Manual, Automatic Interpupillary Distance Measurements Differ Statistically, Not Clinically

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Manual ruler and an automatic pupillometer provide good intersession repeatability, a study shows.

In agreement and intersession repeatability of manual and automatic interpupillary distance (IPD) measurements, binocular and monocular manual and automatic measurements differ statistically, but not clinically, according to a study published in the Journal of Optometry. Distance binocular IPD is approximately 3 mm wider than near IPD, the research shows. 

The investigators looked at IPD, a valuable measurement in developmental anatomy, genetics, design of optical instrumentation, ocular diagnostics, and optical prescribing. It is often measured on different days, using automatic pupillometers (physiological measurement) or a manual ruler (anatomical measurement), making the agreement and intersession repeatability of manual and automatic IPD measurements essential.

The research was divided into 2 parts: An agreement study that included 199 participants (mean age 24.1±5.0 years, range 19-53 years, 58 men, 141 women) and a repeatability substudy that included 30 participants (mean age 27.9±4.5, range 23-39, 6 men, 24 women). Using a millimeter ruler and a pupillometer, clinicians randomly measured monocular distance from the bridge of the nose, binocular distance and near binocular IPD. The researchers performed 4 types of manual IPD measurements: binocular near, binocular distance, monocular right eye distance, and monocular left eye distance. They assessed gender effects with Wilcoxon and Mann-Whitney tests, respectively, and agreement with Spearman correlation and Bland-Altman (B&A) plots. By 1 to 2 weeks, 30 more individuals were tested to find intersession repeatability. 

Men and women monitored in this investigation differed significantly in 2 ways: Age (<2 year mean difference) and IPD (monocular mean difference: <1 mm, binocular mean difference: <2 mm). 

They found significant differences in manual vs automatic measurements for all conditions (mean difference: <1 mm for all) except for left eye monocular PD in men. Researchers noted no significant differences between sessions of both methods.

“Binocular and monocular manual and automatic measurements were significantly different statistically, but not clinically,” they conclude. “Distance binocular IPD was approximately 3 mm wider than near IPD. Male binocular IPD was approximately 2 mm wider than the female IPD. Both methods had good intersession repeatability.”

The study did have some limitations, including cohort size and an imbalanced gender cohort. 

Additionally, IPD values were not normally distributed based on statistical tests. “We would have expected a normal distribution of IPD values in a wide range of ages,” according to the researchers. “Given the age range in our cohort (ages 19-46 for the females and 19-53 for the males), in which the IPD is expected to stabilize without much change, our results are not surprising.”

Finally, the pupilometer used does not provide the individual values of the 4 measurements and only displays the mean of the consecutive measurements.

Disclosure: This research was supported by IRISS Medical Technologies, Ltd. Please see the original reference for a full list of disclosures.


Gantz L, Shneor E, Doron R. Agreement and inter-session repeatability of manual and automatic interpupillary distance measurements. J Optom. 2021;14(4):299-314.doi:10.1016/j.optom.2020.08.010