World Health Organization statistics show that falls are the second leading cause of unintentional injury-related deaths worldwide. Multifocal glasses (bifocals, trifocals, and progressive addition lenses (PALs)) increase the risk of a fall in elderly people but how they do so is unclear. To explain why glasses with a PAL increase the risk of a fall and whether this can be attributed to false projection, this study aimed to 1) map the prismatic displacement of a PAL, 2) test whether this displacement impaired the response to loss of balance, and 3) test whether PALs alter stability.
The reaction time and accuracy of healthy ≥75 year olds (n = 31 participants) were measured when grasping for a bar and touching a black line. These were positioned according to the maximum and minimum prismatic displacement effect through the PALs, mapped using a focimeter. Anterior posterior (AP) deviation was measured while standing on a balance platform. Participants performed each test twice, alternatively wearing their PALs and newly matched single vision (distance) glasses in random order.
Results showed that PALs have large areas of prismatic displacement, especially in the central visual axis. Reaction time was faster for PALs compared to single vision (distance) glasses (mean difference ± SEM, horizontal grab bar in centre -0.101 ± 0.050 s, P = 0.011, repeated measures analysis adjusted for order of glasses, days since participants updated their PALs and amount of prismatic displacement; horizontal black line 300 mm down from centre -0.080 ± 0.016 s, P = 0.007). There were no differences in the balance measures.
PALs have large areas of prismatic displacement, but did not alter stability. Older people appeared to adapt to the false projection of PALs in the central visual axis. This adaptation meant that swapping to new single vision glasses may have affected the visual-spatial stored information. This may lead to a fall, especially in unfamiliar surroundings.
