Prism and Field Enhancement for the Low Vision Rehabilitation of Patients with Hemianopic Field Loss
Stanley Woo, OD, MS, FAAO, DipLV(AAO), Assistant Professor University of Houston, College of Optometry
Hemianopic field loss may result in a significant impairment in independent travel, activities of daily living, and quality of life. Low vision rehabilitation has included a number of techniques to enhance the use of remaining functional vision over the years including prisms, mirrors, amorphic lenses, among others.
In general, there are two principle schools of thought with regards to the use of prisms for field enhancement. One emphasizes the increased dynamic visual field arising from more efficient scanning into the area of field loss. It recommends the use of press-on Fresnel prism to act as a “sign-post” to encourage patients to scan in the direction of the field loss and to provide efficient eye-head coordination.2,3 The second recommends quick and ready access to the prism in order to displace the non-seeing field towards the midline.4 Furthermore, the use of clear sector prisms may achieve the same effect without any degradation in image quality.2,5 It is important to note that these represent a spectrum of options, which are not necessarily mutually exclusive.
To enhance scanning, we typically use a Fresnel prism (15-25 pd) mounted on the posterior aspect of the spectacle lens with the base in the direction of the field loss. For example, for a patient with left hemianopic field loss, the prism would be applied to the left lens on the temporal aspect with the base in the direction of the field loss (base out OS). In order to encourage scanning, the edge of the apex of the prism is placed 2 mm in the direction of the field cut (i.e. temporal, away from mid-line). This offset assures that the prism is not visible in primary gaze, but only accessed when the patient makes an eye rotation in the direction of field loss. The image is displaced towards mid-line proportional to the power of the prism, and there is a scotoma at the apex associated with the image jump at the apex. The patient should be counseled that while viewing through the Fresnel prism the contrast and visual acuity may be decreased.6 However, this does serve to provide feedback to the patient that they are indeed attending to the image through the prism. In addition, if fit monocularly, the patient will likely observe diplopia created by viewing through only one prism segment. Appropriate training is essential to master the technique. At subsequent follow-up visits, the vinyl Fresnel prism can be removed and cut back to increase the extent of eye rotation necessary to access it. Ultimately, with successful and consistent scanning technique, the prism may in fact be removed. Variations of this technique include leaving a 40 pd ribbon of prism mounted either superiorly, inferiorly or both. Developed by Dr. Eli Peli, it has been described as an “expansion prism (EP),” and is now distributed by Chadwick Optical.8.9 The approach is analogous to the “bioptic” strategy adopted for spectacle-mounted telescopes, and avoids the image jump associated with scanning into the apex of the prism.
To maintain a clear image when viewing directly through the prism, an alternative to the press-on Fresnels may be used. The Gottlieb Visual Field Awareness System 5,7 and the InWave system have been popular, though the latter company is now defunct. Although the clear sector and channel prisms are no longer available from InWave, the same systems are produced by Chadwick Optical.9 For patients who have already developed an effective scanning technique, the presence of a clear prism with the apex at the edge of the field cut can promote quick and efficient eye-head coordination when scanning into the area of field loss.2 Generally, the clear sector prism is more appropriate for patients with relatively good visual acuity who are bothered by the decreased contrast and vision through the Fresnel lens.
In summary, hemianopic field loss may arise from stroke, traumatic brain injury, or tumors. It may have a significant and adverse effect on quality of life. Fortunately, there are a number of techniques to enhance the resulting functional visual field some of which have been described herein. Visual field enhancement through canning in conjunction with the use of sector prisms is one of the more common methods used in low vision rehabilitation today. Although we have described some of the technical components in this article, it is imperative to recognize that training is an essential element for the successful low vision rehabilitation of patients with field loss. Although a challenging patient group, the benefits to successful field enhancement can be a boon to both patient and clinician.
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- Lee AG, Perez AM. Improving awareness of peripheral visual field using sectorial prism. J Am Optom Assoc. 1999 Oct;70(10):624-8.
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- Gottlieb Vision Group. Visual Field Awareness System. Available at: http://www.gottliebvisiongroup.com/visual_field_awareness_system.htm Accessed November 30, 2005.
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- Chadwick Optical. Available at: http://www.chadwickoptical.com/prod.htm Accessed November 30, 2005.