Therapy of binocular imaginative and prescient dysfunction made simpler with high-tech measurement software

October 19, 2021

4 min read

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Pervasive digital device use is associated with symptoms such as eye strain, headaches, dry eye disease, and neck and shoulder pain. Together, these symptoms are ubiquitous and known as digital vision syndrome, or DVS.

The steady rise of DVS over the last decade has ramped up these past 18 months, with significant increases associated with COVID-19-related quarantine and social distancing measures. Even as remote work and virtual school directives wane, our patients’ dependence on digital devices will likely remain high. The visual needs of our patients are changing, and the way that we evaluate and treat them needs to change as well.

The DVS-BVD connection

While many DVS symptoms are related to increased screen time, DVS symptoms also overlap with and are exacerbated by those of binocular vision dysfunction (BVD). There are several types of BVD, including amblyopia, strabismus and various forms of eye misalignment. Most patients with DVS exhibit some degree of eye misalignment, and this misalignment is at the root of shared DVS/BVD symptoms. Although the exact mechanism is unclear, it is suspected that eye misalignment overstimulates the trigeminal nerve, and this leads to symptoms such as headache, neck pain and eye strain.

Convergence insufficiency (CI), in which a patient presents with an eye misalignment with greater exophoria at near than far, is the most common BVD. It is often thought that with BVDs, symptomatic patients tend to exhibit large phoria or fixation disparity. The assumption is that these large phoric measurements reflect a breakdown of the binocular vision system, especially the accommodation and vergence mechanisms. But this is not accurate.

Amanda Nanasy

The literature makes it clear that there is no correlation between the severity of BVD symptoms and the severity of the phoria. Data from the Convergence Insufficiency Treatment Trial showed no correlation between the amount of exophoria and the severity of patient symptoms (Bade et al.). And scores from the Convergence Insufficiency Symptom Survey also did not correlate with the severity of the clinical signs, such as near point of convergence or positive fusional vergence limits (Borsting et al.). These studies and my own anecdotal experience indicate that a patient with 1 D of exophoria and a patient with 10 D of exophoria, with no other ocular or extraocular problems related to DVS, might both experience a similar magnitude of eyestrain and need to be treated appropriately.

Measurement techniques

Far too often, only patients with larger phoria are diagnosed and treated while individuals who could benefit from small prismatic corrections of less than 2 D are overlooked. The reason for this may be the imprecise nature of traditional measurement methods. Techniques such as the alternating cover test result in poor repeatability, limiting a clinician’s ability to measure small eye deviations accurately. The smallest phoria value that can be detected with the cover test is about 2 D to 3 D, and other methods, including the Howell near phoria card, the Maddox rod and the von Graefe prism dissociation test, are also hampered by subjectivity and the inability to accurately test prism in the small increments needed of 0.1 D.

In my practice, we recognize the need for precise and reliable phoria measurements, so we utilize the neurolens Measurement Device, Generation 2 (nMD2, neurolens). The nMD2 takes an objective, accurate and repeatable measurement of horizontal and vertical phorias and other alignment information. It acquires more than 10,000 data points per patient and identifies eye misalignment as small as 0.1 D, accomplishing this in less than 3 minutes, according to the manufacturer.

This high-tech screening tool enables me to check my patients’ eye alignment at distance and near and produces a clear chart that is easy to interpret. It provides me with a measurement called the neurolens value, which I use to prescribe contoured prism vision correction. The nMD2 also provides other helpful information, such as accommodative convergence/accommodation ratios and how these are affected by various add powers. A key benefit is that the information is displayed in a pictorial format that I use to easily explain the findings to my patients.

Contoured prism treatment

Once I explain the findings and why the patient is experiencing symptoms, I prescribe a potential solution with a neurolens contoured prism prescription. This lens technology incorporates a patented variable prism correction that helps eye misalignment at different distances, allowing me to customize the lens correction for each patient. The contoured prism design enables variable prismatic treatment at all distances and additional prismatic correction at near where it may be most needed.

With a contoured prism — meaning more base-in prism at near than in the distance — I can give my patients a little bit of extra help for extended near work. Remember that a prism’s efficacy comes from essentially helping to bring the image closer to where it needs to be so the eyes do not have to work as hard to maintain clear, single vision.

For many years, I prescribed conventional prism lenses for patients with concussions to compensate for BVD symptoms, knowing that even small prismatic corrections of misalignment would reduce overstimulation of the trigeminal nerve and ultimately relieve their headache, eye fatigue, and neck and shoulder tension. As digital device use increases, I have more patients than ever with many of the same symptoms as patients with concussions. Now I am able to prescribe a more precise prescription prism lens for all my patients with even small eye misalignment, and I have found that these contoured prism lenses afford the possibility of clear vision and symptom relief with one pair of glasses.

As optometrists, we sometimes get comfortable staying in our lane and focusing solely on what we normally treat. However, given that so many of our patients spend the majority of their day looking at a screen and working at arm’s length, it is imperative to sharpen our knowledge of BVDs and outfit our armamentarium with precise phoria measurement technology so we can provide the most helpful vision solutions.


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Amanda Nanasy, OD, is director of the Florida Institute of Sports Vision at The Eye Center in Pembroke Pines, Fla., and chair of the American Optometric Association’s Sports and Performance Vision Board. She can be reached at [email protected]

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