subjective refraction
DESCRIPTION
Subjective refraction. OP1201 – Basic Clinical Techniques Spherical refractive error Dr Kirsten Hamilton-Maxwell. Today’s goals. By the end of today’s lecture, you should be able to Explain the different methods used to determine the spherical component of your patient’s refractive error - PowerPoint PPT PresentationTRANSCRIPT
OP1201 – Basic Clinical Techniques
Spherical refractive errorDr Kirsten Hamilton-Maxwell
Today’s goalsBy the end of today’s lecture, you should be able to
Explain the different methods used to determine the spherical component of your patient’s 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 toRefine the spherical component of your retinoscopy resultDetermine the BVS of your patients using a variety of techniques
from scratchComplete the task within 10min for both eyes
What is subjective refraction?
Types of spherical refraction
General set up
Subjective refractionAim is correct your patient’s refractive error by
asking them choose what they likeGuided 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 retinoscopyWe will start spherical refraction today
First, let’s see how it all fits together
BE
Today’stopic
Spherical refractionThere are a number of different procedures
Plus/minus test+1.00DS testDuochrome test
All are monocularRule of thumb – “give the most positive lens that
provides maximum vision”Minimise accommodation = increase comfortThere is a twist to this rule when preparing for cross-
cyl
Set upIs the same for all of these proceduresAssume 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 onOcclude the eye that is not being tested
Effect of plus and minusPositive lens
Negative lens
EmmetropiaBoth lenses equally clear(Blur circles same size)
Plus and minus in myopiaPositive lens
Negative lens
Uncorrected myopiaNegative lens is clearer(Blur circles different sizes)
Plus and minus in hypermetropia
Positive lens
Negative lens
Uncorrected hypermetropiaPositive lens is clearer(Blur circles different sizes)
What does the patient see?
Too much minus Too much plus
ClearBlurred Blurred
-0.50DS 0 +0.50DS
-0.25DS Same/smaller and darker-0.50DS Smaller and darker/blurredVision influenced by accommodation
+0.25DS Not as sharp+0.50DS Fainter/blurred
You are expecting a vision change of one line per 0.25DS!
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)
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
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!
That last point again!Check that changing lens power changes the
acuity as wellExpect 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!
How to ask the question!What you say is important as you can lead the
patient towards a particular answer without realising itYou’ll 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?
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
“They look the same”If patient answers “Same” or “No difference” at
first presentationYou 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 presentIncrease lens power and repeat
Small pupilsIncrease power and repeat
Be wary of inducing accommodation!
+1.00DS blur test
Over-plussing should induce a blur circle on the retina, hence reducing visionShould blur by 1 Snellen line per 0.25DS
If distant light currently focussed on retina, +1.00DS should cause 4 lines of blurShould blur back from 6/6 to 6/18
+1.00DS
Performing the +1.00DS blur testInsert +1.00DS and ask patient to read down the
chartIf not 6/18, adjust spherical power
Remember 1 line per 0.25DSRepeat until vision is 6/18
Remove +1.00DS and check that vision is no worse If it has, you have added too much plus!
Let’s look at the optics
+1.00DS – Vision better than 6/18
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
+1.00DS
+1.00DS – Vision worse than 6/18
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
+1.00DS
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 importantReduced 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
Be aware!Not everyone starts from 6/6With greater amounts of blur, 0.25DS per line
relationship breaks down and becomes less accurateVision will generally underestimate spherical error
Results may be unusual if ocular pathologyIf change in power is significant (e.g >0.50DS),
perform +1.00DS again to double check results and confirm with alternatives
When in doubt, use another test to confirm
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
Optical principles of duochrome test
Chromatic aberration
Prismatic effect of lensleads to dispersion
Optical principles of duochrome test
0.50DS
Performing the duochrome testSwitch on duochrome test and establish that the
patient can see the ring targetsAsk “Are the circles sharpest and clearest on the
red or on the green background?”Alter power by 0.25DS according to patient’s
responseMinus if red clearest, plus if green clearest
Repeat until no difference seenBe wary of accommodation and red-preference
Limitations of duochromeThe ring targets are usually constructed of ring
thicknesses equivalent to 6/9 (inner) and 6/12 (outer) Snellen equivalent targetsWill not work if vision is less than 6/12
The difference in focal position due to chromatic aberration is 0.50DSWill not work if prescription is significantly incorrect
Small pupil will reduce size of blur circlesDifference between the clarity of red and green is reducedReduce room lighting for older patients
Always be aware of the alternative tests!
“They look the same”If patient answers “Same” or “No difference” at first
presentationDuochrome is balancedConfirm by using +0.25DS (red now clearest)
Rx too far outUse 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
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
Now what?
What if my ret result is a disaster?If vision is poor after retinoscopy, or you don’t have
a retinoscopy result… Don’t panic - think about what you already know
What did your patient tell you?Vision – distance vs. near blur, pinhole(Current correction)
Check sphere powerYou will need to find the best vision sphere (BVS)
Best vision sphereBest 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 hypermetropesThe 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)
For a myope “My vision is blurred”
“That looks great!”
Simple myopic astigmatism
Interval of SturmDistance between the focal lines
Circle of Least ConfusionFocal lines are equally blurred
Blur is due to combination of…CLC in front of the retinaFocal lines being separated
“It’s very blurred”
With BVSCircle of Least Confusion
Has moved, is now on the retina
Interval of SturmLength unchanged
Reason the vision is still blurred“That’s better but it still isn’t clear”
All blur is now due to uncorrected cyl: We will learn how fix that next week
BVS
BVS = Sphere + ½cyl
The spherical equivalent is calculated the same way
Suggested routineThis will depend on your patient, but a suitable
routine could be…1. Retinoscopy
2. +1.00DS blur test
3. Plus/minus test
4. Duochrome (to confirm that you have found the correct sphere power)
By the end…You have given the patient the best acuity that
you canYou have given the patient the most positive
lens that gives them this visionYou have checked that the addition of +0.25DS makes
vision worseYou have checked that the addition of -0.25DS does
not make vision betterYou have written your result down and recorded
the vision for each eye
Recording results
There is a box for RE and LEYou only “need” to record the final resultBUT you may find it helpful to write down your
results and vision from the individual tests while you are learning
Common errorsForgetting that these tests are monocularNot monitoring vision as you go
Which can result in adding too much minus or plusUsing poor patient instructionsAssuming that 6/6 is the endpointNot listening to the patient and/or listening too
much to the patientNot remembering that this is difficult for your
patient
Read Elliott, Section 4.9-4.12
Review Elliott Online