Subjective refraction

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Subjective refraction

• OP1201 Basic Clinical Techniques
• Spherical refractive error
• Dr Kirsten Hamilton-Maxwell

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Todays goals

• By the end of todays lecture, you should be able
  to
• Explain the different methods used to determine
  the spherical component of your patients
  refractive error
• Explain the concept of Best Vision Sphere (BVS)
  and use several methods to determine it
• Describe the advantages and limitations of the
  different tests in different situations
• By the end of the related practical, you should
  be able to
• Refine the spherical component of your
  retinoscopy result
• Determine the BVS of your patients using a
  variety of techniques from scratch
• Complete the task within 10min for both eyes

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Background

• What is subjective refraction?
• Types of spherical refraction
• General set up

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Subjective refraction

• Aim is correct your patients refractive error by
  asking them choose what they like
• Guided by you, of course
• It is a multi-step procedure that we will cover
  over the next few weeks
• First step is to check that the spherical
  correction is correct following retinoscopy
• We will start spherical refraction today
• First, lets see how it all fits together

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Todays topic
Subjective refraction
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Spherical refraction

• There are a number of different procedures
• Plus/minus test
• 1.00DS test
• Duochrome test
• All are monocular
• Rule of thumb give the most positive lens that
  provides maximum vision
• Minimise accommodation increase comfort
• There is a twist to this rule when preparing for
  cross-cyl

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Set up

• Is the same for all of these procedures
• Assume that retinoscopy has been completed, the
  working distance lens has been removed and the
  vision measured
• (For now, assume that the cylindrical correction
  is correct)
• Turn the room lighting back on
• Occlude the eye that is not being tested

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Plus/Minus Test
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Effect of plus and minus
Positive lens
Emmetropia Both lenses equally clear (Blur
circles same size)
Negative lens
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Plus and minus in myopia
Positive lens
Uncorrected myopia Negative lens is clearer (Blur
circles different sizes)
Negative lens
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Plus and minus in hypermetropia
Positive lens
Uncorrected hypermetropia Positive lens is
clearer (Blur circles different sizes)
Negative lens
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What does the patient see?
Clear
Blurred
Blurred
Too much plus
Too much minus
-0.50DS
0.50DS
0
-0.25DS Same/smaller and darker -0.50DS Smaller
and darker/blurred Vision influenced by
accommodation
0.25DS Not as sharp 0.50DS Fainter/blurred
You are expecting a vision change of one line per
0.25DS!
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Plus/minus technique

• This information can be used to determine the
  refractive error using the plus/minus technique
• Add 0.25DS and ask are the letters clearer,
  more blurred or the same?
• Use the letter chart as a target
• Ask your patient to look at one line bigger than
  current vision
• Use a larger lens power if vision poor (eg.
  0.50DS or 1.00DS)

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Plus/minus technique

• If vision improves or remains the same, exchange
  the current spherical lens in the trial frame for
  one that is 0.25DS higher
• If you used 0.50DS or 1.00DS, then change it by
  0.50DS or 1.00DS respectively
• If you are testing a hypermetrope with
  accommodation, do not remove the trial frame lens
  until the new one is in place
• Repeat until the vision begins to blur
• When it blurs, it is time to stop adding plus

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Plus/minus technique

• Now check with a -0.25DS lens and ask are the
  letters clearer, more blurred or the same?
• If there is an improvement in vision, incorporate
  -0.25DS into the trial frame.
• If there is no improvement in vision, do not
  change the lens power!
• This means that they must read more letters on
  the chart!
• If the patient reports their vision is better but
  they cannot read more letters, ask do the
  letters definitely look clearer, or is it just
  smaller and darker?
• If clearer, add the lens
• If smaller and darker, do not add the lens!

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That last point again!

• Check that changing lens power changes the acuity
  as well
• Expect a change of 1 line per 0.25DS

Never incorporate extra minus into your
prescription unless you can measure an
improvement in vision on the letter chart!
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How to ask the question!

• What you say is important as you can lead the
  patient towards a particular answer without
  realising it
• Youll get the WRONG answer if you are not
  careful
• When adding plus
• Is it better, worse or still the same?
• Compare that to is it better? only
• You could reject a lens that was the same when
  it is actually telling you that your patient had
  been accommodating to make their vision clear
• When adding minus
• Is it clearer, or smaller and darker?

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Plus/minus technique

• In summary
• You are aiming to give the patient as much plus
  (or as little minus) as possible so the lens you
  want is the one in between
• Where adding more plus would cause blur
• Where adding minus does not improve vision

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They look the same

• If patient answers Same or No difference at
  first presentation
• You already have the right lens
• Confirm by changing Rx power to demonstrate a
  definite response
• Vision is too poor due to inaccurate retinoscopy
  result, or pathology is present
• Increase lens power and repeat
• Small pupils
• Increase power and repeat
• Be wary of inducing accommodation!

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1.00DS Blur Test
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1.00DS blur test
1.00DS

• Over-plussing should induce a blur circle on the
  retina, hence reducing vision
• Should blur by 1 Snellen line per 0.25DS
• If distant light currently focussed on retina,
  1.00DS should cause 4 lines of blur
• Should blur back from 6/6 to 6/18

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Performing the 1.00DS blur test

• Insert 1.00DS and ask patient to read down the
  chart
• If not 6/18, adjust spherical power
• Remember 1 line per 0.25DS
• Repeat until vision is 6/18
• Remove 1.00DS and check that vision is no worse
• If it has, you have added too much plus!
• Lets look at the optics

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1.00DS Vision better than 6/18
1.00DS

• If vision with 1.00DS is better than 6/18, then
  blur circle must be smaller than expected
• Explained by focal point behind the retina - you
  have not added enough plus, or you have added too
  much minus
• Action
• Reduce minus or add plus
• By 0.25DS per line better than 6/18

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1.00DS Vision worse than 6/18
1.00DS

• If V/A with 1.00DS is worse than 6/18, then blur
  circle must be bigger
• Explained by focal point in front of retina, so
  you have added too much plus or not enough minus
• Action
• Add minus or reduce plus
• By 0.25DS per line worse than 6/18

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Be aware!

