Fusional vergence

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# Fusional vergence - PowerPoint PPT Presentation

Fusional vergence. Maddox components of vergence. Tonic Fusional Accommodative Proximal. Maddox’s thoughts. 4 components are independent Additive a given vergence movement can be decomposed into components which are added together to produce the full movement

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### Fusional vergence

• Tonic
• Fusional
• Accommodative
• Proximal
• 4 components are
• independent
• a given vergence movement can be decomposed into components which are added together to produce the full movement
• If any one component is insufficient, pts. will have nearpoint complaints
• asthenopia
Specifying the amount of vergence
• Three basic units of measurement
• Angular (°)
• Prism diopters (∆)
• Meter angles
Angular
• Take distance from target and pd in identical units of measurement
• Divide pd in half
• Take inverse tangent of 0.5 pd/distance
• Double that to get the full angle

d (m)

R

R’

pd (cm)

Prism diopters
• most commonly used clinical measure of vergence angle
• 1∆ (prism diopter) is the tangential deviation of 1 linear centimeter at 1 meter’s distance
• For ø ≤ 10 deg., tan ø=ø and ∆ = p/d
• a viewing distance of .4 m( 40 cm) with p = 6 cm (60 mm), the eyes must converge 15∆ (6/.4).
Meter angle
• The meter angle is the amount of vergence required for both eyes to look at an object at 1m distance
• 1 meter angle = 1/d (in m)
• 1 meter angle = 1∆/p.d.
Proximal Vergence
• Maddox called this component “psychic” vergence
• the amount of vergence produced by the awareness of a near object
• In the optometric exam, the phoroptor is a near object that can contribute to a vergence posture.
Tonic Vergence
• difference between the anatomical position of rest and the physiological position of rest
• anatomical position of rest is that which the eyes assume in deep anesthesia, coma, or death
• divergent
Physiological position of rest
• orientation of the two eyes in the absence of any stimulus to fusion
• identical to the distance phoria if the eyes are emmetropic or properly refracted
Distance phoria
• A patient is said to be orthophoric if he/she has no distance phoria.
• If there is a heterophoria, its direction shows whether tonic vergence is excessive or insufficient
• If exophoric (eyes divergent), tonic vergence is probably insufficient.
• If esophoric (eyes convergent), tonic vergence is probably excessive.
Distance phoria
• phoria = position of the two eyes is the position of the visual axes relative to one another when all stimuli to binocular fusion have been eliminated.
• “dissociating” the two eyes
• assumes that the person is emmetropic or properly corrected
Qualifers
• If there are visual stimuli for fusion, the position of the eyes will be determined by both fusional vergence and tonic vergence.
• If the test target is nearer than about 6 m, accommodative vergence will also be present.
Dissociating the two eyes
• two ways
• by covering one eye, as in the cover test
• by placing a dissociating prism in front of one eye and a measuring prism in front of the other
• use a value of vertical dissociating prism that is too great to be overcome by fusional vergence in front of one eye (e.g., 7-8∆).

perception

Exophoria

target

Through dissociating prism

Measuring prism

Distance phoria
• If alignment occurs at 0 ∆, the patient is orthophoric.
• If alignment requires base-in prism, the patient is exophoric.
• If alignment requires base-out prism, the patient is esophoric.
Distance phoria
• Distance phoria measurements assume the target is located at 6 m or greater.
• Tonic vergence is a significant determiner of the distance phoria.
• Other factors contributing to distance phorias
• the position of the eye in the orbit
• the length of the EOM
• the positions of the insertions of the EOM.
Fusional vergence
• also known as disparity vergence
• operates to reduce retinal disparity
• To see an object singly, the image of the fixated object must falls on corresponding points on the two retinas
Corresponding retinal points
• Definition: points on the two retinas which, when stimulated, give rise to perception of identical visual direction
• Fusional vergence movements take place to eliminate noncorresponding retinal stimulation or retinal disparity
• fusional vergence prevents diplopia
Eliciting fusional vergence
• place a prism in front of one eye
• The eye will move (fusional vergence movement) to prevent diplopia.
• uncover a covered eye elicits a fusional vergence movement
• The eye will move from the phoria position to obtain single binocular vision.
Clinical determination of fusional vergence
• Ideally, introduce a small amount of horizontal prism, in equal amounts, before the two eyes
• Inward movements are positive and outward movements are negative
• base-out prism induces positive fusional vergence (convergence)
• base-in prism induces negative fusional vergence (divergence)
Clinical tests
• at both distance and near (40 cm.)
• using 20/20 letters as the test target
• patient is asked to report if the letters blur or become double
• note prism powers where blur (if it occurs) and where doubling (break) are reported
• then reduce prism until the target is seen singly (recovery)--and note that value
Negative fusional vergence at distance
• At optical infinity: base-in prism is added equally before the two eyes as the patient views 20/20 letters
• What does the blur indicate? Is a blur finding expected in this case? Why or why not?
• What does the break represent?
• What does recovery mean?
Negative fusional vergence at distance
• Blur indicates the limits of negative fusional vergence
• now accommodative vergence is called on the supplement fusional vergence
• in this case, we have to diverge so we would have to relax accommodation
• Should we be able to relax accommodation at 6 m?
Negative fusional vergence at distance
• Break indicates that the total ability of the eyes to diverge to avoid diplopia has been reached.
• Recovery indicates that a negative fusional vergence movement has been made to again obtain single binocular vision.
Positive fusional vergence at distance
• adds base-out prism equally before the two eyes while the patient focuses on the test target (20/20 letters)
• patient again reports blur, break and recovery.
• Should we have a blur finding here?
Positive fusional vergence
• Blur -- limits of fusional vergence
• accommodative vergence is being called on to supplement fusional; we should be able to accommodate at distance
• Break -- limits of accommodative vergence (if there is blur
• Recovery occurs after the eyes have converged (positive fusional vergence movement).
Negative and positive fusional vergence at near
• same tests but performed at 40 cm
• expects blur for base-in vergence
• focusing on a target at 40 cm demands 2.5D of accommodation (D=1/.4m)
• accommodation is relaxed to supplement negative fusional vergence
Expected values for fusional vergence
• from Morgan (and Bachman)
• At optical infinity
• Base-in x/7/4
• Base-out 9/19/10
• At 40 cm
• Base-in 13/21/13
• Base-out 17/21/11
Why?
• Test base in before base out?
• Test distance before near?
• There are aftereffects of prism testing
• Effects of base in are less than those of base out
• Effects at distance are less than those at near
• Start where the aftereffects are least for most accurate determination