Optometrists have the ability to carve their own path to practice. During school and residency, it became evident that the way one optometrist practices may be dramatically different from another, in a given day. Some of us can spend the day chasing around little kids while others may spend a considerable amount of time positioning a geriatric patient into the exam chair. At some point, we begin to find a patient demographic that suits us and become fluent in the subset of treatment modalities that follow.

Optometrists have the ability to carve their own path to practice. During school and residency, it became evident that the way one optometrist practices may be dramatically different from another, in a given day. Some of us can spend the day chasing around little kids while others may spend a considerable amount of time positioning a geriatric patient into the exam chair. At some point, we begin to find a patient demographic that suits us and become fluent in the subset of treatment modalities that follow.

In my opinion, one of the best sources of knowledge that we have is our colleagues.

The clinical pearls we learn from one another can help grow our own practice. Sure, continuing education courses offered at major conferences uncover advances in treatment. But let’s face it since we outgrew that student/resident discount it has become increasingly hard to afford to attend every conference. There are already a few platforms such as ODs on facebook, where optometrists can shed insight into some of the cases that they encounter. In doing so, they reveal treatment strategies that we may not have considered otherwise. To promote that spirit of camaraderie, I decided to dedicate this article to share what I have learned from the unique clinicians and patients that I interact with on a daily basis.

I work in a tertiary care setting within a community health clinic where most of the patients are referred for binocular vision disorders. More often than not, these patients require vision therapy. Regardless of the severity of the case, the goal in our clinic is to remediate the visual dysfunction. In other words, we strive to strengthen the connection from the brain to the eye motor systems such that the patient performs as if the problem was never there. We achieve this by expanding upon vergence and accommodative flexibility as well as ensuring that visual response becomes automatic. Our goal of remediation is our tactic to prevent symptoms from resurfacing.

This applies to all ODs…

Some of you may be thinking that since your office does not provide vision therapy, this article has a moot point for you. Well, guess what…in any given case the first step of treatment for any binocular dysfunction, including convergence insufficiency, is to determine an appropriate optical prescription. While your primary care setting may not provide vision therapy, every optometrist does have the necessary tools to prescribe an Rx and monitor progress. In my experience, an appropriate lens prescription has the potential to improve stereopsis and help boost binocularity.

In optometry school, we learned a number of ways to prescribe prism for a binocular vision disorder. Testing included cover test, Von Graefe or modified Thorington phoria measurement, and fixation disparity. There were also a few people like Sheard and Percival who set guidelines for prescribing. One of the alternative ways that I learned to prescribe prism was through “clinical wisdom.” The clinical wisdom method of prescribing is compiled from the experience of various practitioners in their own practice. Robert Kraskin, an acclaimed practitioner in vision development, once wrote: “the base of a prism is academic, the amount is clinical.”(1) In general, as medical professionals, we strive to use evidence-based practice. If a patient has bacterial conjunctivitis, we treat the infection with an antibiotic that has proven to work in the literature. Unfortunately, when it comes to prescribing prism the literature is lacking a consensus. Moreover, the number of controlled studies about how to prescribe prism are few and far between, thus making it the most underutilized tool in our arsenal.

The conventional method of prescribing prism is in a compensatory fashion. This means that the prism is oriented to neutralize the horizontal or vertical deviation. Therapeutic yoked prisms are not as commonly used in practice. Is that because this concept is new? No, in fact in the 1950s Bruce Wolf introduced the concept of yoked prisms to the behavioral optometric community. Ever since, multiple articles have been published that describe the effect of yoked prisms on visual space, posture, and egocentric processing (awareness of where the body is relative to an object in space).(2,3) Yoked prisms have also become emergent in the management of patients with acquired brain injury and those that suffer from visual neglect.(4,5)

Even if you don’t believe in the power of a prism to alter behavior, we can all agree that a prism can produce a change in ocular posture. Changes in behavior are difficult to quantify so let’s focus on the tangible facts. Yoked prisms are prisms of equal power with the bases oriented in the same direction. For the purpose of this article, we will concentrate on the application of vertical yoked prisms. As opposed to conventional prisms, yoked prisms are thought to create a “spatial change.” Martin Birnbaum described yoked base down prisms to “create an upward spatial shift and consequent upward gaze shift associated with divergence, and increased near point working distance” whereas base-up yoked prisms were “spatially compressive, creating decreased size, decreased distance, downward spatial shift, associated with convergence.”(6) With this rationale in mind, the conditions that yoked prisms seem applicable to are convergence insufficiency and convergence excess. They can also be applied to patients with exotropia or esotropia.

