Correlation between Interpupillary Distance, Inner-Outer Canthal Distance and Inter Limbal Distance: A Cross-sectional Study

Abstract

Introduction: Interpupillary Distance (IPD) is the distance between the centres of two pupils which can be anatomical or physiological. There are various inter orbital distances which are important in studying the orbitofacial configuration and also in prescribing a correct spectacle. Among the various measurements IPD is the most important which is routinely measured using AutoRefarctometer (AR) machine or Pupil distance meter. Aim: To find the correlation between IPD and Inner Canthal-Outer Canthal Distance (ICOC) and Right Nasal Limbus to Left Temporal Limbus (RNL-LTL) distance and to derive a regression equation in calculating IPD from measurements like ICOC and RNL-LTL which are easier to measure. Materials and Methods: In this cross-sectional study with a sample of 100 subjects, participants were randomly selected from individuals attending Outpatient Department at Vydehi Institute of Medical Sciences and Research Centre, Bengaluru, Karnataka, India, between March 2021 to August 2021. IPD was measured using Auto-Refarctometer/ Autorefractor Keratometer (AR/ARK) (Model: UNIQUE-RK:UNICOS URK 800F) and Pupillary Distance Metre (PDM) (Essilor). All the other measurements like Inner Inter Canthal Distance (IICD), Outer Inter Canthal Distance (OICD), RNL-LTL distance were measured using a transparent plastic ruler. Data was entered in Microsoft Excel and analysed using Statistical Package for the Social Sciences (SPSS) version 19.0. Categorical variables were presented as frequency and percentage. Multiple Linear regression was performed to find the factors associated with IPD using AR. The p-value <0.05 was considered as statistically significant. Independent sample t-test was performed to compare mean values between males and females. Results: Of 100 subjects, 60 were males and 40 were females, 5 subjects were <20 years and 95 subjects were ≥20 years. The analysis of the various distances found positive correlation between IPD measured using AR and PD meter with the other distances ICOC and RNL-LTL measured with transparent ruler, the strongest correlation found between RNL-LTL and IPD. The results were found to be statistically significant (p-value <0.001 for all these comparisons). A regression equation was derived to calculate IPD using ICOC and RNL-LTL distances and also using other inter orbital variables. Conclusion: Since there was a positive correlation between IPD and ICOC and RNL-LTL distances, these measurements which are easy to perform can be used to calculate IPD in situations where it’s difficult to get access to AR machine as in peripheral community based camps or rural settings and in children and uncooperative patients where it is difficult to use AR machine to measure IPD.