2020–2021 BCSC Basic and Clinical Science Course™
13 Refractive Surgery
Chapter 2: Patient Evaluation
Corneal thickness is an important criterion for determining adequacy for keratorefractive surgery. Corneal pachymetry is usually measured with ultrasound; however, certain non–Placido disk corneal topography and OCT systems can also be used if properly calibrated. Some systems provide a map showing the relative thickness of the cornea at various locations. The accuracy of the pachymetry measurements of scanning-slit systems decreases markedly for eyes that have undergone keratorefractive surgery. Because the thinnest part of the cornea is typically located centrally, a central measurement should always be taken. The thickness of the cornea is an important factor in determining whether the patient is a candidate for refractive surgery and identifying the optimal refractive procedure. In a study of 896 eyes undergoing LASIK, the mean central corneal thickness was 550 μm ± 33 μm (range, 472–651 μm). It has been suggested that an unusually thin cornea (beyond 2 standard deviations) indicates that the patient may not be ideal for any refractive surgery. Many surgeons would not consider LASIK refractive surgery if the central corneal thickness is less than 480 μm, even if the calculated residual stromal bed (RSB) is thicker than 250 μm. If LASIK is performed and results in a relatively thin RSB—for example, around 250 μm—future enhancement surgery that further thins the stromal bed may not be possible. If there is suspicion that endothelial integrity is causing an abnormally thick cornea, specular microscopy may be helpful in assessing the health of the endothelium.
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.