EM 7 Acc-Verg-ZCSBV 2023 Notes

What is accommodative demand, and how is it calculated?

Accommodative demand = 1 / viewing distance in meters.
The result is in diopters (D).

What is the accommodative demand for a far-away target?

For a far-away target, accommodative demand is approximately 0 D because distance approaches infinity:
1 / infinity ≈ 0 D.

Why is the formula “1/distance” only a general estimate of accommodative demand?

It gives the optical demand based on target distance, but the actual demand for a patient also depends on:

• Refractive error

• Refractive correction

So two patients viewing the same object may not have the same actual accommodative requirement.

What is the main rule of accommodation?

The eye can increase accommodation for near targets and relax accommodation for distance, but full relaxation is the lower limit.
You cannot accommodate less than 0 D.

Why can’t accommodation go below zero?

Fully relaxed accommodation already represents the minimum accommodative state.
Since the lens cannot “anti-accommodate,” accommodation cannot be negative.

Why might a patient’s spectacle prescription not exactly match their measured refraction?

The final Spec Rx may differ from refraction because prescribing glasses depends on clinical factors, not just measured refractive error.
Common reasons include:

• Usually accommodation

• Sometimes aniseikonia
  High-yield: Rx’ing glasses requires understanding accommodation.

Why is accommodation important when prescribing glasses?

Accommodation can change the apparent refractive state, especially in younger patients.
If accommodation is not considered, the prescription may over- or under-correct the patient’s true refractive needs.

What assumptions are being made in these accommodation example problems?

The examples assume:

• Blur threshold = 0

• Accommodative response matches accommodative demand whenever possible

• Patient is 10 years old

• No accommodative or vergence dysfunction

• No astigmatism

• No anisometropia

What does “blur threshold is zero” mean in these accommodation problems?

It means the patient is assumed to detect any mismatch between accommodative response and demand.
So, if possible, the eye will accommodate exactly enough to keep the image clear.
This is a simplifying assumption for calculations, not always true clinically.

Why do these examples use a 10-year-old patient?

A 10-year-old generally has strong accommodative ability, so the examples can focus on how accommodation interacts with refractive error/correction rather than limitations from presbyopia or accommodative insufficiency.

What are vergence eye movements, and how do they differ from conjugate eye movements?

Vergences are binocular disjunctive eye movements, meaning the two eyes move in opposite directions.
They are mainly rotations about the vertical axis, producing horizontal eye movements.

• Convergence: eyes rotate inward

• Divergence: eyes rotate outward

What is the zero lateral vergence posture?

Zero lateral vergence occurs when the lines of sight are parallel, such as when viewing an object at infinity. At infinity, there is essentially no need for convergence or divergence.

What is the vergence angle?

The vergence angle is the angle between the two lines of sight.
It depends on:

• pd = interpupillary distance

• D = viewing distance

Basic relationship:

Vergence angle × D = pd

How do you calculate vergence angle in radians?

Vergence angle in radians = pd / D

Examples:

• pd in cm, D in cm

• pd in m, D in m

How do you calculate vergence angle in prism diopters?

Use pd in centimeters and D in meters:

Vergence angle in prism diopters = pd (cm) / D (m)

What are meter angles, and how are they related to accommodation?

Meter angles = 1 / viewing distance in meters

How do you convert meter angles to prism diopters?

Multiply meter angles by the patient’s pd in cm:

Meter angles × pd (cm) = prism diopters

What are vertical vergence movements, and how are they named?

Vertical vergences are binocular disjunctive eye movements where the eyes move in opposite vertical directions.
They are rotations about the horizontal axis.

Naming depends on which eye is relatively higher/lower:

• Hypervergence: one eye moves/positioned upward relative to the other

• Hypovergence: one eye moves/positioned downward relative to the other

What is the most common binocular vision/vergence disorder, and how can it be treated?

Convergence insufficiency is the most common binocular vision disorder.
Treatment options include:

• Orthoptics / vision therapy → improves fusional compensation

• Prism spectacles → can help reduce vergence demand or symptoms

What are the 4 components of horizontal ocular vergence?

Total horizontal convergence is the sum of:

1. Fusional/disparity vergence

2. Accommodative vergence

3. Proximal vergence

4. Tonic vergence

What is fusional vergence, and why is it clinically important?

