Binocular refraction techniques, binocular
balancing & binocular best sphere
Sabina PoudelB.OptometryInstitute of Medicine
Presentation layout
• Introduction • Clinical indications for binocular refraction• Techniques of achieving binocularity during refraction• Binocular balancing• Binocular best sphere• Binocular subjective refraction at near
Subjective RefractionTechnique of comparing one lens against another, using changes in vision as the criterion, to arrive at dioptric lens combination resulting in maximum visual acuity (Polasky 1991)
Monocular Subjective Refraction
Binocular Subjective Refraction
Binocular RefractionClinical procedure in which the subjective refraction is performed monocularly under binocular viewing conditions
Component procedures described monocularly are performed in the same or similar manners
Both eyes are open Unoccluded Views a common target
EXCEPT THAT
Advantages over monocular refraction
Accommodation , convergence , light adaptation more constant
Refractive status evaluated in more nearly normal environment i.e. binocular viewing condition
Detection of suppression Measurement of stereopsis Measurement of fixation disparity
Indications for Binocular Refraction1)Refractive Considerations
Hyperopic anisometropia
Antimetropia
Latent hyperopia
Pseudomyopia
2) Visual Acuity Considerations
Anisooxyopia ( unequal acuities between two eyes)
Unilateral amblyopia
Unilateral reduced acuity as a result of ocular disease
3) Ocular Motility Considerations
Significant horizontal, vertical or cyclo associated phorias
Cyclophoria
Latent nystagmus
Under binocular conditions, the amblyopic eyes accept more of the indicated correction and obtains better acuity
VA in RE 6/6 VA in LE 5/60On retinoscopy: RE +1.00 DS LE +4.00 DS VA in LE with +4.00 DS = 6/60 on monocular subjective refraction LE: 6/24 with +2.50 DS on binocular condition LE: 6/18 with +3.50 DS
• In monocular condition: accommodation of a blurred eye unstable• In binocular condition: vision of non amblyopic eye stabilize the accommodation of amblyopic eye
• In binocular condition : Fixation of the amblyopic eye is steadied by the simultaneous fixation of the non amblyopic eye
Degree of eccentricity diminish under binocular condition compared with when amblyopic eye fixates on its own
Principle of Binocular Refraction
By some means each eye views its respective target, yet both eyes remain open. The right eye views only right portion target, left eye only left portion
Outline of the chart, its borders and sometimes a central “fusion lock” provide visual clues to both eyes that enable the binocular fusion necessary for association of target
This monocular viewing under binocular conditions can be achieved by:
Septum
Polarization
Fogging
Septum
• Concept originally described by Turville in 1927• Each eye views its respective target• Characters on the other side of chart are masked from its view• Both eyes see the border of the chart and the septum, thus single percept is associated
Turville Infinity BalanceApparatus consisted of:
reversed acuity chart containing two vertical columns of letters or test characters
Mirror divided into equal sections by an opaque vertical septum having a width of approximately 3 cm
Acuity chart located superiorly on the wall behind the patient
• Divided mirror placed out in front of patient so that reflection of bordered wall chart can be seen by patient
• With eyes in proper position each eye views characters on its side of chart , characters on other side of chart are masked from its view
• Both eyes see the border of the chart and the septum
• Morgan adapted the Turville principle to projection
• Special slides allowed a chart to be presented at a desired distance in front of the patient
• A septum was located along a track between the patient’s eyes and the distance chart
septum
Clinical procedure for septum technique
• patient seated behind the phoropter • 20/40 or 20/50 line characters on screen• septum in center of patient’s viewing mirror • instructed to hold his or her head in an upright position and not to move head from side to side
• After alignment , patient attention directed to right side of chart
• With retinoscopic findings in place appropriate sized characters projected on screen .
• Best sphere determination made until approximately 20/25 or 20/20 letters are clear.
• Cross cylinder introduced parallel to axis of cylinder present
• This way presence , amount and orientation of astigmatism may be measured .
• Next step is to blur or fog the characters to approx. 20/40 level .
achieved by adding 0.75 or 1.00 sphere more plus or less minus to the sphere power .
From this point power is decreased in 0.25 steps until best or maximum visual acuity is attained .