Let’s hone in on a diagnosis that is resurfacing in this era of smartphones and tablets—convergence insufficiency. The primary factor for diagnosing a patient with convergence insufficiency is a reduced near point of convergence, > 5cm.(7) Additional criteria include exophoria greater at near than distance by at least 4 prism diopters and reduced positive fusional ranges. Symptoms can include headaches, diplopia, loss of place while reading, and loss of concentration. The CITT group devised a questionnaire which quantifies the severity of symptoms for patients with convergence insufficiency. A score of 16 or higher for patients under 18 and a score of 21 or higher for patients over 19 on the survey is considered to be clinically significant.(8,9) This questionnaire is a great tool to have in your practice to screen patients and a way to demonstrate improvement in symptoms before and after treatment.

Based on a randomized multi-center clinical trial, the CITT group concluded that in-office vision therapy is more effective than pencil push-ups and placebo orthoptics in reducing symptoms for patients with convergence insufficiency.(10) As an avid advocate of vision therapy and having personally witnessed its benefit, I believe that convergence insufficiency can truly be remediated with active vision therapy. However, as I mentioned previously, every treatment plan begins with an appropriate optical prescription. Furthermore, in a primary care setting, you may have patients who are unable to undergo an extensive in-office vision therapy program due to time or monetary constraints. For these patients, an intermediate step can be taken by incorporating base up yoked prisms into a reading prescription.

I would like to stress that although prisms create a change in ocular posture, prisms alone do not clinically improve the outcome for a patient with convergence insufficiency. A randomized double-blind clinical trial by Sheiman et.al found that glasses with base in prism alone (amount based on Sheard’s criteria) were no more effective than placebo glasses in reducing the symptoms of pediatric patients with convergence insufficiency.(11) Consider this: accommodation and convergence are linked processes in the brain. If you impact one system, you will create a supplementary change in the other. Since the primary effect of a prism is on vergence and the primary effect of a lens is on accommodation, a combination of a prism with a plus lens is recommended to maximize the visual response. In the past, there was a misconception that prescribing a plus lens to a patient with convergence insufficiency would worsen the condition. Remember, due to a low AC/A ratio lenses do not significantly impact posture in convergence insufficiency as they do in convergence excess. In fact, adding a low plus lens for reading minimizes the stress from accommodation thus further minimizing the stress from convergence. Additionally, a plus lens reduces the working distance thus further promoting convergence.

A 2009 study by Teitelbaum et. al found that progressive reading lenses with base in prism were more effective for symptomatic presbyopes than progressive lenses without base in prism.(12) So, a plus lens alone would not aid a patient with symptomatic convergence insufficiency. A landmark study of the combination of prism and plus lenses was done in the 1990s. A double-blind study by Lazarus sought to find the difference between a combined prism and plus lens vs. a plus lens alone in the management of asthenopia. In this study, the combined prism consisted of base in and yoked base up prism which was shown to be superior in relieving symptoms than a plus lens alone.(13)

So, what are the major takeaways from all these studies?

First, neither prisms nor a plus lens alone can improve the outcome for patients with convergence insufficiency. Base in prisms with combined progressive reading lenses can help relieve symptoms in adult patients, however, their effect on the pediatric population is yet to be discovered. And finally, a combined prism and plus lens for reading maximize the success for patients with asthenopia. In our clinic, we prefer yoked base up prism over lateral base in prism because we believe that the spatial change caused by base up prism promotes egocentric processing and allows the patient to actively redirect the visual response. A future study to evaluate the effect of a combined base in prism with a plus lens vs. yoked base up prisms with a plus lens is warranted to differentiate between the two treatment options.

A recent article in OVP Journal by Joel Warshowsky, director of vision therapy at the Center for Advanced Vision Care, highlighted the use of vertical yoked prisms to manage binocular vision disorders. In this article, a guideline was presented for prescribing yoked base up prisms which ranges from 0.5 prism diopters for pseudo-convergence insufficiency to 3 prism diopters for patients with constant exotropia.(14, Table 1) The amount of prism varies based on body tone, frequency of the deviation and severity of the condition. I have had the opportunity to work alongside Dr. Warshowsky and witnessed successful outcomes for pediatric patients who received prisms based upon these guidelines. In the article, he also mentions that the preferred method for determining the plus power for reading is via monocular estimated method retinoscopy (MEM) or binocular cross cylinder. Results after two months of full-time wear of the progressive reading glasses with yoked base up prisms show objective and subjective improvement. The first symptom to resolve is often diplopia while the primary exam findings, stereopsis and near point of convergence, demonstrate subsequent improvement.

