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Contact lens solutions to binocular vision problems: This article considers clinical scenarios where contact lenses can be used to successfully manage binocular vision anomalies.

Optometrists [??]

Dispensing opticians [??]

Contact lens opticians [??]

(1 CET POINT)

Introduction

Contact lenses and binocular vision may seem like they are opposite ends of the spectrum when it comes to optometric management. However, this article will consider situations where the two may be more intertwined than is immediately apparent. As with any good discussion, let's start with a case.

Case study

A 38-year-old white man presented with diplopia at intermediate and near distances. The patient's ocular history included convergence insufficiency diagnosed 11 years prior, when he started to notice constant diplopia while working on the computer and reading; this caused asthenopia and headaches at work and when performing prolonged near tasks. Since the patient had a history of unsuccessful vision therapy, he had been prescribed 8A net base in spectacles since the diagnosis, which alleviated his symptoms.

However, one month prior to his current visit, the patient shot himself in the face as a result of an accidental gun misfire. The bullet entered the left septum and exited the right septum. Though the patient had no direct ocular sequelae from the incident, he was unable to wear his spectacles due to the damage to his nose. Therefore, the patient's diplopia returned with near tasks, mostly with reading and computer work. The patient is left eye dominant so he had been closing and covering his right eye to alleviate his symptoms of diplopia. The patient is scheduled for five reconstructive surgeries to repair his nose. Pertinent exam findings included orthophoria at distance and a 12-14[DELTA] intermittent alternating exotropia at near. The patient had a history of photorefractive keratectomy in 2003 with a refraction of piano in both eyes at distance.

Due to the patient's history of unsuccessful vision therapy and physical inability to wear spectacles, the patient was fitted with a soft prosthetic contact lens with an occluded pupil (see Figure 1). The lens was fitted for the right eye since the patient is left eye dominant. The lens parameters were: base curve 8.6, diameter 14.0, and 7mm occluded pupil.

Customisable soft prosthetic lenses offer a wide variety of pupil sizes and iris colours. In this case, the patient had dark hides and reported he would only need the lens for computer and near work; therefore, he declined to have any iris colouration on the lens.

At the patient's follow up visit, the lens was noted to fit well, with good comfort, and it provided adequate occlusion to effectively relieve the patient's symptoms of diplopia at near.

Case discussion

Diplopia can be one of the most uncomfortable symptoms for patients to endure and one of the most difficult for practitioners to manage. Often, treatment will include prism of some kind to combine the images and produce binocular vision. However, there are some instances where prism may not be the best option. Some of these include diplopia with a variable magnitude of deviation (as in myasthenia gravis), very large deviations where prism-ground lenses would be cosmetically unappealing visual discomfort with prism, inability for spectacle wear, or in cases of intractable diplopia where patient can neither fuse nor suppress images. (1) In the case presented above, the patient had a history of unsuccessful vision therapy and was physically unable to wear spectacles due to facial deformity, which led to contact lenses being a feasible alternative solution.

Occlusion is a safe and suitable management solution for many conditions including diplopia, other binocular vision issues, (1) or severe visual distortions, with satisfactory results. (2,3) There are many different methods of performing occlusion, ranging from a simple eye patch to an opaque intraocular lens. Opaque contact lenses are a good temporary option since spectacle occlusion is cosmetically unappealing for many patients and an opaque intraocular lens would require surgery. (4)

When it comes to choosing an occluding lens, it would be at the discretion of the provider to fit the patient with the best lens. There are many companies that produce customisable prosthetic soft lenses with occluded pupils like the lens in Figure 2. These lenses have completely customisable parameters, including everything from diameter to base curve, colour, pupil size, and power. A prosthetic soft lens fitting should be approached like any other soft lens fitting with measurements of power, keratometry, visible iris diameter, and anterior eye evaluation.

One concern regarding complete occlusion of an eye is the loss of visual field; occlusion can cause a reduction of anywhere from 48% to 76% of a patient's peripheral vision. (5) This is an important consideration for patients with occupations where a complete field of view or full peripheral vision is needed, for example, professional and commercial drivers.