Fusional vergence is driven by retinal disparity and helps keep images single.
Clinically, it is important because stronger fusional compensation can help patients control vergence disorders like convergence insufficiency.

What drives fusional/disparity vergence?

Binocular disparity drives fusional vergence.
If the two eyes’ images are not aligned, the visual system adjusts vergence to reduce disparity and maintain single binocular vision.

How is binocular disparity calculated conceptually?

Binocular disparity is the difference between where the eyes are aimed and where the target actually is.

Disparity = target position − eye position
or from the slide:

Disparity = b − a

This disparity acts as the error signal that tells the eyes how to adjust.

What is the latency of fusional vergence, and what does that imply?

Fusional vergence latency is about 160 ms.
Because it is feedback-driven, the system detects disparity first, then corrects it after a short delay.

What are the dynamic and static components of fusional vergence?

Fusional vergence has two components:

• Dynamic fusional vergence: changes vergence posture to fixate a target at a different distance

• Static fusional vergence: maintains vergence posture and compensates for a phoria

Which vergence component is controlled by a feedback loop?

Fusional/disparity vergence is the only vergence component controlled by a feedback loop.
It is constantly making adjustments based on binocular disparity to keep vision single.

What is accommodative convergence?

Accommodative convergence is vergence driven by accommodation.
When the eye accommodates for a near target, convergence is linked to that accommodative effort.

What does the AC/A ratio measure?

AC/A = accommodative convergence / accommodation.
It measures how much convergence occurs for each diopter of accommodation. Units are usually prism diopters/diopter

What is the average AC/A ratio, and how is it commonly measured?

The average AC/A ratio is about 4 prism diopters per diopter.

It can be measured using:

• Calculated/far-near AC/A

• Gradient AC/A

What is proximal vergence, and what stimulus drives it?

Proximal vergence is vergence driven by the perceived distance of the target.
It is based on the brain’s awareness that something is near or far, even apart from blur or binocular disparity.

Why can proximal vergence affect clinical testing?

Because the testing setup can change the patient’s perceived target distance.
Examples:

• Mirror room may make a target seem farther away

• Phoropter/instrument testing may make the task feel artificially near or constrained

This can alter vergence posture during testing.

What is tonic vergence?

Tonic vergence is the baseline ocular vergence posture when the other stimulus-driven vergence components are absent.

How do proximal and tonic vergence differ?

• Proximal vergence: driven by perceived target distance

• Tonic vergence: baseline posture when no vergence stimulus is present

What are the 4 components of ocular accommodation?

Total accommodation is the sum of:

1. Blur/defocus accommodation

2. Convergence accommodation

3. Proximal accommodation

4. Tonic accommodation

What is blur/defocus accommodation, and what drives it?

Blur accommodation is accommodation driven by retinal defocus/blur.
It adjusts lens power to reduce blur and bring the image into focus.

Which accommodation component is driven by a feedback loop?

Blur/defocus accommodation is the only accommodation component driven by a feedback loop.
It constantly adjusts based on the amount of retinal blur.

What is convergence accommodation?

Convergence accommodation is accommodation driven by the act of converging.
When the eyes converge, accommodation can be stimulated as part of the near response.

What is tonic accommodation?

Tonic accommodation is the baseline/resting level of accommodation when there is no stimulus to accommodate.

Also called:

• Rest accommodation

• Dark focus

• Empty space myopia

• Dark myopia

What is the average tonic accommodation value?

Tonic accommodation averages about 1.5 D, but varies widely from about 0-4 D.

What is a phoria?

A phoria is the eye’s latent vergence deviation measured when fusional vergence is removed.
It represents where the eyes would naturally rest when they are not using fusion to stay aligned.

What does a phoria tell you clinically?

A phoria shows how close the non-fusional vergence components come to the required vergence angle.

It reflects the combined effect of:

• Accommodative vergence

• Proximal vergence

• Tonic vergence

Whatever alignment error remains must be corrected by fusional vergence.

Why does a larger phoria create more visual strain?

A larger phoria means fusional vergence must work harder to keep the eyes aligned during binocular viewing. So, the bigger the phoria, the greater the fusional vergence demand/strain.

What are the average phoria values at distance and near?

Average values are slightly exophoric:

• Distance: about 1 prism diopter exophoria

• Near: about 3 prism diopters exophoria

How is phoria clinically measured?

