I describe near heterophoria to my patients as where their eyes aim in space – the posture of their vergence system – and esophoria is our key enemy in the myopia control battle. 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.
There are numerous methods to measure heterophoria, and for a full description, refer to my favourite textbook on the topic – Pickwell’s Binocular Vision Anomalies, authored by my BV hero Professor Bruce Evans. In clinical practice, almost all patients will have some amount of heterophoria, but only the decompensated and symptomatic phoria requires treatment.10 The magnitude of the phoria itself may not relate directly to symptoms, but more so whether the phoria exceeds the ability of the fusional reserves to control it. I’d suggest reading this ‘how to’ along with my post on assessing fusional reserves at near.
It is important that the patient can see clearly at near before measuring phoria, as accommodative input is a major influence on vergence control along with proximal and tonic cues.10 It’s also important to pick a target interesting and small enough to stimulate accommodation – I use “Mr Camel” or a small picture stick, both pictured above.
The COVER TEST allows you to evaluate the direction and magnitude of the phoria, and the quality of the recovery. For most clinical purposes, I use a swift alternating cover test and will grade a phoria as small, medium or large with additional information on the speed and ease of recovery – is it swift and smooth; is it slow; is it two-phase where two jerking eye movements are observed; does it break down into a tropia? I’m also interested to see if the cover test changes during fusional reserve measurement, and repeat it several times throughout testing. After convergent (base-out) fusional reserves, does the cover test become more esophoric? This is all useful information to gain a sense of how your patient visually functions in their daily environment. Neutralising the movement with a prism bar (base-in for exophoria and base-out for esophoria) will allow you a quantification of this objective method.11
The HOWELL NEAR PHORIA CARD allows for subjective quantification of the near phoria. Using at least a 6 BD prism in front of the R eye for dissociation, a double image of the numbered line and downward pointing arrow should be seen. Results on the even numbered blue side indicate exophoria, and the odd numbered yellow side esophoria. A continuous presentation where the patient is allowed time to settle on a particular number has shown better inter-examiner reliability than a flashed presentation where the instantaneous result is asked of the patient.12
Normal results for near phoria generally fall slightly on the exo side, with a mean of 3 exophoria and a standard deviation of 5 indicating 68% of the population lie between 2 esophoria and 8 exophoria.10
Other subjective methods include von Graefe’s and Maddox rod which use different methods to dissociate the images of the two eyes and then rely on the patient reporting when the introduction of sufficient horizontal prism produces superimposition of the two images.10 I couldn’t find any comparison of objective cover test to subjective methods in the literature, but did find variation between subjective methods.12 My thoughts on the topic are that since the Howell card provides the patient an extra proximal cue, through holding the card, you may get a smaller phoria result than by cover test. However unless you have three hands, you’ll need the patient to hold the fixation target for prism neutralisation of the cover test, so then the test results may be similar.
Just like your subjective refraction and binocular balancing technique, we can each use slightly different approaches which can influence the outcome, but within an acceptable variability. Your patient’s phoria results will depend on the distance of presentation (33cm for kids and teens, 40cm for adults to reflect habitual working distances), what tests have been done beforehand and even what the patient has been doing that day before attending their eye exam. Overall you are best to develop and persist with a technique which is repeatable, easy to administer and makes clinical sense to you.
Can your patient handle their phoria, or are they likely to become symptomatic? Now that you understand the ‘posture’ of their vergence system, the next step is evaluating their stamina, or ‘petrol in the tank’ to maintain appropriate vergence, through their horizontal fusional reserves at near. After you’ve got this information, you can then read about 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.