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Duke Elder exam > Optics and refractive errors > Flashcards

Optics and refractive errors Flashcards

1
Q

how is axial length of eyeball measured

A

from corneal surface to RPE/Bruch membrane - most lengthening occurs in first 3-6 months of life

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2
Q

axial length of newborn eye

A

16mm

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3
Q

axial length of eye at 3 years

A

22.5mm

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4
Q

axial length of eye at 13-18 years (adult)

A

24mm

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5
Q

length of lens to retina in adults

A

17mm

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6
Q

low, moderate and high myopia is

A

low -6D

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7
Q

2 causes of myopia

A

axial (>24mm - most common)

index (high refractive power - often in keratoconus or nuclear sclerotic cataract)

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8
Q

3 potential methods in slowing myopic progression

A

atropine and pirenzepine drops
outdoor activity
bifocals and progressive lenses

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9
Q

when should contact lens wearers stop wearing them before LASIK/LASEK

A

soft - 14 days

rigid - 1 month

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10
Q

low, moderate and high hypermetropia

A

low +5D

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11
Q

2 causes of hypermetropia

A 
small eyes (<24mm) 
low refractive power (aphakic patients i.e absence of lens, flat cornea)
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12
Q

5 conditions associated with hypermetropia

A 
esotropia 
angle-closure glaucoma
retinoschisis 
uveal effusion syndrome ( nanophthalmos) 
ambylopia in children
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13
Q

how does LASIK/LASEK differ in hypermetropia surgery

A

peripheral corneal tissue ablated = steeper central cornea

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14
Q

how to calculate power of lenses

A

power = 1/f

f = focal length in m

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15
Q

diurnal variation of corneal shape in normal eyes

A

flattest cornea in morning (changes in eyelid pressure and muscle tension)

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16
Q

2 types of astigmatism

A

regular - principle meridians (steepest and flattest) are 90 degrees from each other
irregular - principle meridians are not perpendicular to each other

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17
Q

3 further classifications of regular astigmatism

A
  • with-the-rule astigmatism = vertical meridian (90 degrees) is steepest
  • against-the-rule astigmatism = horizontal meridian (180) is steepest
  • oblique astigmatism = when principle meridians are neither at 90 nor 180
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18
Q

how to correct with-the-rule astigmatism

A

plus cylinder lens between 60 and 120 degrees

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19
Q

how to correct against-the-rule astigmatism

A

plus cylinder lens between 150 and 30 degrees

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20
Q

how to correct oblique astigmatism

A

plus cylinder lens between 31 and 50 and 121-149

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21
Q

when does irregular astigmatism occur

A

conditions such as keratoconus or corneal ulcers

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22
Q

2 ways to manage astigmatism

A 
soft toric lenses (combo of spherical and cylindrical lenses) 
RGP lenses (for irregular astigmatism)
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23
Q

difference between spherical and cylindrical lenses

A 
spherical = same power in all meridians 
cylindrical = power in one meridian only (because focuses light onto a line rather than a point)
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24
Q

what is transposition of prescription lenses/glasses

A

converting a minus cylindrical lens to a plus cylindrical lens and vice versa (doesn’t change the optical properties - often used in toric lens prescriptions)

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25
Q

3 steps of transposition of lenses

A 
1 = add cylinder and sphere power = new sphere power
2 = change sign of cylinder
3 = change axis by 90 degrees (if <90 then add 90, if >90 then subtract 90)
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26
Q

what is presbyopia

A

age-related loss of accommodative ability of eye

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27
Q

2 causes of presbyopia

A

increase in lens size and hardness

ciliary muscle dyfunction

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28
Q

accommodative power at age 8

A

14D - lost starting at age 40 then almost completely lost after age 60

29
Q

what is the amplitude of accommodation

A

maximum increase in diopter power the eye can achieve through accommodation - to achieve comfortable vision, at least 1/3 of amplitude of accommodation should be kept in reserve

30
Q

what is the near point of the eye

A

closest point where the image remains clear

31
Q

2 broad classifications of squints

A

tropias

phorias

32
Q

what is esotropia

A

eye is deviated nasally and moves temporally on cover testing to fixate

33
Q

what is exotropia

A

eye is deviated temporally and moves nasally on cover testing to fixate

34
Q

where should prisms be placed for esotropia/exotropia

A

should be placed for both eyes with power of prisms split evenly between the 2 eyes - base of prism should point away from deviation

35
Q

what is hypertropia

A

eye deviated superiorly and moves inferiorly on cover testing to fixate

36
Q

what is hypotropia

A

eye deviated inferiorly and moves superiorly on cover testing to fixate

37
Q

where should prisms be placed for hyper/hypotropia

A

power split evenly between 2 eyes

base of prism away from deviation

38
Q

what is a prism

A

transparent medium bound by 2 planes that are at an angle to each other - do not focus light but refract it towards the base

