FIXATION DISPARITY

1. Definition

a. purpose

b. indicator of fast vergence activity

i. exophoria: linear relationship, fixation disparity & positive fusional vergence

ii. esophoria: linear relationship, fixation disparity & negative fusional vergence for small esophoria, non-linear for moderate, high esophoria; i.e. more fixation disparity required for a given level of esophoria than in exophoria; i.e. negative fast vergence system is less efficient!

c. not the same as the heterophoria

i. associated versus dissociated conditions

2. Fundamental variables of the forced vergence fixation disparity curve:

a. y-intercept - the habitual actual fixation disparity (0 prism)

b. x-intercept - the associated phoria (prism to neutralization of fixation disparity)

c. slope - measured between 3 base-in & 3 base-out

d. type - I - IV

3. Signs / symptoms / important facts

a. y-intercept - 10' arc exo, almost any eso FD

b. x-intercept - prism correction

c. slope - > 1.0 (i.e. 1' arc / 1 prism diopter)

a steep Type I curve

slope = Dy/Dx

slope = 9/7 = 1.3

 

4. Clinical use of fixation disparity

a. prism Rx determination
i. determine need for prism correction

ii. Rx 50% of the associated phoria [associated phoria = prism to neutralize the fixation disparity; or, the amount of prism that makes the fixation disparity 0].

b. monitor effects of vision therapy

i. increases sensitivity of disparity detectors

ii. increases level of prism adaptation (slow vergence adaptation)

STEREOPSIS

"barometer of binocular vision"

 

1. Clinical factors affecting stereoacuity

a. strabismus

b. amblyopia

c. blur / contrast

1. visual acuity / refractive error

2. accommodative dysfunction

d. fixation disparity

1. not heterophoria directly

2. anisophoria

e. suppression

f. aniseikonia

 

2. Clinical stereopsis tests

a. local, non threshold tests
1. Stereo fly - 40 sec

2. Randot - 20 sec

b. global

1. Random Dot E

3. Expected - 40 sec (local)

SUPPRESSION

1. Loss of sensitivity

a. facultative

b. obligatory

2. Effect on binocularity

a. stereopsis

b. fixation disparity measurements

 

 

3. Presentation

a. strabismus - constant, unilateral, regional

b. non-strabismus - intermittent, alternating (then unilateral), central (foveal)