7: Subjective refraction: refining the sphere

scenario 1: ret indicates there is no astigmatism ( no cylinder was needed to get reversal , there may or may not be a sphere)

scenario 2: ret indicates there is astigmatism ( may or ay not be a sphere from ret)

during subjective refraction, we check if there is any astig (scenario 1)

or refine the correction for astigmatism that we have in place from ret ( scenario 2)

this involves establishing the power of the best mean sphere (1)

or establishing the power of thr best vision sphere (2)

best mean sphere - only a sphere in the trial frame

no cylinder

its the sphere of max plus power, or minus power which gives the best VA without any cylinder

if the eye has no astigmatism, the BMS should give good acuity eg if the eye had -2.00D of myopia, a -2.00D lens should give acuity of VAR 105 snellen 6/4.8 or better 

but if eye has astigmatism the BMS will be reduced as astig not corrected 

if eye had 1.00D of astigmatism, VA with a sphere alone would be around VAR 95 

-2.00/-1.00 × 90 what is BMS 

BMS expected to be around -2.50 DS and VA with tehe BMS will be approx 95

the -2.50 is the mean sphere : phere + ½ cylinder

best vision sphere BVS 

it is the sphere of max plus power or minimum plus power which gives the best VA when a cylinder is in the trial frame 

eg: actual refractive error of the eye is -2.00 / -1.00 × 90 

but ret result is -1.00/-1.00 × 90 

VA will be reduced with the ret result , if we add more minus it will imporve as the sphere power (-1.00) is too low so if we go from -1.00 to -2.00 VA improves better 

VA with BVS will be better if the ret cyl power and axis is correct or nearly correct 

VA with BVS will be reduced if the ret cyl power is incorrect  by a lot, so will need to be refined 

what is BVS

the max plus/minimum minus sphere when a cylinder is in place

what is the first step in subjective refraction

refining the sphere

subj refraction starts with the introduction of sphere power or the refinement of sphere power if already a sphere in place

want to know from the patients perspective how any change in the lens they are looking through affects their vision

need to be asking questions to the patients. bad questions lead to poorer information 

patient responses

patient says its better: do you believe tgem, how do we verify this

if px says its better and we verify that is is because the VA is better, then leave in

if saying its worse: and we verify that is is worse because it reeuces VA dont leave it in

when youre tying to perform subjective refraction…

accom is now what we want

need to relac the patients accomodation

do this by ptting letters 6m away

dont delay swapping lenses

how does active accom affect the pateints responses during subjective refraction

can overminus:

patient uses accom to clear a plus blurthis reports clarity withhextra minus power 

result is less accurate, more myopic prescription 

uncorrected myopes

overpowered

if px is myopic, uncorrected and viewing in the distance.

what will they notice if u add a + lens or a - lens

does it matter if pateint yound or old ( hass accom)

myopic: with plus and minus

worse with a plus lense as light images further from the retina

better with minus as light imaged closer to the retina

keep increasing the minus lens power until no further improvement

which lens is better for the patient 

1: too little minus sphere 

2: right amount of sphere 

3: too much minus power as focus is behind the retina 

4: patient accomodates with the too much minus power so it still is perfect 

myopes

so overminusing in myopes is a porblem but only in pre presbyopic patients

this is because you still have active accom 

so when too much minus power is added their accom kicks in to overcome it 

need the minimum power that gives best VA 

if theres no accom, VA is reduced so over minusing is not an issue 

uncorrected hyperopes

hyperopic eyes are underpowered

so an uncorrected +2.00D hyperopic eye needs to use +2.00 D of its accomoation to see clearly in the distance

but if the eye doesnt have 2.00D of accom it wont see clearly in the distance

we use plus lenses to correct hyperopia

id eye has to accomoate to see clearly in the distance, it might be difficult to get sccom to relax when we examine the px

younf hyperopic patients difficult to refract

hyperopes who have no accom

if use thw wrong sphere power the px VA will get worse

gets better when we offer the correct sign of sphere power

hyperopes with a lot of accomodation 

when eye accomodates distance vision does not reduce 

continue to accommodate when + lenses is in place 

eye accomodates even more with munis lenses in place so px likely to same it is the same if accomodates even more 

hides their true hyperopia as they will say its clear when youve added more minus/plus 

how might we under plus a hyperopic patient 

usually with younf hyperopes as they accomodate 

patients appear to see well with less plus as theyre using 

determining the appropriate sphere power BVS or BMS suring subjective refraction

requires that we are careful to avoid over minusing myopic patients and under plussing

need to ensure giving max plus or minimum minus power as possible 

using plus/minus technique for determining BVS and BMS 

the plus/minus technique for determining the sphere power BVS/BMS

1. dtermine the VA before offering any sphere or refining the sphere

2. ask the right question in the right way using the apporportae sphere powers , with the px looking at the appropriately sized letters

3. interpret px response and then make decision and take this action

4. repeat 1-3 : if you added + ve power in step 3, only offer +ve power. if added - ve power in step 3, only offer -ve power

5. arrive at the endpoint for BVS nd BMS determination

6. measure VA with the BVS/BMS and proceed to address any astig: introduce cyl if none exists or refine if already exists

1: know what the vision/VA if before we offer any new sphere power

direct patients attention to distance letter chart and measure their vision/VA using VAR /snellen

occlude fellow eye

push pateint to read as many as possible an stop once 4 incorrect

record the VA and snellen

2: asking the right questions in the right way , using the appropriate sphere powers 

since we know the currect level of vision/VA we can direct patient to look at appropriately sized letters on the chart 

patient should always be directed to the smallest line of letters they can read correctly 

so always have the patient to concentrate on letters that can just be read

initially, going to offer both +0.50DS and -0.50DS 

always offer plus before minus 

2: continue

1. with a +0.50DS lens in, ask is it better with this lens, or without, or is it the same

