8. Accommodation & vergence assessment - Part 2

[Audio] Assessment of Vergence. Assessment of Vergence.

[Audio] Vergence Vergence is the simultaneous movement of both eyes in opposite directions to obtain or maintain single binocular vision. At is a critical component of the visual system, enabling the eyes to align accurately on a target at various distances. Unlike other eye movements, vergence involves both convergence (inward movement of the eyes) and divergence (outward movement of the eyes), which allow for proper depth perception and focused vision across different viewing distances. Vergence testing assesses the ability of the eyes to perform these movements effectively and maintain single, clear, and comfortable vision. This testing is essential for diagnosing and managing binocular vision disorders, which can manifest as difficulties with reading, double vision, or eye strain. Common vergence tests include measuring the near point of convergence (NPC), assessing vergence ranges (using prism bars or Riley prisms), and evaluating the fusional vergence system through tests like the prism fusion range or vergence facility tests. These tests provide valuable information about the strength, flexibility, and endurance of the patient's vergence system, aiding in the diagnosis and treatment of various visual conditions..

[Audio] Assessment of Vergence includes: Assessment of Near Point of Convergence (NPC) Assessment of Vergence Range Assessment of Fusional Vergence.

[Audio] Assessment of Near Point of Convergence (NPC) The Near Point of Convergence (NPC) is the closest point at which an object can be viewed as single with both eyes working together (Bifoveal vision) In other words, it represents the point where the lines of sight from both foveae converge when maximum convergence is achieved. The test can be performed using a simple ruler or a more specialized device like the Royal Air Force (RAF) Rule..

[Audio] Procedure Using a Ruler: Instruct the patient to fixate on a small target (e.g., a pen tip or a small dot) positioned at arm’s length directly in front of their nose. Gradually move the target closer to the patient’s nose along the midline, asking them to report when they see double (diplopia). Measure the distance from the target to the tip of the nose at the point where the patient first reports diplopia. This distance is recorded as the NPC.

[Audio] Procedure Using the RAF Rule: The patient wears his habitual distance/near correction. Ensure the room is well-lit with normal room illumination. The examiner holds the RAF Rule with the cheek rest placed gently on the patient’s inferior orbital margin. Instruct the patient to focus on the black dot The target is initially held at a distance of 50 cm from the patient’s eyes. Subjective At: The examiner moves the target toward the patient’s eyes at a constant rate of 1-2 cm per second. The patient is instructed to report when the target first appears double (diplopia). This point is known as the subjective break point. The target is then moved back, and the patient is asked to report when they see the target as single again. This is recorded as the subjective recovery point. Objective At: While the target is moved towards the patient, the examiner closely observes the patient’s eyes for any loss of fixation, particularly eye divergence from the target. The moment the examiner notices that one eye deviates or loses fixation is recorded as the objective break point. As the target is moved back, the examiner notes when both eyes regain focus on the target. This is the objective recovery point..

[Audio] The test is repeated 4-5 times as recommended by Which and Mahindra to ensure consistency and accuracy. Record the distance (in cm) from the patient’s eyes at which the subjective and objective break points occur. Similarly, record the recovery points for both subjective and objective measurements. These measurements provide valuable information about the patient’s convergence abilities and can help in diagnosing conditions like convergence insufficiency. Interpretation: Normal NPC: Typically ranges from 5 to 8 cm. Abnormal NPC: A distance greater than 8 cm may indicate convergence insufficiency or other binocular vision anomalies. RAF rule NPC fixation target.

[Audio] Assessment of Vergences Positive Fusional Vergence (PFV) and Negative Fusional Vergence (NFV) Vergence testing evaluates the ability to sustain binocular single vision by measuring the ranges of Positive Fusional Vergence (PFV) and Negative Fusional Vergence (NFV). These tests are conducted while maintaining a constant stimulus to accommodation, ensuring that any changes observed are due to vergence mechanisms alone. Vergence testing is a common method for determining the amplitude of the fusional vergence response for both PFV and NFV. The blur point represents the amount of fusional vergence that occurs without the involvement of accommodation. The break point indicates the total amount of fusional and accommodative vergence before diplopia occurs. The recovery point assesses the ability to reestablish fusion after diplopia has occurred..

[Audio] Equipment 1. Horizontal prism bar 2. Near fixation target 3. Distance fixation target.

