Relationship between working distance and lag of accommodation in myopic and emmetropic children

Abstract

Purpose: To evaluate the relationship between habitual working distance and lag of accommodation in young myopic and emmetropic Chinese children.

Methods: Working distance and lag of accommodation were measured in 24 emmetropic (E: +0.16±0.24 D) and 66 myopic (M: -1.93±0.83 D) Chinese children aged from 7 to 11 (E: 9.6±0.9 y.o., M: 8.6±0.6 y.o.). Myopes were equipped with their best distance correction in spectacles. Near vision posture was recorded continuously at 10 Hz during one reading and one writing task on a desk by means of a Fastrack (Polhemus, USA) electromagnetic motion tracking system. Working distance was defined as the distance between the base of the nose and the center of each line and averaged for the whole duration of the task. Before posture measurements, accommodative response was measured at 33 cm using a Grand Seiko WAM-5500 (Grand Seiko, Hiroshima, Japan) openfield autorefractor and lag of accommodation calculated based on the average of three consecutive readings.

Results: Emmetropic children were reading at closer distance than myopic children (E: 263±56 mm, M: 291±57 mm, p=0.04) while both groups were writing at similar distances (E: 223±66 mm, M: 228±61 mm, p=0.79). Adjusting data for height did not change those results. Lags of accommodation were of similar amount in both groups (E: 1.19±0.32 D, M: 1.36±0.46 D, p=0.12). In emmetropic children, working distance was positively correlated with lag of accommodation for both reading (r=0.50, p=0.02) and writing (r=0.54, p=0.01) tasks. In myopic children however, those parameters were not correlated for any of the two tasks (p>0.10).

Conclusions: Emmetropic children with higher lags of accommodation had greater working distances compared to children with lower lags. This relationship was not observed in myopic children. As lag increases with decreasing distance, it is possible that emmetropic children adjust their posture to keep an acceptable level of retinal blur depending on their initial lag of accommodation while in myopes decreased blur sensitivity may disrupt this relationship.