Ch. 3 - Refractive Status

Front 1

The refractive status of the eye relates to:

Back 1

the manner in which light rays entering the eye are brought to a focus

Front 2

light rays

Back 2

light traveling through space in a straight lines

Front 3

optical infinity

Back 3

when light rays assume a virtually parallel course at a distance of about 20 ft (6 m) and is continuous to infinity

Front 4

When light rays encounter an object, what happens?

Back 4

they are either absorbed, reflected, or refracted

Front 5

The path a light ray takes as it passes through a transparent medium depends on two factors:

Back 5

angle of incidence and index of refraction (n)

Front 6

angle of incidence

Back 6

formed by the light ray and the surface of the medium

Front 7

index of refraction

Back 7

is a number that indicates the speed of light through a medium compared with the speed of light through a vacuum

Front 8

What is the formula for calculating the index of refraction?

Back 8

dividing speed of light in a vacuum, by the speed of light in the medium

n= speed of light in a vacuum/speed of light in the medium

Front 9

speed of light in a vacuum

Back 9

186,000 mps

Front 10

speed of light through water

Back 10

about 140,000 mps

Front 11

midline

Back 11

is that point at which light is traveling along the normal

Front 12

convex

Back 12

-"plus lens"
-thick in center, thinner at edges
-light is converged
-focus point is where the light rays meet at the midline, behind the lens

Front 13

concave

Back 13

-"minus lens"
-thin in center, thicker at edges
-light is diverged
-focus point is at a virtual point in front of the lens

Front 14

diopter

Back 14

How the power of a lens is measured, and is the reciprocal of the focal length of a lens (in meters)

D= 1/focal length in m

Front 15

focal length

Back 15

is measured from the lens to the point at which the light rays meet on the midline

Front 16

The medium must be ________ to allow light through.

Back 16

transparent

Front 17

Refraction of light depends on what?

Back 17

1) the angle of incidence of the light rays

2) the index of refraction of the medium

Front 18

cornea

Back 18

a clear, transparent, steeply curved structure that contributes about 70% of the optical power of the eye

Front 19

Light rays traveling from air into the cornea are refracted as a result of:

Back 19

anterior corneal curvature combined with the change in speed as the light rays pass from air into the cornea

Front 20

The total power of the cornea averages about:

Back 20

+42.00D

Front 21

How is the corneal curvature measured?

Back 21

by either a keratometer or an ophthalmometer

Front 22

Te average corneal curvature measures about:

Back 22

44.00D on the keratometer

Front 23

The cornea and aqueous humor _________ light rays entering the eye.

Back 23

converge

Front 24

crystalline lens

Back 24

a biconvex, normally transparent, resilient structure that lies just behind the iris

Front 25

How is the cyrstalline lens held in place?

Back 25

suspensory ligaments attached at one end to the ciliary muscle, and at the other end to the lens

Front 26

What are the 3 major layers of the lens?

Back 26

1) the highly elastic outer capsule

2) the cortex

3) the nucleus

Front 27

What allows the lens to change shape?

Back 27

changes in tension on the suspensory ligaments

Front 28

accommodation

Back 28

the act of the crystalline lens changing its shape to focus

Front 29

The ability of the eye to accommodate depends on:

Back 29

age, refractive error, and overall physical health

Front 30

amplitude of accommodation

Back 30

the ability of the lens in the eye to focus

Front 31

measuring amplitude of accommodation : "push up" method

Back 31

a near point target (1-2 lines better than BCVA) is presented and brought toward the eyes until it blurs; distance between the target and the eyes is measured in cm and converted to diopters (divide the measurement into 100)

100/cm = accommodative amplitude in D

Front 32

Accommodation is generally greatest:

Back 32

in children, and decreases with age; Hofstetter's formula is able to calculate the average expected amplitude of accommodatoin

Front 33

Hofstetter's formula

Back 33

18.5 - (0.3 * age)

i.e.: 18.5 - (0.3*29)
18.5-8.7=9.8D amplitude of accommodation

Front 34

accommodative reserve

Back 34

the difference between supply and demand

*visual discomfort usually occurs when the accommodative reserve is less than the demand*

Front 35

accommodative insufficiency

Back 35

-insufficient amplitude of accommodation
-symptoms=blurring, headache, sometimes pain
-causes=fatigue, flu, stress, systemic meds, ocular inflammation, head trauma, thyroid disease, or juvenile form of diabetes mellitus
-often associated with convergence insufficiency

Front 36

near point triad

Back 36

near point vision results in accommodation, convergence of the eyes, and miosis of the pupil

Front 37

retina to brain process

Back 37

the retina receives light rays, translates them into electrophysiologic impulses, and transmits the impulses to the brain where they are interpreted as a visual image

Front 38

photoreceptors

Back 38

-"light receivers"
-rods and cones
-specialized cells that respond to the amount and color of light

