The corneal curvature must be evaluated. Although manual keratometry readings can be quite informative, they have largely been replaced by computerized corneal topographic analyses. Several different methods are available to analyze the corneal curvature, including Placido disk-based topography, scanning slit-beam imaging, rotating Scheimpflug photography, high-frequency ultrasound, and optical coherence tomography (OCT) techniques. (See also the discussion of corneal topography in Chapter 1.) These techniques image the cornea and provide color maps showing corneal power and/or elevation. Patients with visually significant irregular astigmatism are generally not good candidates for corneal refractive surgery. Early keratoconus, pellucid marginal degeneration (Fig 2-5), and contact lens warpage are potential causes of visually significant irregular astigmatism. Irregular astigmatism secondary to contact lens warpage usually reverses over time, although the reversal may take months. Serial corneal topographic studies should be performed to document the resolution of visually significant irregular astigmatism before any refractive surgery is undertaken.
Unusually steep or unusually flat corneas can increase the risk of poor flap creation with the microkeratome. Femtosecond laser flap creation theoretically may avoid these risks. When keratometric or corneal topographic measurements reveal an amount or an axis of astigmatism that differs significantly from that determined through refraction, the refraction should be rechecked for accuracy. Lenticular astigmatism or posterior corneal curvature may account for the difference between refractive and keratometric or topographic astigmatism. Most surgeons will treat the amount and axis of the refractive astigmatism, as long as the patient understands that after any future cataract surgery, some astigmatism may reappear (after the astigmatism contributed by the natural lens has been eliminated).
Figure 2-5 A corneal topographic map of the typical irregular against-the-rule astigmatism found in eyes with pellucid marginal degeneration. Note that the steepening nasally and temporally connects inferiorly.
(Courtesy of Christopher J. Rapuano, MD.)
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.