Methods for prism placement for hemianopic visual field loss in adults with low vision.
Prior to prism placement, a full visual field test is completed using a Zeiss, Octopus, or Goldman perimeter, which accurately define the visual field loss. The optometrist and O&M specialist review the results of this test to determine the plan of care for the patient. An Esterman visual field test is completed if the patient is interested in driving again. The Esterman test measures the visual field with both eyes open, as this is required by states that permit driving only if an individual has a visual field of 120 degrees across the horizontal meridian.
Prism placement is first tested with a 20-40D Fresnel prism cut to the shape of the spectacle lens in the area of field loss with the prism's base facing in the direction of vision loss. The prism is placed by the optometrist to bisect the pupil so its edge can be seen without scanning into the prism. The edge is then trimmed with iridectomy scissors to the edge of the pupil. The patient then scans into the edge and is asked to count fingers within 10-15 degrees of the non-seeing field. The individual is then asked to ambulate and to detect objects in the non-seeing field. For example, if the non-seeing field is temporal, the scan would be to the left or right to view a painting on the wall, a doorway, or a person passing to the side. If the prism edge is causing diplopia, it is trimmed again to the edge of the dilated pupil.
In another method of prism placement, an individual is asked to look straight ahead at his or her nose (if the nose can be seen). If they cannot see their nose, then they are asked to look towards the center of their face, then bring a Post-it note (a small self-adhesive piece of paper) along the plane of their glasses from the non-seeing area into the plane of where they can just see the edge of the Post-it note. They then move the Post-it note back 1 millimeter (mm) until they can no longer see it. When viewing straight ahead, they should not see the edge of the Post-it note. A permanent marker pen such as a Sharpie can then be used to mark the eyeglass lens for later placement of the Fresnel prism (Giorgi, Woods, & Peli, 2009).
Choosing the strength of the prism depends on the size of the defect and the individual's tolerance. As a general rule of thumb, the number of degrees the image is moved is half the dioptric power of the prism. Therefore, a 30-diopter prism will move the image approximately 15 degrees. The base of the prism is always placed towards the area of the defect in order to move the image towards the seeing area. The base is marked on the Fresnel prism for orientation. The clinician places the prism to the edge of the defect as described above (Warren & Barstow, 2011).
The clinician escorts patients into the hallway and has them scan into the prism. They will be aware of looking through the prism because they will experience blurry vision caused by the Fresnel lens. The clinician then has the patient look at his or her hand or fingers when they are placed in the area of the visual field loss. The clinician has patients reach out and touch the clinician's fingers. They should exhibit the displacement of the prism and miss the examiner's hand. The patient then ambulates down the hallway a few minutes so they consistently look into the prism, reach for objects, and understand the displacement of the prism with different objects.
The higher the power of the prism, the more the images are displaced and the greater the visual field is enhanced. However, if the lab is asked to grind the prism into the lens later and produce a clearer image in the periphery, there is a limit to that power. This lens is called an Onufryk lens (sector prism), and its limit is 20 diopters. The choice is between a clear image of a ground-in lens versus the greater displacement of a higher-power Fresnel 20-40 diopter lens. The image through the Fresnel prism reduces acuity and produces a blurred image.
Before an individual is sent home to wear the prism for a trial period, the O&M specialist performs a visual function assessment that includes a measure of side vision. The patient is observed in small crowds and in congested areas; and in negotiating stairs, sidewalks, curbs, and street crossings. This observation also helps the O&M specialist determine how information is scanned and processed in various situations. Those who scan efficiently are processing environmental information and will not be good candidates for prisms. If it is determined that a person would benefit from the use of a long cane, it is discussed. Most prefer to test the prism before using a cane. If someone is unable to tolerate the prism, he or she may agree to receive cane training.
The O&M specialist will ask the individual to perform exercises while wearing the prism in both indoor and outdoor environments. A method used to determine whether the person is able to function safely with the prism is to allow them to practice walking around in various environments. Such exercise will help in the acquisition of a system for scanning into the prism while walking. It also provides experience with the spatial displacement of objects. With the placement of a Fresnel prism, the patient with a left or right homonymous hemianopia learns to scan into it, then to turn his or her head towards the periphery to view through the conventional lens to confirm the exact location of the displaced object. This technique is practiced while walking through the hallway and calling out the room numbers on the walls of both sides of the hall as they are seen. When this scanning technique is developed, practice will gradually be developed in navigating in small crowds and congested areas, along with negotiating stairs. Approaching the stairs, the individual will practice visually locating the railing and first step before ascending and descending.
Outdoors, while walking along the sidewalk the individual will scan for objects that may obstruct the pathway, as well as other potential landmarks. Before crossing streets, the intersection should first be analyzed, and visual scanning techniques to increase awareness of turning vehicles should be practiced. Scanning into the prism will be practiced in order to bring vehicles into the field of view and to confirm their location by turning the head and scanning into the conventional lens. These exercises will help the O&M instructor determine whether it is safe for the individual to wear the prism at home. If not, additional training will be needed to ensure safety (Ferraro & Jose, 1983).
The patient is sent home wearing the prism for one month. If the prism can be tolerated and the enhanced field can be appreciated, several more permanent prism options are demonstrated. These include the circular Gottlieb prism, The Eli Peli double prisms, and the half-size Onufryk prism or sector prism (Chadwick Optical, n.d.a, n.d.b).
