Yoke Prisms May Restore Visual Acuity in Low Vision AMD

Mature women on a medical appointment with ophthalmologist
Mature women on a medical appointment with ophthalmologist
The lenses effectively redirect images toward peripheral identifiable preferred retinal location.

Using yoke prisms for image redirection toward a peripheral identifiable preferred retinal location (PRLs) could return potential visual acuity (PVA) to patients with age-related macular degeneration, according to a study published in Journal of Optometry.

The retrospective comparative interventional case series review looked at data from 33 eyes of 33 patients (mean age 80±9.2 years, 24 women). Researchers used low vision rehabilitation (LVR) protocols — including best corrected visual acuity (BCVA) and PVA assessments, topographic PRL identification techniques, and prisms to produce image redirection to the presumed PRL. The primary outcome measure for the study was BCVA for viewing distance targets after using yoke prisms.

With the Early Treatment Diabetic Retinopathy Study (ETDRS) chart, distance mean visual acuity was assessed at baseline as 20/124 (logMar 0.79±0.32) in the better eyes. At baseline, mean PVA was 20/87 (logMar 0.64±0.2) in the better eyes. At 1.5, 3, 6, 12 and 18 cycles per degree and 1.15, 1.37, 1.28, 1.09, 0.45 contrast log unit measurements, researchers obtained contrast sensitivity estimates. The PRL location was recorded as present on all retinal quadrants — upper (73%), lower (12%), temporal (12%), and nasal (3%).

They found that image relocation using prisms in patients with age-related macular degeneration (AMD) produced significantly better BCVA levels (t32=8.57, P <.0001) in the better eye, while distance BCVA levels were nearly identical to PVA levels (t32=0.415, P <.681) (y=-0.136+1.195x, r=0.8333, P <.001). 

“As hoped and expected the use of prisms assisted in redirecting incoming images into the eye towards the retinal periphery and the PRL,” researchers report. “This effect of prisms on moving images on the retina was known for a long time however was demonstrated unequivocally just recently. As seen also from this study images were displaced on the retina towards the existing PRL. The prisms amount used was minimal rep- resenting on the average about 3 spatial angle degrees on the retina, yet sufficient to reach the PRL and facilitate restitution of better distance visual acuity very close to PVA values.” 

“We investigated this intervention only on the better seeing eye. Perfunctory analysis of data on the poorer seeing eye did not allow to obtain meaningful results,” according to the study. 


Markowitz SN, Teplitsky JE, Taheri-Shirazi M. Restitution of potential visual acuity in low vision patients with the use of yoke prisms. J Optom. 2021;14(4):342-345. doi:10.1016/j.optom.2020.10.004.