Subjective refraction OP1201 – Basic Clinical Techniques Binocular balance and final prescription...
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Transcript of Subjective refraction OP1201 – Basic Clinical Techniques Binocular balance and final prescription...
OP1201 – Basic Clinical Techniques
Binocular balance and final prescriptionDr Kirsten Hamilton-Maxwell
The refraction routine so far…Our refraction routine has included
Initial sphere powerCylinder axis and power
In today’s lecture, we will look atFinal sphere powerMaking the vision equal in both eyes (binocular
balance)The final binocular distance prescription
BE
Today’stopic
Final sphereImagine that you have just completed x-cyl for
the REPrior to performing x-cyl, we intentionally left the
patient with green-clearest on duochrome, or equalThey are probably still accommodating!
Final sphere aims to relax accommodation, so need to check the sphere power again after x-cylMonocular test
ProcedureFor 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!
ProcedureInterpretation
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 VASphere(DS)/Cyl(DC)xAxis and VA
PrecautionsRemember that a change in VA must correspond to
the change in lens power, relative to the current correction0.25DS per line of vision0.50DC per line of vision
Be wary of >+0.50D change and any minus lens changeDuochrome for final sphere as it tends to over-minus
Can still use as a confirmation lens
Why is too much minus a problem?Your patient will be required to accommodate to see
clearly, even in the distanceHeadaches, tired eyes, discomfortPotential to induce myopia (?)
We have dealt with how to avoid this already
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 hypermetropeConsider a cycloplegic refraction
From your patient’s point of view, blur!A major cause of needing to remake spectacles
Avoiding over-plussingGenerally applies to elderly patients, but
possible in all patientsSmall 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 VACreates problems detecting 0.25DS change
Binocular balancingWe have only considered one eye at a time BUT
most of your patients will use both of their eyesClear and comfortable vision is the ultimate goal!
So that both eyes can work together, binocular balancing is a technique used to equaliseVisionAccommodative demand
Occlusion can stimulate accommodationRefracting under monocular conditions may not get
out all the plus!So binocular balance also serves to check sphere
under binocular conditions
ProcedureAlways done after the monocular refraction for each
eye has been completedi.e. initial sphere, x-cyl., then final sphere
Many different techniques are available, but fogging techniques are easiest in practice
Humphriss fogging methodOne eye fogged (blurred), other eye clear+0.75DS blur will reduce VA in fogged eye to about
6/12Shifts attention to the unfogged eye
Allows assessment of the spherical refractive error in the unfogged eyeWhile maintaining peripheral fusion (ie. binocularity)
which helps control accommodation
Procedure: Right eyeCheck RE first!After the monocular refraction, blur left eye by
+0.75DSRE is still occluded because you have just finished
monocular refraction of the LEVA should drop to about 6/12 because looking through
the fogging lensThen 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
Procedure: Left eyeNow 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 accordinglyEssentially, 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
Recording resultsRecord the lens power added to the monocular
subjective findingsEg. Binocular balance RE +0.25DS and LE +0.50DS
Include binocular acuity
Unequal binocular balanceBe 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!
Limitations of Humphriss techniqueWill 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 dominantMust check that VA worsens/improves as stated above If this does not occur, then abandon
Other methodsYou will look at this in more detail in your semester 2
coursework, plus year 2The Humphriss method is preferred because it forms part
of the binocular refraction technique that you will learn next year
Occlusion methodsTurville infinity balancePolarisation
Dissociation methods (both eyes open but not truly binocular)Comparison of fogged imagesComparison of duochrome
Successive comparison
What not to do…When not to use binocular balancing
A patient with strabismusAmblyopia or other cause for significant visual reductionUneven acuities of more than one Snellen lineOnly makes sense if patient is using both eyes (has binocularity)!
When to be wary of binocular balancingPatients with compromised binocularity e.g. evidence of a poorly
compensated phoria – this will make sense next semesterAnisometropia (uneven prescriptions), especially on fogging
techniquePerform on patients with no accommodation (???)
For you, still do it as it is a double check of your monocular findings
Doesn’t work well in patients with small pupils due to the increased depth of focus
Final steps of refractionIf 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 binocularlyie. +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
Final steps of refractionAlso need to check for too much plus!So far, we have tried to avoid minus spheres after x-
cylThis is because we are trying to push the plus/relax
accommodation, but can result in over-plussingTo check, offer binocular -0.25DS’sIf patient says letters are definitely clearer (i.e. a
demonstrable improvement in VA) and NOT smaller and darker, then incorporateOften worth double checking thisPatients will often “prefer” a slightly over-minussed
refraction in the consulting room, so check for clarity
Recording results
The extra factor: Vertex distanceThis is the distance between the cornea and the
back of the spectacle lensIt needs to be recorded for all prescriptions that are
more than ±4.00DSThe effective power of a lens changes with distance
from the eyeEstimate by using the scale on the side of your trial
frameYou 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
In summaryOur refraction routine now consists of
RetinoscopyRefinement of sphere prior to x-cylJackson x-cylRefinement of monocular sphere, record monocular
VABinocular balanceRecord final distance refraction, record binocular VA
You’ve now got an entire refraction routine!
Elliott, Section 4.16