title: External factors affecting data acquisition during corneal topography examination.
abstract: OBJECTIVE
  To analyze the factors affecting data acquisition during corneal topography examination with the Medmont E-300 videokeratoscope and to provide strategies to minimize their effects.
  
  
  METHODS
  Sixty eyes from thirty young adults were examined. A second observer registered incidences with the potential to affect data acquisition. Those factors were correlated with the difficulty of measurements as judged subjectively by the practitioner who performed the examination. Measurements of axial curvature were analyzed to evaluate the variability expressed as intrasession and intersession coefficient of variation and the standard error of the mean (SEM).
  
  
  RESULTS
  The level of difficulty rated by the practitioner was in general low, with 70% of the eyes being easy or very easy to measure. For the remaining 30% of the eyes, corneal topography measurements were considered to be difficult (27%) or very difficult (3%). Of the external parameters investigated, only fixation instability (P<0.001, chi2) and the need for head repositioning (P=0.024, chi2) were associated significantly with a higher level of difficulty, as rated subjectively by the practitioner. Further analysis showed that some external factors, including those previously mentioned and others related to tear instability, affect the variability of measurements at certain corneal locations, particularly in the vertical meridian when related to tear instability and in the horizontal meridian when related to the need for head repositioning on the chin rest owing to physiognomy interferences with the keratoscope cone. Intersession SEM improved when three readings from each session were considered.
  
  
  CONCLUSIONS
  The level of subjective difficulty found during videokeratoscopy examination is correlated strongly with fixation instability and the need for head reorientation in the chin rest, whereas tear-related events seem to be less relevant in the practitioner perception of test ease or difficulty. Those factors have relevance in measurement variability.
authors.affiliation: Department of Physics (Optometry), School of Sciences, University of Minho, Braga, Portugal.
authors.forename: José Manuel
authors.initials: JM
authors.lastname: González-Méijome
authors.forename: Antonio
authors.initials: A
authors.lastname: Queirós
authors.forename: Jorge
authors.initials: J
authors.lastname: Jorge
authors.forename: Paulo
authors.initials: P
authors.lastname: Fernandes
authors.forename: Alejandro
authors.initials: A
authors.lastname: Cerviño
authors.forename: José Borges
authors.initials: JB
authors.lastname: de Almeida
mesh_terms: D000328:Adult; D003315:Cornea / Q000503:physiopathology; D019781:Corneal Topography / Q000592:standards*; D005260:Female; D005403:Fixation, Ocular / Q000502:physiology; D006801:Humans; D008297:Male; D011187:Posture / Q000502:physiology; D015203:Reproducibility of Results; D012107:Research Design / Q000592:standards*; D013666:Tears / Q000502:physiology