Peripheral prism glasses for homonymous hemianopia: A systematic review of effects on blind-side detection and mobility, with optical and perceptual mechanisms

Purpose: Homonymous hemianopia (HH) cuts half of the visual field in both eyes, making walking and driving unsafe. The most widely studied optical field-expansion device is peripheral prism glasses; however, researchers have never integrated their clinical effects with the optical and perceptual mechanisms that limit them. Methods: Following PRISMA 2020, we searched OpenAlex and PubMed/MEDLINE including citation searches. The selected studies examined peripheral prisms in subjects with HH or discussed aspects of their optics or perception. We assessed risk of bias using RoB 2 and the JBI checklist, assessed certainty using GRADE, and guided synthesis with SWiM guidance. Results: There were 23 studies (26 reports) included, where three randomized studies, ten non-randomized studies and ten mechanism studies. All studies that conducted a blind side detection test rated prisms as superior, with simulated walking test performance increasing from 26-43% to 92-98% and satisfactory on-road response increasing from 30% (sham) to 80%. In the sham-controlled trial, 64% preferred real prisms (odds ratio 5.3); continued use for 6–12 months was 21–47%. Quality of life did not improve, and headache and visual confusion were very common. As prism power increased (from 20° at 40 Δ to 30° at 57 Δ to 45° multi-periscopic), field expansion increased, whereas TIR, adaptation of perceived direction, and apical scotomas limited functional gain. Conclusions: In the present case, low-certainty evidence suggests that peripheral prisms might improve blind-side hazard detection via an optical field-multiplexing effect rather than a perceptual remapping effect. Real-world mobility end-point sham-controlled trials are required.