7 - accommodation and vergence

what do you need to factor in the accommodative demand for an object?

1. RE

2. refractive correction

why might the spec rx NOT match the refraction?

1. aniseikonia

2. accommodation

what movement is vergence?

eyes go in opposite direction (disjunctive)

convergence: inward rotation

divergence: outward rotation

hypervergence: OD up and OS down → R hypervergence

hypovergence: OD up and OS down → L hypovergence

what is the zero lateral vergence posture?

lines of sight are parallel when viewing object at infinity

what is the vergence angle?

angle between lines of sight

What is the most common binocular vision disorder?

convergence insufficiency

what can be used to tx convergence insufficiency?

1. VT

2. prism

What are the 4 components of vergence?

1. Tonic convergence

2. Accommodative convergence

3. Fusional convergence

4. Proximal convergence

What is the stimulus for fusional vergence?

binocular disparity → cover one eye

what is the latency of fusional vergence?

160 ms

What are the two components of fusional vergence?

1. dynamic: fusional vergence needed to change vergence posture

2. static: holds vergence posture → compensates for the phoria

How do you calculate binocular disparity?

= vergence target demand - vergence angle of the eyes

Positive (vergence) disparity is stimulus for what?

positive fusional vergence

Negative (vergence) disparity is stimulus for what?

NFV

What is the only component of vergence that is controlled by a feed-back loop?

fusional vergence

What is accommodative vergence?

vergence driven by accommodation

what is the average value for accommodative vergence?

4 pd/D

What is proximal vergence?

vergence that occurs due to perceived nearness of target

what is the stimulus for proximal vergence?

perceived distance of the target

what can be affected by proximal vergence?

clinical testing → mirror room, instrument (phoropter)

what is tonic vergence?

ocular vergence posture when the output of the other 3 stimulus driven vergence components is 0

vergence posture in absence of vergence stimulus

What are the 4 types of accommodation/sum of total accommodation?

1. tonic

2. proximal

3. vergence

4. blur

What is the stimulus for blur accommodation?

retinal defocus

What is the only type of accommodation driven by a feedback loop?

blur accommodation

What is normal convergence accommodation (CAC)?

0.1-0.4 D/prism diopter

what is tonic accommodation?

base-line value in absence of any stimulus to accommodation

aka: rest accommodation, dark focus, empty space myopia, dark myopia

What is average tonic accommodation?

1.5D → varies from 0-4D

What is a phoria?

vergence position of eyes in absence of fusional vergence → latent deviation

What components of vergence make up the phoria?

1. accommodative

2. proximal

3. tonic

the bigger the phoria?

the more strain put on fusional vergence → phoria demostrates what fusional vergence must overcome when a person is looking with both eyes

What are the average phoria values for distance and near?

Distance: 1 pd exophoria

Near: 3 pd exophoria

What tests are used to measure phoria clinically?

1. alternating CT

2. V-G phorias

3. Modified Thorington

What kind of fusional vergence is used to compensate for an esophoria?

NFV → turn eyes out

What kind of fusional vergence is used to compensate for an exophoria?

PFV → turn eyes in

What does "opening the vergence feed-back loop" mean?

taking away the stimulus (binocular disparity) for fusional vergence → eliminating fusional vergence

how do you close the "open the vergence feed-back loop"?

1. occlusion (CT)

2. vertical prism (V-G)

What must be done to measure a phoria?

eliminate fusional vergence

What does the phoria angle represent?

how much fusional vergence must be used to maintain fusion → measure how far the covered eye drifted away from fixation target

What is the vergence demand for a 40cm target and a 60mm PD?

15 prism diopters

Describe the steps of eye movements when increasing BO prism (increasing convergence demand)

1. BO prism stimulates PFV

2. PFV creates positive convergence accommodation → potential blur finding

3. @ limit of PFV, patient will increase accommodation to increase AC → blur finding (if not before)

4. when patient runs out of PFV and AC → double (break)

What patients have a hard time overcoming convergence accommodation with blur accommodation?

those with high CA/C (convergence accommodation) → seen in those with PFV deficiency

Describe the steps of eye movements when increasing BI prism (increasing divergence).

1. BI prism stimulates NFV

2. NFV creates negative convergence accommodation → potential blur finding

3. @ limit of NFV, patient will relax accommodation to decrease AC → blur finding (if not before)

4. when run out of NFV and decreased AC → double (break)

what is an abnormal finding for BI prism?

blur at distance → should NOT have any accommodation to relax

what do we consider the limit of fusional vergence?

blur finding (or break in absence of blur)

what is plotted on the Zone of Clear Single Binocular Vision (ZCSBV)?

vergence demand at varying distances

distance and near phoria

vergence ranges

monocular AA

Where are the near and distance phoria data points plotted?

