Binocular Vision FAQs. Your questions answered.

Binocular Vision was always one of my worries as a trainee.  But why should that be?  Is is poorly taught at university perhaps?  Well, the answer to that is probably not.  For some reason it is one of those topics that many have had trouble getting their heads around.  And yet , there are only 6 muscles in the eye..  aren’t they?  Fact is, most of us focus our worries on incomitant deviations.  However, incomitance is a very small (although significant) part of the BV topic.   Indeed, most of the “BV” subject matter on a day to day basis is actually much more simple.  After all, when was the last palsy you encountered?  Quite.

Simon Frackiewicz is both an optometrist and an orthoptist.  There are one or two of them about.  Not many, but in addition he’s a rather good teacher.  Here’s what he has to say..

FAQs. : Binocular Vision

Q:  If a child has exophoria at distance and near, is amblyopic in one eye with no strabismus, and on refraction you find a hypermetropic Rx. Would a cycloplegic refraction be required or is it OK to prescribe from a non-cycloplegic refraction since there is no squint?

A: You are correct to question the prescribing of hyperopic prescriptions to patients with exophoria, particularly bearing in mind that cycloplegic refraction can elicit larger amounts of plus than in the normal state. It follows that as plus lenses reduce accommodation, and thus accommodative convergence, an exo-deviation would increase with a hyperopic Rx.

My first question is to ask why would such a patient be amblyopic? If strabismus is excluded as a cause, and supposedly there is no stimulus deprivation, then a refractive aetiology must be suspected. That is assuming that the reduction in vision is truly due to amblyopia – was the child sufficiently co-operative to ensure the vision was measured accurately? Could there have been a small-angled deviation which is difficult to see due to poor fixation?

Secondly, if we assume that the reduction in vision is genuinely due to amblyopia, and that the patient is asymptomatic, the principal purpose of giving a prescription is to improve the vision rather than to manipulate the BV system. As such, the aim should be to correct any difference in prescription that may have given rise to anisometropic amblyopia, by giving the eye with the lower Rx a plano lens. Alternatively, if there is a significant degree of astigmatism, you could give the necessary correction but give the lowest amount of plus required to keep the mean sphere at zero.

In any case, ensure you check the control of the deviation before and after prescribing a significant amount of plus to ensure you have not caused the deviation to decompensate.

Q: What do I do if I have a Px presenting with a longstanding alternating exotropia? They have good vision in both eyes, say 6/6 R and L, is RE dominant and has never complained of diplopia. Do I,
a) Leave them as they are, with the alternating XOT, as they have no complaints, or
b)Try and use prism to make them binocular again as they have the potential to have BSV?

Also, does the size of the deviation matter when determining the management options?

A: As the patient is asymptomatic, leave well alone. Such patients often have large deviations and would need such a big prism to make them binocular that it becomes totally impractical.
In general size is important when determining management options, particularly when considering prisms or surgery.

Q: We have recently changed our testing charts to electronic screens which are not polarised, thus dissociation is carried out by using coloured filters. I am finding when using fixation disparity on Pxs presenting with large phorias and symptoms of decompensation, the Px finds it very difficult to understand the test. It seems that the filters are not really dissociating the eyes, resulting in too many green, red and black lines.

My question thus is: what other tests can I use to determine the level of compensation after I have prescribed prisms to help control the phoria? If that makes sense!!!

A: The problem with your screen does sound like a mis-match between the colours of the filters and targets. My chart (Test Chart 2000) allows you to alter the colours to get the perfect match.

Failing this, you can use Sheards or Percivals criteria to calculate the optimum prism, by combining results from fusion range and angle of deviation. Alternatively, you can simply compare the subjective comfort of prisms by gradually increasing the power until there is no further increase in comfort, thereby giving the minimum amount necessary. Not very scientific I know, but better than nothing.

Q: If my Px experiences diplopia on one position in motility testing, what do I do next in the exam situation? Will I have time to go on and test ductions? Is it ok to stop there and say you will call Px back for a BV work up, or refer for Hess screening?

A: Personally, I would say it is superfluous to do the Mallett test in the absence of movement on cover test, however, if there are symptoms of asthenopia, it may pick up a tiny uncompensated vertical deviation which is hard to see on cover test, so dont rule it out. Avoid correlating size of deviation with need to do the Mallett unit – a tiny deviation may be symptomatic and show slip whereas a large deviation may be well compensated, so the indication for doing the Mallett unit, in my opinion, should be the presence of symptoms.

Re: diplopia on motility, it is good practise to do a cover test in all positions during motility, thereby effectively testing ductions and versions. If the diplopia is incidental, i.e. not the presenting symptom, it may be appropriate to say you will call back for BV work up. Ultimately, you need to choose how best to spend your time on your routine and do the tests which are appropriate to help you decide how to manage your patient.

Q: When someone has a microtropia, you can check by doing the 4 BO prism test. I was under the impression that putting the prism in from of the normal eye would cause it to deviate, but also the microtropic eye would also deviate through Herrings Law but not make the correcting movement. Having read through Kanski I get the impression that the microtropic eye would make no movement? Is this the case or have I just misunderstood Kanski?

A: Microtropia  with the prism in front of the non strabismic eye, both eyes will make the initial version movement, but only the non strabismic eye will make the second vergence movement. With the prism in front of the strabismic eye, nothing will happen.

Q: Was wondering in the CDM ocular mobility section – it provides you with cover test, NPC and motility results. If their near phoria is greater than their distance phoria but with no problems do you just ignore it? Only recommending orthoptic exercises such as pen to nose and dot card exercises if they are having reading problems? Or if their distance phoria was greater than their near would you recommended lens flipppers for accommodation or prism flippers to increase fusional reserves? Or is it always only if the Px is symptomatic? Also if motility was slow and jerky and over the age of 30 what would I do?

A: In my experience, phorias only require intervention if they are causing symptoms. Admittedly, young children may not be able to elucidate their symptoms, therefore particular attention must always be paid to the speed and quality of recovery on cover test.
The examples given in your question imply you are referring to EXOphoria. Indeed, an exophoria larger at near (convergence weakness exophoria) can be associated with convergence insufficiency, but not in all cases, thus orthoptic exercises would only be indicated in the presence of symptoms. Equally, an exophoria larger in the distance than near (divergence excess exophoria) does not automatically cause any problems to the patient, thus treatment is not always required.
The case of ESOphoria is quite different, as an esophoria greater at near (convergence excess esophoria) in a child may be a precursor to fully accommodative or convergence excess esotropia, and should be investigated accordingly, even if there are no symptoms. Additionally, an esophoria greater in the distance than near (divergence weakness esophoria) can indicate a weakness of the lateral recti, potentially arising from abducens nerve palsy, which therefore require appropriate investigation.
Your final question about slow and jerky ocular motility is harder to answer definitively, thus I would recommend assessing the ocular deviation in all gaze positions and looking for a pattern of underactions and limitations. Defects of the smooth pursuit system per se are generally the result of a higher brain injury and are not common.

Q: Is there a condition in which our vertical fusional reserves are increased more than the normal 4 prism dioptres? If so, do you have any idea what it may be??

A: It is not uncommon to see increased vertical fusional reserves in long-standing vertical eye muscle dysfunction. One common example is the wet phase of Thyroid Eye Disease as the changes to muscle function happen quite slowly, thus the brain is able to adapt to the increasing deviation. One particular patient I treated as an Orthoptist had over 20 dioptres of vertical fusion range, and I am sure there are other documented examples which are even larger.


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