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Duke Elder: Optics (Alice) > Chapter 11: Presbyopia > Flashcards

Chapter 11: Presbyopia Flashcards

1
Q

What are 2 reasons the amplitude of accommodation decreases with age?

A
  1. Sclerosis of fibres of crystalline lens + change in its capsule - reduce spontaneous steepening (= main reason)
  2. Ciliary muscle itself becomes less efficient
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2
Q

What is the accommodation possible lin infancy?

A

+14D

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3
Q

What is the accommodation possile age 45 years?

A

4 D

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4
Q

What is the accommodation possble after age 60?

A

1D or less

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5
Q

What is the remaining 1D of accommodation at age 60 likely due to?

A

depth of field - may be enhanced by senile miosis

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6
Q

How much must the eye accommodate to see an object at reading distance - 25cm?

A

1/0.25 = 4 D

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7
Q

What is essential for comfortable near vision in terms of degree of accommodation?

A

one third of available accommodation must be kept in reserve

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8
Q

At what point will a patient begin to eperience difficulty/discomfort for near vision?

A

when accommodation decays to 6D - 4D used at 25cm and only 2D (third) in reserve

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9
Q

How can comfortable near vision be achieved in presbyopia?

A

supplementary convex lens

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10
Q

What is the definition of presbyopia?

A

age-related inadequacy of accommodation - cannot be determined in terms of remaining ampltiude of accommodation as symptoms vary

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11
Q

How can you calculate the amount of presbyopic correction necessary for a given patient?

A

If remaining amplitue of accommodation is determine (from near point) and desired working distance specified

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12
Q

What presbyopic correction is required for a patient with 3D accommodation (i.e. near point 33cm) to achieve comfortable near vision?

A

Must keep one third in reserve i.e. must only use 2D leaving 1D remaining.
To see clearly at 25cm needs 4D. Therefore needs additional 2D; presbyopic correction =2D

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13
Q

When does onset of presbyopia occur in uncorrected hypermetropia and why?

A

earlier - patient with hypermetropia must accommodate more to achieve near vision (e.g. +3D of hypermetropia needs to exert 3D accommodation to see clearly at infinity).

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14
Q

How much accommodation correction is required for a hypermetrope of +3D to see at 25cm?

A

3D of accommodation to see clearly at infinity
plus 1/0.25 = 4D at reading distance
3+4 = 7D

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15
Q

How much accommodation is used for a patient with -3D of myopia to focus at 25cm?

A

near point is 1/3 = 0.33m =33cm.
to focus at 25cm only 1D of accommodation is used

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16
Q

At what degree of myopia can a patient always read without glasses?

A

-4D or more

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17
Q

Why is it easy to prescribe too strong a presbyopic correction?

A

a patient away from usual surroundings tends to hold the reading test type closer to the eyes than usual

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18
Q

What is a safeguard gainst overcorrection of presbyopia?

A

ensure patient can read N5 at his or her approximate reading distance but also N8 at arm’s length with the proposed correction

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19
Q

In bifocal lenses for distance and near vision, wich portion is usually the larger (major) portion?

A

distance portion

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20
Q

Where is the near portion of bifocal spectacles usually situated?

A

in lower part of lens

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21
Q

What are 4 types/ways to ahieve bifocal lenses?

A
  1. Split (Franklin) bifocals –> segment
  2. Cemented bifocals
  3. Fused bifocals
  4. Solid bifocals
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22
Q

What are Franklin / Split bifocals?

A

distance lens has flat bottom which abuts flat top of separate near lens

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23
Q

How have newer split/Franklin bifocals been modified?

A

segment: near portion constructed by modifying main lens to incorporate near addition, by attaching supplementary lens or changing surface curvature

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24
Q

What are cemented bifocals?

A

near portion constructed by attaching supplementary lens to surface of distance lens of same refractive index; imperceptibily thin wafer added on rear surface

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25
Q

How are fused bifocals made?

A
  • near portion made by heat-fusing a button of flint glass to corresponding depression in a **crown glass main lens **which has a lower refractive index
  • button is ground to marke surface curvature same, but different types of flint glass allow variation in refractive index
26
Q

How are solid bifocals produced?

A

the near addition is produced by a **different curvature of either the front or back surface **of this portion; can be full-width horizontal junction (‘executive’ style)

27
Q

Which type of bifocal spectacles are always of the solid bifocal type?

A

plastic bifocals

28
Q

What are the distance and near visual points (DVP and NVP)?

A

positions on a lens through which it is assumed the visual axis is directed while the spectacles are in use

29
Q

What are the distance and near visual points (DVP and NVP)?

A

positions on a lens through which it is assumed the visual axis is directed while the spectacles are in use

30
Q

Where is the NVP in relation to the DVP?

A

2mm nasal and 8mm below the DVP

31
Q

Where shoult the top of the near portion of solid bifocals be in relation to the eye?

A

top of the near portion should be tangential with the inferior limbus for most purposes

32
Q

When might children be prescribed executive solid bifocals?

A

to overcome convergence excess esotropia

33
Q

What commonly causes intolerance of bifocals?

A

poorly fitting frame which allows near portion to slip too low for comfortable use

34
Q

What are 2 types of optical aberration which can occur with bifocal spectacles?

