EyeWorld India March 2018 Issue

Advances in preoperative testing for refractive surgery by Liz Hillman EyeWorld Staff Writer Determining a patient’s image quality, ruling out keratoconus and ectasia risk, and assessing dry eye P reoperative testing to determine refractive surgery candidacy has dramatically improved to identify early keratoconus, dry eye, and risk for corneal ectasia. More recent technological advances are further fine-tuning the diag- nostic capabilities of patients before corneal refractive surgery. EyeWorld spoke with several experts in the field about what’s new for the preoperative/diagnostic stage for refractive surgery candi- dates as it pertains to quantifying their image quality, ruling out keratoconus, and identifying ocular surface issues. Quantifying quality Patients might be able to describe their own subjective quality of vi- sion but quantifying it is important for objectively accurate refractive surgery. To do this, Karolinne Rocha, MD, assistant professor of oph- thalmology, director of Cornea & Refractive surgery, Medical Univer- sity of South Carolina, Charleston, South Carolina, uses Scheimpflug imaging (Pentacam, Oculus, Arling- ton, Washington) which provides information about lens density (could indicate early cataract) and the double-pass wavefront HD Analyzer (Visiometrics, Costa Mesa, California) which provides the ocu- lar scatter index (could indicate tear film instability and thus dry eye conditions). There are patients who might come in with 20/20 vision complaining of blurry vision, and the HD Analyzer could show a high or changing ocular scatter index over 20 seconds, indicating the blur is from tear film instability. If that were the case, the patient’s vision could improve with dry eye therapy alone or he or she might require dry eye treatment prior to refractive sur- gery. It can also give the patient and the doctor an idea of what to expect postoperatively. “Patients, especially after LASIK, can have dry eye symp- toms,” Dr. Rocha said. “But we know patients usually go back to baseline by 3–6 months. At least we're aware of possible blurry vision postop because of this tear film in- stability. In these cases, it is impor- tant to start the dry eye treatment before surgery.” The point spread function of the HD Analyzer, Dr. Rocha con- tinued, can describe how patients see a source of light (point spread function), which could indicate if they experience coma, halo, or glare in low light situations. Wavefront aberrometry is also useful in that it gives an objective refraction and higher-order aberration measure- ments. “We can sometimes see high spherical aberration or coma that would give us more information in terms of why a patient might have night symptoms. Significant amounts of spherical aberration can cause halos and glare while coma is responsible for monocular diplo- pia,” Dr. Rocha said. “Plus, some- one with higher amounts of coma preoperatively may be a red flag. … They may have irregular astig- matism, keratoconus, or corneal ectasia.” Research has shown that retinal image quality and contrast sen- sitivity can be affected by higher order aberrations. 1 A more recent study analyzed image quality in eyes having LASIK and found those with greater myopic correction were more likely to experience “degrada- tion of their optics” after refractive surgery compared to those with smaller myopic correction due to increased values of higher order aberrations (HOA) with more flat- tening of the cornea. 2 “In conclusion, the increase in HOAs experienced after refractive surgery results in a degradation of peak [image quality (IQ)] and a persistence of this sub-standard IQ over a larger dioptric range when compared to age-matched control eyes or when compared to the same eyes before surgery. Such an increase in optical degradation however appears to have only a minimal impact on psychophysical estimates of spatial visual perfor- mance (high- and low-contrast log- MAR acuity and depth-of-focus),” Sarkar et al. wrote. When it comes to accommo- dation and presbyopia, the study authors wrote that manipulating higher order aberrations could expand depth of focus (DOF) in the hope of improving near and inter- mediate vision. “In this context, patients who have undergone LASER refractive surgery achieve the same opti- cal effect of a multifocal lens in that their expanded DOF might also support useful intermediate and near vision without exerting much accommodative effort—a scenario that is useful with the onset of presbyopia,” Sarkar et al wrote. “However, this might pose a challenge to the binocular near vision in pre-presbyopic ages as the demands on accommodation and its coupled vergence response may be altered due to modification in the eye’s DOF.” Dr. Rocha said a complete eye exam that rules out cataract, retinal diseases, corneal scar, and advanced glaucoma should be performed because “those are all conditions AT A GLANCE • New technologies and techniques in diagnostics can rule out refractive candidates who could be at risk for dry eye, corneal ectasia, or early keratoconus. • Machines such as those that perform wavefront aberrometry or capture point spread function can be helpful in determining a patient’s quantitative image quality. • Epithelial mapping, Placido topography, tomography, corneal OCT, and corneal hysteresis can help identify early keratoconus in patients. • Tear lm and ocular surface analysis is critical to avoid “refractive misses.” March 2018 42 EWAP REFRACTIVE