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Clinical Skills H > Autorefractors > Flashcards

Autorefractors Flashcards

1
Q

what is the definition of an auto refractor

A

any instrument that measures refractive error - it usually excludes those that use trial case lenses

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2
Q

what are 2 other names for auto refractor

A
  • an optometer

- an automated refractor

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3
Q

what is the usual type of target of an auto refractor

A

an image which looks like its at a distance e.g. house at the end of a road, in order to relax the patient’s accommodation

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4
Q

what are the 4 uses of auto refractors

A
  • screening e.g. pre-screening in practice
  • where speed and ease of use is required - widely used in developing world
  • measuring accommodation in research studies
  • basically does the job of a retinoscope - but unintelligent retinoscope
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5
Q

what are the 3 components of a simple optometer

A
  • target
  • optometer lens
  • scale
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6
Q

where is the target placed and how does this differ for a hype rope compare to a myope, in a simple optometer

A

placed on first focal point of optometer lens
- for myope: need to move target in
- for hyperope: need to move target out
for target to be clear

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7
Q

what type of lens is used in a simple optometer and why

A

a +ve lens put in front of the target
it changes most hyperopes into myopes, so that it can measure the patient’s far point (which will now be somewhere in front of px’s eye) & estimate their refractive error

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8
Q

what is the usual type of target of an auto refractor

A

an image which looks like its at a distance i.e. infinity e.g. house at the end of a road, in order to relax the patient’s (proximal) accommodation

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9
Q

what type of lens is used in a simple optometer and why

A

a +ve lens put in front of the target
it changes most hyperopes into myopes, so that it can measure the patient’s far point (which will now be somewhere in front of px’s eye) and estimate their refractive error

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10
Q

how do the dioptre steps on the scale of a simple optometer differ for a myope compared to a hyperope

A

dioptre steps get closer as you get more myopic and further apart as you get more hyperopic

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11
Q

list and explain the 5 main disadvantages of a simple optometer

A
  • large depth of focus: if you move the target over a large range, the patient will still see a clear target
  • target can stimulate accommodation: e.g. from the print of the target
  • scale is non-linear: bigger gaps in dioptres for hyperopes and smaller gaps in myopic range
  • apparent size of target varies: as you bring the target closer, its retinal image size gets bigger, so the target gets easier to see as you move closer to the eye
  • proximal accommodation: px knows the target is not at infinity, so they apply proximal accommodation
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12
Q

what is a scheiner disc

A

a double pinhole disc, one pin hole at top of pupil and one pinhole at bottom pf pupil, a ray goes through the top pinhole and another ray goes through the bottom pinhole

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13
Q

how does the images of the target appear with the scheiner principle for an emmetropic eye

A

the images appear coincident

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14
Q

how do the images of the target appear with the scheiner principle for an emmetropic eye

A

the images appear coincident

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15
Q

how do the images of the target appear with the scheiner principle for a myopic eye

A

the images are crossed - before they hit the retina

the top ray of the pinhole reaches the bottom axis of the eye and the bottom ray of the pinhole reaches the top axis of the eye

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16
Q

how do the images of the target appear with the scheiner principle for a hyperopic eye

A

the images are uncrossed - before they hit the retina

the top ray of the pinhole reaches the top axis of the eye and the bottom ray of the pinhole reaches the bottom axis of the eye

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17
Q

what can you do to check if a patient is myopic with the scheiner principle

A

if you cover the top pinhole, the bottom image will disappear

indicating that the images were crossed and that you need to move the target in

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18
Q

list and explain the 5 main disadvantages of a simple optometer

A
  • large depth of focus: if you move the target over a large range, the patient will still see a clear target
  • target can stimulate accommodation: e.g. from the print of the target
  • scale is non-linear: bigger gaps in dioptres for hyperopes and smaller gaps in myopic range
  • apparent size of target varies: as you bring the target closer, its retinal image size gets bigger, so the target gets easier to see as you move closer to the eye
  • proximal accommodation: px knows the target is not at infinity, so they apply proximal accommodation
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19
Q

what is an advantage of the scheiner principle

A

better assessment of focus - the target is clear over a range of distances, giving a more precise end point for assessing the refractive error

