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objective and subjective refraction: human eyes Flashcards

1
Q

what is objective refraction ?

A

. retinoscopy
. still need basic co-operation
. understand what is happening to light entering the eye
. communicate effectively

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2
Q

what is subjective refraction ?

A

. assessing refractive error by asking Px for their subjective responses
. it can only be done if we have reliable responses from the patient
. understand what is happening to light entering the eye
. communicate effectively

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3
Q

which method of refraction do we use ?

A

. sometimes , we use only subjective methods and other times only objective
. we use objective refraction as a basis for subjective refraction

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4
Q

what else can we use retinoscopy for ?

A

. indication of pupil sizes
. pupil response ( wether accommodation is fluctuating)
. idea of clarity of optical media , are there any lens opacity
. signs of corneal conditions

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5
Q

what are the steps of refraction procedure ?

A
  1. measure the inter-pupillary distance (PD)
  2. adjusting trial frame
  3. measure vision/visual acuity
  4. retinoscopy
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6
Q

how to measure the inter-pupillary distance (PD) ?

A
  • “look into my left /open eye” ( practitioner closes their RE )
  • align zero of ruler scale with tangent to R temporal limbus
  • “look into my right/open eye “ ( practitioner closes LE )
  • read scale from tangent to L nasal limbus
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7
Q

why it important to measure the inter-pupillary distance (PD) ?

A
  • make sure frame is fitting accurately

- make sure that patient is looking through the centre of any lenses that you are using for refraction

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8
Q

what is phoropter ?

A
  • way to do refraction instead of using a trial frame and trial lenses
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9
Q

what is advantage of phoropter ?

A
  • placed in front a patient’s face - you don’t need set of trial lenses
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10
Q

how to adjust trial frame ?

A
  • with the frame off the PX’s face , set PD
  • then put on Px
  • adjust bridge height and tighten
  • adjust side length and angle
  • make fine adjustments when the frame is on
  • ensure Px comfort
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11
Q

what are the steps of measuring vision/visual acuity ?

A

. vision or visual acuity is measured monocularly
. keep room light on
. use an occluder in front of the left eye when measuring the right eye and vice versa
. ask patient to read the the smallest line of letters you can see
. ask if there anything else from the line below that you can read
. record your findings ( Snellen or logMAR)

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12
Q

why do we measure vision/visual acuity first ?

A

. it gives us a clue about the magnitude of refractive error
. gives you some insight before moving on to retinoscopy

. you have an idea of how well/badly the patient can see without correction

. this help you to know what to expect - whether the patient is emmetropic , hyperopic or myopic , astigmatic and whether a high power prescription is likely to be needed

. you also have some indication of the kind of target the patient will be able to look at while you carry out retinoscopy

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13
Q

how is retinoscopy carried out in humans ?

A

. ensure that the patient can view the target with the eye not being tested

. start with RE

. light is shone into one eye while the patient views the target and we ensure their visual axis is clear

. occluding lens is removed

. use your right eye to conduct retinoscopy on the patient’s right eye , and keep away from the visual axis of the left eye

. there may be some accommodation despite your best efforts

. the pupil size will vary between and within patients

. the reflex will also vary

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14
Q

how to control / discourage accommodation during retinoscopy ?

A
  • room light off
  • use a 6m target, appropriate size for level of vision
  • further control for accommodation by using green target on the duochrome
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15
Q

why do we ask patient’s to use green target on duochrome to control accommodation ?

A
  • green wavelengths are shorter than red wavelength
  • when looking at green , the target becomes more blurred with accommodation
  • when looking at red , longer wavelength , the target becomes clearer with accommodation
  • we don’t want patient to accommodate so we asm them to look at circles on green background
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16
Q

what is another way to control/discourage accommodation during retinoscopy ?

A
  • fogging
  • before starting ret on RE, check whether there is a with movement in LE ( hypermetropic ). if so , change this to ‘against’ or ‘reversal’ with a positive lens to discourage Px from accommodating while you carry out RET on RE
17
Q

why is duochrome target test based on?

A

based on principle of longitudinal chromatic aberration - shorter wavelength ( green light ) refracted more than longer wavelength light ( red )

18
Q

what happens when viewing targets on red background ?

A

the longer red wavelengths are relevant . the eye need to accommodate slightly to bring the target into focus

19
Q

what happens when viewing targets on green background ?

A

the shorter green wavelengths are relevant the target would become more blurred with accommodation .

20
Q

how to correct the spherical error in retinoscopy ?

A

. look at the meridian with the slowest with movement or fastest against movement

. this is the meridian with the tendency towards hyperopia/away from myopia ( needs highest positive power or lowest negative power )

. correct this meridian with a spherical lens

21
Q

how to correct the cylinder error in RET ?

A

. should observe an against movement so you can use minus cyl
. slow against = high powered negative cylinder
. fast against = lower powered negative cylinder

22
Q

why is there no need to check for ‘ with ‘ movement in the right eye ?

A

. if you got reversal in right eye - the patient’s refractive error is over plus as we haven’t took into account working distance- patient’s vision is blurry so this acts as fogged lens

23
Q

why do we take working distance into account when conducting RET ?

A
  • you have conducted RET from a working distance which gives the light entering the eye some divergence
  • the divergence disappears when the eye views a distant object, taking the focal point in front of retina
  • reduce positive power / increase negative power equivalent to the working distance to take focal point back to retina
24
Q

what to do after RET ?

A

subjective refraction - light ON

  • adjust your estimate to allow for the working distance
  • check VA monocularly and record it - not always good
  • do subjective refraction RE first , with LE occluded
  • when RE subjective refraction is complete, occlude the RE and do subjective on LE
25
Q

how to find BVS?

A
  • modify the spherical power to improve VA as much as possible ( max plus , min minus )
  • done with the highest positive / lowest negative
  • aim for spherical correction at which more positive power makes vision worse and more negative power makes no difference
  • BVS places circle of least confusion on retina
26
Q

why do we need to do subjective refraction- BVS ?

A
  • estimate of final refractive error from RET might be close to or far from the true refractive error, so the object viewed by PX might be imaged on or off the retina
  • if we corrected any astigmatism there may be 2 focal point
  • if 2 focal points circle of least confusion will be between two focal points
27
Q

what are the steps to find best vision sphere (BVS) ?

A
  1. ask PX to read the smallest line of letters
  2. is there anything from line below you can read
  3. place +0.25DS in front of the eye ‘ is that just as good , or is it more blurred ?
  4. just as good : keep adding +0.25DS and repeat step 3 till blurred
  5. if more blurred with +0.25DS , then place a -0.25DS in front of the eye “ till makes no difference can you read any more letters “
  6. repeat step 5
28
Q

why do we present +0.25DS first ?

A
  • we want to find the most positive

- negative sphere will cause PX to accommodate

29
Q

what to do after you found BVS ?

A
  • duochrome - to check the circle of least confusion is on the retina or close to it
  • targets should be either :
    1. equally clear
    2. if Px young <40 clearer on green ( means Px is able to accommodate )
30
Q

explain the RET procedure ?

A

. binocular ( no occluding lens )
. ensure Px is looking at a non-accommodative distance target to promote accurate and steady fixation
. maintain correct height and visual axis
. fog LE before doing ret on the RE
. do not remove ret results of RE when moving to LE
. correct for working distance after you have finished 2nd eye
. check VA monocularly after RET on both eye
. subjective refraction first on RE ( so LE is occluded), then LE( with RE occluded )
. check BVS- allows us to refine the spherical component of the prescription and have CLC on retina
. determine astigmatism

cs (18 decks)

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