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Duke Elder exam > Optics and refractive errors > Flashcards

Optics and refractive errors Flashcards

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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
3 steps of transposition of lenses
``` 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) ```
26
what is presbyopia
age-related loss of accommodative ability of eye
27
2 causes of presbyopia
increase in lens size and hardness | ciliary muscle dyfunction
28
accommodative power at age 8
14D - lost starting at age 40 then almost completely lost after age 60
29
what is the amplitude of accommodation
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
what is the near point of the eye
closest point where the image remains clear
31
2 broad classifications of squints
tropias | phorias
32
what is esotropia
eye is deviated nasally and moves temporally on cover testing to fixate
33
what is exotropia
eye is deviated temporally and moves nasally on cover testing to fixate
34
where should prisms be placed for esotropia/exotropia
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
what is hypertropia
eye deviated superiorly and moves inferiorly on cover testing to fixate
36
what is hypotropia
eye deviated inferiorly and moves superiorly on cover testing to fixate
37
where should prisms be placed for hyper/hypotropia
power split evenly between 2 eyes base of prism away from deviation
38
what is a prism
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
what is Snell's law
when light moves from one transparent medium of higher density to another of lower density. the light refracts (applies to eyes)
40
what is the angle of incidence
angle the light travels as it hits the boundary of another medium
41
what is the critical angle
when the angle of refraction is equal to 90 degrees (happens when the angle of incidence increases)
42
when does total internal reflection occur
when the angle of incidence > critical angle = light doesn't pass through medium and is completely reflected
43
what does one prism diopter (PD) produce
a deviation of a light ray of 1cm measured at 1m from the prism
44
2 equations to calculate PD
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
how to interpret Snellen chart readings
6/6 - first 6 = distance chart is away from patient second 6 = distance at which an average person can read
46
what is a duochrome test
test using chromatic aberrations of the eye to refine the best vision sphere following optical correction black letters on red and green background
47
how does duochrome test work
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
what is ishihara chart used for
to screen for red-green colour blindness
49
what is logmar chart used for
for research purposes instead of Snellen chart
50
what is 0 and 1 LogMAR equivalent to
``` 0 = 6/6 Snellen 1 = 6/60 Snellen ```
51
how does a logMAR chart work
- 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
how does retinoscopy work
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
how to achieve the perfect vision correction value from retinoscopy
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
age at which we develop full colour vision
5 months
55
VA of newborns, 3 mo, 6 mo, 9 mo, 1-2 years
``` 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
how to test VA in children <1 year
keeler or cardiff charts - forced preferential looking charts
57
how to test VA in children 6mo-2 years
Cardiff cards
58
how to test VA in children 2-3 years
Kay pictures
59
how to test VA in children 3-5 years
Sheridan-Gardiner
60
when to use Keeler LogMAR or illiterate E test
for preschool children and illiterate adults
61
5 types of illumination techniques using slit lamps
- direct (focal) illumination = most common - diffuse illumination - retroillumination - specular reflection - sclerotic scatter
62
when is diffuse illumination used
general examination of external eye strutures - where illumination light is out of focus
63
when is retroillumination used
uses light reflected from iris to look for corneal opaci of from fundus to examine red reflex, patency of iridotomies and lens opacities
64
when is iris transillumination (type of retroillumination) used
uses light reflected from retina in an undilated pupil to view iris abnormalities e.g. pigment dispersion or pseudoexfoliation
65
when is specular reflection used
to view corneal endothelium e.g. Fuch's corneal dystrophy
66
hwen is sclerotic scatter used
to evaluate general corneal opacities - light directed at limbus which is scattered through cornea
67
magnification of image produced by direct ophthalmoscopy
x15 (area of about 2 disc diameters) virtual and erect
68
magnification of image produced by indirect ophthalmoscopy
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
when is the field of illumination in indirect ophthalmoscopy largest vs smallest
largest = high myopia smallest = hypermetropia

Duke Elder exam (5 decks)

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