Fusional vergence

1. Fusional vergence

2. Maddox components of vergence • Tonic • Fusional • Accommodative • Proximal

3. 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

4. Specifying the amount of vergence • Three basic units of measurement • Angular (°) • Prism diopters (∆) • Meter angles

5. 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)

6. 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).

7. 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.

8. 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.

9. 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

10. 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

11. 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.

12. 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

13. 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.

14. 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∆).

15. perception Exophoria target Through dissociating prism Measuring prism

16. 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.

17. 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.

18. 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

19. 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

20. 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.

21. 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)

22. 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

23. 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?

24. 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?

25. 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.

26. 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?

27. 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).

28. 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

29. 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

30. 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