Another occlusion option includes a novel contact lens with a monocular central scotoma, dubbed a scotogenic contact lens (see Figure 3), which eliminates central diplopia while maintaining peripheral viewing capabilities. (6) The lens also includes a gradient of occlusion to match the gradient of sensitivity of the retina from the most sensitive area (the fovea, 0[degrees] from centre), sloping outwards (see Figure 4). Using self-reported scores in comparison with eye patch occlusion, the scotogenic contact lens has been shown to be as effective in eliminating diplopia as complete occlusion while simultaneously allowing increased acceptability due to the reduced impact on peripheral vision. (6)

Monovision

Another contact lens consideration for patients with binocular vision problems is monovision, thereby allowing one eye to be used for distance viewing and the other eye to be used for near viewing; this allows suppression of the image of one eye at all times to eliminate any binocular vision or diplopia issues, especially in cases where the images are competitive and infusible, while still maintaining adequate visual acuity. (7) Monovision is also a great alternative to occlusion as it allows the patient to maintain better peripheral vision, which is a concern for occlusion. Yet like occlusion, monovision allows the same benefits of adequate monocular acuity and improved cosmesis. (8)

Aniseikonia and anisometropia

The use of contact lenses can also be very beneficial for patients with anisometropia--both axial and refractive--and its resulting aniseikonia. In these situations, spectacle correction results in the retinal images presented to each eye being different in size resulting in difficulty fusing the cortical images. (9) As a result, binocular vision can become unstable, resulting in asthenopia, diplopia, or other binocular vision issues. Contact lenses help to reduce the amount of disparity between the retinal images as the lens is in contact with the cornea, causing reduced image magnification changes. They are also more cosmetically appealing and more visually comfortable, as spectacle lenses can be heavy and lopsided when one lens is significantly stronger and thicker than the other.

Anisometropic and aniseikonic patients are especially susceptible to strabismus, despite being able to pass some stereopsis tests. (10) Certain types of stereo testing, such as Wirt circles, have monocular cues and can even be passed with one eye covered. Therefore, these tests are not sensitive enough to pick up more subtle binocular vision problems. The introduction of random dot stereogram (RDS) testing has been helpful in the further testing of binocular vision, as these tests require bifoveation. However, patients with aniseikonia have been shown to still pass RDS testing, demonstrated in patients with monocular aphakia with a large amount of aniseikonia. An estimated 10-15% of patients who pass the random dot stereogram (RDS) test with aniseikonia still become strabismic. (11) Even in cases of small amounts of anisometropia and aniseikonia, elimination of the retinal image disparity can be beneficial. Keep in mind that a positive RDS result does not imply protection from strabismus. Fusing the cortical images so that both eyes can be used equally is still the ultimate goal. Contact lenses can be utilised with good results in these situations.

Prism

Contact lenses are not commonly used to replace spectacle prism, as the lenses rotate and move on the ocular surface, causing any prism to be unstable. However, certain customised lenses can have prismatic designs. Therefore, in very specific situations, specialty contact lenses are a possible option for prism, if necessary. A case study by Carballo-Alvarez in 2015 describes a custom-design lathed hydrophilic toric contact lenses with 1.82[DELTA] base down prism in the right eye and 0.84[DELTA] base down prism in the left eye, therefore a net prism of ~1[DELTA] base down over the right eye. (12) The lenses were used to successfully treat 1[DELTA] of right hypertropia. The results showed stable and well-centred lenses with good visual acuity, binocularity, and comfort.

Though contact lenses are not a first line choice for prism correction, it is possible to correct phorias and tropias with hydrophilic customised contact lenses with prism as an alternative to prism spectacle correction. (12) However, the fitting can be challenging due to the rotating nature of a contact lens and it would be highly dependent on the particularity of the patient. The practitioner should consider other options before trying a specialty contact lens with prism for a patient.

Other considerations

Contact lens induced binocular vision problems?

As with any management option, there are pros and cons. Though contact lenses can be a good option for treating many conditions, they can also contribute to increased dry eye and asthenopia in some cases, which can lead to further binocular vision complications. Accommodative lag, which leads to accommodative insufficiency and pseudo-convergence insufficiency, is the most common binocular vision issue seen in soft contact lens wearers, especially in myopic soft contact lens wearers. (12) This also provides further evidence to the understanding that myopic patients have to accommodate more with contact lens wear compared to spectacle wear. (13)

