Fusional reserves are easy to assess using a prism bar, and with some keen observation can become an objective measure of your patient’s ability to maintain the correct balance of convergence and divergence at near. There is a reported association between higher levels of esophoria and accommodative lag at near in myopic children and young adults as compared to emmetropes.1-5 Studies of progressive spectacle lens prescription for myopia control show a link between faster progression and hence better treatment results in children with esophoria and accommodative lag,6,7 and myopic children with esophoria specifically selected for bifocal soft lens myopia control have shown strong results.8,9 Myopia development and progression is multifactorial, so it’s important to assess all contributory factors – the myopia profile is designed to guide you through this clinical decision making and communication process.
Assessing horizontal fusional reserves at near gives you the measure of your patient’s stamina, or ‘petrol in the tank’ to be able to manage their phoria, or ‘posture’. I describe this to patients using the analogy of a bad back – if I have a bad back posture (big esophoria), but strong muscles to control it (normal base-in divergent fusional reserves) then I may not be symptomatic. But if I have a bad posture (big esophoria) and weak muscles – a low amount of ‘petrol in the tank’ (reduced base-in divergent fusional reserves) then my system will fatigue readily. Extending this further – a borderline posture (small-moderate esophoria) with weak muscles (low base-in fusional reserves) can also be an issue.
Grab your horizontal prism bar and an interesting near target – I use “Mr Camel” or a small picture stick. What, you say you don’t have a horizontal prism bar? Gasp! Please order one immediately! Binocular vision workups are infinitely easier using a prism bar, but until your new favourite tool arrives, you can use the rotary prism in your phoropter, which measures response to increasing vergence demand in a smooth presentation, as opposed to a jump presentation with the prism bar. It’s hard to see what’s happening though, for both you and your patient – with your prism bar you can observe objective fusion in free space making it simple to measure fusional reserves on very young patients…. You really should get a prism bar. Here's my loving my prism bar, below. 🙂
In almost all cases, I start by holding the prism bar in my right hand and the fixation target (hereby called Mr Camel, as it’s less letters than ‘fixation target’) in my left. This puts the prism bar over the patient’s left eye, base-in, to measure divergent fusional reserves. You have two measurement options now:
- Jump vergence – move the prism bar up, up, up while asking the patient if Mr Camel has one head or two.
- Jump facility vergence– hold the prism bar up, watch for fusion, then pull it off for the eyes to recover back to normal demand, then hold the next strongest step up, watch for fusion, then pull away again for recovery and repeat.
Watch for smooth divergence and maintenance – if you see a case of wobbly eyes, flicking horizontally between two images, you know that your patient has diplopia even if they’re not reporting it. Continue until you see wobbly eyes and/or the patient reports seeing two, in which case you have got your BREAK point. From there step back until they achieve one again, which is your RECOVERY point.
Switch the prism bar to your left hand and Mr Camel to your right hand, hold the prism bar up base-out over the right eye, and you’re now ready to measure convergent fusional reserves. Jump up and up and up until the patient reports two for your break point, then reduce the prism demand until they regain fusion for your recovery point.
I perform a quick cover test between each component of the fusional reserves test – diverge eyes (base-in), cover test, converge eyes (base-out), cover test, then repeat divergence (base-in) and cover test to assess for any changes. The latter gives you clinical information on how your patient may function with fatigue – if they become more esophoric after only a short amount of convergence (your base-out test), then on repeating your divergent (base-in) test you get a much lower value than what you got initially, this patient may be more likely to fatigue significantly throughout a day at school or work.
You can also ask the patient to tell you when Mr Camel starts go fuzzy – your BLUR point. Fusional reserve results can be recorded as your blur / break / recovery results, or more often I record break / recovery. Blur is a trickier one to assess in younger children and your accommodation tests can give you further information, so I don’t usually ask my patients for this.
The only case in which I won’t follow this divergent, convergent, divergent assessment pattern (prism bar on left eye base-in, on right eye base-out, then back to left eye base-in again) is if I’ve already seen an enormous exo-deviation on prior cover test. I always measure near phoria before measuring fusional reserves, and if there’s a large exophoria, there’s not a lot of use in diverging the eyes even more before then seeing how well the patient can converge to control the exo. In this case, I’ll start with the prism bar held over the patient’s right eye, base-out, then move to base-in over their left eye. You still need to assess both convergence and divergence to check if it’s just an exo problem or, if both show low results, a vergence infacility.
Sheard’s criterion describes whether a phoria is likely to become decompensated (symptomatic) in reference to the opposing fusional reserves, which should be at least twice the value of the phoria.
- An esophoric ‘posture’ requires at least twice as much ‘petrol in the tank’ as assessed on divergent (base-in) fusional reserves
- An exophoric ‘posture’ requires at least twice as much ‘petrol in the tank’ as assessed on convergent (base-out) fusional reserves.10
From the myopia control point of view, we are especially interested in divergent (base-in) fusional reserves, to balance any esophoria which carries a higher risk for myopia development and progression. However keep in mind the patient with a low phoria who shows low fusional reserve results – they may satisfy Sheard’s criterion but in fact have binocular instability or vergence infacility. To delve more into this detail, invest in my favourite BV textbook Pickwell’s Binocular Vision Anomalies – you can purchase chapter PDF’s online or buy the book from Amazon or Elsevier.
Normal results will be influenced by your method of testing, and I use the jump facility vergence method which places extra demands on the vergence system and hence elicits reductions and fatigue more easily. Being a primary care topic of little controversy, it’s near impossible to find any comparative research on methods so I’ve included the ‘normals’ I’ve determined clinically over more than a decade and applied to diagnostic and management decisions.12
You can read more on my clinical approach to prescribing adds for near esophoria, from a myopia control point of view.
To delve more into my simplified two-system approach to BV diagnosis and management, you can watch this video entitled Binocular Vision - easier than you think. This one hour lecture includes cases; details easy use of prism correction for vergence disorders; changes to BV in contact lens wear; and why BV matters - for reading and learning in kids, clinical problem solving, and myopia management. You can also download my lecture notes here.