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Animal Studies

Animals Become Myopic

  • Barrett (1932)  demonstrated differences in refractive error between domestic and wild animals
  • 75% of caged cats become myopic, while 85% of wild animals are hyperopic (farsighted) (Belkin et al 1977)
  • Young (1964) demonstrated that laboratory, hooded monkeys became more myopic than non-hooded animals (confined visual environment

Animal Studies: Young (1977)

  • N=8 pigtail prepubescent monkey were placed in near visual environment, i.e. could not see further than 10 inches, for 6 months
  • the furthest distance the monkeys could see was 10 inches which induced 2.25D of myopia
  • They were given 1% atropine twice a day for 6 months
  • This caused a regression of 0.5D of myopia while still viewing at near.
  • Conclusion: accommodation and/or near viewing can create myopia which can be reduced with a drug that eliminates accommodation or focusing.

Experimentally Induced Myopia

  • Neonatal form deprivation and defocused light induced significant myopic changes in animals
    Wallman et al 1978, Raviola and Wiesel (1985)
  • Monkeys, chicken or tree shrews have one eye lid sutured, or have a translucent occluder placed over an eye
  • Local axial change occurring in the sclera at the specific site (VF) where deprivation takes place.  Occurs in the presence of a severed optic nerve.
  • Conclusion:  the environment can change the length of the eye with resultant myopia

Form Deprivation

  • Sensitive period for form deprivation induced myopia
  • Greatest with neonates
  • Similar effect with humans with myopia
  • Myopia did not occur when animals were raised in darkness, therefore, myopia is induced by altered visual/environmental input
  • Responses varied per animal
  • Greatest with chickens
  • Strong with tree shrews (mammal)
  • Less with monkeys                  

Blur Induced Refractive Error

  • Schaeffel et al (1988) used both plus and minus lenses to induce refractive changes in the chick (one eye +, other -, or control)
  • Eye with plus becomes pseudo myopic and developed hyperopia (farsighted)
  • Eyes with minus become pseudo hyperopic and develop myopia (nearsighted)
  • Measurements were made under cycloplegia
  • Conclusion: the direction of the defocused light caused the eyes to become either farsighted or nearsighted.  the amount of refractive error induced was related to the power of the lens.  The change was with the length of the eye.

Blur Induced Refractive Error

Fairly linear changes in refractive power from -10 to +20 D

  • Choroid thickens to reduce blur in with plus
  • CNS is not necessary for the response
  • Happens with optic Nerve severed
  • Happens with the brain removed
  • Happens if ganglionic cell activity is blocked (tetrodotoxin)
  • These results suggest that the ocular system can determine the direction of defocused light
  • Thus growth regulating systems mechanism changes the size of the vitreous cavity
  • Change occurs in the sclera

Atropine and Animals

  • Atropine stopped the progression of myopia in stump tailed monkeys but not rhesus monkeys
  • Thus accommodation per say was not the mechanism
  • Chicks demonstrated regional myopia depending on the area of the retina deprived – thus, not accommodative nor central
  • Chicks have striated muscle in the ciliary body, thus, accommodation occurs by nicotinic action, not muscarinic

Pirenzepine

  • Leech et al (1995) intravitreal injection of pirenzepine, a M1 muscarinic antagonist stopped the progression of myopia in lid sutured chickens
  • Effected local choroidal and retinal mechanisms
  • M3 muscarinic receptors mediate contraction
  • Schaeffel et al (1990) destroyed Edinger Westphal’s nucleus and still induced myopia

Muscarinic Effect

  • Tiggs et al (1999) occluded the right eye of 20 rhesus monkeys
  • Three groups – atropine (7), pirenzipine (7), and control (6)
  • After 36 wks of treatment – myopia progressed in the control, but not in the treated group
  • Leech et al (1995) showed similar effects in tree shrews
  • Other Drugs Reduce Myopia

    Dopamine receptor agonist apomorphine reduced deprivation induced myopia – Kaymak et al (1997)