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20
Q

list 4 disadvantages of the scheiner principle

A
  • target can stimulate accommodation
  • scale is non-linear: bigger gaps in dioptres for hyperopes and smaller gaps in myopic range
  • apparent size of target varies: as you bring the target closer, its retinal image size gets bigger, so the target gets easier to see as you move closer to the eye
  • proximal accommodation: px knows the target is not at infinity, so they apply proximal accommodation
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21
Q

what causes longitudinal chromatic aberration

A

light of different wavelengths are refracted by different amounts

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22
Q

which wavelengths correspond to myopic eyes

A

shorter (blue) wavelengths

23
Q

which wavelengths correspond to hyperopic eyes

A

longer (red) wavelengths

24
Q

which wavelengths correspond to emmetropic eyes

A

medium (green) wavelengths

25
Q

how much D of longitudinal chromatic aberration is between 480 - 700nm (visible spectrum)

A

~1.6D

26
Q

how much D of longitudinal chromatic aberration is between 480 - 700nm (visible spectrum)

A

~1.6D

27
Q

what needs to be done with the difference in ocular refraction between visible light and IR wavelength used with auto refractors

A

an allowance needs to be made for the difference in ocular refraction

in auto refractors usually 800-900nm is used

28
Q

how much D is 800-900nm worth, which is used in auto refractors and what needs to be done to make it correspond to the visible wavelength of 550-555nm

A

0.75 - 1.00D hyperopic compared with 550nm

so need to take off about 0.75D e.g. if reading in auto refractor gives +3.00D, it will actually be +2.25D (0.75 off)
how much the power changes depends on what wavelength is used in the optometer. but you must take some D off the result in the auto refractor, so that the power corresponds to 550-555nm

29
Q

how much D is 800-900nm worth, which is used in auto refractors and what needs to be done to make it correspond to the visible wavelength of 550-555nm

A

0.75 - 1.00D hyperopic compared with 550nm

so need to take off about 0.75D e.g. if reading in auto refractor gives +3.00D, it will actually be +2.25D (0.75 off)
how much the power changes depends on what wavelength is used in the optometer. but you must take some D off the result in the auto refractor, so that the power corresponds to 550-555nm

LCA has low inter subject variability, so its the same for everyone

30
Q

list 4 main point about infrared optometers

A
  • transmission of ocular media is good: % of light that gets through to the cornea, aqueous, lens, vitreous to the back of the eye is about 80-90% gets through to the fundus at about 500-800nm and at about 800nm transmission is still goof at about 80%
  • removes stimulus to accommodation: since IR is invisible to the patient, there is no stimulus to accommodate to & can’t see target
  • reflectance of fundus ~40% (820nm): 40% of light that goes into the eye is reflected back from the fundus which is good enough amount for detection at 820nm
  • needs electronic detector: to decide if the images are crossed or uncrossed as the patient and practitioner can’t see the IR
31
Q

list 4 disadvantages of the scheiner principle

A
  • target can stimulate accommodation
  • scale is non-linear: bigger gaps in dioptres for hyperopes and smaller gaps in myopic range
  • apparent size of target varies: as you bring the target closer, its retinal image size gets bigger, so the target gets easier to see as you move closer to the eye
  • proximal accommodation: px knows the target is not at infinity, so they apply proximal accommodation
32
Q

list the 2 advantages of infrared optometers

A
  • better assessment of focus

- target cannot stimulate accommodation (as IR is invisible)

33
Q

list 3 disadvantages of infrared optometers

A
  • scale is non-linear: bigger gaps in dioptres for hyperopes and smaller gaps in myopic range
  • apparent size of target varies: as you bring the target closer, its retinal image size gets bigger, so the target gets easier to see as you move closer to the eye
  • proximal accommodation: px knows the target is not at infinity, so they apply proximal accommodation
34
Q

what 4 problems does the badal optometer solve

A
  • better assessment of focus
  • target can stimulate accommodation
  • scale is non-linear
  • apparent size of target varies
35
Q

what is the one thing that the badal optometer cannot solve

A

proximal accommodation

36
Q

where in the fundus is IR reflected compared to visible radiation

A

from deeper in the fundus

37
Q

where in the fundus is IR reflected compared to visible radiation

A

from deeper in the fundus (from sclera) than visible radiation

38
Q

which layer of the fundus is visible light reflected from, e.g. from ret and why is this a disadvantage

