visual systems 2

Question 1

what is refraction?

Answer 1

light bends as it goes from one medium to another
it is faster in less dense mediums (light bends towards the normal when going into a more dense medium)

refraction index (n)
n= speed of light in a vacuum (c) / speed of light in a medium (Vm)

n=c/Vm

the denominator will always be smaller, it is unit-less

all substances have a unique refraction index

some light is also reflected at the boundary

angle of incidence = angle of reflection

angle of incidence > than angle of refraction

Question 2

what are the two types of lenses?

Answer 2

convex:
converging
gathers light rays and brings them to a point
real focal point, focal length is on other side to incident light

concave:
diverging
takes light rays and spreads them
virtual focal point, focal length is on same side as the lens

Question 3

what is emmetropia?

Answer 3

when in a healthy eye adequate correlation between axial length and refractive power

parallel light rays fall on the retina (no need for accommodation)

Question 4

what is ametropia?

Answer 4

refractive error

mismatch between axial length (length of eye) and refractive power

parallel light rays dont fall on the retina (without accommodation)

near sightedness (myopia) (most common)
far sightedness (hyperopia)
astigmatism
presbyopia

Question 5

what is myopia?

Answer 5

short sightedness

parallel rays converge at the focal point ANTERIOR to the retina

image doesnt fall on the surface of the retina

aetiology: not clear, genetic factor

causes:
excessive long globe (axial myopia)- more common
excessive refractive power (refractive myopia)

Question 6

what are the symptoms of myopia?

Answer 6

blurred distance vison

squint in attempt to improve uncorrected visual acuity when gazing into the distance

headache

Question 7

how is myopia treated?

Answer 7

1. correction with a diverging (concave) lens (negative lenses)
2. correction with contact lenses
3. correction by removing the lens to reduce the refractive power of the eye

Question 8

what is hyperopia?

Answer 8

parallel rays converging at a focal point posterior to the retina

aetiology: not clear, inherited

causes:
excessive short globe (axial hyperopia) - more common
insufficient refractive power (refractive hyperopia)

Question 9

what are the symptoms of hyperopia?

Answer 9

visual acuity at near tends to blur relatively early
nature of blur varies from inability to read fine print to clear vision with a sudden intermittent blur
blurred vision is more noticeable if the person is tired, printing is weak, or low light

asthenopic symptoms: eye pain, headache in frontal region, burning sensation in the eyes, blepharoconjunctiveitis

amblyopia - uncorrected hyperopia > 5D

unlike myopia, these people cant really read

Question 10

how is hyperopia treated?

Answer 10

1. correction with converging (convex) lenses (positive lenses)
2. correction with positive lens and cataract extraction(no lens)
3. correction with contact lens
4. correction with intraocular lens

Question 11

what is astigmatism?

Answer 11

parallel rays come to focus in 2 focal lines rather than a single focal point

aetiology: heredity

cause: refractive medium is not spherical (cornea is irregular) ->
refracts differently along one meridian than along meridian perpendicular to it ->
two focal points (punctiform object is represented as 2 sharply defined lines)

between the two focal lines (perpendicular), there is a “circle of least confusion” (Kr), the location with the least loss of image definition

Question 12

what are the symptoms of astigmatism?

Answer 12

asthenopic symptoms (headache, eye pain)
blurred vision
distortion of vision
head tilting and turning

Question 13

what is the treatment of astigmatism?

Answer 13

regular astigmatism: cylinder lenses with or without spherical lenses (convex or concave), Sx

irregular astigmatism: rigid cylinder lenses, surgery

Question 14

what is the near response triad?

Answer 14

adaptation for near vision

1. pupillary miosis (sphincter pupillae) to increase depth of field
2. convergence ( medial recti from both eyes) to align both eyes towards near object
3. accommodation (circular ciliary muscles) to increase the refractive index power of lens for near vision (more curved). (zonules relax)

Question 15

what is presbyopia?

Answer 15

naturally occurring loss of accommodation (focus for near objects). due to loss of elasticity

onset from 40 years

distant vision intact

corrected by reading glasses (convex lenses) to increase the refractive power of the eye

Question 16

how is presbyopia treated?

Answer 16

convex lenses in near vision:

reading glasses
bifocal glasses
trifocal glasses
progressive power glasses

Question 17

why might contact lenses be used instead of glasses?

Answer 17

higher quality of optical images and less influence on the size of retinal image than spectacle lenses

indication: cosmetic, athletic activities, occupational, irregular corneal astigmatism, high anisometropia, corneal disease

Question 18

what are different types of optical correction?

Answer 18

contact lenses:
disadvantages - careful daily cleaning and disinfection, expense
complications - infectious keratitis, giant papillary conjunctivitis, corneal vascularisation, severe chronic conjunctivitis

intraocular lenses:
replacement of cataract crystalline lens
gives best optical correction for aphakia (no lense), avoid significant magnification and distortion caused by spectacle lenses

Question 19

what is surgical correction?

Answer 19

ketorefractive surgery: RK, AK, PRK, LASIK, ICR, thermokeratoplasty

intraocular surgery: clear lens extraction (with or without IOL), phakik IOL

Question 20

what is the general process of (laser) surgical correction?

Answer 20

1. pre operative eye
2. initial cutting of corneal flap
3. cutting of corneal flap
4. flipping of corneal flap
5. photorefractive treatment (laser)
6. corneal stroma reshaped post laser
7. corneal flap back in position
8. treatment completed

Question 21

what is the staar intra-collamer lens?

Answer 21

ICL

inserted into the eye in front of real lens

correction of myopia and astigmatism

Question 22

what is clear lens extraction and intraocular lens?

Answer 22

clear lens extraction + IOL

same as cataract extraction (except lens is softer)

implantation of artificial lens

lose accommodation (patient will need reading glasses)