Phoria is measured by removing fusional vergence and observing the latent deviation. Common methods include:

• Alternating cover test

• Von Graefe phorias

• Modified Thorington

In esophoria, what is the eye’s tendency and what fusional response compensates?

In esophoria, the eyes have a latent tendency to turn inward.

To compensate during binocular viewing, the patient uses:

Negative fusional vergence = divergence / turning eyes outward

In exophoria, what is the eye’s tendency and what fusional response compensates?

In exophoria, the eyes have a latent tendency to turn outward.

To compensate during binocular viewing, the patient uses:

Positive fusional vergence = convergence / turning eyes inward

What vergence components are isolated when measuring phoria?

To measure phoria, fusional vergence is removed, so the remaining vergence posture reflects mainly:

• Accommodative vergence

• Proximal vergence

• Tonic vergence

These should be kept stable during testing so the measured phoria reflects the patient’s latent deviation.

Why must fusional vergence be eliminated to measure a dissociated phoria?

A phoria is the eye position when fusional vergence = 0.
To measure it, you must “open the vergence feedback loop” by removing the stimulus for fusional vergence: binocular disparity.

How do clinicians eliminate fusional vergence during phoria testing?

They create dissociation, meaning the two eyes no longer have usable binocular disparity. This can be done with:

• Occlusion, as in the cover test

• Vertical prism, as in Von Graefe phoria testing

This makes fusional vergence drop toward zero, though it may take some time.

After fusion is broken, what does the measured eye drift represent?

The amount the covered/dissociated eye drifts away from the fixation target represents the amount of fusional vergence required during normal binocular viewing.

How do you assess whether a phoria is clinically significant?

Measure the patient’s vergence ranges to see whether they have enough fusional ability to compensate for the phoria. A large phoria is more symptomatic if the patient has limited compensating vergence reserves.

What is graphical analysis used for in binocular vision?

Graphical analysis evaluates the workload placed on the visual system at different viewing distances.
It helps determine the patient’s zone of clear single binocular vision: the range where vision remains:

• Clear

• Single

• Binocular

What happens when base-out prism is introduced during vergence testing?

Base-out prism increases convergence demand, so the patient must use positive fusional vergence (PFV) to keep the target single.

How can positive fusional vergence affect accommodation?

Using positive fusional vergence can stimulate convergence accommodation (CA).

Formula:

CA = (CA/C) × FV

So as fusional convergence increases, accommodation may also increase. This extra accommodation may be partly counteracted by negative blur accommodation (-BA) to keep the image clear.

Why can base-out prism sometimes cause blur before diplopia?

As PFV increases, some patients generate extra convergence accommodation, which can over-accommodate and blur the target.
This is more likely in patients with a high CA/C ratio, especially those with positive vergence deficiency.

What happens at the limit of positive fusional vergence?

Near the limit of PFV, many patients increase accommodation to drive more accommodative convergence.
This may help maintain single vision briefly, but it often causes blur.
When the patient runs out of both PFV and accommodative convergence, they experience diplopia.

In graphical analysis, what does the blur finding represent during PFV/base-out testing?

The blur point represents the limit of clear single binocular vision.
At this point, the patient is still fusing, but accommodation has changed enough that the target becomes blurry.

Sequence during BO/PFV testing:

Clear + single → blur → break/double → recovery

Clinically, what finding is considered the limit of fusional vergence?

The blur finding is considered the clinical limit of fusional vergence.
If no blur occurs, then the break point is used instead.

Why is blur considered close to the limit of fusional vergence?

Blur usually occurs when the patient is near the edge of their ability to maintain fusion while keeping the target clear.

What happens when base-in prism is introduced?

Base-in prism decreases convergence demand, so the patient must use negative fusional vergence (NFV) to keep the target single.

How does negative fusional vergence affect accommodation?

Negative fusional vergence can create negative convergence accommodation.

Formula:

CA = (CA/C) × FV

Because FV is negative during NFV, CA becomes negative:

NFV → −CA

This may be partially counteracted by positive blur accommodation (+BA) to keep the image clear.

Why can base-in prism cause blur?

With base-in prism, the patient uses negative fusional vergence, which may reduce convergence accommodation.
In some patients, this creates enough accommodative change to cause blur.

What happens at the limit of negative fusional vergence?

At the limit of NFV, many patients decrease accommodation to release accommodative convergence and help diverge more.
This can make the target blurry.
When the patient runs out of both NFV and accommodative help, they experience diplopia.

