Refraction Flashcards
Describe the problem in myopia?
Eyeball is too long
In any eye the nearer objects come towards the eye, the further back their image falls.
In myopia, only closer objects focus on the retina (short sightedness) while far objects focus infarct of the retina
What are causes of myopia?
Genetic
In normal growth changes in eyeball and lens curvature compensate for the eye getting longer as it grows but in myopic children such compensation may not be occurring so myopia worsens with age.
Most do not become myopic until 6 years.
Myopia will continue to worsen until the late teens when changes stop below 6 diopters in most people
It is important there fore for children with myopia to have regular eye checks as spectacle changes are expected.
Over-correction can make myopia worse.
In later life, increasing myopia can indicated cataracts development.
What is pathological myopia a risk factor for?
Myopia above 6 diopters
Secondary degeneration of vitreous and retina can lead to renal detachment, choiroidoretinal atrophy and macular bleeding
How is myopia managed?
Concave lens spectacles
Contact lenses
LASIK
What is astigmatism?
Cornea does not have the same degree of curvature.
Cornea becomes aaa irregular surface
Usually one half is flatter or steeper than the other half.
This means when light rays strike the cornea, they do not focus together in one point and produce a blurred image either longitudinally or vertically.
Correcting lenses can compensate
What is hypermetropia?
Eye is too short
Distant objects are focused behind the retina
Ciliary muscles contract and the lens gets more convex to focus the object on the retina.
This can produce tiredness of gaze and sometimes esotropic strabismus in children.
How is hypermetropia managed?
Convex lenses to bring image forward onto retina.
What is presbyopia?
Ciliary muscle reduces tension in the lens allowing it to get more convex for close focusing.
Young lenses can go from far to near in 0.4sec.
With age, the lens stiffens and hence the need for glasses for reading.
These changes start in the lens at 40 years and are complete by 60.