[Audio] Procedure: Distance Step Vergences Ensure the patient is adequately corrected for refractive error. In moderate room lighting, project an isolated letter two lines above the patient's best-corrected visual acuity (BCVA) onto the wall screen or monitor. Position the prism bar in front of one of the patient’s eyes, beginning with the lowest power placed either base-in (BI) or base-out (BO). Gradually increase the BI or BO power of the prism bar at a rate of 2 Δ per 1-2 seconds, allowing time for the patient to fuse if possible. Vergence ranges can be determined either objectively or subjectively. Objective Measurement (BI) Instruct the patient to keep the target clear and single for as long as possible. Observe the eyes behind the prism as you increase the prism amount. The eye will make a series of small abduction movements (BI prism will cause the eyes to diverge) until it makes a large adduction movement, indicating the occurrence of diplopia and the break in fusion. Note the amount of prism that causes the break point. Decrease the BI power at the same rate that you increased it and observe for a fusion movement. This movement may occur in the eye with the prism, or as a version-vergence movement involving both eyes. The version-vergence movement is seen as an initial version (conjugate eye movement) followed by a vergence movement (deconjugate eye movement of one or both eyes). Some of these movements are subtle, so observe closely. Record the break and recovery findings. (A blur point cannot be objectively determined.).

[Audio] Subjective Measurement (BI) Instruct the patient to keep the chart as clear as possible. Instruct them to notify you when the target becomes blurred, double, or starts to move. Begin to increase the power of the prism bar before one eye. When the patient reports a blur, give them a few seconds to clear it before moving on to the next incremental increase in prism. The recorded blur is the first sustained blur. Remember, a blur at distance during negative fusional vergence measurement means the accommodation was not entirely relaxed. Note: while a blur finding may be recorded for step vergences, there are no expected norms for blur. Continue to increase the prism power until the patient reports double vision or the target starts to move to the left or right. Note: If the prism bar is placed over the right eye and the target moves to the right, then the left eye is suppressing. If the target moves to the left (with the prism bar over the right eye), then the right eye is suppressing. Record the magnitude of the prism as the break point. Begin to decrease the power of the prism until the patient reports that the two images have become one (single), and record this as the recovery finding..

[Audio] Objective Measurement (BO) and Subjective Measurement (BO) Position the prism bar base-out (BO) in front of one of the patient’s eyes. Increase the BO prism at the same rate that you increased the BI prism. You may perform either objective or subjective measurements. Continue increasing the BO prism power until one of the eyes (usually the eye under the prism) makes an abduction movement. Record this point as the break point. Reduce the prism power until the eyes are seen to recover fusion. This recovery may be a simple vergence movement, or it may involve a version followed by a vergence movement of one or both eyes. Note the break and recovery points. Some movements are small and subtle, so observe closely..

[Audio] Procedure: Near Step Vergences Place the target, such as a Gulden Fixation Stick with a single column of 20/30 letters, at a distance of 40 cm from the eyes, positioned directly in the primary gaze. Ensure that both ambient and overhead lighting provide adequate illumination for the target. Follow the standard steps used for distance step vergence testing. To test negative fusional vergence, place a base-in prism over the right eye and gradually increase the prism power until the patient reports double vision or you observe a loss of fusion. For positive fusional vergence testing, place a base-out prism over the left eye and similarly increase the prism power until the patient reports double vision or you observe a loss of fusion..

[Audio] Vergence Facility Test The purpose of vergence facility testing is to assess the performance of the vergence system under time-dependent conditions, evaluating both speed and resistance to fatigue. At is particularly useful in identifying issues with vergence dynamics, such as latency and velocity, which may contribute to symptoms even when standard vergence amplitude tests appear normal. Advantages Vergence facility testing is the only test that measures the dynamics and stamina of the vergence system. At helps differentiate between symptomatic and asymptomatic patients who might have normal vergence amplitudes but struggle with vergence facility. Equipment Near fixation target (e.g., Gulden fixation stick) 3 Δ BI/12 Δ BO combination prism.

[Audio] Procedure Position the target (e.g., Gulden Fixation Stick with a single vertical column of 20/30 letters) in the primary gaze at 40 cm from the eyes. Ensure ambient and overhead lighting provide adequate illumination. Instruct the patient to focus on the fixation target while wearing their habitual correction. Begin by placing the 3 Δ BI (Base-In) side of the prism in front of the patient's eyes. Ask the patient to make the letters single and clear as quickly as possible, and to say "now" once the letters are clear. After the patient reports clarity with the BI prism, quickly switch to the 12 Δ BO (Base-Out) side and repeat the process, instructing the patient to report when the letters are again single and clear. Once the patient understands the task, start timing the test for one minute using a stopwatch. Alternate between the BI and BO sides of the prism throughout the minute, counting the number of times the patient successfully makes the target single and clear. Record the number of prism cycles as cycles per minute (cpm), which is the total number of times the patient cleared both BI and BO prisms divided by two. The test can be performed either subjectively (based on patient feedback) or objectively. When performed objectively, the examiner observes the patient's eyes to ensure they diverge and converge appropriately in response to the prisms. The patient can complete 18 flips in 1 minute. This equals 9 cpm. If the patient cannot do either base-in or base-out, record as 0 cpm, fails base-out or base-in. If the patient cannot do both, record as 0 cpm, cannot do base-in or base-out. Normal Results: Approximately 15 cycles per minute (cpm) ± 3 is considered within the normal range..