The Eli Peli prism system is preferred by the patients and the O&M specialists at the CMCVAMC, who report that this system provides a more natural way of viewing the environment (Bowers, Keeney, & Peli, 2008; Bowers, Keeney, & Peli, 2014). Although eye movement is needed to use this prism system, it does not require scanning into the prism as is required by other systems. It was noted that many patients who wear the Eli Peli prisms adapt quickly and do not require additional training (Peli, 2000).
The Gottlieb prism is a button Fresnel-type prism that comes in two sizes, 16 mm and 19 mm, and is placed in the temporal portion of the visual field loss with the base towards the defect. Because of its location and small size, it can allow for a full-segment bifocal so the patient can continue to read while wearing the prism. This aspect allows someone who has a hemianopia and cannot locate words in a sentence to track these words and read evenly. It can also be ground into the patient's lens as a clear prism lens. This is referred to as a button prism (see Figure 1). It is thicker and heavier but provides a clearer image for the patient. Note that any time a prism is ground into the lens, it will be more costly for the patient.
Peli Press-on Prisms, which are available in 40 diopter and 57 diopters, are made up of two Fresnel strips that are fitted 9 to 12 mm apart above and below the pupil. These are designed to be viewed simultaneously while the patient looks straight ahead. It also has the advantage of a high prism power of 40-57 diopters. Peli Press-on Prisms are sometimes tested during the initial phase if the patient does not tolerate the traditional Fresnel type of prism, offering an alternative to a conventional prism. If the press-on prisms are successful, Chadwick Optical can place them as a permanent prism (see Figure 2).
The Onufryk prism or sector prism is a ground-in prism for enhancement of the visual field. It begins where the original Fresnel prism was placed, with the base towards the defect. The highest power for Onufryk is 20 diopters, due to the thickness of the prism and keeping in mind the individual's comfort. Depending on the size of the eyeglasses frames, the Onufryk prism can be heavy and will weigh more than the Gottlieb or Eli Peli prisms. It is the most cosmetically acceptable, however, because the edge of the prism is behind the lens and only a line bisecting the lens can be seen (see Figure 3).
Having the patient experience prism displacement with the traditional hemifield prism for a month seems to help with the training and success of using the smaller specialty prisms. O&M specialists at the CMCVAMC in Philadelphia instruct veterans, but there is no access to an O&M specialist at the Bucks County Association for the Blind and Visually Impaired, where a physical therapist provides mobility instruction. A team approach and the utilization of rehabilitation specialists are highly recommended when possible. The key is to be flexible to the needs of patients and to be open to utilizing all of the prism types that are available.
Enhancing an individual's visual field can bring great benefits for a person's mobility and quality of life. Working in conjunction with an O&M specialist can also increase the success rate of adaption to the prism.
Bowers, A., Keeney, K., & Peli, E. (2008). Community-based trial of peripheral prism visual field expansion device for hemianopia. Archives of Ophthalmology, 126(5), 657-664.
Bowers, A., Keeney, K., & Peli, E. (2014). Randomized crossover clinical trial of real and sham peripheral prism glasses for hemianopia. JAMA Ophthalmology, 132(2), 214-222.
Brilliant, R. L. (1999). Essentials of low vision practice. Woburn, MA: Butterworth-Heinemann.
Chadwick Optical. (n.d.a). Other hemianopic products. Retrieved from http://www.chadwickoptical.com/index_files/otherhemianopicproducts.htm
Chadwick Optical. (n.d.b). The Peli lens: Products for fitting and screening. Retrieved from http://www.chadwickoptical.com/index_files/pelilensforhemianopia.htm
Ferraro, J., & Jose, R. T. (1983). Training programs for individuals with restricted fields. In R. T. Jose (Ed.), Understanding low vision (pp. 365-372). New York: American Foundation for the Blind.
Giorgi, R. G., Woods, R. I., & Peli, E. (2009). Clinical and laboratory evaluation of peripheral prism glasses for hemianopia. Optometry and Vision Science, 86, 492-502.
Peli, E. (2000). Field expansion for homonymous hemianopia by optically induced peripheral exotropia. Optometry and Vision Science, 77, 453-464.
Warren, M., & Barstow, E. A. (2011). Occupational therapy interventions for adults with low vision. Bethesda, MD: AOTA Press.
Denise T. Wilcox, P.T., O.D., Ph.D., low vision optometrist, Advanced Low Vision Clinic, Corporal Michael J. Crescenz Veterans Affairs Medical Center, 3900 Woodland Avenue, Philadelphia, PA 19104; e-mail: <firstname.lastname@example.org>. Connie L. Chronister, O.D., M.S., professor of optometry, Salus University, 8360 Old York Road, Elkins Park, PA 19027; and low vision optometrist, Advanced Low Vision Clinic, Corporal Michael J. Crescenz Veterans Affairs Medical Center, Philadelphia; e-mail: <email@example.com>. Muriel R. Savage, M.S., COMS, orientation and mobility specialist, Corporal Michael J. Crescenz Veterans Affairs Medical Center, Philadelphia; e-mail: <firstname.lastname@example.org>.
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|Title Annotation:||Practice Perspectives|
|Author:||Wilcox, Denise T.; Chronister, Connie L.; Savage, Muriel R.|
|Publication:||Journal of Visual Impairment & Blindness|
|Date:||Jul 1, 2016|
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