title: The effects of astigmatism and working distance on optic nerve head images using a Heidelberg Retina Tomograph scanning laser ophthalmoscope.
abstract: OBJECTIVE
  To determine effects of astigmatism and working distance on optic nerve head images in normal patients using the Heidelberg Retina Tomograph.
  
  
  METHODS
  The optic disks of 51 normal healthy subjects, aged 19 to 44 years, were imaged through dilated pupils. Subjects with 0.75 DC or less of astigmatism were imaged without correction at a working distance of 15 mm. They were then re-imaged with a cylindrical correction of +3.00 DC at 90 degrees axis (n = 20). Naturally astigmatic subjects with more than 1.00 DC were imaged without correction and then re-imaged once this was neutralized with their appropriate spectacle prescription (n = 15). The effects of working distance were studied using subjects with 0.75 DC or less (n = 16). Two working distances were used, 15 and 25 mm. At each session the means of three topographic images were taken from which standard deviations and parameters were recorded. Parameters analyzed included cup shape measure, rim area, and inferior temporal rim volume. Z-profile full width at half maximum was calculated from one image per subject for each condition.
  
  
  RESULTS
  No significant difference was found in the measured parameters of the optic disk for any astigmatic condition or changes in working distance (P >.05), (paired t test). Both the standard deviation of the mean topographic images and the Z-profile half-maximum width of the axial intensity profile were significantly greater with induced astigmatism of +3.00 DC (P values 0.3 and.00, respectively).
  
  
  CONCLUSIONS
  Optic disk parameters are not significantly affected by uncorrected astigmatism (up to 2.50 DC) or working distance. The algorithm used by the Heidelberg Retina Tomograph to generate topographic maps is sufficiently robust that astigmatism up to 2.50 DC does not require correction.
authors.affiliation: Department of Optometry and Vision Sciences, Cardiff University, Cardiff, United Kingdom.
authors.forename: N J
authors.initials: NJ
authors.lastname: Sheen
authors.forename: C
authors.initials: C
authors.lastname: Aldridge
authors.forename: N
authors.initials: N
authors.lastname: Drasdo
authors.forename: R V
authors.initials: RV
authors.lastname: North
authors.forename: J E
authors.initials: JE
authors.lastname: Morgan
mesh_terms: D000328:Adult; D001251:Astigmatism / Q000503:physiopathology*; D006801:Humans; D007834:Lasers; D009887:Ophthalmoscopy / Q000379:methods*; D009898:Optic Disk / Q000473:pathology*; D012016:Reference Values; D014054:Tomography / Q000379:methods*