• End point may be monitored by red green balance to avoid overminusing
• Same procedure repeated for left eye
• Then each eye is blurred or fogged by +0.75 or +1.00 D • This amount reduced binocularly until best visual acuity is attained
Polarization• Analyzer and polarized target have same axis of polarization , allowing right eye to view right half of the target , but blocking left half of target from view• Variation of this technique is to polarize only the letters , leaving background normal
Polarized Targets and Polarizing Filters
Vectographic Slides for Projection
Polarized Targets and Polarizing Filters Simplified binocular subjective refraction
Proper positioning of septum not necessary Distance acuity chart divided into two halves, each covered by polarizing films
An analyzer before right eye polarized at angle parallel to right half of chart permitted right eye to view only the right half of chart
An analyzer before left eye parallel to the polarization of left half of chart permitted left eye to view only the left half of chart
Both eyes could see the unpolarized border of entire chart and area surrounding the chart
Cowen introduced polarization of only the letters on an unpolarized illuminated background
Amount of light emitted by polarized letters is reduced to 50% by polarization
Analyzer at right angle to polarization angle of letters
For right eye
Light from the letter in right half totally excluded
But 50% of the light from the white background is transmitted
Black symbols in white background
50 % light from the letters in left half transmitted
50 % light from the background also transmitted
White blank field
Analyzer at right angle to polarization angle of letters
For left eye
Light from the letter in left half totally excluded
But 50% of the light from the white background is transmitted
Black symbols in white background
50 % light from the letters in right half transmitted
50 % light from the background also transmitted
White blank field
Binocular percept is that of a uniform white
background with black letters on it
Both eyes are allowed to view the unpolarized
borders and boxes of the chart in which polarized letters are contained : serve as fusion lock
Vectographic Slides for Projection Its perception is identical to that for polarized symbols
The slide presents 1. VA chart visible only to RE2. VA chart visible only to LE3. Radial chart to determine astigmatism
4. Equalization chart5. Suppression reading chart6. Binocular acuity chart7. Fixation disparity chart8. Stereopsis chart
Vectographic Chart
Stereo Test
Associated Phoria Test (fixation disparity)
Astigmatic Test
VA Test: OD
VA Test: OS
Polarized Fan Dialo split vertically so that left half visible to LE & right half visible to RE
o a common vertical bar and two dots near center of target unpolarized- serve as fusion lock for BE
oFusion lock hold BE in normal binocular position, whether cyclophoria is present or not
o thus permitted eastimation of astigmatism for each eye in normal viewing condition
Fogging
• Slightly blur the central vision in the eye not under test
• This slight blurring suspends foveal vision , such that each eye is refracted under conditions of binocular vision with peripheral fusion
Immediate Contrast TechniqueHumphriss used +0.75 DS over the non tested eye for the performance of astigmatic tests and Bichrome tests on the tested eyeThis unilateral fogging also called a psychological septum
Centrally suppressed the foveal image of the blurred eye but permitted the peripheral images to be fused
Outline & summary of binocular subjective refraction
1) Starting point- Identical to that of a monocular refraction, except that the eyes must be set up for binocular fusion- Lenses determined by objective methods- Habitual spectacle correction- Previous subjective refraction
2) Control of accommodation- As untested eye is unoccluded, accommodation need to be controlled in that eye- Both eyes are initially fogged to at least +0.75 DS- Degree of fog maintained in untested eye- Unfogging done in tested eye till the spherical equivalent is placed at the retina
3) Astigmatic correction- Similar techniques as used for monocular testing except that both eyes remain unoccluded
4) Monocular spherical end points- Separately determined for one eye and then for the other under binocular viewing condition- Similar techniques as used for monocular refraction
Spherical equalization or binocular balancing
Purpose To balance the state of accommodation of two eyes
NOT the visual acuities of two eyes Allows both eyes have the retinal images simultaneously in focus
Difference in accommodative response between two eyes
Inequality in clarity or size of retinal images
Reduce stereoacuityReduce fusional amplitudes Cause discomfortCreate visual inefficiency
Methods for binocular balancing
Alternate Occlusion Prism Dissociated Blur Balance Prism Dissociated Red-Green
(Bichrome) Balance
Alternate OcclusionTarget: smallest acuity letters patient can read through the spherocylindrical correctionAlternately occlude the patient’s eye with cover paddleIf the images equally clear- balance is correct
If not, two options-add plus sphere in +0.25 DS steps before the eye with better image till both the images are equally blurred -add minus sphere in 0.25 DS steps before the eye with poorer image till both images are equally clearFinally to confirm, add +0.25 DS before each eye. If the balance is correct BE will be equally blurred
Disadvantages - The subject compares a visible object with a previous one remembered but no longer visible- Endpoint is equal acuity, and no allowance is made for patients having amblyopia or unequal maximum visual acuities in two eyes- Each eye may assume its monocular accommodative status when alternately unoccluded
Prism dissociated blur balance
RE – 3 ∆ Base Down LE – 3 ∆ Base Up
Two charts will be seen separated vertically
Procedure
• Fog vision in both eyes to 20/40 (6/12), on to spherocylinder lenses found monocularly;- assuming eyes capable of 6/6 acuity
• If one eye has reduced VA;-A row of letters at least two rows larger than that of the power eye of best acuity
• Dissociate the chart by equal amounts of vertical prism 3 or 4 Δ BU in one eye 3 or 4 Δ BD in other eye
• Equal magnitude: ensures that images are affected by equal prismatic distortions
• Upon removal of occlusion patient should be able to see two identical targets.