Why should there usually be no blur with base-in testing at distance?

At distance, accommodation should already be relaxed near 0 D.
Because accommodation cannot relax below zero, the patient should not have much accommodation available to decrease.

Compare base-out vs base-in prism during fusional vergence testing.

• Base-out prism: increases convergence demand → patient uses PFV → may increase accommodation

• Base-in prism: decreases convergence demand → patient uses NFV → may decrease accommodation

What does the phoria line represent in ZCSBV graphical analysis?

The phoria line predicts the patient’s phoria at any viewing distance.
Distance and near phorias are measured relative to the target demand, so they are plotted relative to the demand line and then connected.

How is the phoria line used to estimate AC/A?

The slope of the phoria line relates to the patient’s calculated far-near AC/A ratio.

Relationship: 1 / slope = calculated far-near AC/A

How are fusional vergence limits plotted in ZCSBV?

Distance and near vergence ranges are measured relative to the target demand.
So clinically measured vergence ranges are actually relative fusional vergence ranges, and they are plotted relative to the demand line.

What do fusional vergence limits tell you clinically?

They show the boundaries of the patient’s ability to maintain single binocular vision.
If the visual demand or phoria compensation exceeds these limits, the patient may experience diplopia, eyestrain, or loss of fusion.

What does amplitude of accommodation represent in ZCSBV?

Amplitude of accommodation (AA) represents the maximum amount of accommodation the patient can produce. In ZCSBV, it forms the ceiling of the zone.

What do fusional vergence limits tell you clinically?

They show the boundaries of the patient’s ability to maintain single binocular vision.
If the visual demand or phoria compensation exceeds these limits, the patient may experience diplopia, eyestrain, or loss of fusion.

What does amplitude of accommodation represent in ZCSBV?

Amplitude of accommodation (AA) represents the maximum amount of accommodation the patient can produce. In ZCSBV, it forms the ceiling of the zone.

What are the major boundaries/components used to build the ZCSBV?

The ZCSBV is built from:

• Demand line → accommodative + vergence demand at each distance

• Phoria line → latent deviation across distances

• Fusional vergence limits → limits of single binocular vision

• Amplitude of accommodation → ceiling for clear vision

Why should the phoria line never leave the wall of the ZCSBV?

The phoria line represents the eye position with fusional vergence removed.
The ZCSBV walls are created by the patient’s fusional vergence ranges, which expand outward from the phoria line.

What does it mean if the demand line exits the ZCSBV?

If the demand line exits the ZCSBV, the patient can no longer maintain clear, single, binocular vision at that demand level.

Possible symptoms:

• Blur

• Diplopia

• Eyestrain

• Difficulty sustaining single clear vision

For which patients may the demand line never enter the ZCSBV?

Patients with strabismus may have a demand line that is never inside the ZCSBV because they do not have normal bifoveal fusion.

Do prisms or lenses change the actual ZCSBV?

No. Prisms and lenses do not change the patient’s actual zone.
They only change the visual demand placed on the system.

How do prisms affect ZCSBV analysis?

Prisms alter the vergence demand. They can shift the demand line closer to the patient’s available fusional vergence range, making binocular vision easier.

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How do lenses affect ZCSBV analysis?

Lenses alter the accommodative demand, which can also affect vergence through the AC/A relationship.

Example:

• Plus lenses decrease accommodative demand

• Minus lenses increase accommodative demand

What is the difference between relative vergence and fusional vergence in ZCSBV?

• Relative vergence is measured from the demand line to the zone boundary.

• Fusional vergence is measured from the phoria line to the zone boundary.

How are PRV and NRV defined in ZCSBV?

• Positive relative vergence (PRV): measured from the demand line to the right-hand boundary of the ZCSBV

  • Clinically corresponds to base-out vergence finding

• Negative relative vergence (NRV): measured from the demand line to the left-hand boundary of the ZCSBV

  • Clinically corresponds to base-in vergence finding

How are PFV and NFV defined in ZCSBV?

• Positive fusional vergence (PFV): measured from the phoria line to the right-hand boundary of the ZCSBV

• Negative fusional vergence (NFV): measured from the phoria line to the left-hand boundary of the ZCSBV

Why does the ZCSBV not guarantee comfortable binocular vision?

