The distribution of near point of convergence and its association with age, gender and refractive error: a population-based study

Abstract

Purpose: The aim was to determine the distribution of the near point of convergence (NPC) and its association with age, gender and refractive errors through a population-based study. Methods: In this cross-sectional study, samples were randomly selected from the population of Mashhad city in the northeast of Iran through stratified cluster sampling. In each of the 120 randomly selected clusters, 10 households were systematically chosen. Examinations included visual acuity testing, refractive and binocular vision examination. NPC was measured using a 6/12 single target on a Gulden fixation stick. Positive and negative fusional vergences were measured by the step method at far and near fixation distances. Results: Of the 4,453 invitees, 3,132 people participated (response rate of 70.4 per cent) and after applying the exclusion criteria, analysis was conducted on data from 2,433 individuals whose mean age was 31.2 ± 15.1 years (range: 10 to 86 years). Mean NPC was 8.59 ± 4.82 cm (range: 2 to 40 cm) in the total sample, 6.95 ± 3.87 cm in the 10–19 years age group and 13.06 ± 5.2 cm in those over 70 years. The relationships of NPC with the variables ages, gender and refractive errors (spherical equivalent) was assessed in a multiple linear regression model. Based on this model, older age (p < 0.001, coefficient = 0.106), male gender (p = 0.002, coefficient = 0.608) maintained a significant correlation with NPC, while the association with refractive error was no longer significant (p = 0.109, coefficient = 0.128) after adjusting for age and gender. Conclusion: This is the first population-based study to examine the normal ranges of NPC and also the first to describe normal values in a presbyopic population. The results of this study indicate an age-related change in NPC. These changes should be noted in the interpretation of tests for the diagnosis and treatment of problems with binocular vision.