Comprehensive Exam Notes on Accommodation, Convergence, and Phorias

Accommodation Testing

Purpose

• Determine if patients have adequate accommodation and enough reserve to see clearly at their preferred working distance.

When to Test?

• Pre-presbyopic patients only (different rules for presbyopes).
• Usually after a subjective refraction.
• Patients with symptoms indicating accommodative problems: blur, difficulty changing focus, eyestrain, headaches.
• Can be included in preliminary tests, especially if the patient is hyperopic or having their first eye exam.

How to Test?

• Amplitude of Accommodation (AoA): Measures the maximum amount of accommodation a patient can use.
• Other measurements include assessing:
  • Accuracy of accommodation (lead or lag).
  • Flexibility of accommodation (facility).

Expected Accommodation Amplitudes (in Diopters)

Donders’ Table

Duane’s Table

Sheard’s Table

Duane-Hofstetter Formula

• Maximum: $25.0 – 0.40 \times age$
• Average: $18.5 – 0.30 \times age$ (around 12-13D for a 20-year-old)
• Minimum: $15.0 – 0.25 \times age$

Patients can sustain 1/2 to 2/3 of their AoA.
Symptoms/adaptations likely if they require more than this to see clearly.

Recording Results

• Record each eye individually and then both together.
• Units are diopters; record as a whole number.
• Format: Amp accom (push-up): RE …D, LE …D, BE …D
  • Example: RE 10D LE 10D BE 12D

Optometric Actions Based on Results

• Offer visual hygiene advice:
  • Working distance.
  • Rest breaks.
  • Lighting.
  • Print size.
• Prescribe a correction that reduces accommodative demand:
  • Reduce/eliminate hyperopia if present.
  • Reading add.
• Prescribe eye exercises to improve accommodation.
• Refer to an orthoptist.

Interpretation

• Monocular vs. binocular: Binocular usually slightly higher.
• A difference of more than 1D RE vs LE or between monocular and binocular usually indicates:
  • The distance Rx is wrong (especially balance).
  • Binocular vision anomaly.

Low Amplitude of Accommodation

• Getting older (presbyopia).
• Examples of accommodation disorders:
  • Unable to accommodate properly (accommodative insufficiency).
  • Already using some accommodation to see clearly (i.e., a hyperope).
  • Accommodative excess.

Limitations of Push-Up/Pull Away Method

• Angular size of letters increase as target moves closer, overestimating accommodation.
• Patients vary in their ability to notice blur; results can be variable.

Common Errors

• Forgetting to use an appropriate correction.
• Using an inappropriate target - needs to be an accommodative target.
• Moving the target:
  • Too quickly; may overestimate.
  • Too slowly; may underestimate.
• Not encouraging accommodation ("can you make it clear again?").
• Using the wrong endpoint (sustained blur, not unable to read).
• Testing in primary gaze or above the midline.

Convergence

Types of Convergence

• Tonic: Baseline amount of convergence due to tonus in extraocular muscles; no stimulus required.
• Accommodative: Vergence change due to a change in accommodation; ideally, same amount of convergence per diopter of accommodation, but varies.
• Proximal: Convergence induced by the knowledge of the object's location/distance; closer the object, more proximal convergence elicited; does not depend upon accommodative state or retinal disparity.
• Fusional: Involuntary movement to correct retinal disparity.
• Voluntary: Ability to converge beyond tonic position with no stimulus; subject visualizes a near object and uses proximal and/or accommodative convergence.

What Can We Measure?

• Amplitude of convergence: Nearest point where vision is single (measured in cm).
  • Under monocular and binocular conditions…
  • Tonic + accommodative + proximal.
  • Both eyes converge for a near object, even if one eye cannot see it.
  • Fusional contributes under binocular conditions only.

When and How to Test?

• Purpose: Determine if patients have adequate convergence and reserve.
• When: Usually during preliminary testing to screen for convergence insufficiency.
  Symptoms: Diplopia, eyestrain, headaches, avoiding near work.
  Not effective with a trial frame or phoropter.
• How:
  • Near point of convergence (NPC): What is the maximum convergence?
  • Jump convergence.

What Do We Expect?

• Below 8/11cm is normal, but 5/7cm is a more recently proposed cutoff.
• Smooth and effortless, does not deteriorate if repeated.
• Break >12-15cm abnormal.
• Examples of convergence disorders: Convergence insufficiency, convergence excess.

Recording NPC

• Record break/recovery in cm, note which eye lost fixation first.
• NPC 8/15 cm RE out.
• If the patient can converge to their nose, record TTN ("to the nose").
• Note if you had to repeat it or do it more slowly.

