BV5 - fixation disparity

What is fixation disparity?

ocular deviation less than the size of the diplopia threshold

What is the maximum FD size?

when point of regard is at edge of Panum's area

What does fixation disparity do?

creates retinal disparity for the point of regard to create stimulus for fusional vergence

what is a strabismus?

ocular deviation larger than size of diplopia threshold

What does the amount of fixation disparity proportional to?

amount of fusional vergence

What does the presence of fusional disparity indicate?

indicates the patient is using fusional vergence

How much fusional vergence is someone using if they have no fixation disparity?

0

What is the gain of fusional vergence equal to?

gain = ratio of fusional vergence to fixation disparity

= fusional vergence/FD

what is the Nonius line method?

subjective method to measure fixation disparity → monocular vertical lines allow subject to visualize the oculocentric direction of the eye

What instruments is fixation disparity measured using the nonious line method?

1. disparometer

2. borish card

how to use disparometer to measure fixation disparity?

patient adjusts physical position of lines until they are aligned

how to use a borish card to measure fixation disparity?

fixed target → patient must report appearance

Why/how does the nonius lines appear to someone with an exo fixation disparity?

patient will be underconverged → fixating behind target, therefore target will have crossed disparity

R line to the left and L line to the right

Why/how does the nonius lines appear to someone with an eso fixation disparity?

patient will be overconverged → fixating in front of target, therefore target will have uncrossed disparity

R line to the right and L line to the left

How will the nonius lines look after someone with an exo FD aligns them?

UNCROSSED

R line to the right

L line to the left

How will the nonius lines look after someone with an eso FD aligns them?

CROSSED

R line to the left

L line to the right

What is the forced vergence fixation disparity curve (Ogle curve)?

the relationship between fixation disparity and fusional vergence

How does a standard forced vergence fixation disparity curve (type 1 curve) change as different prism is added?

↑ BI prism added → ↑ eso FD

↑ BO prism added → ↑ exo FD

what is habitual fixation disparity?

how much fixation disparity without any prism in place

What is the associated phoria?

amount of prism to get fixation disparity to 0

What FD does someone have if they see the right horizontal line higher on a Borish card?

right hypo → need R BU

what does associated and dissociated phorias both indicate?

convergence position of the eyes where no fusional vergence is needed to fuse:

dissociated phoria → remove stimulus to fusional vergence so eyes assume posture where fusional vergence is 0

associated phoria → find position of eyes where input to maintain fusional vergence is 0

When do you measure associated phoria?

when eyes are fused/associated

When do you measure dissociated phoria?

when eyes are dissociated/not fused

What is the relationship between associated and dissociated phorias? Why?

low positive correlation between them because of prism adaptation

What is a type 2 fixation disparity curve?

↑ BO prism → does not increase exo disparity (BO adaptation)

↑ BI prism → ↑ eso fixation disparity

What is a type 3 fixation disparity curve?

↑ BO prism → ↑ exo disparity

↑ BI prism → does not increase eso disparity (BI adaptation)

What is a type 4 fixation disparity curve?

↑ BO or BI prism → does not increased disparity (has both BI and BO prism adaptation)

How does the phoria look after looking through prism in a type 1 FD curve patient?

quickly goes back to phoria position

How does the phoria look after looking through different prism in a type 2 FD curve patient?

BI prism → quickly goes back to phoria position

BO prism → adapts to prism and does not go back to phoria position, stays converged

How does the phoria look after looking through different prism in a type 3 FD curve patient?

BO prism → quickly goes back to phoria position

BI prism → adapted to prism and does not go back to phoria position, stays diverged

How does the phoria look after looking through different prism in a type 4 FD curve patient?

adapts to both types of prism and does not go back to phoria position → stays converged and diverged

What FD curve patient is least likely to be symptomatic?

type IV curve → have prism adaption for BO and BI

What is the work of fusional vergence?

fast fusional vergence

What component of vergence is vergence adaptation?

slow vergence

Is vergence adaptation/prism adaptation good or bad?

good for patient → reduces amount of work from fast system and therefore likely to be asymptomatic

hard for doctor to prescribe prism as CT only measures fast fusional vergence → can miss larger phoria + patient will adapt to prism