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Ocular Physiology #2 > Accommodation and Presbyopia > Flashcards

Accommodation and Presbyopia Flashcards

2
Q

what happens to the lens in accommodation

A

the optical power increases (focal length decreases) by increasing the surface curvature and thickness

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3
Q

what is accommodation measured in

A

diopters (1/meter)- a measure of the vergence of light

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4
Q

what is accommodation

A

a dynamic change in the power of the eye- change in the focus from distance to near

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5
Q

what mediates accommodation

A

contraction of the ciliary muscle, release of zonular tension at the equator and “rounding up” of the lens (force exerted by capsule)

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6
Q

can a person with myopia focus at near or distance without accommodation

A

they can focus clearly on objects closer than optical infinity without accommodation

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7
Q

what has a higher refractive index: the cornea or aqueous humor

A

the cornea (n= 1.376 and aqueous humor n= 1.336)

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8
Q

can a person with hyperopia focus at optical infinity without accommodation

A

no, they are only able to focus clearly on objects at optical infinity only through an accommodative increase or with a (+) lens

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9
Q

where does the optical power of the cornea come from

A

the change in refractive index and the positive radius of curvature

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10
Q

do both the anterior and posterior sides of the lens increase in curvature with accommodation

A

yes- although historically it was thought the posterior did not move

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11
Q

what is depth of field

A

the range over which a target can be moved toward or away from the eye without a perceptible change in the blur or focus of the image

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12
Q

what is depth of focus

A

the focusing error that can be tolerated without an appreciable decrease in acuity or change in blur or focus of the image on the retina

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13
Q

what is depth of focus dependent on

A

the pupil size

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14
Q

what size pupil is the depth of focus larger

A

a small pupil has a larger depth of focus but a smaller field of view

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15
Q

what happens to the depth of focus during accommodation

A

the pupil size decreases during accommodation, which will increase the depth of focus

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16
Q

why would there be a decreased need for presbyopic correction as a person ages

A

their pupil size decreases with aging (senile miosis) which give a pinhole effect and increases the depth of focus

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17
Q

where is the eye focused when the eye is at rest

A

for distance

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18
Q

what are the lens and zonule positions when the eye is at rest

A

the zonules apply an outward tension on the lens and it is in a flattened and unaccommodated state

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19
Q

name the 3 parts of the accommodative triad or near reflex

A
  1. pupils constrict
  2. eyes converge
  3. eyes accommodate
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20
Q

what is blur-driven accommodation

A

place a (-) lens in front of an eye and the eyes will accommodate to overcome the blur/defocus

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21
Q

what is tonic accommodation

A

when the eye is at rest it has some residual or resting level of accommodation- about 1.5 D due to muscle tone

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22
Q

how can you induce blur with a BO prism

A

it increases the vergence- causing pupil constriction, convergence, and accommodation occurs

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23
Q

how can you induce accommodation with pharmacologic stimulation

A

use of a topical muscarinic cholinergic agonist (pilocarpine) results in stimulation of the ciliary muscle causing pupil constriction but not convergence

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24
Q

what is accommodative esotropia

A

occurs in uncorrected hyperopes because they need to accommodate to see distance objects and it over-converges the eyes

25
Q

what drops can block accommodation

A

cycloplegia drops- atropine, cyclopentolate, tropicamide

26
Q

how do cycloplegic drops work

A

they competitively bind to the same muscarinic receptors as the agonists: preventing the agonist from binding and block accommodation

27
Q

what is the push-up method

A

a way to measure accommodation: the patient reports when a near letter chart is no longer in sharp focus as it is brought closer to the eye

28
Q

what is a downside of the push-up method for measuring accommodation

A

it is a subjective evaluation of best image focus and can overestimate the amount of accommodation

29
Q

what can influence the push-up method

A

depth of focus, visual acuity, contrast sensitivity of the eye and contrast of the image

30
Q

what is the minus lens to blur method

A

a subjective measurement of accommodation amplitude by placing a (-) lens in front of one or both eyes to blur a distant letter- stimulating accommodation

31
Q

what is a problem with the minus lens to blur method

A

it minifies the letters and can underestimate the amount of accommodation

32
Q

what does accurate objective measurement of accommodation require

A

static or dynamic refractometers

33
Q

when does a person have complete loss of their accommodative ability

A

usually by age 50 (presbyopia)

34
Q

how many diopters of accommodation are lost per decade

A

2.3 diopters

35
Q

when is 2/3 of our accommodation amplitude lost

A

between ages 15 and 35

36
Q

what are 3 surgical options for presbyopia

A

corneal refractive surgery, cataract removal, and accommodative IOL’s

37
Q

what is a controversial surgical inventions to restore natural accommodation

A

scleral expansion surgery

38
Q

what is being researched as an option to restore natural accommodation

A

a deformable polymer- an IOL that can bend just like the natural lens

39
Q

what happens to the ciliary muscle as we age

A

there is a loss of muscle fibers and an increase in connective tissue, it is less able to hold or pull the flattened and unaccommodated configuration

40
Q

do the ciliary muscle contractile forces decrease as we age

A

no it actually increases and is at maximum during presbyopic age

41
Q

why does the configuration of the older unaccommodated ciliary muscle appear like a young accommodated ciliary muuscle

A

the inner apex of the unaccommodated ciliary muscle resides further forward and inward toward the anteroposterior axis of the aging eye

42
Q

how much does the lens capsule increase in thickness from birth to 60 years old

A

11 microns at birth to 20 microns at 60 (then decreases slightly)

43
Q

what happens to the lens capsule as we age

A

it becomes more brittle, gets thicker and less extensible (elasticity)

44
Q

what is the Fisher theory of the lens capsule

A

that the force that can be transmitted per unit of thickness of the capsule decreases by half after age 60 but the increased thickness offers some compensation

45
Q

what increases in the lens as it grows throughout life

A

thickness, increased surface curvatures and equatorial diameter

46
Q

what is the linear increase in mass over the human life span

A

more than 1.5 times

47
Q

what happens to a young lens if you remove it from the eye (no zonular forces)

A

it becomes accommodated (older lenses don’t change shape)

48
Q

how much does the lens increase in hardness during the human life span

A

more than a 4 fold increase

49
Q

what happens to the anterior chamber depth as the lens ages

A

it decreases (due to the increase in lens thickness)

50
Q

does the anterior segment length change as we age

A

no it stays constant

51
Q

what is lenticular sclerosis theory of presbyopia

A

the most common theory: the crystalline lens gets harder with age

52
Q

what is the geometric theory of presbyopia

A

that presbyopia is a consequence of the altered geometry of the lens/zonular relationship

53
Q

what is the explanation for the geometric theory of presbyopia

A

the anterior zonular connections are near the lens equator in a young lens and as we age the connections are farther forward so there is not effective relaxation of the anterior zonular/capsular force with an accommodative ciliary muscle contraction

54
Q

what is the disaccommodation theory of presbyopia

A

suggests that presbyopia is not caused by a failure of the lens to accommodate, but by a gradual failure of the lens to be held in an unaccommodated form at rest

55
Q

what does the multifactorial theory of presbyopia suggest

A

that presbyopia doesn’t result from a single causal factor, but through global deterioration of accommodative function of several aspects of the accommodative apparatus

56
Q

what are some optical compensations used for presbyopia

A

spectacle lenses, CL, corneal refractive surgical procedures and IOL’s

Ocular Physiology #2 (7 decks)

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