Refractive error and Emmetropisation Flashcards
define refractive error in two ways
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
human eye refracts at which 2 ocular structures and how?
cornea- fixed focussing power
lens- accommodation changes shape
name the 3 basic elements that determine the eyes ability to focus light?
shape of cornea
power of the lens
length of the eyeball
what is the total power of the eye?
60D
Cornea accounts for how much of the total eye refraction?
2/3 so 40D
Lens accounts for how much of the total eye refraction?
1/3 so 20D
what is the refractive error?
mismatch between the power of the eye’s optical system and the length of the eye
refractive error results in what symptoms?
blurred vision
headache
eyestrain
where do the light rays focus in a refractive error?
light rays do not focus on the retina
name the three types of refractive errors?
myopia
hyperopia
astigmatism
what is the difference between ammetropia and emmetropia?
ammetropia is a refractive error (myope/ hyperope/ astigmatism)
emmetropia is no refractive error
where do the light rays form a focal point in an emmetrope?
on the fovea of retina
where do the light rays form a focal point in an myope?
focal point infront of retina
where do the light rays form a focal point in an hyperope?
focal point behind retina
what is refractive ammetropia and axial ammetropia?
axial ammetropia= Rx due eyes being long/short
refractive ammetropia= Rx due to cornea of lens too powerful/weak
what can cause myopia?
- cornea too curved
- lens too powerful
- eye too long
- or a combination
what can cause hyperopia?
- cornea too flat
- lens too weak
- eye too short
- or a combination of these
what is astigmatism?
Rx, usually occurs with myopia or hyperopia
- irregular curvature of the cornea or lens
- light rays focus in different locations
where does light form focal points in an astigmatic?
1st focal point before retina
2nd focal point behind retina
what is meant by
a) against the rule
b) with the rule
in astigmatism
a) against the rule= cornea more curved in the horizontal meridian
b) with the rule= cornea more curved in the vertical meridian
when looking at a near object, you need to __________ to bring the object into ______.
accomodate
focus
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.
converge
diverge
reciprocal
focal
The correction of Rx is the amount of _______ in dioptres, needed to bring the _____ of light back to _____ on the _______.
power
rays
focus
retina
how can you correct
a) emmetrope
b) myope
c) hyperope
d) astigmatic
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
describe the distribution of mean sphere refractive error known as the Theoretical Gaussian Distribution.
(bell shaped)
- more emmetropes than predicted
- fewer moderate errors (+2.00D)
- more high errors (-6.00D)
- The function is LEPTOKURTOTIC
what is the RX of an average newborn?
average newborn is hyperopic so Rx is +2.00DS
what happens to RX of infants aged between 6 months and 2 years
there is a rapid decline in hyperopia
what gradually decreases unto 6 years of age?
Refractive error in infants
define emmetropisation?
the rapid reduction in refractive error over the first few years of life towards emmetropia as coordination of ocular components starts to occur.
in emmetropisation:
human eye is programmed to achieve __________ in childhood. The eye maintains ________ in childhood.
emmetropia
emmetropia
what are the 3 phases of emmetropisation ?
rapid infantile phase
slow juvenile phase
growth phase
when is the rapid infantile phase in emmetropisation?
from birth to 2-3 years (axial length from 18-23mm)
when is the slow juvenile phase in emmetropisation?
from 3 years to puberty
what happens in the growth phase of emmetropisation?
a co-ordinated process ‘emmetropisation’ which ceases around 14-15 years
fill in the blanks;
Astigmatism ________ over the first few years of life.
It is not associated with _______ patterns.
It can change with ________.
Decreases
growth
age
