For Binocular px’s with 6/12 or better – WHY?! Because if the “fog” or “blur” gets too heavy the fogged eye will not serve as a binocular cue/fusion lock/accommodation lock, and the whole technique breaks down. Binocular vision is not maintained.
- It’s a technique to ensure the accommodation of the eyes is equal & zero – we aim to equalise the accommodation between both eyes at zero.
- Both eyes will always accommodate by the same amount binocularly (when working together), even if they don’t under monocular conditions.
- If the right & left eyes are not “balanced” (have the same accommodation), the eyes will accommodate by the least amount possible to produce a clear retinal image in one eye – the other eye may have a slightly blurred image & eye strain may result.
- g. – Px aged 20, Rx is actually:
- RE: +1.00DS 6/5 LE: +1.00DS 6/5
But refractionist gives:
RE: +1.00 DS LE: +0.50DS
- Monocular VA would be:
RE: 6/6 no accommodation in either eye
LE: 6/5 0.50D accommodation in both eyes
- Binocular VA would be:
no accommodation in either eye
RE: 6/5 LE: 6/9
- This may tend to alternate between eyes & vary the accommodation or Px tries to use one eye for near – results in poor quality binocular vision & discomfort.
- Binocular balancing is necessary following right & left monocular refractions. (after F&B /cross cyl).
- Assume equal/similar VA
- Several techniques include:
- Turville Infinity Balance
- Humphriss Immediate Contrast
- Polarised Duochrome
- Prism Dissociation
- Turville Infinity Balance (TIB):
- F & L targets separated by a septum (see only one letter with each eye)
- Try to achieve equality by simultaneous comparison
- RE can see L but not F, LE can see F but not L (these letters are directly in front of each eye)
- Set up by covering LE (with cover) & moving septum until F is covered, then occlude RE & check that now L is covered. (Then remove cover from both eyes).
- Both eyes see rest of chart & room
- If correction is not balanced F & L cannot be clear at the same time, even if vision is 6/5 R&L monocularly.
- Ask Px if F & L are equally clear – if equal that’s the end of the procedure.
- If there is a difference,
- Add +0.25DS to worse eye, continue until VA starts to reduce (still looking at L & F)
- If VA reduces with first +0.25DS add +0.25DS to better eye until VA starts to reduce
- If VA reduces with first +0.25DS on better eye, add -0.25DS to worse continue until VA stops improving.
- g. Px’s true correction: RE: +2.00DS LE: +2.00DS
With present Rx in place, L (RE) is clearer than F (LE). There are three ways you can get it wrong to give this result:
- L>F because left eye is under-plussed
e.g. you have RE: +2.00DS LE: +1.50
- each eye is 6/5 monocularly but right eye controls accommodation. Left VA binocularly is ~6/9 (0.50DS blur). (so if you’re missing 0.50DS for accom. off left eye, VA reduces by about 2 lines because 1/0.50 = 2?)
- So you add +0.25DS to left eye, left VA increases to ~6/6 so F becomes more similar to L. Add a further +0.25DS, left VA increases to ~6/5, F & L become equally clear
- (N.B. this is the only occasion where adding small amounts of plus will improve a young person’s VA, normally in young Px plus makes VA stay same or get worse).
- If adding first +0.25DS makes F even worse, then cause is not as described above.
- L>F because RE is under plussed but controlling accommodation for both eyes
e.g. you have RE: +1.50DS LE: +2.00DS
- Binocularly, because RE is controlling accommodation, both eyes accommodate by +0.50DS making RE 6/5. The “correct” LE also accommodates by 0.50DS reducing its acuity to ~6/9 (what makes L>F)
- Add +0.25DS to better eye (RE) & ask what effect it has on right VA, then left VA. If 2) is correct reason right VA will not be affected, left VA will improve. (RE (6/5) relaxes accommodation by 0.25DS (still 6/5), also relaxes LE’s accommodation by 0.25DS, left VA increases from ~6/9 to ~6/6 & F becomes more similar to L)
- Add further +0.25DS to RE for as long as right VA does not reduce & left VA continues to improve. (RE relaxes accommodation by another 0.25 (still 6/5), also relaxes LE’s accommodation by 0.25DS. Left VA increases from ~6/6 to ~6/5 & F becomes equal to L
- (Adding further +0.25DS would blur RE from 6/5 to 6/6. Left eye would remain 6/5 – F becomes worse than L)
- If adding plus to better eye decreases better eye’s VA then 2) cannot be correct.
- L>F because LE is under-minused
e.g. you have RE: +2.00DS LE: +2.50DS
- Add -0.25DS to worse eye (LE). Left VA increases from ~6/9 to ~6/6
- Add further -0.25DS to LE. Left VA increases to 6/5 & F = L
- Add further -0.25DS to LE. Either LE blurs or LE starts to accommodate & makes RE go blurred & F ≠ L
- 3) should not happen – LE was never at max VA
- If none of the above work, check the cylinder &/or use the pinhole
- B. the idea is NOT to make F = L by blurring the better eye to the level of the poorer eye
- We monitor the level of accommodation by noting the way VA of each eye changes with lenses. VA is fixed, determined by health of the eye.
- If VA.s are slightly different it is still essential to balance the accommodation – we can still do the TIB.
Rather than seeing F = L as objective, we ensure that each eye is achieving its optimum VA with minimum accommodation (i.e. so that after adding plus to either eye makes the VA worse)
- Patients NOT suitable for TIB:
- Strabismic patients (squints)
- Large difference in VA between the eyes
- Very unstable phoria (poorly compensated heterophoria) making the F & L move around excessively