2. regardless of response , try the -0.50DS and ask, is it better with, without or the same

3. the-0.50DS lens should be offered for a shorter period compared to the +0.50DS lens

4. if the patient responses to +- 0.50 are contradictory or overly hesitant, offer +0.75 first then -0.75

5. if contradictory with +-0.75 then offer +1.00DS then -1.00DS

6. eg. assumes the px gives clear responses to +-0.50S 

3: interpret px responses and make decision and take this action

if when offered a +0.50DS , the px says its better with, offer the full +0.50DS to the trial frame sphere

if when offered +0.50DS and px says the same, only add +0.25 to trial frame sphere

if when offered -0.50DS px says definitely better,minus lens power should only be added if more letters can be read

px must say better/worse and they must read more letters before consudering adding the -ve

if adding minus poer, the amount added must commensurate with the improvement in VA: what does this mean

if the acuity improves by around 1 line , only -0.25DS should be added to the trial frame sphere

if improvement is 2 lines or more then add the full -0.50DS

3: continue 

if you had to use the +-0.75 or +-1.00 same rules apply 

if offering +0.75 and px says better with then add the full +0.75 to trial frame sphere 

if with +0.75 px says its the same then only add +0.50DS 

if px says deffo better wit -0.75, only add the minus power if more letters can be read 

amount of minus power should commensurate with improvement on visiual acuity 

if improves by 1 line add -0.25

improve by 2 then add -0.50

imrpove by 3 full -0.75

step 4: repeat 1-3

if added +ve power after px says better with is, continue to only offer +ve until max reached

if added -ve power because px said better with, only now offer -ve power

eg: if added + sphere power, continue to add positive power until the pateint is certain that is is definitiely clearer without this lens

if added negative sphere in step 3, continue to add minus sphere power until the patient says its better without or same , or better but no improvement in VA

continue adding -ve until no further imporvement in VA

5: endpoint for BMS/BVS determination

• plus

now switch to the weakest power flippers, ie +- 0.25DS

now back to offering both + an - ve

offer +0.25 first , then -0.25 asking the same questions

as prev, add +0.25 to trial framw provided that it doesnt reduce VA

the -0.25 should only be added if the patient can actually read more letters

how do you know youve reached an end point

reached end point for BMS/BVS when a +0.25DS produces blur (px says worse) and a -0.25DS makes no difference in VA or makes VA worse MAX PLUS, MINIMUM MINUS

5: continued

the sphere in the trial frame at the end point is therefore max positive power or minimum minus sphere which gives the best VA

if only sphere is present in the trial frame, this lens is the BMS

if there is quite a lot of astig the VA with the BMS is going to be poor

but if low or no astig them BMS would give high VAR

if the sphere sits alongside a cylinder in trial frame, the sphere at the endpoint is called BVS

if cyl is correct VA shoul dbe good, bad if cyl is wrong

6: measure VA with BMS/BVS and proceed to address any possible astig. ie introdce cyl if none exists or refine one that exists from ret

measure VA with BMS if no cyl exists

measure VA with BVS if cyl present in the trial frame

apply termination rule

eg1: 

patient age: 60 

starting point: no lens in place (sph or cyl)

1. carefully measure unaied vision : VAR 85 snellel 6/12

2. offer a +0.50DS: px responds worse. confirm with VA check and VA fails 

3. offer a -0.50DS: px responds with better; big line imporvement in VA 

4. now add the -0.50 into trial frame and etermine new VA level: VA improves to VAR 95 6/7.5 

5. we have added -ve power and seen VA improvement,so continue with -ve 

6. again, offer -0.50DS: px responds no better, worse 

7. switch to +-0.25DS step size since a further -0.59 did not lead to further VA improvement 

8. offer +0.25DS first: px responds worse, confirm with VA and theyre right 

9. next offer -0.25DS, px responds same 

10. endpoint: so correct sphere power for now is -0.50DS so thats the BMS 

11. since VA with BMS is only VAR 95 we expect that there may be 1DC of astig 

eg2

patient age: 10

starting point (ret result): +2.00 /0.50 × 120

1. carefully measure VA with final ret result: VAR 1-5 snellen 6/4.8

2. offer +0.50DS: px says same and confirms theyre right as VAR stays 105

3. offer -0.50DS , px says same and confirmed

4. add +0.25 to rial frame sphere. lens in trail frame are: 2.25/-0.50 × 120

5. +ve added and VA maintained, so continue with +ve offering +0.50DS and adding +0.25 where px says its the same

6. offer +0.50DS: px says same ( VAR 105 snellen 6/4.8 ), and confirmed so add +0.25 to trial frame lens

7. lens in trial frame now +2.75 /-0.50 × 120

8. offer +0.50S: px says worse, but not worse (VAR 103 6/4.8-2)

9. add +0.25DS to trial frame lens

10. lens trial frame now +3.00/-0.50 × 120

11. offer further +0.50 : px saus worse and confirmed by lower VAR 99 and snellen 6/6-1

12. switch to +-0.25DS step size since further 0.50 led to VA reduction

13. offer +0.25: px says worse

14. offer -0.25: px says better and confirmed: VAR 106 snellen 6/4.8+1

15. lenses in trial frame now : +2.75/-0.50 × 120

16. double check: offer +0.25 px responds worse and VA falls, offer -0.25 and px responds same 

17. reached endpoint: so BVS for now is +2.75DS

still check if cyl is correct