It is important to screen symptomatic contact lens patients for binocular vision disorders, as they can be misdiagnosed with a dry eye issue. Since the symptoms of dry eye and binocular vision can often be concurrent or overlapping, it is vital to tease out whether symptoms are due to multiple problems, such as dry eye and binocular vision concerns, or a standalone issue. A significant, positive correlation between the ranks of scores on the Ocular Surface Disease Index (OSDI) and Convergence Insufficiency Symptom Survey (CISS) demonstrates the similarity of symptoms between dry eye and binocular vision disorders. (14)

Cosmesis

Many of the above conditions in which contact lenses can be used as a management option mention cosmesis is a benefit, which can easily be dismissed. As practitioners, we sometimes forget how significant I a benefit of good cosmesis is, especially for younger patients. Yet one of the most important benefits of contact lenses is the improved cosmesis compared to spectacle lenses and the significant psychosocial benefits. Younger patients can be especially susceptible to development of poor self-esteem and fear of peer rejection, which can be further increased by poor cosmesis due to spectacle wear. (14) Therefore, improved cosmesis for a patient should never be dismissed as an important benefit for the use of contact lenses as a therapeutic option.

Conclusion

As discussed in this article, there are many ways to approach contact lenses for therapeutic and cosmetic uses for binocular vision issues. Contact lenses can be a powerful tool to help patients with a variety of conditions, from diplopia to aniseikonia. As practitioners, it is important to explore various treatment options with patients to provide optimal care.

Exam questions

Under the enhanced CET rules of the GOC, MCQs for this exam appear online at www.optometry.co.uk. Please complete online by midnight on 26 December 2018. You will be unable to submit exams after this date. Please note that when taking an exam, the MCQs may require practitioners to apply additional knowledge that has not been covered in the related CET article. CET points will be uploaded to the GOC within 10 working days.

You will then need to log into your CET portfolio by clicking on 'MyGOC' on the GOC website (www.optical.org) to confirm your points. Visit www.optometry.co.uk, and click on the 'Related CET article' title to view the article and accompanying 'references' in full.

* Dr Linda Shi is a graduate of the State University of New York College of Optometry and completed a residency in primary care and ocular disease at the Veterans Affairs Health Care System in Portland, Oregon. She also received a concurrent master's degree in Business Administration. Dr Shi currently serves as parttime clinical faculty at Marshall B. Ketchum University's Southern California College of Optometry in Fullerton, California. Her primary optometric and academic interests include ocular disease, specialty contact lenses, and international optometry.

* Dr Kirk Halvorson completed a primary care/ geriatric optometry residency at the Veterans Affairs Puget Sound Health Care System in Tacoma Washington. He spent several years in private practice and as a faculty member with Pacific University before joining the Veteran Affairs Portland Health Care System. Currently he works at the Portland and Vancouver hospital facilities.

* Dr Jonathon Thomas is a staff optometrist at the Veterans Affairs Portland Health Care System in Portland, Oregon. He is also an adjunct assistant clinical professor for the Pacific University College of Optometry, a clinical instructor at the University of California Berkeley School of Optometry and an adjunct clinical faculty member at the New England College of Optometry.

Course code: C-61042 Deadline: 26 December 2018

Learning objectives

* Be able to explain to patients about the use of contact lenses for managing binocular vision anomalies (Group 1.2.4)

* Understand the use of contact lenses for managing binocular vision anomalies (Group 8.1.2)

* Be able to explain to patients about the use of contact lenses for managing binocular vision anomalies (Group 1.2.4)

* Understand the use of contact lenses for managing binocular vision anomalies (Group 5.1.1)

* Be able to explain to patients about the use of contact lenses for

managing binocular vision anomalies (Group 1.2.4)

* Understand the use of contact lenses for managing binocular vision anomalies (Group 5.5.3)

Caption: Figure 1 Image of prosthetic soft contact lens with 7mm occluded pupil

Caption: Figure 2 Example of customisable, layered colours available in a prosthetic soft lens

Caption: Figure 3 Diagram of the design of a scotogenic contact lens (6)

Caption: Figure 4 Diagrammatic representation of the variation in spatial acuity across the visual field (black line) and the consequently optimal pattern of degradation of the perceived image produced by a scotogenic lens designed to abolish binocular diplopia with minimal effect on the peripheral visual field (white line) (6)
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Title Annotation:Binocular vision
Author:Shi, Linda; Halvorson, Kirk; Thomas, Jonathon
Publication:Optometry Today
Date:Dec 1, 2018
Words:2407
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