The effect of albinism on emmetropization.

A study using chickens as an animal model for human albinism.

Presenter:Christine Wildsoet

University of California - Berkeley

Funded by NIHhttp://vision.berkeley.edu/VSP/CW/

High refractive errors & Congenital vision

abnormalities?

Alternative explanations Impaired/disrupted emmetropization,

due to poor vision? Genetic abnormality?A peculiarity of humans?

The albino example

Human albinismAmetropia is the rule!

Wildsoet et al., 2000

Mean: +1.07 D

1. Wider than normal distribution of refractive errors

2. Refractive errors are axial

3. High WTR astigmatism

Does the link between high refractive errors & human

albinism generalize to other animals?

Is melanin &/or its precursorsimportant?

Subjects

Albino White-Leghorn#

oculocutaneous tyrosinase-negative albinism(deletion in exon 1 of tyrosinase gene)

tyrosinase-like molecule produced but inactive

Normal White-Leghorns Smokey White-Leghorns#

Bitgood & Smyth, Poultry Sciences,1991, 70 (9): 1861-3.

# obtained as fertilized eggs from WS Payne, Michigan State Uni.

Eye, including RPE devoid of melanin

Very poor visionPhotophobicNo nystagmus

Ocular features of the albino chicks at hatching

Also very fragile!!

Questions asked about the albino chicks?

Do they have significant congenital refractive errors?

Do they show developmental emmetropization?

Do they show active emmetropization?

Answers?

? Do they have significant congenital refractive errors? (probably)

Do they show developmental emmetropization? (but abnormal)

Do they show active emmetropization?(but abnormal)

Albinos & “normal” refractive

development?

Higher than normal refractive errors

Overall myopic biasSome

developmental emmetropization

Albinos & astigmatism

Higher than normalIncreases with agePredominantly ATR

Albinos & “normal”

ocular growth?

Shallower than normal anterior chambers

Longer than normal vitreous chambers

Overall growth pattern

Active emmetropization:The effects of diffusers &

defocusing lenses

Two treatment ages:15 days35 days

Treatments:+5, +10 D lenses-7, -10 D lensesDiffusers

Albino

Smokey

Emmetropization is poorer than normal

√ Responses to diffusers

√ Response to negative lenses

? Response to positive lenses

Treatment period

Younger albinos & active emmetropization

Refractive changes have an axial basis

Young

Older

Retinal morphology

& visual acuity

Albino retina

RPE devoid of melanin All photoreceptor subtypes presentINL slightly thinner than normalVA near normal by 29 days of age (preliminary finding)

Summary & Conclusions

Albino chicks show early abnormal refractive errors (at hatching?)

Emmetropization is abnormal Reduced visual acuity may partly underlie the

post-hatch changes Eye shape abnormalities resemble light-

induced avian glaucoma & may represent the precursor to glaucoma as reported in albino quail

Acknowledgements to: Kai, Ajay, Michel, Chun-Mei, Vivian, Sigrid, Nina, Kevin, Sabrina & Young