seg given to most +ve eye

  • works on the principle that a larger segment will induce the most base down prism - most +ve and -ve lens produces more base down

  • based off the formula;

    • segment radius (cm) x near ADD = amount of vertical prism overcome

  • to find out how much difference in seg to give;

    • difference in seg (mm) = 20 x differential prism / ADD

Slab-off in +ve and -ve lenses

SLABBED OFF;

• -ve lenses - base down removed

• +ve lenses - base up added

What are the different types of slab-offs?

• Conventional slab-off

  • most common option

  • base up is grounded into the lens

• Reverse slab-off

  • rarely used but induced base down prism