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

• 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

Ø

d (m) d (m)

pd (cm)R R’

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∆).

Exophoria

perception

Measuring prism

target

Through dissociating 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. – The eyes return to the phoria position at

break.

• 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