title: Quantitative electroretinogram measures of phototransduction in cone and rod photoreceptors: normal aging, progression with disease, and test-retest variability.
abstract: OBJECTIVE
  To determine (1) reference values for cone and rod phototransduction variables derived from the a-wave of the electroretinogram, (2) their dependence on age, (3) the progression in cone and rod variables in patients with X-linked retinitis pigmentosa (XLRP), and (4) the test-retest variability in these a-wave measures compared with the variability in cone and rod b-wave measures.
  
  
  METHODS
  One hundred control subjects aged 5 to 75 years and 24 patients with XLRP aged 5 to 38 years.
  
  
  METHODS
  High-intensity stimuli were used to elicit electroretinograms in the dark and in the presence of a rod-saturating background. Computer averaging and computer subtraction of cone components from mixed rod-cone responses were used to derive rod-only and cone a-waves. Rod and cone phototransduction variables were derived by computer fitting physiologically based computational models to the leading edges of a-wave ensembles.
  
  
  RESULTS
  Phototransduction efficiency, as indexed by the sensitivity variable (S), decreased with age for cone and rod-only responses, whereas maximum cone and rod photoresponses (Rm(P3)) remained constant. In patients with XLRP tested annually for 4 years, Rm(P3) for rods and, to a lesser extent, cones declined with disease progression, whereas S remained stable. The test-retest variability in the a-wave Rm(P3) is lower than previously reported measures of the variability in b-wave peak-to-peak amplitude.
  
  
  CONCLUSIONS
  The leading edge of the a-wave of the electroretinogram can be related to rod and cone phototransduction variables through quantitative models. Rm(P3), rather than S, should be the outcome measure of choice when using the a-wave to follow photoreceptor function in prospective studies and treatment trials.
authors.affiliation: Retina Foundation of the Southwest, Dallas, TX 75231, USA. dbirch@retinafoundation.org
authors.forename: David G
authors.initials: DG
authors.lastname: Birch
authors.forename: Donald C
authors.initials: DC
authors.lastname: Hood
authors.forename: Kirsten G
authors.initials: KG
authors.lastname: Locke
authors.forename: Dennis R
authors.initials: DR
authors.lastname: Hoffman
authors.forename: Radoul T
authors.initials: RT
authors.lastname: Tzekov
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000375:Aging / Q000502:physiology*; D002648:Child; D002675:Child, Preschool; D003623:Dark Adaptation; D018450:Disease Progression; D004596:Electroretinography; D005260:Female; D008040:Genetic Linkage; D006801:Humans; D008297:Male; D008875:Middle Aged; D020419:Photoreceptor Cells, Vertebrate / Q000502:physiology*; D012016:Reference Values; D015203:Reproducibility of Results; D012174:Retinitis Pigmentosa / Q000235:genetics / Q000503:physiopathology*; D014785:Vision, Ocular / Q000502:physiology*; D014960:X Chromosome