Subjective refraction

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 • 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

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

Subjective refraction • Aim is correct your patient’s 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, let’s see how it all fits together

BE Today’s topic Subjective refraction

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

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

Plus/Minus Test

Effect of plus and minus Positive lens Emmetropia Both lenses equally clear (Blur circles same size) Negative lens

Plus and minus in myopia Positive lens Uncorrected myopia Negative lens is clearer (Blur circles different sizes) Negative lens

Plus and minus in hypermetropia Positive lens Uncorrected hypermetropia Positive lens is clearer (Blur circles different sizes) Negative lens

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!

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 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!

How to ask the question! • What you say is important as you can lead the patient towards a particular answer without realising it • You’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 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!

+1.00DS Blur Test

+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

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! • Let’s look at the optics

+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

+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

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

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 >0.50DS), perform +1.00DS again to double check results and confirm with alternatives • When in doubt, use another test to confirm

Duochrome Test

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

Chromatic aberration Optical principles of duochrome test Prismatic effect of lens leads to dispersion

Optical principles of duochrome test 0.50DS

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 patient’s response • Minus if red clearest, plus if green clearest • Repeat until no difference seen • Be wary of accommodation and red-preference

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!

“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

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

My ret was a disaster 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 power • You will need to find the best vision sphere (BVS)

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)

For a myope “My vision is blurred” “That looks great!”

Simple myopic astigmatism Circle of Least Confusion Focal lines are equally blurred “It’s very blurred” Blur is due to combination of… CLC in front of the retina Focal lines being separated Interval of Sturm Distance between the focal lines

With BVS Circle of Least Confusion Has moved, is now on the retina Interval of Sturm Length 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

Routine

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)

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

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

Things that can go wrong

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

Further reading Read Elliott, Section 4.9-4.12 Review Elliott Online

Subjective refraction