[Audio] AC/A ratio (Accommodative Convergence/Accommodation Ratio) AC/A ratio, is a measurement of changes in accommodative convergence in prism diopters induced when the patient exert or relax 1diopter of accommodation. Changes in the accommodation are either evoked by placing plus lens which relaxes accommodation or by minus which activate accommodation or when a person fixates near the target. The assessment of the AC/A ratio is significantly important. AC/A ratio plays a crucial role in reaching a final diagnosis and is an important factor to consider in determining the appropriate treatment approach. E.g., in a High AC/A ratio patients respond well to lens treatment whereas, in the case of a normal or low AC/A ratio, the appropriate treatment approach should be prism and/or vision therapy..

[Audio] There are two methods used clinically to determine the AC/A ratio Calculated heterophoria method Gradient method.

[Audio] Calculated Heterophoria method Calculated heterophoria method is based on the difference in deviation between distance phoria & near phoria and this method takes account of patient Interpupillary distance (IPD), which is an important factor if accommodative changes are induced by placing the target at near. As convergence require for a patient with a larger IPD is more compared to a patient with a small IPD, when the target is placed 40cm far away from the eyes. AC/A ratio is determined by using the formula: AC/A Ratio = IPD (cm)+NFD (meters)×(Hn−Hd) Note: Esophoria is plus, Exophoria is minus. Were; AC/A ratio (Accommodative convergence/ Accommodation ratio) IPD: Interpupillary Distance in centimeters NFD: Near Fixation Distance in meters (typically 0.4 meters for a 40 cm target) Hn: Near Phoria (in prism diopters) Hd: Distance Phoria (in prism diopters).

[Audio] Procedure: Make sure the patient is wearing their appropriate correction (distance prescription). Measure the patient’s binocular interpupillary distance (IPD) in millimeters. Ask the patient to fixate on a target 6 meters away and measure the patient’s distance phoria. Ask the patient to fixate on a near target at 40 cm and measure the patient’s near phoria. Example Calculation: Patient IPD: 70 mm (7 cm) Distance Phoria: 2 PD esophoria Near Phoria: 6 PD esophoria at 40 cm Calculate the AC/A Ratio: Convert IPD to Centimeters: IPD=7 cm Near Fixation Distance: NFD=0.4 meters Difference in Phoria (Hn - Hd): Difference=6 PD (near)−2 PD (distance)=4 PD Apply the Formula: AC/A Ratio=7+0.4×4=7+1.6=8.6 AC/A Ratio: AC/A Ratio=8.6:1.

[Audio] Gradient Method The gradient AC/A ratio measures the amount of convergence produced by a diopter of accommodative effort. This ratio is determined by inducing changes in accommodation using plus or minus lenses and observing the corresponding changes in convergence (phoria). AC/A ratio is determined by placing +1.00 DS or +2.00 DS in frontof each eyes. Determine the AC/A ratio using the formula: AC ratio = Phoria with additional minus lenses – Baseline phoria A Power of additional minus lenses Note: Esophoria is plus, Exophoria is minus.

[Audio] Procedure: Ensure the patient is wearing their appropriate distance correction. Measure the horizontal near phoria without any additional lenses to establish a baseline measurement. Place +1.00 DS or +2.00 DS lenses in front of both eyes to induce a change in accommodation. Measure the horizontal near phoria again with the additional plus lenses in place. Example Calculation: Baseline Phoria: 2 exophoria Phoria with Additional Lenses (+2.00 DS): 12 exophoria Calculate the AC/A Ratio: Change in Phoria: Change in Phoria=12 exophoria−2 exophoria=10 prism diopters Change in Accommodation (Diopters): Change in Accommodation=2.00 D AC/A Ratio:10/2=5 AC/A Ratio Result: 5:1.

[Audio] REFERENCES Zhu-Tam, L., & Chung, I. (Eds.). (2024). The Pediatric Eye Exam: Quick Reference Guide: Office and Emergency Room Procedures. Citywide Eye Care Optometry, USA; College of Optometry, Western University of Health Sciences, USA. Hussaindeen, J. R. (2021). Keep It Single and Simple: Binocular Vision Testing Made Easy. Scheiman, M., & Wick, B. (2023). Clinical Management of Binocular Vision: Heterophoric, Accommodative, and Eye Movement Disorders (5th ed.). Pennsylvania College of Optometry at Salus University; University of Houston College of Optometry..