Management of Anisometropia

Question 1

What is isometropia?

Answer 1

Equal refractive error between eyes, difference is < 1.00 D

Question 2

What is the aetiology of aniseikonia?

Answer 2

Optical - axial - due to the difference in ocular axial length. Refractive due to the difference in corneal curvatures

neurological - due to unequal division of the primary visual cortex

Question 3

What is Aniseikonia?

Answer 3

Condition of the binocular visual system in which there exists a relative difference in size or shape of two ocular images.

Question 4

What are symptoms of aniseikonia?

Answer 4

• HA
• Photophobia
• reading difficulties
• nausea
• diplopia
• dizziness and vertigo
• depth perception issues

Question 5

Why is the aetiology of anisometropia?

Answer 5

• congenital
  refractive errors: hyperopic or astigmatism
• acquired
  developmental myopia

Question 6

What is amblyopia?

Answer 6

An uncorrectable decrease in vision in one or both eyes with no apparent structural abnormality seen to explain it

Can occur from any condition that prevents the eye from focusing clearly:
- physical/deprivation - cataract, corneal opacities
-optical - anisometropia
- abnormal retinal correspondence, i.e, strabismus

Question 7

When does aniseikonia occur?

Answer 7

Usually occurs when correcting anisometropia due to
= anatomical physiological factors
= receptive field size
= retinal receptor density
= cortical mapping

Question 8

What are the optical consequences of correcting anisometropia with specs?

Answer 8

Difference levels of magnification between both eyes

Difference in induced prism (by decentration)

Difference in stimulus to ocular accommodation between both eyes: spectacle vs ocular accommodation

Question 9

What happens to spectacles that induce different levels of magnification?

Answer 9

• Leads to incongruous retinal images
• Reduces Binocularity
• Amblyopia

Question 10

What does prism disparity (imbalance) do?

Answer 10

Disparity in rx results in difference in induced prismatic effect between eyes

SV wearers can turn into ‘head movers’ - always look through optical centres - no prismatic effect

Multifocal wearers - downgaze - vertical prism imbalance = differential prismatic effect

Question 11

How do you correct Aniseikonia with specs?

Answer 11

Use of iseikonic lenses to reduce magnification.
Changes in shape factor for lens.
Changes in lens thickness - low powered thicker
Changes in base curve - lower powered higher base curve

Question 12

What is Knapp’s Law?

Answer 12

• Axial and refractive anisometropia require different types of optical correction in order to equate retinal image size
• Axial anisometropes require a lens places in the anterior focal plane of the eye
• Refractive anisometropia requires a lens placed as close as possible to the nodal point