Indications for Corneal Imaging in Refractive Surgery
Corneal topography is an essential part of the preoperative evaluation of refractive surgery candidates. About two-thirds of patients with normal corneas have a symmetric astigmatism pattern that is round, oval, or bow-tie shaped (see Fig 1-10). Asymmetric patterns include asymmetric bow-tie patterns, inferior steepening, superior steepening, skewed radial axes, or other nonspecific irregularities.
Corneal topography detects irregular astigmatism, which may result from abnormal tear film, contact lens warpage, keratoconus and other corneal ectatic disorders, corneal surgery, trauma, scarring, and postinflammatory or degenerative conditions. Repeat topographic examinations may be helpful when the underlying etiology is in question, especially in cases of suspicious steepening patterns in patients who wear contact lenses or who have an abnormal tear film. Contact lens wearers often benefit from extended periods without contact lens wear prior to preoperative planning for refractive surgery; this period allows the corneal map and refraction to stabilize. Patients with keratoconus or other ectatic disorders are not routinely considered for ablative keratorefractive surgery because the abnormal cornea may exhibit an unpredictable response and/or progressive ectasia. Forme fruste, or subclinical, keratoconus typically is considered a contraindication to ablative refractive surgery. Studies are under way to determine the suitability of some keratorefractive procedures in combination with corneal crosslinking as alternative therapeutic modalities for these patients (see also Chapter 7).
Corneal topography and tomography can also be used to demonstrate the effects of keratorefractive procedures. Preoperative and postoperative maps may be compared to determine the refractive effect achieved (difference map; Fig 1-14). Corneal mapping can also help explain unexpected results, including undercorrection and overcorrection, induced astigmatism, and induced aberrations from small optical zones, decentered ablations, or central islands (Fig 1-15).
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De Paiva CS, Harris LD, Pflugfelder SC. Keratoconus-like topographic changes in keratoconjunctivitis sicca. Cornea. 2003;22(1):22–24.
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Rabinowitz YS, Yang H, Brickman Y, et al. Videokeratography database of normal human corneas. Br J Ophthalmol. 1996;80(7):610–616.
Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.