How to measure fusional reserves in clinical practice

Understanding the dynamics of fusional vergence is an important part of optometric testing. The fusional reserves – the range or amplitude of fusional vergence, indicates the vergence system’s ability to maintain binocular vision.

Heterophorias represent the underlying tendency for the eyes to misalign when they are dissociated, for example when one eye is covered. A person must apply enough motor fusion to overcome this misalignment and restore the correct balance between convergence and divergence. Large phorias come with high vergence demand, and correspondingly, higher fusional reserves are needed to keep this balance in check.

If the degree of fusion is not enough to compensate for the phoria, it may result in decompensation, and lead to asthenopia, blur, diplopia, and sometimes suppression. Moreover, patients with intermittent deviations are likely to be near their motor fusion threshold and identifying their reserves provides valuable information.1 

Binocular vision anomalies have been associated with higher risk of developing myopia. Children with higher accommodative convergence (AC/A) ratios, typically seen with esophoria, have over 20-fold increased risk of myopia development within one year.2 Higher accommodative lag may be observed in individuals who undergo myopic shifts, regardless of whether they were originally myopic or non-myopic.3 Intermittent exotropia (IXT) has been linked with myopia onset, as more than 90% of children with IXT are predicted to develop myopia by 20 years of age.4 

Binocular vision disorders can have a significant impact on visual comfort. Reading, learning, other near visual tasks require comfortable, functional vision. If abnormal binocular vision is unaddressed, this may affect a child’s quality of life, psychological wellbeing, and potentially their academic achievement.5 

Binocular vision status can influence prescribing choices too. Some myopia control modalities, like orthokeratology, have been shown to alter binocular vision parameters. For example, esophoria and accommodative lag are likely to be improved in orthokeratology wear.6,7 On the other hand, several of the latest myopia control treatments (MiSight 1 day, Hoya MiYOSMART, Essilor Stellest) do not alter accommodation or vergence.8-10

Fusional reserves can be directly measured using either smooth or step (jump) vergence testing. 

Whereas smooth vergences involve a gradual, continuous increase in prism, step vergences introduce prismatic demand in larger, discrete steps.11 Although both are designed to assess the patient’s vergence amplitude, the findings are not interchangeable due to their low level of agreement.12 

Step vergence testing is performed outside the phoropter using a horizontal prism bar. This method allows you to observe objective fusion in free space, making it a practical technique with young patients or verbally uncooperative patients.(13) Otherwise, smooth vergence testing can be performed using the rotary prism in the phoropter. 

A small, detailed fixation target at near is also required. A picture stick or similar target with fine detail works well, as it provides a clear stimulus for fusion and is engaging for younger patients.

Positive (base-out) and negative (base-in) fusional reserves are measures of the amount of prism that can be placed in front of the eyes before the patients begins to experience sustained blur. Past this point, the patient will begin to use accommodative vergence alongside fusional vergence to maintain single binocular vision.13 

It is recommended to start with measuring the heterophoria, then testing negative fusional vergence (NFV) in the relaxed state, followed by positive fusional vergence (PFV). This helps prevent the impact of vergence adaptation on subsequent findings.14 For example, testing PFV initially may reduce the base-in amplitude when NFV is tested afterwards.

Steps:

1. Start by holding the prism bar in your right hand, and the fixation target in the left. This results in the prism bar being held in front of the patient’s left eye.
2. Measure NFV (base-in fusional reserves) with one of two methods:
   • Jump vergence: increase the prism demand step by step by moving the prism bar upwards, asking the patient at each step whether the target appears single or double.
   • Jump facility vergence: hold the prism bar at a given prism step, watch for fusion, then remove the prism to allow recovery, then advance to the next step and repeat.
3. Increase the prism demand until the patient reports diplopia or until wobbly, horizontal eye movements are seen – this is the break point. Patients may not spontaneously report diplopia, so objective observation is important.
4. Reduce the prism demand until single vision is restored – this is the recovery point. 
5. The blur point (when the target first becomes unclear) can also be recorded, though this is less reliable in younger children and accommodation testing can provide complementary information.
6. Record results as break/recovery or blur/break/recovery.
7. To test PFV (base-out fusional reserves), swap the prism bar to your left hand and the fixation target to your right. Repeat steps 1–6.

If a cover test is performed between each component of testing (NFV – cover test – PFV – cover test – NFV – final cover test), this sequence helps detect changes heterophoria and fusional reserves after fatigue. For example, if the patient becomes more esophoric after PFV, this suggests that they may struggle with fatigue throughout a day at school or work.

The standard assessment sequence of NFV (divergence), PFV (convergence), NFV (divergence) is appropriate in most cases. 

However, if a large exophoria has already been identified on phoria measurement (e.g. cover test), begin with PFV (base-out fusional reserve) instead. Diverging the eyes even further before assessing convergence range is of limited value, as the priority is to see how well the patient can overcome their exophoric deviation. 

• In this case, start with the prism bar held over the patient's right eye, measure base-out prism range, then move to base-in over their left eye. If both NFV and PFV are reduced, consider vergence infacility as a differential rather than a basic phoria finding.

Sheard's criterion specifies that for sustained comfort, the fusional reserves should be at least twice the demand (i.e. the heterophoria).15 If the fusional reserve is insufficient, this suggests the patient is likely to be symptomatic.

• A patient with esophoria requires NFV (base-in fusional reserve) of at least twice the esophoria magnitude.
• A patient with exophoria requires PFV (base-out fusional reserve) of at least twice the esophoria magnitude.

From the myopia management point of view, NFV (base-in fusional reserves) are of particular interest. Given the established link between esophoria and myopia risk, having enough base-in fusional reserve is important to balance any esophoria present.

• Assessing fusional reserves in addition to heterophoria provides a more complete picture of binocular stability than either measurement alone.
• The most practical way to measure fusional reserves is through step vergences using a prism bar. This allows you to observe the patient’s eyes for break in fusion and recovery, rather than relying on subjective reporting.
• Apply Sheard's criterion to interpret results – opposing fusional reserves should be at least twice the phoria magnitude.

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