Subjective refraction

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Subjective refraction

• OP1201 Basic Clinical Techniques
• Binocular balance and final prescription
• Dr Kirsten Hamilton-Maxwell

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The refraction routine so far

• Our refraction routine has included
• Initial sphere power
• Cylinder axis and power
• In todays lecture, we will look at
• Final sphere power
• Making the vision equal in both eyes (binocular
  balance)
• The final binocular distance prescription

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Subjective refraction
Todays topic
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Final (monocular) sphere
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Final sphere

• Imagine that you have just completed x-cyl for
  the RE
• Prior to performing x-cyl, we intentionally left
  the patient with green-clearest on duochrome, or
  equal
• They are probably still accommodating!
• Final sphere aims to relax accommodation, so need
  to check the sphere power again after x-cyl
• Monocular test

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Procedure

• For all patients, the procedure is the same
• Go back to the plus/minus test and push the
  plus
• Direct subject to smallest Snellen line
  achievable and ask Is it clearer with or
  without? while presenting/removing 0.25DS
• Make sure lens is clean!

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Procedure

• Interpretation
• If clearer or no difference with 0.25DS,
  incorporate and repeat
• If/when clearer without do not incorporate this
  final blurring 0.25DS because it means you have
  reached the end
• Record final lens power and VA
• Sphere(DS)/Cyl(DC)xAxis and VA

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Precautions

• Remember that a change in VA must correspond to
  the change in lens power, relative to the current
  correction
• 0.25DS per line of vision
• 0.50DC per line of vision
• Be wary of
• gt0.50D change and any minus lens change
• Duochrome for final sphere as it tends to
  over-minus
• Can still use as a confirmation lens

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Why is too much minus a problem?

• Your patient will be required to accommodate to
  see clearly, even in the distance
• Headaches, tired eyes, discomfort
• Potential to induce myopia (?)
• We have dealt with how to avoid this already

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Why is more plus a problem?

• The cyl findings are probably incorrect
• Circle of least confusion needs to be on, or
  slightly behind, the retina
• So if more minus needed in the final stages, your
  patient was over-plussed on cross-cyl
• Your patient may be a latent hypermetrope
• Consider a cycloplegic refraction
• From your patients point of view, blur!
• A major cause of needing to remake spectacles

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Avoiding over-plussing

• Generally applies to elderly patients, but
  possible in all patients
• Small pupils
• 0.25DS sphere will make minimal difference to
  blur circle consider 0.50DS pendulum
• 1.00DS test will not blur back as far as 6/18,
  so encourages you to add more plus
• Media opacification or other pathology causing
  poor VA
• Creates problems detecting 0.25DS change

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Binocular balancing
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Binocular balancing

• We have only considered one eye at a time BUT
  most of your patients will use both of their eyes
• Clear and comfortable vision is the ultimate
  goal!
• So that both eyes can work together, binocular
  balancing is a technique used to equalise
• Vision
• Accommodative demand
• Occlusion can stimulate accommodation
• Refracting under monocular conditions may not get
  out all the plus!
• So binocular balance also serves to check sphere
  under binocular conditions

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Procedure

• Always done after the monocular refraction for
  each eye has been completed
• i.e. initial sphere, x-cyl., then final sphere
• Many different techniques are available, but
  fogging techniques are easiest in practice

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Humphriss fogging method

• One eye fogged (blurred), other eye clear
• 0.75DS blur will reduce VA in fogged eye to
  about 6/12
• Shifts attention to the unfogged eye
• Allows assessment of the spherical refractive
  error in the unfogged eye
• While maintaining peripheral fusion (ie.
  binocularity) which helps control accommodation

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Procedure Right eye

• Check RE first!
• After the monocular refraction, blur left eye by
  0.75DS
• RE is still occluded because you have just
  finished monocular refraction of the LE
• VA should drop to about 6/12 because looking
  through the fogging lens
• Then remove occluder from RE
• VA should improve, indicating that the RE is
  being used
• If it does not, stop here!
• Push the plus in right eye as described earlier