If you are up to date with the literature you may have come across a couple of recent articles that, at first glance, seem to refute the application of vertical yoked prisms. Asper et. al looked into the immediate effect of vertical yoked prisms on heterophoria and Schmid et. al studied their direct effect on accommodation and fusional ranges.(15,16) Both studies found no direct correlation of yoked prisms to changes in binocularity. However, there are a few important considerations with these studies. First, one of the studies used higher amounts of yoked prisms than suggested in the Warshowsky guidelines. Second, both studies looked for immediate results after prism administration only. Finally, both studies were performed on a group of normal pre-presbyopic patients. To this date, a clinical study of the combined effects of prism and a plus lens on symptomatic patients with convergence insufficiency has not been performed.

Be bold, you may only have one chance!

Time and time again, I have learned that as a doctor you may only have one chance to create an impact on a patient. In a city like New York, where there is no shortage of optometrists a patient may disappear into the abyss after the initial appointment. So why not take the first and maybe the last opportunity you have to help them. Sometimes we hesitate because we are afraid of making too many recommendations or straying from their current (20/happy) prescription. We are ready to prescribe an add for a patient with accommodative insufficiency but are reluctant to add prisms for patients with vergence dysfunction. I believe the lack of consensus about which method to use to prescribe prisms and the uncertainty about the outcome accounts for the oversight. It is also a fact that there need to be more clinical studies that look into the application of prisms for patients with vergence dysfunction. With that said, we must remember that every leap in medicine began with a little experimentation. If we fear the unknown, the profession would not move forward. If you don’t incorporate new techniques into your clinical practice, you won’t know if it works with your patients or not. So, the next time you have a pediatric patient with convergence insufficiency, be bold and consider incorporating yoked base up prisms in a progressive lens as the first step of management.


References

Kraskin RA. Lens Power in Action. Santa Ana, CA: Optometric Extension Program, 2003
Begotka B. Improving Orientation with Yoked Prism. Optom Vis Perf 2014;2(2):83-4

Sheedy JE, Parsons SD. Vertical yoked prism–patient acceptance and postural adjustment. Ophthalmic Physiol Opt 1987;7(3):255-57

Padula WV. Neuro-optometric Rehabilitation, 3rd Edition. Santa Ana CA: Optometric Extension Program, 2000

Bansal S, Han E, Ciuffreda KJ. Use of yoked prisms in patients with acquired brain injury: a retrospective analysis. Brain Injury 2014;28(11):1441-6

Birnbaum MH. Optometric Management of Near Point Visual Disorders. Stoneham, MA: utterworth-Heinemann 1993.

Maples W, Hoenes R. Near Point of Convergence Norms Measured in Elementary School Children. Optom Vis Sci 2007; 84: 224 – 228

Borsting EJ, Rouse MW, Mitchell GL, et al. Validity and Reliability of the Revised Convergence Insufficiency Symptom Survey in Children Aged 9 to 18 Years. Optom Vis Sci 2003; 80(12):832-8

Rouse MW, Borsting EJ, Mitchell LG, et al. Validity and reliability of the revised convergence insufficiency symptom survey in adults. Ophthalmic Physiol Opt 2004: 24: 384-390
Convergence Insufficiency Treatment Trial Study Group. A Randomized Clinical Trial of Treatments for Symptomatic Convergence Insufficiency in Children. Arch Ophthalmol 2008 October; 126(10):1336-1349

Scheiman M, Cotter S, Rouse M, et al; Convergence Insufficiency Treatment Trial Study Group. Randomised clinical trial of the effectiveness of base-in prism reading glasses versus placebo reading glasses for symptomatic convergence insufficiency in children. Br J Ophthalmol. 2005 Oct;89(10):1318-23

Teitelbaum B, Pang Y, Krall J. Effectiveness of Base in Prism for Presbyopes with Convergence Insufficiency. Optom Vis Sci 2009: 86(2):153-156

Lazarus SM. The use of yoked base-up and base-in prism for reducing eye strain at the computer. J. Am. Optom. Assoc. 67, 204–208

Warshowsky J. Rationale for a behavioral clinical approach to routinely prescribed prism. Optom Vis Perf 2019;7(2):113-24

Asper L, Leung A, Tran C. The Effects of Vertical Yoked Prism on Horizontal Heterophoria. Optom Vis Sci. 2015 Oct;92(10):1016-20

Schmid KL, Beavis SD, Wallace SI, et al. The Effect of Vertically Yoked Prisms on Binocular Vision and Accommodation. Optom Vis Sci. 2019, Jun;96(6):414–423