Optics and refractory errors

Question 1

Myopia

Answer 1

a refractive error, with a defect in the ability of the system to focus the images. While in the emmetropic eye the parallel rays of the light arriving from infinity focus on the retina, in a myopic eye the parallel rays are focused before the retina, ‘in front’ of it, with accommodation being at rest.

Question 2

Risks for myopia

Answer 2

long time with smartphone, tablet; not enough time outside

Question 3

4 main refractive errors

Answer 3

myopia, hyperopia, astigmatism and presbyopia

Question 4

refractive error leading to vergence of 5D

Answer 4

see 20 cm around

Question 5

3 main reasons for a myopic eye:

Answer 5

1. AXIAL: increased anterior/posterior length of eyeball
2. CURVATURE: increased curvature of cornea
3. INDEX: increased refractive index of lens with nuclear sclerosis

Question 6

normal eyeball length

Answer 6

24mm; +1mm - 3 diopters

Question 7

2 reasons why an eye may be longer than necessary

Answer 7

Excessive stimulus to the growth; Usually the eye growth ends at the age of 20-25 years.

2) Bulging/Yielding of the sclera’s structure; the sclera is a rigid structure, but it is formed by collagen fibers that could be of low resistance giving rise to a budging of the sclera, called ‘staphyloma’

Question 8

degenerative myopia

Answer 8

never stops, reaches 20-30D

Question 9

Etiology of hyperopia

Answer 9

1. Short eyeball - <24mm. This is the most frequent etiology.
2. Cornea and lens less steep - instead of 43D the cornea could have 41-42D
3. Lens refractive index changes - especially in the peripheral (cortical) parts of the lens.
4. Abnormal position of the lens in the eyeball - could be a luxated lens (abnormal position) or aphakia (absence of lens).

Question 10

hyperopia in children

Answer 10

physiological hyperopia ;
since the eyeball is <24mm), but normally with growth there is a gradual reduction of hyperopia that disappears with puberty

Question 11

Latent hyperopia

Answer 11

asymptomatic thanks to accommodation

Question 12

astigmatism

Answer 12

shape of cornea, normally spherical, here asymmetric - football shape

Question 13

irregular astigmatism - keratoconus

Answer 13

due to progressive thinning of the cornea

Question 14

regular astigmatism

Answer 14

curvature uniformly different in meridians at right angles to each other (the two main meridians are perpendicular to each other and the power along each meridian remains constant)

Question 15

Difference between regular and irregular astigmatism

Answer 15

In regular astigmatism we have the same diopters in all the meridians of the cornea. In irregular astigmatism (e.g. after ocular trauma) we have different dioptric powers in different points of the cornea.

Question 16

anisometropia

Answer 16

a condition in which there is a difference in the refractive power of the two eyes (different diopters between the two eyes). It is a fairly common condition

Question 17

antimetropia

Answer 17

One myopic and the other hyperopic

Question 18

Aniseikonia

Answer 18

When the difference between the two eyes is more than 3-4 diopter, the different power of the lenses of the glasses causes the perception in the brain of images of different size in the two eyes; need surgery

Question 19

Amblyopia

Answer 19

Lazy eye (amblyopia) is reduced vision in one eye caused by abnormal visual development early in life.

Question 20

Presbyopia

Answer 20

accommodation disorder secondary to the physiological reduction in the ability to focus, with resulting difficulty in visual activities to close.

Question 21

accommodation

Answer 21

is the main mechanism that allows to focus on the retina objects placed between the ‘far point’ and the ‘near point’. The far point is the most far point you can see, while the near point is the closest point you can see a clear image. The distances between the far point and the near point vary between individuals

Question 22

LASIK

Answer 22

(laser in situ keratomileusis) is another commonly used technique that is assisted by excimer laser. It uses the same laser to perform ablation of the corneal stroma, but it needs a microkeratome to make a cut in the cornea to produce a flap of <120microns in thickness (thinner than a sheet of paper).

Question 23

PRK photo refractive keratectomy

Answer 23

It allows permanently changing the shape of the cornea (it makes the cornea less convex than before). It involves the use of an excimer laser

Question 24

LASEK

Answer 24

(Assisted Sub-Epithelial Keratectomy) it involved removal of the epithelium