1. Subjective refraction
OP1201 – Basic Clinical Techniques
Spherical refractive error
Dr Kirsten Hamilton-Maxwell

2. 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

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

4. 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

5. Subjective refraction
Check sphere
Check cyl
(Axis and power)
Recheck sphere LE
Today’s
topic
Subjective refraction BE
Binocular balance
Final prescription (Rx)

6. 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

7. 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

8. Plus/Minus Test

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

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

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

12. What does the patient see?
Clear
Blurred
Blurred
Too much minus
Too much plus
-0.50DS
+0.50DS
-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!

13. 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 DS or +1.00DS)

14. 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

15. 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!

16. 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!

17. 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?

18. 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

19. “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!

20. +1.00DS Blur Test

21. +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

22. 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

23. +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

24. +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

25. 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

26. 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

27. Duochrome Test

28. 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
Relies upon longitudinal chromatic aberration
Whichever focal point is nearest to the retina will be seen as clearest
Emmetrope = equal
Myope = red clearer
Hypermetrope = green clearer

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

30. Optical principles of duochrome test
0.50DS

31. 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

32. 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!

33. “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

34. 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

35. My ret was a disaster
Now what?

36. 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)

37. 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)

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

39. 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

40. With BVS Circle of Least Confusion Interval of Sturm
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

41. BVS
BVS = Sphere + ½cyl
The spherical equivalent is calculated the same way

42. Routine

43. 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)

44. 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

45. 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

46. Things that can go wrong

47. 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

48. Further reading
Read Elliott, Section
Review Elliott Online