October 2008

 

OPHTHALMOLOGY NEWS

 

Peripheral prism glasses & hemianopia


by Rich Daly EyeWorld Contributing Editor

 

 

Carefully fitted eyewear helped users avoid obstacles while inspiring long-term use

Hemianopia patients may receive long-term benefits from the use of carefully fitted peripheral prism glasses, according to recent research.

Stroke survivors and others afflicted by homonymous visual field defects (HVFDs) that stem from lesions in the postchiasmal visual pathways have more evidence about a prominent optical aid used to help them detect obstacles on the side of the loss. Such patients, who are 0.8% of the general population older than 49 years, are often left with impaired mobility.

To address this frequently stroke-induced damage, the authors of a study in the May 2008 issue of Archives of Ophthalmology examined the efficacy of an optical aid that fits low-to-moderate powered prism segments as binocular or monocular sector corrections and covers only part of the lens. The prisms aim to provide field relocation or expansion of less than 10 degrees.

The large-scale, community-based, multi-center study of 43 participants with homonymous hemianopia, who were fitted with temporary press-on Fresnel peripheral prism segments of 40 prism diopters, found positive long-term results.

Through follow-up telephone interviews after a median of 12 months, the study authors, led by Alex R. Bowers, Ph.D., M.C.Optom., Schepens Eye Research Institute, Department of Ophthalmology, Harvard Medical School, Boston, found that 32 participants (74%) continued to wear the prisms after six weeks and 20 participants (47%) continued after 12 months. The median time the prisms were worn each day was 8 hours.

The authors used a point scale on which participants rated the prism glasses as very helpful for obstacle avoidance and reported significant benefits for obstacle avoidance in a variety of mobility situations. Although success rates varied among clinic groups—from 27% to 81%—they were more likely to have success if they were fitted by the experienced eye centers.

The prism lenses are one of several types of optical aids used to assist such patients, although none of them have been rigorously tested in large, randomized controlled trials, wrote Victoria S. Pelak, M.D., associate professor of neurology and ophthalmology, University of Colorado Health Sciences Center, Aurora, in a review of the technology published in the January 2007 issue of the journal Current Treatment Options in Neurology.

The current study findings could have significant impact because prism lenses are one of the most commonly prescribed treatments for homonymous hemianopia by American neuro-ophthalmologists.

Samuel N. Markowitz, M.D., associate professor of ophthalmology, Faculty of Medicine, University of Toronto, is one ophthalmologist who has incorporated into his practice a modified technique of the one described in the study.

“There is merit to the ideas behind the technique and I witnessed positive results in most of my patients fitted with a variety of prism glasses,” he said.

The need for such treatments is largely underestimated by many clinicians, he said. Stroke, trauma, and tumor patients with hemianopia are very common in the community, and perhaps as common as the age-related macular degeneration group, according to Dr. Markowitz’s experience. However, the first group of patients tends to receive much less vision rehabilitation care.

“Responsible for this status of affairs is poor awareness among practitioners and patients of the fact that hemianopia qualifies for low vision status, as well as lack of awareness of the existence of effective vision rehabilitation methods for this group of patients,” Dr. Markowitz said.

He described the approach of the Harvard researchers as “a very original and sensible approach for HVFD patients” and an effective modality for peripheral field expansion. Another benefit is that the method described is relatively inexpensive, due to its use of prism glasses that are adaptable for introduction into the practice of community-based low vision practitioners.

Widespread use of the technology is unlikely to be affected by any need for a faster way to determine the minimum interprism separation that could be tolerated when walking. Dr. Markowitz described interprism separation as just one of the technical details that has to be sorted out when fitting a patient with such prisms. It can be addressed as an additional technical detail when specific patients are fitted with the prisms.

Future developments

Among the advances that the study authors suggested could follow from solid research support for the technology was the use of peripheral prism glasses for additional visual field expansion to supplement visual restoration or visual search training.

Dr. Markowitz cast doubt on that approach since other methods mentioned involve voluntary eye movements directed into the peripheral fields. It is inevitable that patients with the prisms would have their central gaze run into the prism segments and create instant central diplopia.

Further research is needed on vision rehabilitation among stoke patients, in general, Dr. Markowitz said. Such research might help in the creation of guidelines on the most appropriate vision rehabilitation program or approach suitable for each of the various sub-groups of stroke patients. “Further research to clarify the needs and methods required for vision rehabilitation in the sub-groups of stroke patients will bring some order and logic into the field,” Dr. Markowitz said.

Editors’ note: Drs. Bowers, Pelak, and Markowitz did not disclose any financial interests related to their comments.

Contact information

Bowers: 617-912-2512, alex.bowers@schepens.harvard.edu

Pelak: victoria.pelak@uchsc.edu

Markowitz: snm1@rogers.com

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