Refractive error and Emmetropisation

Question 1

define refractive error in two ways

Answer 1

1) change in direction of a path of light as it passes obliquely from one medium to another having a different refractive index.
2) process of measuring and correcting the refractive error of the eye

Question 2

human eye refracts at which 2 ocular structures and how?

Answer 2

cornea- fixed focussing power

lens- accommodation changes shape

Question 3

name the 3 basic elements that determine the eyes ability to focus light?

Answer 3

shape of cornea
power of the lens
length of the eyeball

Question 4

what is the total power of the eye?

Answer 4

60D

Question 5

Cornea accounts for how much of the total eye refraction?

Answer 5

2/3 so 40D

Question 6

Lens accounts for how much of the total eye refraction?

Answer 6

1/3 so 20D

Question 7

what is the refractive error?

Answer 7

mismatch between the power of the eye’s optical system and the length of the eye

Question 8

refractive error results in what symptoms?

Answer 8

blurred vision
headache
eyestrain

Question 9

where do the light rays focus in a refractive error?

Answer 9

light rays do not focus on the retina

Question 10

name the three types of refractive errors?

Answer 10

myopia
hyperopia
astigmatism

Question 11

what is the difference between ammetropia and emmetropia?

Answer 11

ammetropia is a refractive error (myope/ hyperope/ astigmatism)

emmetropia is no refractive error

Question 12

where do the light rays form a focal point in an emmetrope?

Answer 12

on the fovea of retina

Question 13

where do the light rays form a focal point in an myope?

Answer 13

focal point infront of retina

Question 14

where do the light rays form a focal point in an hyperope?

Answer 14

focal point behind retina

Question 15

what is refractive ammetropia and axial ammetropia?

Answer 15

axial ammetropia= Rx due eyes being long/short

refractive ammetropia= Rx due to cornea of lens too powerful/weak

Question 16

what can cause myopia?

Answer 16

• cornea too curved
• lens too powerful
• eye too long
• or a combination

Question 17

what can cause hyperopia?

Answer 17

• cornea too flat
• lens too weak
• eye too short
• or a combination of these

Question 18

what is astigmatism?

Answer 18

Rx, usually occurs with myopia or hyperopia

• irregular curvature of the cornea or lens
• light rays focus in different locations

Question 19

where does light form focal points in an astigmatic?

Answer 19

1st focal point before retina

2nd focal point behind retina

Question 20

what is meant by
a) against the rule
b) with the rule
in astigmatism

Answer 20

a) against the rule= cornea more curved in the horizontal meridian
b) with the rule= cornea more curved in the vertical meridian

Question 21

when looking at a near object, you need to __________ to bring the object into ______.

Answer 21

accomodate

focus

Question 22

a dioptre (D) is a measurement of the ability of a lens to ________ or _________ light. It is also a _______ of distance between the lens and its ______ point.

Answer 22

converge
diverge
reciprocal
focal

Question 23

The correction of Rx is the amount of _______ in dioptres, needed to bring the _____ of light back to _____ on the _______.

Answer 23

power
rays
focus
retina

Question 24

how can you correct

a) emmetrope
b) myope
c) hyperope
d) astigmatic

Answer 24

a) cannot correct, as rays focus on retina
b) correct with a negative lens
c) correct with a positive lens
d) correct one meridian with a sphere and the 2nd meridian with a cylinder

Question 25

describe the distribution of mean sphere refractive error known as the Theoretical Gaussian Distribution.

Answer 25

(bell shaped)

• more emmetropes than predicted
• fewer moderate errors (+2.00D)
• more high errors (-6.00D)
• The function is LEPTOKURTOTIC

Question 26

what is the RX of an average newborn?

Answer 26

average newborn is hyperopic so Rx is +2.00DS

Question 27

what happens to RX of infants aged between 6 months and 2 years

Answer 27

there is a rapid decline in hyperopia

Question 28

what gradually decreases unto 6 years of age?

Answer 28

Refractive error in infants

Question 29

define emmetropisation?

Answer 29

the rapid reduction in refractive error over the first few years of life towards emmetropia as coordination of ocular components starts to occur.

Question 30

in emmetropisation:

human eye is programmed to achieve __________ in childhood. The eye maintains ________ in childhood.

Answer 30

emmetropia

emmetropia

Question 31

what are the 3 phases of emmetropisation ?

Answer 31

rapid infantile phase
slow juvenile phase
growth phase

Question 32

when is the rapid infantile phase in emmetropisation?

Answer 32

from birth to 2-3 years (axial length from 18-23mm)

Question 33

when is the slow juvenile phase in emmetropisation?

Answer 33

from 3 years to puberty

Question 34

what happens in the growth phase of emmetropisation?

Answer 34

a co-ordinated process ‘emmetropisation’ which ceases around 14-15 years

Question 35

fill in the blanks;

Astigmatism ________ over the first few years of life.
It is not associated with _______ patterns.
It can change with ________.

Answer 35

Decreases
growth
age