Methods for Prism Placement for Hemianopic Visual Field Loss in Adults with Low Vision

Abstract

Several adult patients with low vision have come to the Advanced Low Vision Clinic at the Corporal Michael Crescenz Department of Veterans Affairs Medical Center (CMCVAMC), in Philadelphia, Pennsylvania, and to the Bucks County Association for the Blind and Visually Impaired, in Newton, Pennsylvania, with a diagnosis of hemianopic loss secondary to a stroke in the visual cortex; brain tumors; or traumatic brain injury. Prism placement for peripheral field loss allows patients to have enhanced peripheral awareness and an increased peripheral field depending on the type of prism used (Brillant, 1999). To determine prism placement, a low vision optometrist evaluates the patient. The evaluation consists of trials of different types of prism systems, including Fresnel, Gottlieb, Onufryk, and Eli Peli. When a preference for a particular prism system is shown, it is temporarily placed. The optometrist confers with the orientation and mobility (O&M) specialist to discuss patient goals and a plan of care. The O&M specialist performs an evaluation and provides visual scanning training as well as instruction in the functional use of prisms. This report describes the process used in these centers for assessing and placing the prism for these individuals. 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). …