Improving interpupillary distance accuracy in the plusoptix A12R: A distance-dependent correction

Handheld photorefractors are increasingly used in paediatric populations, where patient cooperation and examination time are often limited. Given this widespread use, it is essential to characterise potential sources of measurement error. This study aimed to investigate the dependence between interpupillary distance (IPD) measurements and measurement distance (MD) when using Plusoptix A12R.

Variability in interpupillary distance (IPD) measurements with the Plusoptix were observed during a paediatric vision screening programme (n = 67). A theoretical model predicting a linear IPD - MD relationship was developed, including a correction factor. The model was validated under controlled conditions (n = 3) using a bench testing. Then, a pilot study (n = 8) was carried out under habitual clinical conditions to assess the applicability of the correction factor. IPD measurements obtained with the Plusoptix before and after correction were compared with those obtained using a pupilometer. Agreement between methods was assessed using Bland-Altman analysis and Cohen's effect size.

In 42% of children, IPD measurements were relatively stable (SD ≤ 1 mm), whereas 9% showed fluctuations exceeding 4 mm. Bench testing data confirmed a strong linear dependence of IPD on MD (slope = 0.99), with larger IPD values obtained at shorter MDs. The pilot study showed that applying the proposed correction factor reduced IPD variability and improved agreement with pupilometer measurements.

IPD measurements obtained with the Plusoptix A12R are affected by MD, introducing a systematic and predictable error. The application of a correction factor effectively compensates for this effect improving its clinical reliability.