Fixation Disparity

1. under binocular conditions

2. allows analysis of groups of data

3. easy to determine tentative prism rx

4. easy to use

what are some advantages for fixation disparity?

1. no direct info about accommodative or oculomotor problems

2. analysis can be clinically bulky

what are some disadvantages of fixation disparity?

fixation disparity

measure of changes in ocular alignment in response to binocular vergence stimulation &/or accommodative stress

small misalignment of the eyes under binocular conditions

no

does diplopia occur if the pt has fixation disparity?

min of arc

how is fixation disparity measured?

eyestrain/discomfort

fixation disparity can result in what clinical sx?

no

can fixation disparity testing be used in strabismic pt?

higher

the higher the pt’s dissociated phoria, the _____ the demand is on fusional vergence

more

the higher the demand on fusional vergence, the _______ likely there is error making in the vergence system

more

pt w/ high dissociated phorias are ______ likely to have fixation disparity

same

fixation disparity & dissociated phoria are typically in the _______ direction

exo

under convergence results in a _______ FD

eso

over convergence results in a _________ FD

prism adaptation (vergence adaptation, fusional after-effects)

shift to tonic vergence after the use of fusional vergence

clinically shown as a change in dissociated phoria before & after extended prism viewing

forced vergence fixation disparity curve

test to measure the robustness of the binocular vision system

determined by measuring FD when a pt looks through varying amounts of prism

divergence, NFV, eso

when BI prism is introduced, the pt induces _______, must use __________, and results in ________ FD

convergence, PFV, exo

when BO prism is introduced, the pt induces _______, must use __________, and results in ________ FD

type 1

• FVFDC type

• equal adaptation to BI & BO

• typically asymptomatic

• 55% of the pop

• “normal”

type 2

• FVFDC type

• more adaptation to BO

• most commonly eso pt

• 30% of the pop

type 3

• FVFDC type

• more adaptation to BI

• most commonly exo pt

• 10% of the pop

type 4

• FVFDC type

• unstable binocularity

• abnormal sensory or motor fusion (strabismus)

• 5% of the pop

eso

up on FVFDC is ____

exo

down on the FVFDC is ____

BO

right on the FVFDC is ____

BI

left on the FVFDC is ____

y intercept

the start of the FD test on the curve, when there is no prism in the system, what the pt sees just walking around in everyday life

x intercept (associated phoria)

amount of prism it takes to bring the pt’s FD to 0

same

associated phoria is typically in the ______ direction as the pt’s dissociated phoria

(FD at 3BO - FD at 3BI) / 6^

slope of an FVFDC =

shallow, better

a ______ slope means the pt is less symptomatic, _____ tonic vergence adaptation

steep

a ________ slope means the pt is more symptomatic

center of symmetry

• area where the greatest vergence adaptation to changes in fusional vergence occur

• flattest central region of the FD curve

• useful for prescribing prism

Sheedy’s criterion

• substituted for center of symmetry

• easier to determine

• useful for prescribing prism

1. identify the flattest segment of the curve

2. choose the point closest to the y axis (lowest amount of prism)

how do you find Sheedy’s criterion?

down & left

plus lenses shift the FD curve ______

esos

lenses are more beneficial for ______ (exos/esos)

flattens the curve (useful for steep slopes)

what are the effects of VT?

VT

what is the only way to change the slope of the FVFDC?

1. type of curve

2. y intercept

3. x intercept

4. slope

5. center of symmetry

6. Sheedy’s criterion

what are the 6 key FVFDC parameters?

type 1

type 2

type 3

type 4