The ZCSBV only tells whether a point is theoretically single and clear.
It does not prove that fusion is comfortable or sustainable.

A point inside the zone may still cause symptoms if the patient has to work too hard to maintain fusion.

What are the two major clinical criteria used to assess adequacy of binocular function?

The two criteria are:

1. Percival’s criterion

   • Often a better predictor of symptoms with esophoria

2. Sheard’s criterion

   • Often a better predictor of symptoms with exophoria

What does Percival’s criterion state?

Percival’s criterion says asymptomatic binocular vision occurs when the vergence demand lies within the middle third of the total fusional vergence range.

What does Percival’s criterion evaluate clinically?

It evaluates whether the patient’s vergence demand is centered enough within the total fusional vergence range to allow comfortable binocular vision.

What is the key limitation of Percival’s criterion?

Percival’s criterion does not adequately consider phoria.

It does not account for:

• The magnitude of the phoria

• The direction of the phoria

• Whether the phoria is adequately compensated by fusional vergence

Why can Percival’s criterion miss symptomatic patients?

A patient may satisfy Percival’s criterion but still be symptomatic if:

• Their fusional vergence ranges are small overall

• Their phoria is large

• Their phoria requires too much compensation

• Fusion is possible but not comfortable or sustainable

What are the major limitations of Percival’s criterion?

Percival’s criterion has two main limitations:

1. It does not consider the amount of fusional vergence range present

   • Limited ranges are more likely symptomatic

2. It does not consider the magnitude or direction of phoria

   • The phoria must still be adequately compensated

What does Sheard’s criterion state for comfortable binocular vision?

Asymptomatic binocular vision occurs when the compensating fusional reserve is at least 2× the magnitude of the phoria.

What is the compensating vergence for exophoria vs esophoria?

Compensating vergence is always in the direction opposite the phoria:

• Exophoria → eyes tend outward → needs PFV / convergence

• Esophoria → eyes tend inward → needs NFV / divergence

What does “fusional reserve” mean in Sheard’s criterion?

In Sheard’s criterion, fusional reserve refers to relative vergence, meaning the clinical prism finding measured from the demand line, not from the phoria line.

What are treatment options for inadequate positive vergence?

Inadequate positive vergence means the patient lacks enough PFV/convergence to compensate, often seen with exophoria.

Management options:

1. Vision therapy / orthoptics

   • Most common and very successful

   • Increases compensating fusional vergence range

2. Base-in prism

   • Reduces convergence demand

   • Patient does not have to converge as much

3. Minus lenses for reading

   • Increase accommodative demand → increase blur accommodation → increase accommodative convergence

   • Works best if AC/A is normal to high

Why are minus lenses usually less ideal for patients with inadequate positive vergence?

Minus lenses only help if accommodation can drive enough accommodative convergence through the AC/A relationship. But patients with inadequate positive vergence may not have a high enough AC/A for this to work well.

What are treatment options for inadequate negative vergence?

Inadequate negative vergence means the patient lacks enough NFV/divergence to compensate, often seen with esophoria.

Management options:

1. Plus lenses

   • Decrease accommodative demand → decrease accommodation → decrease accommodative convergence

   • Usually very effective, especially with near esophoria + high AC/A

2. Vision therapy

   • Increases compensating NFV range

   • More difficult than increasing PFV

3. Base-out prism

   • May reduce symptoms but may not be effective in many cases

Why are plus lenses often effective for near esophoria with inadequate NFV?

Plus lenses reduce the need to accommodate at near.
Less accommodation produces less accommodative convergence, which helps reduce the eso posture.

Pathway:

Plus lens → ↓ accommodative demand → ↓ accommodation → ↓ accommodative convergence

Compare management of inadequate PFV vs inadequate NFV.

• Inadequate PFV / convergence problem:

  • Best: vision therapy

  • Prism: BI prism

  • Lens option: minus lenses, if AC/A normal-high

• Inadequate NFV / divergence problem:

  • Best lens option: plus lenses, especially high AC/A near eso

  • Vision therapy possible but harder

  • Prism: BO prism, often less effective

What is the purpose of the unilateral cover test?

The unilateral cover test determines whether a manifest deviation is present.

Manifest deviation = tropia / strabismus.

During the unilateral cover test, which eye should you watch?

Watch the eye that is not being covered.

You are looking for a movement/saccade by the uncovered eye to take up fixation.