39
Q

what is Snell’s law

A

when light moves from one transparent medium of higher density to another of lower density. the light refracts (applies to eyes)

40
Q

what is the angle of incidence

A

angle the light travels as it hits the boundary of another medium

41
Q

what is the critical angle

A

when the angle of refraction is equal to 90 degrees (happens when the angle of incidence increases)

42
Q

when does total internal reflection occur

A

when the angle of incidence > critical angle = light doesn’t pass through medium and is completely reflected

43
Q

what does one prism diopter (PD) produce

A

a deviation of a light ray of 1cm measured at 1m from the prism

44
Q

2 equations to calculate PD

A

P = Fd - P = PD, F = lens power (D) and d = distance (cm) of pupil from optical centre)

P = 2 x angle of deviation (measured in degrees)

45
Q

how to interpret Snellen chart readings

A

6/6 - first 6 = distance chart is away from patient

second 6 = distance at which an average person can read

46
Q

what is a duochrome test

A

test using chromatic aberrations of the eye to refine the best vision sphere following optical correction

black letters on red and green background

47
Q

how does duochrome test work

A

red = longer wavelength than green therefore focuses behind retina

green = shorter wavelength therefore focuses before retina

seeing no differene = focusing on retina and perfect sphere correction

clearer red = hypermetropia, clearer green = myopia

48
Q

what is ishihara chart used for

A

to screen for red-green colour blindness

49
Q

what is logmar chart used for

A

for research purposes instead of Snellen chart

50
Q

what is 0 and 1 LogMAR equivalent to

A 
0 = 6/6 Snellen 
1 = 6/60 Snellen
51
Q

how does a logMAR chart work

A
  • patient placed 4m away
  • 5 letters on each row with equal spacing
  • letter of spacing = one letter width
  • row spacing = height of a letter from row below
  • each correct letter is worth 0.02 log units - one correct line = 0.1 log units
  • scoring is from 1 to 0, so that each letter read correctly will result in subtraction of 0.02 from 1
52
Q

how does retinoscopy work

A

light from retinoscope shone into retina at certain distance - observe patient’s red reflex while adding plus/minus lenses until a complete red reflex is observed

myopic = direction of reflex against direction of light

hypermetropic = direction of reflex with direction of light

53
Q

how to achieve the perfect vision correction value from retinoscopy

A

once perfect reflex is observed, examiner should subtract the dioptric equivalent of their working distance from the correcting lens

e.g. if working distance is 67cm (2/3m), examiner should subtract 1.5D (power = 1/f = 1/0.67) from lens power

54
Q

age at which we develop full colour vision

A

5 months

55
Q

VA of newborns, 3 mo, 6 mo, 9 mo, 1-2 years

A 
newborn = 6/200-6/60
3 mo = 6/90-6/60
6 mo = 6/30
9 mo = 6/24 
1-2 years = 6/18-6/6
56
Q

how to test VA in children <1 year

A

keeler or cardiff charts - forced preferential looking charts

57
Q

how to test VA in children 6mo-2 years

A

Cardiff cards

58
Q

how to test VA in children 2-3 years

A

Kay pictures

59
Q

how to test VA in children 3-5 years

A

Sheridan-Gardiner

60
Q

when to use Keeler LogMAR or illiterate E test

A

for preschool children and illiterate adults

61
Q

5 types of illumination techniques using slit lamps

A
  • direct (focal) illumination = most common
  • diffuse illumination
  • retroillumination
  • specular reflection
  • sclerotic scatter
62
Q

when is diffuse illumination used

A

general examination of external eye strutures - where illumination light is out of focus

63
Q

when is retroillumination used

A

uses light reflected from iris to look for corneal opaci

of from fundus to examine red reflex, patency of iridotomies and lens opacities

64
Q

when is iris transillumination (type of retroillumination) used

A

uses light reflected from retina in an undilated pupil to view iris abnormalities e.g. pigment dispersion or pseudoexfoliation

65
Q

when is specular reflection used

A

to view corneal endothelium e.g. Fuch’s corneal dystrophy

66
Q

hwen is sclerotic scatter used

A

to evaluate general corneal opacities - light directed at limbus which is scattered through cornea

67
Q

magnification of image produced by direct ophthalmoscopy

A

x15 (area of about 2 disc diameters)

virtual and erect

68
Q

magnification of image produced by indirect ophthalmoscopy

A

x2-5 - as power of the biconcave aspheric lens (15-40D) increases the magnification decreases

20D lens = x3 magnification
30D lens = x2 magnification

(area of about 8 disc diameters)

inverted vertically and horizontally

69
Q

when is the field of illumination in indirect ophthalmoscopy largest vs smallest

A

largest = high myopia

smallest = hypermetropia

Duke Elder exam (5 decks)

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