• If patient cannot see two targets simultaneously, alternately cover two eyes while directing patient to regard target with uncovered eye
• Upon removal of occlusion, patient should be able to see two targets simultaneously
• Ask pt. to compare the legibility of two acuity lines or charts
• If neither is readable; -reduce plus or increase minus until letters are visible but slightly fogged
• If lines appear equally distinct in both eyes, add +0.25 in each eye-Targets appear equally distinct, though slightly blurred; equalization exists
• If acuity line in one eye is clearer than in other;-Add plus power in +0.25D steps in clearer eye until acuity is equal in both eyes-Continue change in power until a reversal
• End point –balance at equality of acuity• Alternative - leave dominant eye with better acuity
• When end point of balance is reached; remove prism and find binocular spherical end point
Prism Dissociated Red-Green (Bichrome) Balance • Bichrome test or Duochrome test- Based on eye’s natural chromatic aberration- Uses distance VA chart with black letters, splitted equally into two halves- Letters on one half on red background- Letters on other half on green background
- Similar to dissociated blur balance except fogging lenses are not added- Instead of seeing two blurred charts, patient will see two relatively clear charts, with each chart half red and half green- Method of choice when maximum acuities for two eyes differ by several letters
• Patient views the full size split R-G chart with 6/6 letters on the bottom
• Occlude both eyes and dissociate the chart using 3 ∆ BD in RE and 3 ∆ BU in LE
• The patient must be capable of perceiving diplopia
Procedure
• Attention first on upper chart (seen by right eye )
• Asked to report which those on red or green side ; appear sharper , blacker or more distinct
• Sufficient plus power is placed in RE till letters on red side are clearer
• Reduce plus power in 0.25 DS steps until both sides are equally clear (equality) OR green side first is clear ( first green)
• Repeat the process for lower chart (left eye)
• The balance is complete when the patient reports the same red-green end point for each eye (i.e if the criterion used for lower chart is appearance of first green, same criterion should be used for upper chart)
• It is not necessary for the upper and lower charts to be equally clear as long as both have the same end point
Special considerations in balancing techniques
1) Dissociative Techniques- Prism value need to be increased for patients with large phorias- Otherwise prism may only compensate and diplopia will not occur
2) Reduced Best Corrected Acuities- If BCA is reduced but equal, larger acuity lines should be used- If BCA is unequal, Dissociated Red Green balance is preferred
3) Presbyopes - For presbyopes with some amount of accommodation, balancing is needed- For full presbyopes, balancing not needed
Binocular Best Sphere• The eyes are simultaneously fogged and then unfogged until maximum binocular acuity is attained
• The spherical endpoints are the maximum plus or minimum minus power providing maximum binocular visual acuity
• These endpoints are usually about +0.50 DS more plus or less minus than those found monocularly
Binocular subjective refraction at near
• Near refraction refers to the measurement of the refractive state when the patient is fixating at near point
• Although near refraction may be performed monocularly, small amount of cyclodeviation will generally occur with near fixation and may result in different cylinder axis. So binocular near refraction is preferred
• Useful in conditions that significantly alter the refractive error when patient fixates at near
Important in:1) Presbyopes2) Persons with binocular vision problems3) Accommodative abnormalities4) Greater spherical or cylindrical power of
spectacle Rx which cause greater change in the stimulus to accommmodation during near fixation ( > +/- 4 DS , > +/- 1.50 DC)
5) Persons with special near task requirements
Techniques of achieving binocularity during refraction at near
1.Turville (septum ) and related methods :• Septum mounted 10 cm in front of a dual or split chart at near served to combine respective images separately to each eye• Outer border of the chart and peripheral field served as fusion locks .
2. Vectographic near-point cards
• Consisted of duplications of identical targets that composed projected vectographic distance chart.• Distance chart miniaturized to a separate near card .• Need for different near point add powers for each eye could be determined
Binocular versus monocular refraction at near
Presbyopic near addition:
• To designate identical add powers for both eyes .• Eyes don’t act identically when accommodated for near .• Less accommodation demanded - fully corrected myopic for distance • More accommodation-fully corrected hyperopic
• Two eyes having different optical corrections –have different accommodative demands at near • The imbalance requires unequal accommodative efforts by two eyes• When near add determined by binocular refraction at distance and at near, imbalance is minimized
Astigmatism
• Difference between power of astigmatic correction at far compared to near due to – change in accommodative demand due to power effectivity
• Accommodative demand for more minus meridian – less than more plus meridian
• When binocular fixation altered from distance to near, lines of sight converge and most often depressed- eyes undergo excyclovergence• Axis of cylinder is not same for binocular distance and near refractions• A monocular assessment of cylinder axis ignore this phenomenon because cyclofusional eye movements are then not possible at distance or at near
SummaryBinocular refraction is superior to monocular – better control of accommodation, separate binocular balancing not requiredBinocular balancing important as unbalanced accommodation between two eyes result in asthenopia , discomfortIf patient is not binocular (suppression, strabismus), monocular best sphere is the end point of subjective refraction
Subj
ecti
ve R
efra
ctio
n RE
Monocular sphere
Astigmatism
(axis/power)
Monocular best sphere
LE
Monocular sphere
Astigmatism (axis/power)
Monocular best sphere
BE Binocular best sphere
Binocular balancing
References • Borish’s Clinical Refraction, 2nd edition• Clinical Procedures in Optometry• Primary Care Optometry, 2nd edition• ICEE Module 3 – The Refractive Examination• Internet