Refractive Errors Flashcards
What is acuity measured in
Dioptres
What refractors of light does the eye have + what are their strengths
cornea - 40D
lens - 20D
What is emmetropic vision
Perfect vision - 60D refraction + 22.22mm axial length
what is myopia and how is it fixed
short sightedness - due to focal point being in front of the retina - fixed with a concave lens
what is hypermetropia and how is it fixed
long sightedness - due to focal point being behind the retina - fixed with the convex lens
What does a visual assessment consist of
distance vision retinoscopy subjective refraction near vision assessment unit magnification
what are measurers of acuity
LogMAR
Snellen charts
what is the best measurer of acuity + why
logMAR - equal number of letters all the way down
what is a refractive error called
ametropia
what are 2 main causes of ametropia
axial - where the axial length changes
refractive ametropia - changes in refractive index
what is an astigmatism
asymmetrical eye shape, leading to bent light rays, leading to 2 points of focus on retina
how do you fix an astigmatism
toric and cylindrical lenses
what is presbyopia
loss of accommodation, corrected by +ve lenses
what are visual indications for contact lenses
anisometropia (unequal refractive powers)
high myopia
aphakia - absence of lens due to surgery (congenital cataracts)
irregular cornea
refractive surgery failure
what are occupational indications for contact lenses
theatre
film
armed forces
sports
what are requirements for contact lenses
stable, accurate visual acuity good comfort throughout wear easy to insert/remove easy to clean/replace/reproduce reasonable cost maintain corneal physiology minimal lid/teat film inteference
what needs assessing before contact lenses
prescription + patient needs
keratometry performed - any astigmatism identified
slit lamp assessment - corneal health, lids and tear film
Trial lens fitted
what is contact lens aftercare
correct cleaing/insertion/removal
gradual build up to every day wear
1 month of wear -> aftercare appointment (check refraction, fit and eye health)
repeat 3 months and 6 months then annually
what are complications of contact lenses
giant papillary conjunctivitis corneal abrasion infecting keratitis corneal ulcers neovascularisation corneal hypoxia solution hypersensitivity
what types of contact lenses are there
soft
rigid-gas-permeable
what are the advantages and disadvantages of soft contact lenses
Adv - flexible, good confort, good VA, large diameter so its a safer sit, safer for sport, easier to fit, suitable for extended wear
Disadv- split, tear deposition, expensive, can dehydrate
what are advantages and disadvantages of rigid-gas-permeable lenses
Adv - fixed shape, durable, easy to clean, good for all day wear, decreased diameter = less likely for hypoxia, smooth surface = better visual result for irregular corneas and high astigmatism
Disadv - poor initial comfort, small diameter, more prone to falling out
