7: Refractive States of the Eye

Question 1

in an unaccommodated myopic eye, light from a distant object is image:

Answer 1

anterior to the fovea

Question 2

in an unaccommodated hyperopic eye, light from a distant object is imaged:

Answer 2

posterior to the fovea

Question 3

in myopia, the eye’s power is too ____ for its axial length

Answer 3

great

Question 4

in hyperopia, the eye’s power is too ____ for its axial length

Answer 4

low

Question 5

define the far point of the eye (R)

Answer 5

it is the object point conjugate with the fovea (R’) of the unaccommodated eye

Question 6

define the far point distance (k)

Answer 6

is that from the first principal point (P) of the eye to the far point (R)

Question 7

where is the far point of the myopia eye located?

Answer 7

is located in real space somewhere closer than infinity

Question 8

are the far point distances (k) negative or positive for myopia and hyperopia

Answer 8

myopic eye: k is negative (-)

hyperopia eye: k is positive (+)

Question 9

where is the far point of the hyperopic eye located?

Answer 9

the far point is a virtual object point located somewhere behind the corneal vertex

Question 10

what is the ocular refraction (K)?

Answer 10

it is the reciprocal of the far point distance k

Question 11

ocular refraction depends on a balance between the:

Answer 11

eyeball length and the eye power

Question 12

total hyperopia is

Answer 12

the amount of hyperopia determined with ‘adequate’ cycloplegia

Question 13

manifest hyperopia is

Answer 13

the portion of the total that can be obtained without cycloplegia

Question 14

latent hyperopia is

Answer 14

the portion of the total that remains hidden (or latent) without cycloplegia
(determined by calculation)

Question 15

facultative component of hyperopia

Answer 15

can be overcome with accommodation

this includes the latent component

Question 16

absolute component of hyperopia

Answer 16

cannot be overcome with accommodation

Question 17

in young patients, the amplitude of accommodation far exceeds total hyperopia,
total= ? and absolute=?

Answer 17

total=facultative

absolute=zero

Question 18

regular astigmatism is when

Answer 18

the eye power is maximal and minimal at two meridians separated by 90 degrees

Question 19

irregular astigmatism can occur when

Answer 19

• the meridians of maximal and minimal power are not separated by 90 degrees
• when the eye cannot be corrected by a regular spherocylinder lens

Question 20

example of irregular astigmatism and how it occurs

Answer 20

keratoconic eyes develop markedly irregular astigmatism, due to a highly aberrated corneal shape

Question 21

with the rule astigmatism

Answer 21

• the meridian of highest positive ocular power is closer to vertical
• the eye is corrected with a negative cylinder device that has its axis in the range of 180

Question 22

against the rule astigmatism

Answer 22

• the meridian of highest positive ocular power is closer to horizontal
• the eye is corrected with a negative cylinder device that has its axis in the range of 90

Question 23

oblique astigmatism

Answer 23

the axis of the correcting cylinder is neither with-the-rule nor against-the-rule

Question 24

anisometropia

Answer 24

the mean spherical equivalent ocular refraction (M) is different in the two eyes

Question 25

what occurs in an individual with anisometropia

Answer 25

they are unable to obtain a clear focus in both eyes simultaneously

Question 26

what is ametropia and how can it occur

Answer 26

when one eye is hyperopia and the other is myopic, occurs if the eye is too short or too long for its dioptric power

Question 27

the process of emmetropization occurs in which

Answer 27

1. the distribution mean moves toward emmetropia or low hyperopia 2. the standard deviation of the distribution decreases

Question 28

with time, the distribution of ocular refraction becomes

Answer 28

leptokurtotic and skewed towards myopia

Question 29

ocular diffraction has a ______ distribution in neonates, but this distribution changes with development

Answer 29

Gaussian

Question 30

leptokurtotic distribution is one that has

Answer 30

relatively lower proportions in the shoulders and relatively higher proportions near the peak and in the tails

Question 31

skewed distribution is one in which

Answer 31

the mean and mode do not coincide | in the case of ocular refraction, the mean is more myopic than the mode

Question 32

many individuals are classified as having correlation ametropia. this is when

Answer 32

their components are within the usual range for emmetropes, but those components do not balance correctly

Question 33

what is component ametropia?

Answer 33

``` when one (or more) components fall outside the usual range for emmetropes -there are two types ```

Question 34

describe the two types of component ametropia?

Answer 34

axial ametropia occurs when the axial length is abnormal, and refractive ametropia occurs when one or more of the refractive components are abnormal

Question 35

Javal's rule proposed what relationship

Answer 35

between corneal astigmatism (Cc), presumed lenticular astigmatism, and total astigmatism (Ct) Ct= (1.25)Cc - 0.5

Question 36

Javal's rule sign conventions are

Answer 36

+ indicates with the rule astigmatism | - indicates against the rule astigmatism