Common Errors

• Failing to observe eyes, relying on patient report.
• Carrying out the test only once in patients with symptoms.
• Moving the target too quickly or slowly.
• Not encouraging single vision ("can you make it single again?").
• Testing in primary gaze or above the midline.
• Testing unsuitable patients (e.g., strabismus).

Optometric Actions

• Visual hygiene advice: Working distance, working height, rest breaks.
• Prescribe correction to reduce convergence demand:
  • Directly, with prism.
  • Indirectly, through effect on accommodation.
• Eye exercises.
• Referral to an orthoptist.

Link Between Accommodation and Convergence

• Neurologically linked: If we accommodate, we converge; if we converge, we accommodate.
• The amount of each varies between patients.
• If one is affected, the other may show abnormal results.
  • Uncorrected hyperopia may report diplopia because they converge too much (convergence excess).
  • Poor accommodation may hold reading material close, trying to use the boost from convergence to improve accommodation.

Cover Test

Today's Goals

• Understand why ocular misalignment occurs and causes symptoms
• Explain how to conduct the cover test
• Explain and classify heterophoria and heterotropia
• Conduct and record the cover test at distance and near appropriately and efficiently.

Components of Ocular Alignment

• Tonic: Natural tone of extraocular muscles (physiological position of rest, no stimulus required)
• Proximal: Individual's ability to "guess" distance
• Accommodative: Accommodative convergence at near
• Fusional: Corrects remaining misalignment to ensure binocular fixation

Scenarios

• Clear and single vision: All components work together
• Single but strain, blur, headaches: Too much fusional convergence required
• Double but clear: Not enough fusional vergence, eyes cannot align, and a tropia occurs
• Single but suppression: One eye suppresses the image.

Phoria

$Tonic + Proximal + Accommodative + Fusional$
We want to know how much of this is needed.

Compensated

Clear and single Vision

Decompensated

Single but strain, blur, headaches

Phorias Definition

Both eyes see target, then one eye sees target (esophoria), then eyes return to natural alignment when binocularity is interrupted, then both eyes see target

Definitions

• Heterophoria (Phoria): Misalignment only present when binocular vision is prevented.
• Heterotropia (Tropia): Misalignment under binocular conditions (manifest deviation); also known as eye turn, strabismus, squint.

Cover Test

• Determines ocular alignment by preventing binocular vision.

Advantages

• Quick
• Powerful
• No special equipment needed
• Objective
• Differentiates between phoria and tropia

Disadvantages

• Requires practice

Cover Test Procedure

• Performed before (optionally after) assessing refractive error
• Lights on, spectacles on
• Accommodative target, one line worse than the poorer eye
• Position yourself for a good view

Distance

• Check at both $6m$ and $40cm$
  phorias can be a different size at distance and near.
  A tropia might be present at distance but only a phoria is present at near

Direction of Deviation

• Eso: Convergence of visual axes
• Exo: Divergence of visual axes
• Ortho: No deviation
• Specify if phoria or tropia (e.g., exotropia, exophoria)

Vertical Deviations

• Hyper: Given eye is higher
• Hypo: Given eye is lower
• Only the hyper eye is recorded

Cyclo Deviations

• Excyclo: Outward rotation of the upper limbus
• Incyclo: Inward rotation of the upper limbus

Magnitude

• Measured in prism dioptres (Δ)
• $1 \Delta =$ the distance in cm that a prism displaces an image over a distance of 1m
• $7\Delta = 4 deg$ (roughly)
• $x\Delta = 1m \times cm$
• Estimate deviation in prism dioptres.
• Measured using prisms or prism bars.

Recovery

• Movement quick or slow?
• Slow or jerky recovery indicates decompensating phoria

Decompensating Phoria

• Compensated: No symptoms (headaches, diplopia, tired eyes, blurred vision) and no sensory adaptions (suppression)
• Decompensated: Symptoms or sensory adaptations present

Types of Cover Tests

• Cover/Uncover Test
• Alternating Cover Test
• Subjective Cover Test

Using Cover Tests to Estimate Size

|| >4Δ | 3-4Δ | 2-3Δ | 0.5-1Δ |
| - - - - - - - - - - - | - - - - - - - | - - - - - - - | - - - - - - | - - - - - - - |
| Cover/Uncover | Yes | Yes | No | No |
| Alternating | NA | Yes | Yes | No |
| Subjective | NA | NA | Yes | Yes |

Tropias

• Double but clear with double vision
  OR
• Single but suppression with compensatory single vision

Tropias: Types

• Esotropia
• Exotropia
  Visual axes are in different directions

Tropias: Frequency

• Constant: Present all the time
• Intermittent: Not there all the time, specify how often as a percentage

Tropias: Laterality

• Right or left eye: Deviation always present in the same eye
• Alternating: Either eye is able to fixate

Tropias: Age of Onset

• Congenital: First occurred within the first few years of life
• Acquired: Occurs after that (serious in adults)

Influence of Accommodation

• Link between accommodation and convergence can result in esotropia in uncorrected hyperopes.
• Accommodative esotropia: Deviation disappears with full refractive correction.
• Partially accommodative esotropia: Deviation partially disappears with correction.
• Non-accommodative deviations: Not improved by correction.

