title: The distribution of near point of convergence and its association with age, gender and refractive error: a population-based study.
abstract: OBJECTIVE
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Refractive Errors Research Center, School of Paramedical Sciences, Mashhad University of Medical Sciences, Mashhad, Iran.
authors.forename: Hadi
authors.initials: H
authors.lastname: Ostadimoghaddam
authors.affiliation: Noor Research Center for Ophthalmic Epidemiology, Noor Eye Hospital, Tehran, Iran.|Noor Ophthalmology Research Center, Noor Eye Hospital, Tehran, Iran.
authors.forename: Hassan
authors.initials: H
authors.lastname: Hashemi
authors.affiliation: Noor Research Center for Ophthalmic Epidemiology, Noor Eye Hospital, Tehran, Iran.
authors.forename: Payam
authors.initials: P
authors.lastname: Nabovati
authors.affiliation: Department of Optometry, School of Sciences, Mashhad University of Medical Sciences, Mashhad, Iran.
authors.forename: Abbasali
authors.initials: A
authors.lastname: Yekta
authors.affiliation: Department of Medical Surgical Nursing, School of Nursing and Midwifery, Shahid Beheshti University of Medical Sciences, Tehran, Iran.
authors.forename: Mehdi
authors.initials: M
authors.lastname: Khabazkhoob
mesh_terms: D000065:Accommodation, Ocular; D000293:Adolescent; D000328:Adult; D017677:Age Distribution; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D003290:Convergence, Ocular / Q000502:physiology*; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D007492:Iran / Q000453:epidemiology; D008297:Male; D008875:Middle Aged; D009017:Morbidity / Q000639:trends; D011159:Population Surveillance / Q000379:methods*; D011305:Presbyopia / Q000175:diagnosis / Q000453:epidemiology / Q000503:physiopathology*; D017678:Sex Distribution; D014787:Vision Tests; D015348:Vision, Binocular / Q000502:physiology; D014792:Visual Acuity; D055815:Young Adult
keywords: distribution; epidemiology; near point of convergence