title: Varying difficulty of Snellen letters and common errors in amblyopic and fellow eyes.
abstract: OBJECTIVE
  To investigate the varying difficulty of Snellen letters in children with amblyopia.
  
  
  METHODS
  We tabulated the letter-by-letter responses of amblyopic and nonamblyopic fellow eyes on random, computer-generated Snellen lines. Participants were 60 children, aged 5 to 13 years, with a history of amblyopia. Main outcome measures were relative difficulties of Snellen letters and common misidentifications.
  
  
  RESULTS
  Errors were 7.5 times more common with certain letters (B, C, F, S) than with others (A, L, Z, T), this difference increasing to 17.6-fold at threshold. Similar relative letter difficulty was demonstrated at lines above and at visual acuity thresholds, and both difficult and easy letters were the same for amblyopic and nonamblyopic fellow eyes. Specific misidentification errors were often repeated and were often reciprocal (eg, B for E and E for B).
  
  
  CONCLUSIONS
  Since therapeutic decisions in amblyopia management are often based on small differences in visual acuities, the relative difficulties of letters used in their measurement should be considered. The Early Treatment Diabetic Retinopathy Study system should be considered for use in this clinical setting.
authors.affiliation: Department of Ophthalmology/Lions Eye Institute, Albany Medical College, 1220 New Scotland Road, Slingerlands, NY 12159, USA.
authors.forename: Julia A
authors.initials: JA
authors.lastname: Mathew
authors.forename: Sabah A
authors.initials: SA
authors.lastname: Shah
authors.forename: John W
authors.initials: JW
authors.lastname: Simon
mesh_terms: D000293:Adolescent; D000550:Amblyopia / Q000175:diagnosis* / Q000503:physiopathology; D015858:Anisometropia / Q000175:diagnosis / Q000503:physiopathology; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D008297:Male; D010364:Pattern Recognition, Visual; D011446:Prospective Studies; D011932:Reading; D013285:Strabismus / Q000175:diagnosis / Q000503:physiopathology; D014787:Vision Tests / Q000295:instrumentation*; D014792:Visual Acuity / Q000502:physiology