Cosmesis

• Rate cosmesis as good, average, or poor.
• Base decision on whether the deviation is obvious to other people

Record Keeping

• A useful tip!
  when I see movement in the eye that has been covered, I ask myself “where must the eye. be coming from the explain the direction it is moving?”
  for example, it I see the eye moving “in”, I know that it had been “out” under the cover, which indicated an exo deviation

Recoding Results

Examples are

• Dist unaided 3, XOP smooth recovery
• Near aided 15 constant L XOT
• Dist aided 4 RHP jerky recovery
• +(this means ortho horizontal and vertical)
  DO not write “eso” alone - is it a tropia or a phoria?

Expected Findings

• At distance, $1\Delta$ exophoria $+- 2\Delta$
  So most people will have between 3 esophoria to 5 exophoria.
• At near, $3\Delta$ exophoria $+- 3\Delta$
  So most people will have between 3 esophoria and 9 exophoria near.
• Deviations of 2Δ or less cannot be seen, but can be detected by the subjective cover test.

Accepted Abbreviations

• XO: Exo
• SO: Eso
• Hyper: Hyper
• ⊕: Ortho
• P: Phoria
• T: Tropia
• Δ: Prism dioptre

Subjective Phorias

Goals

• Explain principles of subjective phoria measurement.
• Explain what a patient sees during cover/uncover test.
• Confirm cover test results with Maddox rod and Maddox wing.

Concepts

• When both eyes are fixated on the same object, the retinal image is formed at corresponding locations in each eye
• What happens if a phoria is allowed to become manifest, so that the eyes no longer point in the same direction?

Crossed Diplopia

EXO

Uncrossed Diplopia

ESO

Subjective Phoria Measurement

• A phoria is defined as a misalignment of the eyes when binocularity is prevented
• Removal of binocularity is referred to as dissociation
  We have been doing this by covering an eye
  Can also be achieved by any other method that causes the image in the RE to differ from the image in the LE

Dissociation Techniques

• Distortion
  The view of one eye is distorted so that fusion is eliminated e.g Maddox Rod
• selective screening
  Selective parts of the visual field are seen by each eye independently with no common objects e.g Maddox wing
• Prismatic dissociation
  Large enough prism to overcome fusion perpendicularly to the phoria of interest will make the phoria become manifest
• Independent objects
  each eye is presented with an entirely different image e.g Synoptophore

Maddox Rod: Distance Phoria

• Vertical cylinders (about +60 DC) refract light in the horizontal direction only; therefore, transforms a spot source into a horizontal streak
• Red filter aids in easier recognition of the streak and greater dissociation

Expected Results and Recording

• The results should correspond to the cover test and are recorded in the same way, but in the “muscle balance” box instead
• Don't forget to record the type of test you used - in this case “Maddox Rod”

Reminders

• Record horizontal or vertical
• record the size and type of the deviation
• cyclo deviations cannot be assigned to either eye
• Horizontal, vertical and cyclo deviations can also appear in combination
• A cyclo deviation is almost always associated with a vertical deviation
• Describe and measure each component separately

Limitations of Maddox Rod

• Control of accommodation is poor because there is no accommodative target
• Cannot discriminate between phoria and tropia (use cover test for this)
• Recovery information not available

Common Errors

• Induced vertical phoria because the trial frame is not fitted correctly
• Forgetting that accommodation control isn't good with this technique (which may influence results)
• Allowing prism adaptation by not removing prism in between presentations.

Maddox Wing

• Patient holds the Maddox wing via a handle on the bottom and looks through the eyepiece
• The septum means that RE sees only the arrows, and the LE sees only the numbers = dissociation

Limitations of Maddox Wing

Testing distance is short

• Large print reduces accommodative demand, thus accom/conv is minimised
• patient not holding the target, thus proximal convergence is minimised
• Can be awkward with spectacles/trial frame on.
• No recovery information

Common Errors

• Forgetting that this test is not performed at 40cm
• Forgetting to use the near correction
• Not checking that each eye is seeing the correct target
• Letting the patient tilt the Maddox Wing to the side
• Not clarifying between an exo and an eso deviation