MULTIPLEXING PRISMS CORRECT LIMITATIONS OF FRESNEL PRISMS

THE GOAL of managing a glaucoma patient is to preserve his vision for the remainder of his life. In many cases of advanced glaucoma, patients will maintain a small central area of VA, as little as 1% to 2% of their full VF. This loss of peripheral vision can cause tripping and colliding with objects — or even other people — in their homes and public places.

These actions occur because these patients do not scan or look from side to side in front of them, but, instead, look straight ahead using the small island of vision that remains, reports a study in the Journal of Vision. The study’s researchers also identified the highest risk area of collision for these patients: a 45° angle from their walking path.

Here, I discuss the limitations of the Fresnel prism spectacle lens for glaucoma patients and a multiplexing prism spectacle lens, that, once available, could reduce the tripping and colliding these patients often experience.

FRESNEL LIMITATIONS

Although conventional Fresnel prism spectacle lenses have been used as rehabilitative tools for years in these patients, they come with the following constraints:

• Apical scotoma. Conventional Fresnel prisms have identical prisms arranged base to apex. The apex of a prism creates a scotoma in the field of view called the apical scotoma. Eliminating the apical scotoma is as important for safety and mobility for patients suffering from extensive VF loss as the view provided by the prisms.
• Binocular visual confusion. To achieve true VF expansion, the visual scene from the unseen and the seen portions of the field of view must be superimposed on one another. This results in visual confusion, reveals a study in Translational Vision Science & Technology. If a patient has two functional eyes, but only a small island of central field of vision, toleration of visual confusion can be difficult. Other issues that can occur with binocular visual confusion: binocular rivalry and suppression.
• Monocular vision patients. If a patient is monocular, it is not possible to create the visual confusion necessary to expand the field of view with a conventional Fresnel prism spectacle lens.

MULTIPLEXING PRISM

Researchers at the Schepens Eye Research Institute, in Boston, have designed a prism called a multiplexing prism to address the issues discussed previously. Multiplexing prisms have prisms alternating with flat elements in a single segment. (See: http://bit.ly/2EnaKSX .)

Specifically, the prism is placed in front of one eye, and the shifted and the see-through view are seen simultaneously. The field of view is expanded with no field lost to an apical scotoma.

The multiplexing image addresses suppression and binocular rivalry as well. Since the visual confusion that occurs with the multiplexing prism is monocular, it is uncommon for suppression or rivalry to occur. Once this goes from research to market, our advanced glaucoma patients can look forward to safer vision. OM

DR. SCHWEITZER specializes in advanced glaucoma, refractive surgical clinical care and anterior segment pathology at Vance Thompson Vision. He has had multiple articles published and has delivered lectures on glaucoma, anterior segment pathology, refractive surgical clinical care and surgical management. Email him at justin.schweitzer@vancethompsonvision.com.