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Procedure Left eye

• Now check the LE!
• Add 0.75 DS in front of right eye and ensure VA
  is worse (is there sufficient fog?)
• Remove 0.75DS from left eye and ensure VA
  improves (check attention has shifted)
• Push the plus in left eye and adjust accordingly
• Essentially, you are repeating what you did
  earlier to determine the monocular final sphere,
  but you are pushing the plus with the other eye
  fogged rather than occluded

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Recording results

• Record the lens power added to the monocular
  subjective findings
• Eg. Binocular balance RE 0.25DS and LE 0.50DS
• Include binocular acuity

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Unequal binocular balance

• Be wary of unequal findings
• This almost never happens if monocular refraction
  went well at most, there is 0.25DS difference
• So use this as a double check of your monocular
  findings!

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Limitations of Humphriss technique

• Will not work if unfogged eye VA is worse than
  6/12
• Cannot shift attention to the unfogged eye.
  Abandon.
• May not work if there is unequal acuity
  (particularly if VA in the unfogged eye
  approaches 6/12)
• Increase fogging power or abandon?
• Will not work if fogged eye VA is worse than
  6/12
• Lose binocularity and simulates monocular
  refraction. Reduce fogging power.
• May not work if one eye is heavily dominant
• Must check that VA worsens/improves as stated
  above
• If this does not occur, then abandon

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Other methods

• You will look at this in more detail in your
  semester 2 coursework, plus year 2
• The Humphriss method is preferred because it
  forms part of the binocular refraction technique
  that you will learn next year
• Occlusion methods
• Turville infinity balance
• Polarisation
• Dissociation methods (both eyes open but not
  truly binocular)
• Comparison of fogged images
• Comparison of duochrome
• Successive comparison

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What not to do

• When not to use binocular balancing
• A patient with strabismus
• Amblyopia or other cause for significant visual
  reduction
• Uneven acuities of more than one Snellen line
• Only makes sense if patient is using both eyes
  (has binocularity)!
• When to be wary of binocular balancing
• Patients with compromised binocularity e.g.
  evidence of a poorly compensated phoria this
  will make sense next semester
• Anisometropia (uneven prescriptions), especially
  on fogging technique
• Perform on patients with no accommodation (???)
• For you, still do it as it is a double check of
  your monocular findings
• Doesnt work well in patients with small pupils
  due to the increased depth of focus

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Final prescription
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Final steps of refraction

• If all has gone to plan, the vision is now equal
  in both eyes and excess accommodation has been
  neutralised
• The final step of refraction is to push the plus
  binocularly
• ie. 0.25DS over each eye simultaneously
• This is the final double check for
  over-minussing!
• In my experience, the final Rx will usually be
  0.25DS more in each eye than the monocular
  subjective findings

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Final steps of refraction

• Also need to check for too much plus!
• So far, we have tried to avoid minus spheres
  after x-cyl
• This is because we are trying to push the
  plus/relax accommodation, but can result in
  over-plussing
• To check, offer binocular -0.25DSs
• If patient says letters are definitely clearer
  (i.e. a demonstrable improvement in VA) and NOT
  smaller and darker, then incorporate
• Often worth double checking this
• Patients will often prefer a slightly
  over-minussed refraction in the consulting room,
  so check for clarity

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Recording results
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The extra factor Vertex distance

• This is the distance between the cornea and the
  back of the spectacle lens
• It needs to be recorded for all prescriptions
  that are more than 4.00DS
• The effective power of a lens changes with
  distance from the eye
• Estimate by using the scale on the side of your
  trial frame
• You will be shown other methods in Dispensing
• There is no box for this so you will need to
  remember to measure and record it, when
  appropriate

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In summary

• Our refraction routine now consists of
• Retinoscopy
• Refinement of sphere prior to x-cyl
• Jackson x-cyl
• Refinement of monocular sphere, record monocular
  VA
• Binocular balance
• Record final distance refraction, record
  binocular VA
• Youve now got an entire refraction routine!

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Further reading

• Elliott, Section 4.16