subjective refraction

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