Laser Vision Correction Pre-Screening Tests Demystified

  1. Topography is usually the first corneal test performed. In this test, we look for inferior steepening, skewed axis and other possible irregularities.
  2. Tomography is performed to evaluate anterior corneal surface further, to determine corneal thickness, including the exact location of the thinnest spot of the cornea, to evaluate posterior cornea, and to perform ectasia risk assessment with advanced software that compares multiple parameters of the patient’s cornea to those found in a general population of people with normal corneas.
  3. Epithelial Thickness Mapping (ETM) is performed to evaluate a patient’s corneal epithelium to rule out subtle pre-ectatic conditions, contact lens warpage, and epithelial basement dystrophy. It is not uncommon for subtle focal abnormalities on topography and tomography to be misdiagnosed as pre-ectatic conditions, whereas they are truly epithelial basement membrane or contact lens warpage (Schallhorn, JM, et al. Distinguishing between contact lens warpage and ectasia: Usefulness of optical coherence tomography epithelial thickness mapping. J Cataract Refract Surg. 2017;43(1):60–66.) Topography and tomography simply do not show the “whole picture” and often lead the examiner to recommend PRK for patients who are great LASIK candidates based on ETM or recommend no procedure to patients who are excellent PRK candidates. I have seen patients for second and third opinion whose diagnosis and treatment plan becomes unequivocally clear after performing and analyzing their ETM.
  4. Ocular Response Analyzer (ORA) is a sophisticated technology designed to test the elasticity of the cornea and its ability to remain strong and stable after laser vision correction. ORA is also helpful in sorting out different opinions about a patient’s diagnosis and treatment options and resolving split opinions. Moreover, a recent review of literature by Moshirfar M, et al has concluded that technologies such as ORA are essential for screening of corneal ectasia because “changes in biomechanical properties may occur before disease becomes apparent via tomography or topography.” (Moshirfar M, et al. Advances in biomechanical parameters for screening of refractive surgery candidates: A review of the literature, Part III. Med Hypothesis Discov Innov Ophthalmol. 2019;8:219–240).
  5. CONTOURA® Vision Analysis: is performed to generate a highly detailed map of corneal higher order aberrations by measuring elevation profile of 22,000 unique data points on the cornea. This creates a highly detailed contour scan of microscopic peaks and valleys to screen out pre-ectatic conditions and to determine if the patient has been out of contacts for a sufficient length of time to establish a precise treatment plan.

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Founder of Pacific Vision Institute in San Francisco.

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Dr. Ella Faktorovich

Dr. Ella Faktorovich

Founder of Pacific Vision Institute in San Francisco.