title: Refractive changes during 72-hour exposure to high altitude after refractive surgery.
abstract: OBJECTIVE
  The authors prospectively analyzed refractive and pachymetric parameters during exposure to high altitude after radial keratotomy (RK) and photorefractive keratectomy (PRK).
  
  
  METHODS
  The authors measured manifest and cycloplegic refraction, keratometry, computed video keratography, and central and peripheral pachymetry in six subjects who have undergone RK (11 eyes), six who have undergone PRK (12 eyes), and nine with myopia (17 eyes) at sea level and on three consecutive days at 14,100 feet. All measurements were repeated 1 week after subjects returned to sea level.
  
  
  RESULTS
  Subjects who have undergone RK demonstrated a significant and progressive increase in spherical equivalence (+0.30 +/- 0.50 diopters on day 1 and +1.52 +/- 1.01 diopters on day 3; P < 0.001) and a decrease in keratometry values during exposure to altitude when compared with control subjects with myopia. Healthy subjects and those who have had PRK demonstrated no significant change in refractive error. Pachymetry measurements demonstrated significant peripheral corneal thickening in all three groups (RK, P < 0.004; PRK, P < 0.007; control subjects, P = 0.0006) by day 3 at high altitude. Refraction, keratometry, and pachymetry returned to baseline (P = 1.000) after return to sea level.
  
  
  CONCLUSIONS
  Seventy-two-hour exposure to high altitude in subjects who have had RK induces a significant, progressive, and reversible hyperopic shift in refraction with corresponding video keratographic and keratometric changes. The authors hypothesize that the high-altitude hypoxic environment causes increased corneal hydration in the area of the RK incisions, which may lead to central corneal flattening and a hyperopic shift in refractive error. Subjects who have had PRK and those with myopia are not susceptible to this refractive shift. The authors' RK data suggest that the time since surgery and the amount of surgery are related to the degree of hyperopic shift during altitude exposure.
authors.affiliation: Madigan Army Medical Center, Ophthalmology Service, Tacoma, Washington, USA.
authors.forename: T H
authors.initials: TH
authors.lastname: Mader
authors.forename: C L
authors.initials: CL
authors.lastname: Blanton
authors.forename: B N
authors.initials: BN
authors.lastname: Gilbert
authors.forename: K C
authors.initials: KC
authors.lastname: Kubis
authors.forename: S C
authors.initials: SC
authors.lastname: Schallhorn
authors.forename: L J
authors.initials: LJ
authors.lastname: White
authors.forename: V C
authors.initials: VC
authors.lastname: Parmley
authors.forename: J D
authors.initials: JD
authors.lastname: Ng
mesh_terms: D000328:Adult; D000531:Altitude; D001274:Atmospheric Pressure; D003315:Cornea / Q000473:pathology / Q000503:physiopathology / Q000601:surgery*; D006801:Humans; D006956:Hyperopia / Q000209:etiology* / Q000473:pathology / Q000503:physiopathology; D007091:Image Processing, Computer-Assisted; D007429:Intraocular Pressure; D007646:Keratotomy, Radial; D054018:Lasers, Excimer; D008875:Middle Aged; D009216:Myopia / Q000473:pathology / Q000503:physiopathology / Q000601:surgery*; D018903:Photorefractive Keratectomy; D011183:Postoperative Complications; D011446:Prospective Studies; D012029:Refraction, Ocular; D013997:Time Factors