refractive errors Flashcards
Refractive powers of cornea and lens?
What would happen if one of these parameters is less than perfect?
Cornea=40Dioptres
Lens= 20D
Axial length another key determinant (22.22mm)
Cornea therefore the most powerful
Leads to ametropia (emmetropia = optically perfect)
What is the near vision triad?
Accommodation
Pupil contraction
Convergence
How does the eye accommodate?
Ciliary muscle contract– zonules of zinn relax and the crystaline lens assumes a more convex shape.
What is presbyopia?
The lens becoming sclerosed with age means that accommodation ability is reduced
Accommodation reduces from age 30
Can you have normal VA in glaucoma?
yes
How do you correct myopic vision?
Divergent lens (these are concave with negative power)
living in caves is for people who love books and are really negative. You diverge because you’re adrift from people
What is long-sightedness known as ?
Describe what’s happening
Hypermetropia
Light from a distance is focussed behind the retina
How do you treat hypermetropia?
Correct with convergent lens (convex and positive)
How do you treat astigmatism?
The issues is a second point of focus being formed from the non-spherical shaped eye
First correct the main focussing error with a convergent or divergent lens
Second correct the astigmatic error with a cylindrical or tonic lens that has power in one direction only.
Why use a pinhole?
Pinholes can help diffrentiate between reduction in VA due to uncorrected refractive error, and pathology (mac degen). It reduces the blur circle and induces a point focus within the eye.