Front 39

rods

Back 39

-the prevalent photoreceptor
-about 130 million in the retina
-mainly responsible for scotopic vision (dark-adapted/dim light), peripheral vision, and for movement detection

Front 40

cones

Back 40

-about 7 million in the retina
-provide photopic vision (light-adapted/bright light), acute visual acuity, color vision, and central vision

Front 41

fovea centralis

Back 41

the area of the retina that provides the most acute vision; marked by a small depression in the center of the macula lutea

Front 42

axial length

Back 42

the length of the eyevall from the cornea to the posterior pole; average axial length = 23-25mm (about the size of a quarter)

Front 43

a-scan ultrasonography

Back 43

may be used to measure axial length; usually performed to calculate the power of an IOL before cataract surgery

Front 44

emmetropia

Back 44

a state of proper correlation between the refractive system of the eye and the axial length of the eyeball; rays of light entering the eye parallel to the optic axis being brought to a focus exactly on the retina

Front 45

ametropia

Back 45

*can be either refractive or axial*
a condition of the eye in which images fail to come to a proper focus on the retina, due to a discrepancy between the size and refractive powers of the eye
-when accommodation is relaxed, parallel light rays don't focus on the retina, resulting in myopia, hyperopia, and/or astigmatism

Front 46

refractive ametropia

Back 46

axial length is normal and the refractive power (either/both the cornea and the lens) is either too strong or too weak to focus light rays on the retina

Front 47

axial ametropia

Back 47

refractive power of the eye is normal and the axial length is either too long or too short to place the retina at the point of focus

Front 48

myopia

Back 48

the refractive condition in which, when accommodation is relaxed, parallel light rays entering the eye focus in front of the retina (nearsighted)

Front 49

physiologic myopia

Back 49

myopia is usually physiologic - refers simply to the shape involving particularly either/both a steep corneal curvature and long axial length

Front 50

pathologic myopia

Back 50

causes include diabetes (change in blood sugar level affects index of refraction), and degenerative changes in the eyeball (cataract formation)

Front 51

orthokeratology

Back 51

-often called Corneal Refractive Therapy (CRT)
-myopia treatment
-theory is that using a series of progressively flatter contact lenses will flatten a steep cornea

Front 52

hyperopia

Back 52

the refractive condition in which, when accommodation is relaxed, parallel light rays entering the eye focus behind the retina; farsighted/ hypermetropia (closely connected to accommodation)

Front 53

latent hyperope

Back 53

a hyperopic individual (usually child/adolescent), who accommodates constantly to compensate for refractive error

Front 54

Accommodation and convergence are related in that:

Back 54

with every diopter of accommodation, there is a specific amount of convergence (explained in the AC/A ratio)

accommodative convergence/accommodation

Front 55

amblyopia

Back 55

-"lazy eye"
-the loss of one's ability to see details

Front 56

astigmatism

Back 56

a refractive condition in which parallel light rays entering the eye do not focus at a single point but instead form two line images at different meridians, generally at right angles to each other

Front 57

interval of Sturm

Back 57

the distance between the 2 lines of focus in an astigmatism

Front 58

circle of least confusion

Back 58

at the midpoint between the two focus lines (in an astigmatism) a circle is formed, the circle of least confusion

Front 59

residual astigmatism

Back 59

the difference between ocular astigmatism and corneal astigmatism

Front 60

regular astigmatism

Back 60

two the principal meridians are 90 degrees apart

Front 61

with-the-rule astigmatism

Back 61

refers to a steeper curvature in the vertical meridian (like a football lying on its side)

Front 62

against-the-rule astigmatism

Back 62

refers to a steeper curvature in the horizontal meridian (like a football standing on end)

Front 63

oblique astigmatism

Back 63

occurs when the meridians are between 30-60 degrees and 120-150 degrees

Front 64

irregular astigmatism

Back 64

-is not common in non-traumatized eyes
-occurs when meridians aren't 90 degrees apart or when they aren't uniformly positioned from point to point along a meridian

Front 65

Why is it impossible for an astigmatism to be axial?

