title: The Effects of Vertical Yoked Prism on Horizontal Heterophoria.
abstract: OBJECTIVE
  Vertical yoked prisms (VYPs) have been recommended for the remediation of vergence dysfunction, but the evidence base for their use is mostly low level. This study investigates the effect of VYP on horizontal subjective dissociated heterophoria.
  
  
  METHODS
  Phoria measurement was performed in primary gaze with the Modified Thorington technique at 3 m and at 40 cm on 40 nonpresbyopic young adults. Subjects were seated and head position was held constant. Baseline measures (without yoked prism) of distance and near phoria were measured. Phorias were measured again through the following range of VYP, randomly presented: 2 prism diopters (Δ) base up (BU), 2Δ base down (BD), 5Δ BU, and 5Δ BD. Twenty-six subjects also had their phorias measured with control lenses of +0.125 DS OU (because of unavailability of Plano trial lenses) randomly presented along the other conditions.
  
  
  RESULTS
  There was no significant difference overall between phoria measured in any of the yoked prism conditions, including the baseline measure, at distance or near. Neither was there evidence of a predictable esophoric or exophoric shift with either BU or BD prism.
  
  
  CONCLUSIONS
  Vertical yoked prisms did not exert any immediate effect on horizontal phoria in young adults when posture was controlled. This suggests that, if VYPs do indeed improve horizontal vergence problems, they do not do so by a direct or immediate impact on horizontal phoria.
authors.affiliation: *PhD, OD †BOptom, BSc(Hons) ‡PhD, MCOptom §BOptom(Hons) aDeceased School of Optometry and Vision Science, University of New South Wales, Sydney, New South Wales Australia (all authors); and Optometry and Visual Science, School of Health Sciences, City University London, United Kingdom (CMS).
authors.forename: Lisa
authors.initials: L
authors.lastname: Asper
authors.forename: Andrew
authors.initials: A
authors.lastname: Leung
authors.forename: Catrin
authors.initials: C
authors.lastname: Tran
authors.forename: Catherine M
authors.initials: CM
authors.lastname: Suttle
authors.forename: Kathleen
authors.initials: K
authors.lastname: Watt
mesh_terms: D000293:Adolescent; D000328:Adult; D004948:Esotropia / Q000503:physiopathology / Q000628:therapy; D005099:Exotropia / Q000503:physiopathology / Q000628:therapy; D005139:Eyeglasses; D005260:Female; D006801:Humans; D008297:Male; D012030:Refractive Errors / Q000503:physiopathology / Q000628:therapy; D013285:Strabismus / Q000503:physiopathology* / Q000628:therapy*; D015348:Vision, Binocular / Q000502:physiology; D014792:Visual Acuity / Q000502:physiology; D055815:Young Adult