Fixation disparity basics

Type 3 curve

(0% distance/10% near) ◦ Exo fixation disparity ◦ Flattened BI region ◦ Poor adaptation in BO regio

Type 2 curve

(25% distance & near) ◦ Eso fixation disparity ◦ Flattened BO region ◦ Poor adaptation in BI regio

Type IV curve

(5% distance & near) ◦ Rare, whole curve is relatively flat

Type 1 curve

(70% distance/60% near) ◦ Flattened central area, steep peripheral tail

fixation disparity angle

<30 minutes of arc

Typical starting prism

1/2 associated phoria

Saladin card

OD sees right line of vertical measurment and upper line for horizontal

Measuring fixation disparity

Requires a fusion lock (seen by both eyes) Uses polarized nonius lines (OD blue, OS red) Mallet box Bernell test lantern AO vectographic slide Borish card Wesson card Saladin card Sheedy disparometer

what is the horopter centered around?

The horopter is centered around intersection of monocular lines of sight - NOT the target

why would Purposeful misalignment be used to maintain alignment

because fusional vergence is leaky -exophoria w/ exo disparity uses fusional convergence to prevent diplopia -exophoria with eso fixation disparity, allows for larger slip before diplopia

Esophoric patients typically have _____________ fixation disparities

eso

Exophoric patients typically have _____________ fixation disparities

eso or exo

exo fixation disparity

eyes intersect behind the target crossed disparity space smaller crossed area

eso fixation disparity

eyes intersect in front of the target target in uncrossed disparity space smaller uncrossed area

Patients with flat central regions are (more/less) likely to be symptomatic and (more/less) likely to adapt to prism

less more

Plotting the fixation disparity curve

measured by plotting fixation disparity with prism by alternating (BI/BO in 3D steps) till the pt reports diplopia X axis: BI prism demand left/BO prism demand right X Intercept is "associated phoria" Y axis: Eso fixation disparity up/exo fixation disparity down ◦ Y Intercept is fixation disparity without prism

Patients with steep central regions are (more/less) likely to be symptomatic and (more/less) likely to adapt to prism

more less larger fixation disparity=larger symptoms; vision therapy flattens the central area

Sheedy Disparometer

multipe sets of offset nonius lines that can be dialed in to detirmine the fixation disparity with various amounts of prism attaches to near rod, acc controlled by reading words on the cards around the lines Upper: vertical, OD sees right line (right line up = R hypo) Lower: horizontal, OD sees upper line (Upper line to the right = eso fixation disparity)

Target must be in _____________________ to avoid becoming dipolic

panums fusional area

Fast vergence

quick response leaky/ decays quickly tails of forced duction fixation disparity curve

Reasons for fixation disparity

random errors of alignment response to binocular stress Purposeful misalignment used to maintain alignment

Slow vergence

responsible for prism adaptation Shifting of deviation even with correcting prism 10 eso with 5 BO stays 10 eso Central portion of forced duction fixation disparity curve

BI prism and minus lens effect on fixation disparity

shifts fixation disparity more "eso" and less "exo"

BO prism and plus lens effect on fixation disparity

shifts fixation disparity more "exo" and less "eso"

when does a fixation disparity happen

when the monocular lines of sight do not coincide with the target

Can vergence demand alter fixation disparity?

yes