Yoked Prisms
by Dr. April Eryou
Optical prisms are lenses that are able to deviate the direction in which light rays are travelling. Optical prisms have a thicker end (the base) and a thinner end (the apex). The direction in which the light is deviated is based on the position of the base as light deviates in the direction of the apex of the lens. The degree to which the light rays are deviated is measured in prism diopters. Yoked prisms is a term used to describe a pair of prisms (one in front of the right eye and the other in front of the left eye) either used therapeutically in glasses or as a tool in neuro-optometric rehabilitation therapy, where the bases of the prisms are in the same direction: left, right, up or down. In some of these cases, it is not the ability to deviate light towards the apex that is their purpose, it is the ability of the base end of the prism to compress space in the lateral direction and expand space in the anterior/posterior direction compared to the apex, that provides their therapeutic effect. The ability of yoked prisms to deviate light in front of both eyes can be used for patients with a homonymous visual field loss. In these cases, the prisms are used to deviate the light to try to centre the patient’s visual field. The ability of yoked prisms to deviate light along, with a patient’s ability to adapt to prisms, can be used to treat unilateral spatial Inattention (USI). When presented with a target and asked to ballistically point at the target, the patient will overshoot towards the left if they have right USI and left if they have right if they have left USI. The patient then wears prisms glasses to further deviate the light away from the true center for 10 minutes. During this time, the patient adapts and adjust their motor output for the given visual information and will begin to accurately hit the target. Following the removal of the yoked prisms glasses the patient will now point to the opposite side of the target compared to prior to the treatment. Overtime this is an effective way to treat USI. Finally, the main use of yoked prisms in neuro-optometric rehabilitation is actually to adjust spatial perception. Neuro-optometric rehabilitation patients frequently present with post trauma vision syndrome (PTVS) and/or visual midline shift syndrome (VMSS). Post Trauma vision syndrome is a dysfunction in how the ambient visual process and sensorimotor system integrates in the midbrain. This ambient-sensorimotor integration system is first developed in infants and children as they move though space and interact with their visual and physical environment. The use of yoked prisms for visual motor tasks is a way to promote changes in this system for patients suffering from PTVS. Promoting change allows for this system to rehabilitate. For patients with VMSS, they can experience lateral expansions/contractions of space and/or anterior/posterior expansions/contractions of space. These spatial distortions can cause patients to unevenly distribute their weight when standing and sitting, can negatively affect their gait and ability to walk straight and can cause posture changes. Yoked prisms can help neutralize these perceived spatial changes and immediately significantly improve the balance and mobility of patients with VMSS. Observing how a patient stands, sits and walks can provide clues as to how they process visual space. Immediate changes in posture and gait can be seen when the proper yoked prisms are worn by the patient. Challenges in attention, vision posture and gait may be noted by many professions involved in the care of a neuro-optometric rehabilitation patient, however, it is only optometrists and ophthalmologists who are licensed to prescribe prism glasses.