Back 65

because an eyeball cannot be of two different lengths

Front 66

mixed astigmatism

Back 66

one focal line is in front of the retina, and the other is behind the retina

Front 67

simple astigmatism

Back 67

one line is on the retina, the other is either in front of the retina (simple myopic) or behind the retina (simple hyperopic)

Front 68

compound astigmatism

Back 68

both focus lines are either in front of the retina (compound myopic) or behind the retina (compound hyperopic)

Front 69

cylindrical lens

Back 69

a lens with power in only one meridian

Front 70

spherocylinder

Back 70

used for compound or mixed asitgmatism; has a different power in each of its principal meridians

Front 71

RGP

Back 71

has a rigid spherical back surface that rests on the corneal tear film; the tear film fills in the spaces

Front 72

bitoric contact lenses

Back 72

have toric curves on both the front and back surfaces; used in cases of combined significant residual astigmatism and corneal astigmatism

Front 73

radial keratotomy

Back 73

performed by making spoke-like incisions into the cornea to flatten the center

Front 74

photorefractive karatectomy (PRK)

Back 74

uses a custom-designed laser to reshape the cornea by scraping away the top layer of the cornea (epithelium) and vaporizing a portion of the middle layer (stroma)

Front 75

LASIK

Back 75

-Laser Assisted In Situ Kertomileusis
-most common procedure used today
-permanently changes the shape of the cornea using an excimer laser
-knife (microkeratome) is used to cut a flap in the cornea; pulses from a computer-controlled laser vaporize a portion of the stroma and the flap is replaced without sutures

Front 76

presbyopia

Back 76

-is a reduction in the ability to accommodate that occurs normally with age
-may result from a reduction in lens elasticity or in the strength of the ciliary muscle
-factors may include: general health, age, visual demand, ametropia, environmental conditions, stature, and self image

Front 77

aphakia

Back 77

the absence of the crystalline lens

Front 78

cataracts

Back 78

-an opacity of the cyrstalline lens or its capsule
-usually results from changes in the tissues or fibers of the lens
-may be attributed to trauma, genetic or metabolic disorders, effects of radiation, drugs or infection, or the aging process

Front 79

senile cataract

Back 79

*most prevalent type of cataract*

the term simply refers to cataracts that form as a natural part of the aging process

Front 80

nuclear cataract

Back 80

-may cause an increase in myopia or a decrease in hyperopia
-positioned on the visual axis
-may cause monocular diplopia

Front 81

subcapsular cataracts

Back 81

-often appear at the posterior pole of the lens
-may cause glare and greater reduction of near vision

Front 82

cortical cataracts

Back 82

-are cataracts within the cortex itself
-often go unnoticed unless the cataract is prominent

Front 83

treatment for aphakia

Back 83

involves one of or a combo of three methods:
glasses
contact lenses
IOL implant

Front 84

spectacle lens correctin for aphakia

Back 84

-"aphakic spectacle lenses"
-front curve is usually aspheric (curves are slightly flatter toward the periphery)
-are either full-field or lenticular

Front 85

full-field aphakic spectacle lenses

Back 85

has power throughout the lens that decreases from center to edge

Front 86

lenticular aphakic spectacle lenses

Back 86

-reduces thickness and weight
-confines power to a 38-40mm "bowl", surrounded by a thin, nonoptical carrier

Front 87

contact lens correction for aphakia

Back 87

afford the patient a comfortable, nonmagnified, wide-field vision

Front 88

IOL implant to correct for aphakia

Back 88

-most commonly used method to treat aphakia
-surgical insertion of intraocular lens implant (IOL)

Front 89

anisometropia

Back 89

a condition of unequal refractive state of the two eyes, usually at least 1 diopter different

Front 90

What are the 3 types of anisometropia?

Back 90

simple
compound
mixed

Front 91

simple anisometropia

Back 91

one eye is emmetropic the other is ametropic

Front 92

compound anisometropia

Back 92

refers to both eyes being myopic, hyperopic, or astigmatic, but in differing amounts

Front 93

mixed anisometropia (antimetropia)

Back 93

one eye is myopic, the other is hyperopic

Front 94

refractive anisometropia

Back 94

attributed to a marked difference in corneal curvature of the two eyes, or monocular aphakia; best corrected with contact lenses

Front 95

axial anisometropia

Back 95

attributed to difference in axial length between the two eyes along with fairly equal curvature (resulting in a marked difference in total refractive error); best corrected with glasses

Front 96

aniseikonia

Back 96

a difference in size of the two retinal images; two major subdivisions are inherent and acquired

Front 97

inherent aniseikonia

Back 97

refers to different image sizes caused by the refractive condition of the two eyes

Front 98

acquired aniseikonia

Back 98

occurs as a result of the effects of corrective lenses

Front 99

eikonometer

Back 99

clinical measurement of aniseikonia (eikonometry is performed with the eyes disassociated, either with polarized filters, or independently adjustable optical systems for each eye)

Front 100

iseikonic lenses

Back 100

lenses specifically designed to alleviate aniseikonia

Front 101

amblyopia

Back 101

-"lazy eye"
-reduced visual acuity with no apparent cause and not correctable by refractive means
-functional amblyopia is divided up into 3 categories: strabismic, refractive, and stimulus deprivation

Front 102

strabismic amblyopia

Back 102

-is also called amblyopia ex anopsia
-results from abnormal binocularity
-conflicting images of the eyes may result in one eye suppressed for a significant amount of time, it may turn in or out

Front 103

refractive amblyopia

Back 103

-associated with uncorrected refractive errors
-uncorrected refractive errors for a significant amount of time prevents normal visual stimulation leading to a habitual state of reduced VA

Front 104

stimulus deprivation amblyopia

Back 104

-results when sensory receptors do not receive input (i.e. retina doesn't receive light), due to some factor, such as a congenital cataract
-receptors may not be developed properly in the absence of stimuli

Front 105

The eye is said to have a refractive error when ______________.

