title: Estimate and measure of the region of view and myopia resulting from vitreous gas.
abstract: OBJECTIVE
  After vitreous gas injection, patients notice better acuity in downward gaze than in horizontal gaze. The authors evaluated the refractive error and the size of the region within which vision improves.
  
  
  METHODS
  For the vitreous fluid-gas interface, the authors calculated the angle of total internal reflection and the expected myopic shift and then measured them in nine consecutive patients. The volume of gas, declination angle at which perception of small targets occurred, and preoperative and postoperative refractive error in downward gaze were measured.
  
  
  RESULTS
  Total internal reflection occurs at 41.5 degrees declination. Patients perceived a region of improved acuity below 41 degrees (+/-5 degrees) declination. The difference between the calculated and clinically measured gas-induced myopia was less than 25% for five of nine patients. The largest measured induced myopia was -23.9 diopters (60% gas volume). In eight eyes, patients read 5-point type or smaller.
  
  
  CONCLUSIONS
  Patients accurately perceive that their acuity improves in downward gaze; the boundary of this region corresponds with the angle of total internal reflection. Calculations predict that vitreous gas produces a myopic shift and aberration. These data support the notion that ocular positioning by patients with vitreous gas can be enhanced by instructing them to regard near targets in downward gaze.
authors.affiliation: Department of Ophthalmology and Visual Sciences, The University of Iowa, Iowa City 52242-1091, USA.
authors.forename: S R
authors.initials: SR
authors.lastname: Russell
authors.forename: K M
authors.initials: KM
authors.lastname: Gehrs
authors.forename: C L
authors.initials: CL
authors.lastname: Hess
mesh_terms: D005740:Gases / Q000009:adverse effects*; D006801:Humans; D009216:Myopia / Q000209:etiology* / Q000503:physiopathology; D012167:Retinal Perforations / Q000601:surgery*; D014792:Visual Acuity / Q000502:physiology; D014796:Visual Perception / Q000502:physiology; D014822:Vitreous Body
chemical_list: D005740:Gases