title: Photorefractive keratectomy for hyperopia: long-term nonlinear and vector analysis of refractive outcome.
abstract: OBJECTIVE
  To characterize the refractive changes after excimer laser photorefractive keratectomy for the correction of hyperopia over a follow-up up to 3 years and to assess refractive stability and changes in astigmatism.
  
  
  METHODS
  Noncomparative, nonrandomized, retrospective, interventional case series.
  
  
  METHODS
  Thirty-eight hyperopic eyes of 28 patients (age range, 33-62 years) with refraction in the range +1.00 to +8.00 diopters (D). Mean attempted correction was +3.33+/-0.98 D (range, +1.00 to +4.00 D). Data were compared to those from 216 eyes treated for myopia in the range -1.00 to -12.70 D.
  
  
  METHODS
  The hyperopic correction was made using an erodible mask inserted in the laser optical pathway to produce a circular ablation measuring 6.5 mm in diameter. An axicon was then used to create a blend transition zone from 6.5 mm up to 9.4 mm in diameter. Eyes were evaluated 3 to 11 times (5.5+/-2.4) over a 3- to 34-month follow-up (16.8+/-8.4 months).
  
  
  METHODS
  Vector analysis of refractive error, applying a nonlinear statistical model fitting the spherical equivalent, and the sphere component data. The fit parameters were the long-term error at stabilization (epsilon(infinity)), the amount of regression (epsilon0), being the difference of refractive errors immediately after surgery and at stabilization, and the time constant (T1/2) giving the temporal scale length by which the overcorrection halves (regression half-life). Cylinder was analyzed by a linear regression.
  
  
  RESULTS
  The initial overcorrection was much larger after hyperopic treatments than myopic ones (epsilon0 = -3.26+/-0.35 D vs. +1.43+/-0.35 D), and it takes typically four times longer to regress (T1/2 = 3.30+/-0.91 months). Refractive stabilization is reached after more than 1 year, with a satisfactory refractive result. The hyperopic treatment induces a mean astigmatism of 1.00 D, which remains constant throughout the follow-up, and tends to be aligned along the with-the-rule meridian.
  
  
  CONCLUSIONS
  The advantages of a reasonably well-designed algorithm to correct hyperopia (epsilon(infinity) = +0.20+/-0.23 D) are counterbalanced by the long time to refractive stabilization and by the induced astigmatism.
authors.affiliation: Department of Ophthalmology and Visual Sciences, Scientific Institute San Raffaele Hospital, University of Milan, Italy.
authors.forename: F
authors.initials: F
authors.lastname: Corones
authors.forename: P G
authors.initials: PG
authors.lastname: Gobbi
authors.forename: L
authors.initials: L
authors.lastname: Vigo
authors.forename: R
authors.initials: R
authors.lastname: Brancato
mesh_terms: D000328:Adult; D000465:Algorithms; D001251:Astigmatism / Q000209:etiology / Q000503:physiopathology*; D003315:Cornea / Q000503:physiopathology / Q000601:surgery*; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D006956:Hyperopia / Q000503:physiopathology / Q000601:surgery*; D054018:Lasers, Excimer; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D018903:Photorefractive Keratectomy / Q000009:adverse effects / Q000379:methods; D012029:Refraction, Ocular / Q000502:physiology; D012189:Retrospective Studies; D016896:Treatment Outcome; D014792:Visual Acuity