The Role of Corneal Topography in Refractive Surgery
Corneal topography is one of the key evaluative technologies in refractive surgery, crucial not only in preoperative screening but also in postoperative evaluation of patients with unexpected results. Topographic analysis should be undertaken in all patients being considered for refractive surgery in order to identify patients who should not undergo the procedure. Although refractive surgery has numerous contraindications (see Chapter 2), some of the most important to recognize are the corneal ectatic disorders: keratoconus and pellucid marginal degeneration (see BCSC Section 8, External Disease and Cornea, for further discussion).
Keratoconus (KC) and pellucid marginal degeneration (PMD) are generally progressive conditions in which thinning occurs in the central, paracentral, or peripheral cornea, resulting in asymmetric corneal steepening and reduced spectacle-corrected visual acuity. These 2 conditions may be separate entities or different clinical expressions of the same ectatic process; in either case, they are currently contraindications for excimer laser surgery. The topographic pattern in keratoconic eyes usually demonstrates substantial inferonasal or inferotemporal steepening, although severe central and even superior steepening patterns may occur (Fig 1-16). The classic topographic pattern in PMD is inferior steepening, which is most dramatic between the 4 and 8 o’clock positions, with superior flattening. This inferior steepening often extends centrally, coming together in what has been described as a “crab-claw” shape (see Chapter 10, Fig 10-2). There may be substantial overlap in the topographic patterns of KC and PMD.
Figure 1-16 Corneal topography in keratoconus. Placido imaging showing distorted corneal mires (A) and axial and elevation topography in keratoconus (B).
(Courtesy of M. Bowes Hamill, MD.)
The patient who poses the greatest difficulty in preoperative evaluation for refractive surgery is the one in whom KC ultimately develops but who shows no obvious clinical signs at the time of examination. Corneal topography may reveal subtle abnormalities that should alert the surgeon to this problem. Although newer screening indices take into account various topographic and corneal biomechanical factors that may indicate a higher likelihood of subclinical KC, none of these indices is definitive. In addition to topographic metrics, substantial displacement of the thinnest area of the cornea from the center as revealed by corneal tomography is also suggestive of KC. Normal corneas are substantially thicker peripherally than centrally (by approximately 50–60 μm), and corneas that are not thicker peripherally suggest an ectatic disorder. Newer technologies such as high-resolution anterior segment OCT, ultra-high-frequency ultrasound, and hysteresis analysis may be helpful as screening tests for keratoconus by aiding in evaluating the relative position of the posterior and anterior apex, epithelial thickness, and corneal biomechanical properties; however, these technologies have yet to be validated.
Ambrósio R Jr, Alonso RS, Luz A, Coca Velarde LG. Corneal-thickness spatial profile and corneal-volume distribution: tomographic indices to detect keratoconus. J Cataract RefractSurg. 2006;32(11):1851–1859.
Lee BW, Jurkunas UV, Harissi-Dagher M, Poothullil AM, Tobaigy FM, Azar DT. Ectatic disorders associated with a claw-shaped pattern on corneal topography. Am J Ophthalmol. 2007;144(1):154–156.
Rabinowitz YS. Videokeratographic indices to aid in screening for keratoconus. J Refract Surg. 1995;11(5):371–379.
Rabinowitz YS, McDonnell PJ. Computer-assisted corneal topography in keratoconus. Refract Corneal Surg. 1989;5(6):400–408.
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.