A
  1. Prismatic effect / pristmatic jump- increases towards junction of distance and near
    2.Astigmatic aberrations
35
Q

What causes prismatic jump in bifocals?

A

prism power at the junction of near and distance portions will suddenly change when patient looks from one to the other and image will suddenly change position

36
Q

What is the prism power in bifocals proportional to?

A

dioptric power of each portion and the distance of the interfae from its optical centre

37
Q

What causes the prismatic effect at the NVP?

A

sum of the prismatic effect due to the main lens and that due to the segment at this point

38
Q

Why may bifocals not be tolerated by patients with anisometropia?

A

prismatic effect on each eye will be different

39
Q

What degree of vertical prismatic imbalance may be insuperable and make binocular vision uncomfortable/cause diplopa?

A

1.5 PD

40
Q

What is the advantage of fused or segmented bifocals in terms of prismatic aberration?

A

for most of these 2 types, optical centre of the segment overlies the NVP of the main lens and produces no extra prismatic effect beyond that caused by looking through an eccentric point on the distance lens

41
Q

Why are prismatic jump and excessive prismatic effect at the NVP most troublesome in higher power lenses?

A

higher refractive errors magnify the imbalance and may require uncomfortable degree of down gaze to read due to large prismatic effect

42
Q

How can prismatic jump in bifocal spectacles be reduced? 2 ways

A
  1. if the optical centres of the lens lie at or near the junction of the two portions
  2. by incorporating a base-up prism in the near segment
43
Q

Where do the optical centres coincide in excutive bifocal lenses?

A

at the junction fot he distance and near portions - so no prismatic jump

44
Q

How can shaped segments in bifocals reduce jump?

A

segment top is only a short distance from its optical centre

45
Q

What is the prismatic effect of a down-curve circular segment?

A

base-down prismatic effect at NVP because optical centre is always below it

46
Q

In what situations could the base-down prismtic effect of a down-curve circular segment be a) beneficial b) problematic?

A

a) counters base-up effect of hypermetropic distnce correction
b) adds to base down effect of myopic distance correction - may make it intolerable

47
Q

What is meant by ‘prism controlled’ bifocals?

A

meant to refer to any bifocals designed to counter prismatic effect, but the term is often used only where this is achieved by incorporation of a prism

48
Q

What are the 3 steps to incorporating prismatic correction with a prism in bifocals?

A
  1. may be pre-cast or (for straight top fused or cemented bifocals) biprism (slab off) process
  2. prismatic correction added to near and distance portions of less prismatic lens so prismatic effect at NVP of each is equal
  3. base up prism removed from upper part of modified lens - original prismatic modification affects only near portion
49
Q

When is the biprism process useful for bifocal spectacles?

A

when anisometropia causes excessive vertical prismatic imbalance at the NVP

50
Q

What causes astigmatic aberrations from bifocal lenses?

A

the near visual axis does not correspond with the optic axis of the near portion of the lens - astigmatic aberrations occur when light passes obliquely through a lens when the near portion is used

51
Q

How can astigmatic astigmatism be reduced when using the near portion of the bifocal spectacles?

A

bifocals should be made so the top of each lens is tilted forward to 10 -20 degrees (pantoscopic tilt) making the visual axis more perpendicular to the near portion of the lens when reading

52
Q

Why is it important to consider the needs of the individual patient when prescribing bifocals?

A

large near portion if do lot of work at near distance, can reduce presbyopic prescription is this is slightly greater than usual reading distance e.g. typist or supermarket cashier. if work outdoors can be smaller near segment

53
Q

What occupations are a contraindication for bifocals?

A

working at height e.g. scaffolder - down-gaze will be dangerous through near portion
also if elderly, unsteady on feet, vertigo

54
Q

When are trifocal lenses useful?

A

if little or no accommodation remains, they are useful to focal at middle distance (if accommodation intact can use distance portion of bifocals and accommodate for middle distance)

55
Q

What are 2 situations when trifocals may not be tolerated?

A
  1. anisometropic pt
  2. prisms required for near work
56
Q

What are progressive addition lenses?

A

lens power changes gradually between distance and near visual points so that a single pair of spectacles suffices for all distances

57
Q

What are 2 advantages of progressive addition lenses (PALs)?

A
  1. cosmetic advance - no visible interfae between distance and near portions
  2. power progression corridor between DVP and NVP - lens is optically true and focuses for intermediate distances, avoids sudden step in power/rpsim
58
Q

What is a disadvantage of PALs?

A

to each side of the progression corridor, **aberration and astigmatism increase peripherally **and may be intolerable for prescriptions with a large cylinder or high reading addition

59
Q

Why might you specify an extra +0.50D added to the near addition prescription in PALs?

A

sometimes the full reading adition is only achieved near the lower edge of the lens, so this ensures sufficient reading power is present at the NVP

60
Q

What are ‘hard’ PALs?

A

wide distance and near portions but narrow progression corridor (aberrations close to it)

61
Q

What are ‘soft’ PALs?

A

smaller distance and near portions allowing wider progession corridor, less pronunced aberrations to each side

62
Q

How can you calculate the amount of presbyopic correction necessary for a given patient?

A

If remaining amplitue of accommodation is determine (from near point) and desired working distance specified

Duke Elder: Optics (Alice) (19 decks)

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