29/04/2013

1

Higher order aberrations in asymmetric

refractive error development

Stephen Vincent Queensland University of Technology

Singh

•Background – Refractive error development

• Genetic influence

- twin studies Sorsby (1962), Dirani (2008), genealogical studies Zadnick (1994), Ip (2007)

• Environmental influence (near work theories)

MYOPIA (axial elongation)

OPTICAL

(hyperopic defocus)

MECHANICAL

(force)

NEAR WORK

Lag of accommodation Abbot (1998), Allen (2006)

Corneal aberrations Buehren (2003), Collins (2006)

Total aberrations Collins (1995), He (2002)

IOP Abdalla (1970), Quinn (1995)

Convergence Goss (1996), Bayramlar (1999)

Ciliary muscle/choroid van Alphen (1986)

Other: diet, outdoor activities

Animal models: altered visual experience

CHOROIDAL

THINNING

CHOROIDAL

THINNING

CHOROIDAL

THICKENING

Axial elongation

(myopia)

Axial elongation

(myopia)

Axial retardation

(hyperopia)

Hyperopic blur

(minus lens)

Myopic blur

(plus lens)

Diffuser

Wildsoet

Form

deprivation

Image

manipulation

• Emmetropisation - passive & active components

• Proposed mechanisms - accommodation Wildsoet (1991)

- neural link to brain Troilo (1990)

- image mediated retinal control of eye growth Wallman (1987)

Not essential?

Human examples: altered visual experience

- Corneal scarring Tabbara (1999)

- Congenital cataract Kugelberg (1996)

- Vitreous haemorrhage Mohney (2002)

- Partial occlusion (ptosis) Hoyt (1981)

Disrupted emmetropisation – form deprivation

Monovision Phillips (2005)

Retinal image manipulation – imposed defocus

Ortho-K

Cho (2005)

Kakita (2011)

Multifocal SCL Anstice (2011)

“What could be the reason for the mismatch in

the growth rates of the eyes of children with

anisometropia?

One explanation could be that subtle changes in

lenticular or corneal properties could result in

different image qualities in the two eyes. In the

future, studies should investigate possible

asymmetry in the visual images between eyes,

such as those due to astigmatism and higher

order aberrations.”

Anisometropia: asymmetric visual experience? Anisometropia

• Interocular difference in axial length Sorsby (1962)

- two different endpoints within one visual system

• Typically regresses during childhood

- emmetropisation Abrahamsson (1990)

- if persists amblyopia

• Mechanism unknown

- independent emmetropisation? Abrahamsson (1990)

• Increases in later decades

- unilateral cataract (refractive)? Attebo (1999)

- breakdown of binocular vision? Weale (2006)

29/04/2013

2

Studies with animals suggest hyperopic defocus & reduction in retinal

image quality stimulates axial elongation.

Wildsoet

OD -6.00

(6/6)

OS -3.00

(6/6)

Potential mechanisms of asymmetric blur in humans

Unequal:

• Total higher order aberrations

• Eyelid forces ± corneal aberrations

• Accommodation ± lag/hyperopic defocus

Sivak

Classification of higher order aberrations (optical imperfections)

Lower order aberrations

Higher order aberrations

Higher order aberrations (optical imperfections) and visual performance

1 mm 2 mm 3 mm 4 mm 5 mm 6 mm 7 mm

Perfect Eye

Typical Eye

Eyelid forces and myopia development

Eyelid pressure on the cornea

Previous studies:

Buehren (2003, 2005) & Shaw (2008)

Changes in corneal topography have been

observed following near work due to eyelid

pressure.

The magnitude of corneal change is proportional

to the angle of downward gaze, duration and

type of task and palpebral aperture morphology.

Myopes show larger changes compared to

emmetropes, suggesting a possibly link between

near work and myopia development.

Interocular symmetry has not been investigated.

Eyelid pressure on the cornea

29/04/2013

3

• Aim:

To examine the interocular symmetry of changes in corneal optics

following a short period of near work in a cohort of myopic

anisometropic subjects.

• Participants: ­ 34 myopic anisometropes ≥ 1.00 D

­ Mean SER anisometropia 1.70 ± 0.74 D

­ Pre presbyopic 18-34 years of age: Mean 24 ± 4 years

­ 31 East Asian and 3 Caucasian, 22 female and 12 male

­ No systemic or ocular disease, history of patching or ocular trauma

­ VA 0.10 logMAR or better in each eye and equal VA between the eyes

• Methods:

Digital photography of the anterior eye

(primary and down gaze)

10 minute reading task

2.5 D accommodation

25 degree downward gaze

Pre-reading

corneal topography

Post-reading

corneal topography

Right

Right

Left

Left

PRIMARY

GAZE

DOWNWARD

GAZE

• Results: Symmetry of anterior eye morphology

Right

Right

Left

Left

Symmetry of anterior eye morphology

More myopic Less myopic

More myopic Less myopic

PRIMARY

GAZE

DOWNWARD

GAZE

Symmetry of anterior eye morphology

Primary Down

29/04/2013

4

• Results: Typical change in corneal topography

Sub

ject

22

MORE MYOPIC LESS MYOPIC SCALE

-9.25/-2.50 x 2 -7.25/-2.50 x 168

PR

E TA

SK

PO

ST T

ASK

D

IFFE

REN

CE

(PO

ST –

PR

E)

25

DEG

REE

DO

WN

WA

RD

Anterior morphology

& pre/post reading

corneal topography

Change in corneal topography following reading

Less myopic More myopic Scale (D)

Change in HOA and image quality

Point Spread Function

- Measure of optical quality

- The degree of image distortion

(from a point source of light)

- In this case due to the change in

corneal optics following reading

No change

1 min arc

Less myopic More myopic

Change in aberrations and image quality

Less myopic

More myopic

For 6 mm pupil, there

was a significantly

greater reduction in

image quality for the

more myopic eyes.

Due to greater

changes in lower

order aberrations

(astigmatism) rather

than higher order

aberrations.

•Summary

• Following a short duration reading task we observed:

- Changes in corneal optics corresponding with downward gaze palpebral aperture

morphology due to eyelid pressure

- Similar changes in higher order aberrations, but significantly greater changes in corneal

astigmatism in the more myopic eye, despite symmetrical anterior eyelid morphology

• These findings lend some support to the hypothesis that image

degradation following near work may contribute to myopia/anisomyopia

development.

• However, cross-sectional study of short duration. Requires a longitudinal

study examining symmetry of optical changes during myopia

development.

29/04/2013

5

Acknowledgements

• QUT

– Professor Michael Collins

– Dr Scott Read

– Professor Leo Carney

• HKPU

– Professor Maurice Yap

– Percy Ng

2010