• British standard Snellen chart is missing the
  6/7.5 and 6/15 lines of the LogMAR chart, so 4
  lines of blur can sometimes appear to be only 3
• Pupil size is important
• Reduced pupil size can also reduce the size of
  the blur circle
• For example, an emmetrope may only be blurred by
  2 lines despite 1.00DS of uncorrected refractive
  error
• If you added plus to blur the extra two lines,
  you will overplus!
• Be wary is elderly patients with small pupils
• Large pupil has opposite effect will blur back
  too quickly

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Be aware!

• Not everyone starts from 6/6
• With greater amounts of blur, 0.25DS per line
  relationship breaks down and becomes less
  accurate
• Vision will generally underestimate spherical
  error
• Results may be unusual if ocular pathology
• If change in power is significant (e.g gt0.50DS),
  perform 1.00DS again to double check results and
  confirm with alternatives
• When in doubt, use another test to confirm

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Duochrome Test
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Duochrome test

• Uses longitudinal chromatic aberration to
  determine the refractive error
• Whichever colour is focussed nearest to the
  retina will be seen as clearest
• Emmetrope equal
• Myope red clearer
• Hypermetrope green clearer

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Optical principles of duochrome test
Prismatic effect of lens leads to dispersion
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Optical principles of duochrome test
0.50DS
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Performing the duochrome test

• Switch on duochrome test and establish that the
  patient can see the ring targets
• Ask Are the circles sharpest and clearest on the
  red or on the green background?
• Alter power by 0.25DS according to patients
  response
• Minus if red clearest, plus if green clearest
• Repeat until no difference seen
• Be wary of accommodation and red-preference

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Limitations of duochrome

• The ring targets are usually constructed of ring
  thicknesses equivalent to 6/9 (inner) and 6/12
  (outer) Snellen equivalent targets
• Will not work if vision is less than 6/12
• The difference in focal position due to chromatic
  aberration is 0.50DS
• Will not work if prescription is significantly
  incorrect
• Small pupil will reduce size of blur circles
• Difference between the clarity of red and green
  is reduced
• Reduce room lighting for older patients
• Always be aware of the alternative tests!

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They look the same

• If patient answers Same or No difference at
  first presentation
• Duochrome is balanced
• Confirm by using 0.25DS (red now clearest)
• Rx too far out
• Use other tests, when vision 6/12 or better,
  return to duochrome
• Small pupils
• If no change in response with lens change, move
  to another test (though the effectively of all is
  reduced)
• Vision too poor due to pathology
• Abandon duochrome try plus/minus test with
  large steps instead

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Limitation?

• What if your patient is one of the 8 of the
  population that have a red/green colour vision
  deficiency?
• It still works! Why?
• Refer to top/bottom of the chart instead of
  red/green

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My ret was a disaster

• Now what?

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What if my ret result is a disaster?

• If vision is poor after retinoscopy, or you dont
  have a retinoscopy result
• Dont panic - think about what you already know
• What did your patient tell you?
• Vision distance vs. near blur, pinhole
• (Current correction)
• Check sphere power
• You will need to find the best vision sphere
  (BVS)

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Best vision sphere

• Best vision sphere is literally the lens that
  gives the best vision with a sphere only!
• Use the plus/minus test described above, but
  using larger steps (0.50DS or greater)
• Provides
• Crystal clear vision for simple myopes and
  hypermetropes
• The best possible vision for an astigmat because
  the circle of least confusion will be on the
  retina the remaining blur is due to the cyl
  alone
• Record your result and vision, then check for
  astigmatism (as described next week)

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For a myope
My vision is blurred
That looks great!
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Simple myopic astigmatism
Circle of Least Confusion Focal lines are equally
blurred
Its very blurred
Interval of Sturm
Blur is due to combination of CLC in front of
the retina Focal lines being separated
Distance between the focal lines
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With BVS
Circle of Least Confusion Has moved, is now on
the retina
Interval of Sturm
Length unchanged Reason the vision is still
blurred
Thats better but it still isnt clear
All blur is now due to uncorrected cyl We will
learn how fix that next week
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BVS
BVS Sphere ½cyl
The spherical equivalent is calculated the same
way
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Routine
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Suggested routine

• This will depend on your patient, but a suitable
  routine could be
• Retinoscopy
• 1.00DS blur test
• Plus/minus test
• Duochrome (to confirm that you have found the
  correct sphere power)

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By the end

• You have given the patient the best acuity that
  you can
• You have given the patient the most positive lens
  that gives them this vision
• You have checked that the addition of 0.25DS
  makes vision worse
• You have checked that the addition of -0.25DS
  does not make vision better
• You have written your result down and recorded
  the vision for each eye

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Recording results

• There is a box for RE and LE
• You only need to record the final result
• BUT you may find it helpful to write down your
  results and vision from the individual tests
  while you are learning

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Things that can go wrong
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Common errors

• Forgetting that these tests are monocular
• Not monitoring vision as you go
• Which can result in adding too much minus or plus
• Using poor patient instructions
• Assuming that 6/6 is the endpoint
• Not listening to the patient and/or listening too
  much to the patient
• Not remembering that this is difficult for your
  patient

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Further reading

• Read Elliott, Section 4.9-4.12
• Review Elliott Online