What is aniseikonia?
Difference in perceived image size or shape between the two eyes
Aniseikonia can lead to visual discomfort and difficulties in binocular vision.
What are the types of aniseikonia?
- Static: difference at optical centre (straight ahead)
- Dynamic: difference when looking away from optical centre (prism-induced)
Dynamic aniseikonia is typically greater than static.
What are the causes of aniseikonia?
- Physiological
- Optical (e.g. spectacles)
- Retinal
- Neurological
Each cause can contribute differently to the perception of image size.
What is the difference between static and dynamic aniseikonia?
- Static: due to magnification differences at primary gaze
- Dynamic: caused by prismatic effects when viewing off-axis
Dynamic aniseikonia is typically greater than static (~1.5×).
How does Prentice’s Rule relate to dynamic aniseikonia?
Prism (Δ) = d × F
d = decentration (cm), F = lens power (D). Off-axis viewing creates unequal prism, leading to image size difference.
Why is dynamic aniseikonia worse in up and down gaze?
Vertical gaze induces greater prismatic effects due to lens decentration → more image size difference
This can exacerbate visual discomfort.
What is ocular aniseikonia?
Caused by anisometropia (unequal refractive error) → different retinal image magnification between eyes
This condition can lead to difficulties in fusion and depth perception.
How do spectacles contribute to aniseikonia?
Spectacles cause spectacle magnification, which differs between eyes → induces or worsens aniseikonia
This can complicate visual processing.
What is the formula for spectacle magnification?
Spectacle magnification = Shape factor × Power factor
This formula determines how much image size is altered by lenses.
At what level does aniseikonia become symptomatic?
- 2% → asthenopia (eye strain)
- 5% → fusion often breaks → diplopia
Symptoms can significantly impact daily activities.
What is the Robertson technique for measuring aniseikonia?
Maddox rod used monocularly, patient looks up/down, measure separation between line and light
1 cm displacement = 1Δ at 1 m.
How do afocal lenses help in aniseikonia?
They provide magnification without changing refractive power → correct image size difference
This can help in achieving better visual comfort.
How does the Awaya test measure aniseikonia?
Patient views red/green semicircles (with R-G glasses), determines which image appears smaller, adjust size until subjective equality
Calculate ratio → convert to %.
How do you calculate aniseikonia using the Awaya test?
Example: Red = 92 mm, Green = 97 mm, Ratio = 97 / 92 = 1.054, Invert = 1 / 1.054 = 0.948
Aniseikonia = −5.15% (LE image smaller than RE).
What are the main management options for aniseikonia?
- Full anisometropic spectacle correction (adaptation)
- Bicentric lenses
- Isogonal / Iseikonic lenses
- Contact lenses
- Partial correction (balance lens)
- Occlusion (last resort)
Management must be tailored to individual patient needs.
Why can full anisometropic correction be used, especially in children?
Allows neural adaptation to image size difference, essential in anisometropic amblyopia → must give full correction
Prevents suppression and improves visual development.
What are bicentric lenses and when are they used?
Lenses with two optical centres, include vertical prism segment
Used to compensate for induced vertical prism in anisometropia.
What is the difference between isogonal and iseikonic lenses?
- Isogonal: minimise spectacle magnification differences
- Iseikonic: specifically designed to correct subjective aniseikonia
Modify shape factor (thickness, base curve, vertex distance).
Why are contact lenses preferred in refractive anisometropia?
- Reduce spectacle magnification differences
- Minimise dynamic aniseikonia (less prismatic effect)
- Better cosmesis
Sit closer to eye → less image size difference.
What are the limitations of contact lenses in aniseikonia?
- Patient compliance and motivation
- Ocular surface health (dry eye, etc.)
- Infection risk
- Need backup glasses
These factors can affect the success of contact lens use.
What is Knapp’s Rule and when does it apply?
- Axial anisometropia: spectacles minimise aniseikonia
- Refractive anisometropia: contact lenses preferred
Based on where magnification difference originates.
What is a balance lens approach in aniseikonia?
Partial correction in higher refractive error eye
Reduces symptoms but does not fully correct refractive error.
When is occlusion used in aniseikonia?
Last resort, used when symptoms are severe and cannot be managed
Eliminates diplopia but sacrifices binocular vision.
Why is dynamic aniseikonia often more symptomatic than static?
Patients can adapt to static differences, but dynamic changes (with gaze shifts) are harder to compensate → more symptoms
This can lead to greater discomfort.
