title: OCT-derived comparison of corneal thickness distribution and asymmetry differences between normal and keratoconic eyes.
abstract: OBJECTIVE
  The aim of this study was to investigate the corneal thickness asymmetry indices in a large pool of patients with keratoconus derived using anterior segment optical coherence tomography (AS-OCT) in comparison with established Scheimpflug imaging-derived keratoconus classification indices.
  
  
  METHODS
  Six specific indices were comparatively investigated in this study encompassing 2 groups: keratoconic group A (175 eyes) and age- and gender-matched control group B (175 eyes). AS-OCT was used for corneal focal thinning and irregularity indices, namely the superior nasal minus inferior temporal (SN-IT), superior minus inferior (S-I), minimum minus median (Min-Med) focal thinning, and thickness range, defined as the minimum minus maximum (Min-Max). Scheimpflug imaging was used for keratoconus grading and for investigating anterior surface irregularity index of height decentration (IHD) and index of surface variance (ISV).
  
  
  RESULTS
  In study group A, SN-IT was 74.6 ± 38.9 μm; S-I, 77.3 ± 42.3 μm; Min-Med, -60.1 ± 36.8 μm; and Min-Max, -117.7 ± 55.4 μm. In control group B, SN-IT was 23.32 ± 11.47 μm; S-I, 22.55 ± 12.56 μm; Min-Med, -19.69 ± 5.20 μm; and Min-Max, -55.24 ± 12.96 μm. In group A, IHD was 0.077 ± 0.055 and ISV was 84.24 ± 48.61. In group B, IHD was 0.031 ± 0.038 and ISV was 31.82 ± 29.72. A statistically significant relationship was identified between SN-IT, S-I, Min-Med, and Min-Max indices with IHD and ISV (P < 0.01 in all pairs; coefficients of determination 0.649, 0.663, 0.481 and 0.483 versus IHD, and 0.690, 0.722, 0.551, and 0.562 versus ISV, respectively).
  
  
  CONCLUSIONS
  This study presents a novel and comprehensive investigation of corneal asymmetry and focal thinning indices for keratoconus by AS-OCT. These indices correlate with established Scheimpflug-derived anterior surface irregularity indices, particularly in the early and milder stages. AS-OCT may hold promise as a helpful screening and diagnostic tool for suspect, early, and clinical keratoconus.
authors.affiliation: *Laservision.gr Eye Institute, Athens, Greece; and †Department of Ophthalmology, New York University Medical School, New York, NY.
authors.forename: Anastasios J
authors.initials: AJ
authors.lastname: Kanellopoulos
authors.forename: George
authors.initials: G
authors.lastname: Asimellis
mesh_terms: D000293:Adolescent; D000328:Adult; D003315:Cornea / Q000473:pathology*; D063171:Corneal Pachymetry; D005260:Female; D064368:Healthy Volunteers; D006801:Humans; D021621:Imaging, Three-Dimensional; D007640:Keratoconus / Q000145:classification / Q000175:diagnosis*; D008297:Male; D008875:Middle Aged; D009929:Organ Size; D041623:Tomography, Optical Coherence