RELATIVE TO VERGENCE DEMAND OF TARGET

how to find calculated ACA on ZCSBV?

reciprocal of slope of phoria → conv/accom

Where are the near and distance vergence ranges data points plotted?

RELATIVE TO VERGENCE DEMAND OF TARGET

What is the ceiling of the ZCSBV?

amplitude of accommodation

What should never leave the wall of the ZCSBV?

phoria line

What patients may have the demand line never in their zone?

strabismic patients

How is the ZCSBV affected with prism or lenses?

it is NOT affected → we are only changing the vergence demand

while the vergence demand line helps sets the walls of ZCSBV relative to its position without lenses/prism, the vergence demand is what is changed not the walls of the ZCSBV

what does the zone of clear single binocular vision paralellogram tell you?

whether any combo of accommodative and vergence demand allows for single clear binocular vision

if point falls within zone → see single and clear

if point falls outside of zone → see single and blurry, double and clear or double and blurry

How does BI prism shift the demand line?

shifts it to the left (i.e. negative convergence)

BI for exos → moves image out towards eye → requires more NFV to view (or negative convergence) or less PFV (or convergence)

How does BO prism shift the demand line?

shifts it to the right

How do minus lenses shift the demand line (zscbv plot)?

shifts it up

How do plus lenses shift the demand line (zscbv plot)?

shifts it down

What is positive relative vergence (PRV)?

clinical BO finding → measured from demand line to right hand boundary

What is negative relative vergence (NRV)?

clinical BI finding → measured from demand line to left hand boundary

What is positive fusional vergence (zscbv plot)?

clinical BO finding + exo/- eso → measured from phoria line to right hand boundary

What is negative fusional vergence?

clinical BI finding + eso/- exo → measured from phoria line to left hand boundary

What are Morgan's Norms at distance ?

BI: x/7/4

BO: 9/19/10

What are Morgan's Norms at near?

BI: 13/21/13

BO: 17/21/11

Symptoms for what phoria is better predicted with Percival's criteria?

esophoria

Symptoms for what phoria is better predicted with Sheard's criteria?

exophoria

What is Percival's Criteria?

asymptomatic BV occurs when vergence demand is within the middle 1/3 of total fusional vergence

What is the limit of Percival's criteria?

when the larger relative vergence range is double the smaller range

What are the downsides to Percival's criteria?

1. doesn't take the magnitude or direction of phoria into account (if large phoria not compensated for → symptomatic)

2. doesn't consider the range of fusion vergence present (someone with narrow ranges would still be symptomatic)

What is Sheard's Criterion?

asymptomatic binocular vision occurs when the compensating vergence range is at least double the magnitude of the phoria

what are the compensating vergence ranges?

in the direction opposite to the phoria

exophoria: PFV (BO)

esophoria: NFV (BI)

what is the limit of Sheard's criterion?

when size of the phoria is 1/3 the compensating fusional vergence

or

compensating fusional vergence needs to be twice the phoria

What are ways to manage patients with inadequate PFV (exophoria)?

1. increase compensating vergence range with VT

2. BI prism

3. prescribe minus lenses for reading (unlikely to work, need high AC/A)

What are ways to manage patients with inadequate NFV (esophoria at near)?

1. prescribe plus lenses

2. VT to increase NFV ranges (hard)

3. prescribe BO prism (not effective in many cases)

what is the unilateral cover test used for?

to determine if there is a manifest deviation present (tropia/strabismus)

What are the unilateral CT steps of a right esotrope fixating target with left eye?

1. left eye fixating, right eye deviated in

2. cover OD, OS stays fixating

3. cover OS, OD is deviated IN

a. a saccade to right is needed for OD to fixate

4. uncover OS (if preferred) will saccade to left, otherwise OD continues to fixate

Steps the COVER of unilateral CT with esophoria?

Patient is bifoveally fixating on target

1. cover OD → stimulus for fusional vergence is gone

2. no fusional vergence → eyes converge

3. object of regard off fovea of OS → pursuit to left

4. results in left eye fixating target and OD deviated in

How is fixation accomplished on the cover of unilateral cover test with esophoria?

convergence eye movement followed by a pursuit (conjugate) eye movement

Steps the UNCOVER of unilateral CT with esophoria ?

OD covered, OS fixating target:

1. uncover OD: left eye on target, OD fixated in

2. diplopic target stimulates fusional vergence → eyes diverge

3. now OS if off target → need right saccade

4. bifoveal fixation

How is fixation accomplished on the uncover of unilateral cover test with esophoria?

divergence (disjunctive) eye movement and a saccadic (conjugate) eye movement