Back 105

light rays don't bend properly to form a single-point focus on the retina

Front 106

The index of refraction is abbreviated by the symbol _____________.

Back 106

n

Front 107

A light ray traveling from one medium to another medium that has a higher index of refraction will be _____________.

Back 107

bent toward the normal

Front 108

Divergence occurs when light rays ________________.

Back 108

bend away from the midline

Front 109

There is no refraction of light at __________________.

Back 109

the optical center

Front 110

Light rays assume a virtually parallel course, "optical infinity", at approximately _____________.

Back 110

20 ft/6m

Front 111

Convex lenses are also referred to as ________________.

Back 111

plus lenses

Front 112

A lens in which the center is thinner than its edges is known as _________________.

Back 112

concave

Front 113

Light rays entering the eye must first pass through the ________________.

Back 113

cornea

Front 114

Which structure contributes approximately 70% of the optical power of the eye?

Back 114

the cornea

Front 115

Combining front and back surface powers, the total power of the cornea averages approximately how many diopters?

Back 115

42.00D

Front 116

Which part of the eye is biconvex, normally transparent, and fits just behind the iris?

Back 116

the crystalline lens

Front 117

Accommodation is defined as ________________.

Back 117

the act of the crystalline lens at varying distances

Front 118

The number of diopters of power is equal to the reciprocal of the focal length of a lens in ______________.

Back 118

meters

Front 119

A 3.00D lens has a focal length of ________________.

Back 119

33 cm

Front 120

The ability of the crystalline lens to focus is known as

Back 120

amplitude of accommodation

Front 121

Accommodative reserve is the difference between _________________.

Back 121

supply and demand

Front 122

There are approximately 130 million of what in the human retina?

Back 122

rods

Front 123

Cones in the human retina provide which of the following?
a. peripheral vision
b. scotopic vision
c. color vision
d. depth perception

Back 123

c

Front 124

The area of the retina that provides the most acute vision is the _________________.

Back 124

fovea centralis

Front 125

What is the length of the eyeball from the cornea to the posterior pole known as?

Back 125

axial length

Front 126

Axial length discrepancies will results in producing a __________________.

Back 126

refractive error

Front 127

In an emmetropic eye, where are parallel light rays refracted?

Back 127

on the retina

Front 128

Myopia is commonly called ______________.

Back 128

nearsighted

Front 129

Concave lenses would be used to correct ________________.

Back 129

myopia

Front 130

Minus lenses may cause ____________________.

Back 130

barrel distortion

Front 131

How are light rays refracted in hyperopia?

Back 131

behind the retina

Front 132

Hyperopia is closely connected with what?

Back 132

accommodation

Front 133

Treatment for hyperopia is usually with what type of lens?

Back 133

convex lenses

Front 134

What is the distance between two line of focus called?

Back 134

Interval of Sturm

Front 135

The difference between total ocular astigmatism and corneal astigmatism is called __________________.

Back 135

residual astigmatism

Front 136

In mixed astigmatism, the focal lines are __________________.

Back 136

one in front of the retina, one behind the retina

Front 137

What type of lens is used for correction of compound astigmatism?

Back 137

spherocylindrical

Front 138

A normal reduction in the ability to accommodate is known as ______________.

Back 138

presbyopia

Front 139

What is the distinction between hyperopia and presbyopia?

Back 139

accommodative amplitude

Front 140

Who is credited with inventing bifocals?

Back 140

Benjamin Franklin

Front 141

What is the most prevalent type of cataract?

Back 141

senile

Front 142

What is aphakia?

Back 142

the absence of the crystalline lens

Front 143

What is the condition of an unequal refractive state of the two eyes known as?

Back 143

anisometropia

Front 144

How is refractive anisometropia best corrected?

Back 144

contact lenses

Front 145

What instrument is used in the clinical measurement of aniseikonia?

Back 145

eikonometer

Front 146

The difference in the size of the two retinal images is known as ___________________.

Back 146

aniseikonia

Front 147

What is the small depression in the center of the macula lutea, which is located at the back of the retina?

Back 147

fovea centralis

Front 148

What are the major categories of amblyopia?

Back 148

-strabismic amblyopia
-refractive amblyopia
-stimulus deprivation

Front 149

Strabismic amblyopia ("lazy eye") is most likely the result of what?

Back 149

abnormal binocularity