Refractive error Flashcards
Emmetropia =
No refractive error
What are the 3 ocular determinants of whether light focuses on the retina?
Cornea (most powerful)
Lens (variable power)
Axial length of the eyeball
What is accommodation?
Increase in lens power
e.g. when looking at a near object - lens more convex and powerful
What is presbyopia?
Lack of accommodation (lack of increase in lens power)
Point of closest focus gets further from eye
Happens with increasing age
Image formed behind retina
What is hypermetropia?
Long-sightedness
Refractive power too weak and image focuses behind retina
Management of hypermetropia?
Correct with a convex (+ve) lens
If young with low level hypermetropia then can correct yourself but prolonged accommodation can cause eye strain and headache
What is myopia?
Short-sightedness
Ocular system to powerful so distant targets are focussed in front of the retina
Patients can focus near objects
Management of myopia?
Correct with concave (-ve) lens
What is astigmatism?
Eye has different refractive powers along different meridians
Can occur alongside emmetropia, hypermetropia or myopia
Need lens with different powers along different meridians to bring multiple focus points to sharp focus on retina
What is keratoconus?
Corneal collagen disorder
Causes corneal astigmatism and often myopia
Central corneal thinning and steepening of curvature
Signs of keratoconus:
Abnormal red reflex (oil droplet)
Retiniscope reflex shows scissors movement
Bulging of lower lid on down gaze
Vertical lines (striae) in the corneal stroma
Management of keratoconus:
Contact lenses, fit with rigid gas permeable
Photorefractive surgery contraindicated
Corneal grafting in late stages for scarring
What do bifocals correct?
Both distance and near vision in one eye (near in lower half for reading)
Feature of varifocals?
Gradually increase from distance to near correction from top to bottom
PRK:
Laser slices off anterior slice of cornea to decrease power and correct myopia