A

internal limited membrane = first layer of the fundus

not great for measuring refractive error as want to get reflectance from deeper structures

39
Q

what happens as a result of IR radiation being reflected from the sclera and what needs to be done to correct it

A

eye appears to be longer i.e. more myopic than it really is

correction from 800nm to 550nm = approx -0.50DS, this cancels out with the 0.75D hyperopia from LCA

40
Q

how does an auto refractor measure sphere-cylindrical errors/astigmatism and what is the advantage to this method

A

the variation of refraction in different meridians is sinusoidal i.e. power changes along different axis, so is not just a 2 principle meridian, and also refractive error doesn’t go in 0.25D steps but are actually continuous

  • in practice, the auto refractor measures refractive error from 6 different meridians
  • this makes the estimate of refractive error significantly more accurate
  • it takes these 6 measurements very quick
41
Q

how does an auto refractor measure sphere-cylindrical errors/astigmatism and what is the advantage to this method

A

the variation of refraction in different meridians is sinusoidal i.e. power changes along different axis, so is not just a 2 principle meridian, and also refractive error doesn’t go in 0.25D steps but are actually continuous.
in theory, an auto refractor only needs to measure the sphere-cylinder component of refractive error in 3 meridians to calculate the sphere/cyl axis refractive error.

  • in practice, the auto refractor measures refractive error from 6 different meridians
  • this makes the estimate of refractive error significantly more accurate
  • it takes these 6 measurements very quick
42
Q

how is small pupils a problem with auto refractors

A

auto refractors can’t get enough IR back out of the eye i.e. low light intensity (same problem with ret)

43
Q

how much DS can a typical auto refractor get a reading up till

A

± 25DS

44
Q

how much DC can a typical auto refractor get a reading up till

A

± 8DC

45
Q

what element does a typical auto refractor have in order to relax accommodation

A

autofogging to prevent proximal accommodation

46
Q

what subjective refraction facilities do some typical auto refractors have

A

internal targets:

  • duochrome
  • x cyl
  • fan and block
  • subjective refraction
47
Q

as well as detecting refractor error, what other measurement can most auto refractors carry out

A

keratometry

48
Q

how do targets in auto refractors cope with proximal accommodation

A

go in and out of focus

49
Q

other than automatic or progressive fogging, what other method can auto refractors use to control proximal accommodation

A

some try to control convergence

50
Q

list the steps of how an auto refractor uses progressive fogging to work out a persons refractive error which is +1.50D

A
  • first time auto refractor measures +1.00D
  • then fogs the eye with a +1.75D to relax accommodation
  • it then removes the +1.75D fogging lens, the eye will accommodate, but not quick enough
  • the auto refractor takes a measurement before the eyes can accommodate, if +1.25D
  • so the eye is now a bit more +ve as the accommodation is a bit more relaxed
  • it then again adds the +1.75D fogging lens to relax accommodation
  • it removes the fogging lens
  • and gets a reading of +1.50D (more +ve so can see more relaxing of accommodation)
  • it adds the +1.75D fogging lens
  • removes the fogging lens
  • gets a reading of +1.50D - accommodation not relaxed anymore, so auto refractor gets as much +ve power as it can get from the eye
  • end point = +1.50D
51
Q

what must be done after taking auto refractor readings of a patient’s refractive error

A

must do subjective refraction before prescribing

52
Q

name 3 hand held auto refractors

A
  • nikon retinomax
  • nidek palm arc
  • welsh allyn suresight
53
Q 
what is the:
weight
measurement wavelength
time for each measurement 
and 
cost 
of the nikon retinomax hand held auto refractor
A
  • weight: approx 1KG
  • measurement wavelength: 860nm (IR)
  • time for each measurement: 0.01ms
  • cost: £10,000
54
Q

list 5 drawbacks of IR optometers

A
  • pupil diameters of 3mm or less produce an undetectable output
  • saturated output if patient links
  • cost
  • proximal accommodation remains an issue, even with all the tricks of automatic fogging etc
  • they will always remain an unintelligent ret

Clinical Skills H (18 decks)

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