Binocular Balancing

Binocular Balancing

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
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Colonoscopy is an objective technique (most common method)

(so up makes lens stronger)

  • The vergence of the illumination affects the speed of the reflex (rets have slide to vary vergence)
  • Hence light can be convergent, divergent or parallel
  • Change is vergence does not affect value of neutral but does affect direction & speed of reflex
  • Varying vergence can be useful:
    • To speed up a slow reflex when there is a large refractive error by making the beam parallel. Remember the case of The Skip hire Glasgow company.
    • To slow down a fast reflex to find neutral more accurately by making the beam divergent
  • The smaller the sight hole the more closely defined neutral is & the more accurate the retinoscope. But a small sight hole makes it harder to see a dim, out of focus reflex
  • Astigmatism – different speed reflexes in different meridians, can only occur when one meridian is close to neutral
  • Should end up with neutral in all meridians
  • Using minus cyls are more forgiving so use those. If see two with movements initially, correct slowest, thickest & dullest with a plus lens (sphere) causing other meridian to become an against movement, therefore needing a minus cyl to neutralise it
  • If see against movements both meridians initially, correct fastest, thinnest & brightest to still leave the other as an against movement
  • Finally if see a with movement in one meridian & an against in other, neutralise with movement first
  • Before starting make sure there is an against movement in the other eye to make sure they are not accommodating – the fixing eye controls the accommodation of both
  • Retinoscopy – what can it tell you about your Px?
  • Refractive error
  • Media changes – cornea, lens, vitreous
  • Cataract vitreous opacities
  • Retinal detachment
  • Active accommodation
  • Ability to fixate
  • Independent of subjective availability of Px
  • Still requires some Px cooperation
  • Need to control accommodation & fixation
  • Results used:
  • As the starting point for subjective tests
  • As the Rx if subjective refraction is unreliable e.g. infants
  • In some Px’s this might be the only way of assessing refractive error
  • Stroke victims
  • Cerebral palsy
  • Downs syndrome
  • Young children
  • Uncooperative patients
  • Two types of ret are available – a spot ret & a streak ret
  • The ret contains halogen bulb, rheostat to control brightness & oblique mirror with central aperture – these can reduce brightness of reflex
  • Ret technique:
  • A cone of light is shone into the eye
  • Some of the light passes through the pupil & is reflected off the Px’s retina
  • Remainder of light falls on ocular adnexa forming a facial light patch
  • Practitioner observes movement of light reflected from Px’s retina, called reflex
  • When performing ret movement of retinal reflex is compared to movement of facial light patch
  • Three movement options:
    • No movement (parallel beam – pupil is ‘filled’ with light) – emmetropia
    • Against movement (convergent beam) – myopia
    • With movement (divergent beam) – hypermetropia
  • Observer interprets light reflex seen in Px’s pupil
  • Ret places patch of light on Px’s retina – acts as a ‘secondary source’ & light from this source is refracted by Px’s eye
  • Ret method:
  • Define working distance
  • Dim the lights – NOT off
  • Ask Px to look at green on duochrome (distant target) – distant as correcting distance vision so don’t want them to be accommodating
  • If Px has a strabismus, occlude good eye while ret-ing strabismic eye
  • Use RIGHT EYE & RIGHT HAND to test Px’s RIGHT EYE, vice versa
  • Move beam side to side & note reflex movement
  • Ret technique:
  • Endpoint = neutralisation of the eye (neutral = no movement of reflex)
  • Split reflex – moves in both directions (scissor movement), only apparent when neutral so not a problem
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The Morlocks

When the Time Traveller is face to face with the Morlocks he says, “I snatched at my matches and, hastily striking one, I saw three white creatures.”  This is the first time the Time Traveller meets the Morlocks. The fact that the Time Traveller uses matches instead of a gun or another weapon shows us how the fancy, expensive technology we have developed doesn’t matter or help us in the future and that then basic method of survival is all due to mother nature. Fire has always been an important symbol in literature – it is the source of man kinds knowledge. When the Morlocks see the fire, they can’t bare it due to them living in darkness and rarely ever seeing light. Since fire represents knowledge this shows how the upper class had harmed the lower class so much that they feared the light and how they were banished from the light in the first place due to their status. Wells is trying to tell us that the future will not be man-made as humans won’t be able to survive with the technology that we put more importance in, rather than helping other countries that are struggling. The success of technology today will not show in the future.