Peripheral prism glasses use high power prism segments embedded in a regular spectacle lens and can expand the upper and lower visual fields of patients with hemianopia by as much as 30 degrees. The prisms optically shift objects from the blind side of the visual field to the seeing side, alerting people with hemianopia to objects and obstacles otherwise invisible to them.
A masked multi-centre study published online in JAMA Ophthalmology used a “crossover” design in which each participant wore two pairs of glasses: a pair of real peripheral prism glasses, and a pair of sham peripheral prism glasses with less than two degrees of field expansion.
Of the 73 people randomised, 61 completed the crossover period. After four weeks, 64 per cent chose to continue with the real prisms because they were helpful for obstacle avoidance when walking. Thirty-six per cent selected the sham prisms for comfort and ease of wearing. Six months later, 41 per cent of patients continued to wear the real peripheral prism glasses.