Step Vergences; Sheard’s Criterion, Vergence Facility

Horizontal Step Vergence

Overview

• This test is an objective rather than subjective assessment of the ocular vergence system, designed to measure the ability of the eye muscles to maintain single vision under varying prism induced conditions.

• The test is conducted outside of the phoropter using prism bars, and is performed at both distance and near settings to assess the full range of eye muscle function.

Set-up

• Prism Selection: Select the appropriate prism bar with varying magnitudes placed in front of one eye, being sure to position it close to the patient's eye for accurate measurements.

• Lighting Conditions: Ensure that the room lights remain on to provide adequate visibility for the target letters.

Target

• Distance Testing Target: Use an isolated letter 1-2 lines above the patient's Best Corrected Visual Acuity (BCVA) to ensure that the target is challenging but still within the patient's capability.

• Near Testing Target: Similarly, utilize an isolated letter 1-2 lines above BCVA for near assessments, typically employing a fixation stick to focus attention on the letter.

Procedure

Initial Steps

• Prism Alignment: Verify that the patient is focusing through the prism rather than the lines of the prism magnitudes themselves to avoid confusing visual inputs.

Instructions to Patient

• Clearly communicate, "This test measures how well your eye muscles can keep objects clear and single. I will adjust lenses and you may notice the letter appearing doubled. Please let me know when that happens and try to keep the letters as clear as possible."

Testing Steps

1. Begin with Zero Prism: Start by placing no prism over the patient's right eye (OD) to establish a baseline measurement.

2. Introduce Base In (BI) Prism: Gradually introduce Base In prism over OD at a speed of approximately 2/3Δ per second, allowing for smooth adjustments.

3. Monitor for Blur: As the patient reports seeing blur, record the prism amount at which this occurs without pausing the movement. This is critical in understanding the patient's initial vergence capacity.

4. Identify Break Point: Continue to increase the prism until the patient reports seeing diplopia (double vision), marking this point as the Break Point, indicative of the limit of the patient's fusional ability to maintain single vision.

5. Identify Recovery: To establish the Recovery Point, overshoot the Break Point slightly and then gradually decrease the prism until the patient reports perceiving a single image again. This assists in understanding the dynamic range of recovery in the patient's binocular vision.

6. Repeat for Base Out (BO): Follow the same methodology for introducing the Base Out prism over the OD. This dual assessment is necessary to evaluate both convergence and divergence responses of the ocular muscles.

Observations During Testing

• Be attentive to the patient's eye movements; during the testing with BO prism, there should be a consistent movement of the eye under the prism towards convergence.

• Subtle changes in eye movements with small BO introduced prisms become more pronounced as the amount of prism increases.

• The objective break-point is recognized by a swift, large outward movement of either eye, indicating a critical limit of fusional control.

Patient Suppression

• Patients may suppress one eye during the test; similar to smooth vergences, if suppression occurs, the patient will depend on the opposite eye to focus on the target.

• During step vergence tests, the suppressed eye may appear straight, which indicates the influence of a large prism magnitude. This may disrupt expected convergence or divergence effects, indicating more complex binocular issues.

Recording Results

• It is vital to accurately document the final prism values for blur, break, and recovery points:

  • At Near: BI = X / 8 / 6; BO = 10 / 18 / 8

  • At Distance: BI = 12 / 18 / 14; BO = 18 / 24 / 16

• If the available prism runs out before the patient reports diplopia, temporarily remove the prism, reassess, and reintroduce it to ensure accurate measurements.

Vertical Step Vergence

Overview

• This section mirrors the setup and execution of horizontal step vergence tests but focuses on vertical adjustments to assess upward and downward vergence responses effectively.

Set-up

• Use the same prism bar in front of one eye close to the patient's eye, maintaining similar lighting conditions to ensure consistency in the testing environment.

Procedure

• Both the instructions given to the patient and the testing methodology are aligned closely with those previously described for horizontal testing, with specific emphasis on upward and downward movements in the eyes.

Compensating Fusional Vergences

Definition

• This type indicates the necessary vergence to effectively counterbalance a patient's phoric demand, crucial for understanding personalized treatment strategies.

Calculation of Vergence

• For Exophoria: Measure the amount of Positive Fusional Vergence (PFV) [Base Out] required before blur occurs.

• For Esophoria: Measure the amount of Negative Fusional Vergence (NFV) [Base In] before the introduction of blur.

• For Hyperphoria: Calculate the required amount of infravergence (Base Up); for Hypophoria, assess the amount of supravergence (Base Down).

Sheard's Criterion

Application

• To achieve comfort during binocular tasks, the blur and break points of the compensating vergence must be at least twice that of the existing phoria level.

Example

• For instance, if a patient has a phoria at near of 5Δ Esophoria and their smooth vergences show BI = 8 / 15 / 13 and BO = 12 / 16 / 10, they would require 10Δ of BI for optimal comfort. However, if their measurements only support 8Δ, the patient fails to meet Sheard’s criterion, indicating discomfort during vision tasks.

Vergence Facility

Purpose

• This metric evaluates the dynamics and flexibility of the vergence system, particularly in patients experiencing symptomatic binocular vision issues, which can help inform treatment paths.

Test Setup

• Testing requires the patient's habitual prescription, measuring targets at the level of Best Corrected Visual Acuity (BCVA).

• The prisms used include 3Δ Base In and 12Δ Base Out, which have been shown to be effective in distinguishing symptomatic patients from those without issues.

Recording Results

• Results should be documented based on the number of cycles achieved within one minute: formatted as "Vergence Facility at distance: x cycles/minute (12BO/3BI)." Normal values typically hover around 15 cycles per minute at near.

Clinical Considerations

• It is vital that the tests are explained to patients in a straightforward manner, and all ocular movements are closely observed during the assessment.

• Careful management of prism introductions is essential for obtaining accurate and meaningful data.

• Be vigilant for whether the patient experiences single but blurry vision, which can indicate insufficient fusional reserves necessitating accommodative assistance for improved visual clarity.