Eyeworld

EW FEATURE 76 Laser vision correction • February 2018 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 topogra- phy, tomography, corneal OCT, and corneal hysteresis can help identify early keratoconus in patients. • Tear film and ocular surface analysis is critical to avoid "refractive misses." by Liz Hillman EyeWorld Staff Writer Advances in preoperative testing for refractive surgery Determining a patient's image quality, ruling out keratoconus and ectasia risk, and assessing dry eye P reoperative testing to de- termine 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 diagnostic capabilities for patients before corneal refractive surgery. EyeWorld spoke with several experts in the field about what's new at the preoperative/diagnostic stage for refractive surgery candidates 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 ophthal- mology, director of cornea and re- fractive surgery, Medical University of South Carolina, Charleston, uses Scheimpflug imaging (Pentacam, Oc- ulus, Wetzlar, Germany), which pro- vides information about lens density (could indicate early cataract), and the double-pass wavefront HD Analyzer (Visiometrics, Costa Mesa, California), which provides the ocular 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 surgery. It can also give the patient and the physician an idea of what to expect postop. "Patients, especially after LASIK, can have dry eye symptoms," Dr. Rocha said. "But we know patients usually go back to baseline by 3–6 months. At least we're aware of pos- sible blurry vision postop because of this tear film instability. In these cases, it is important to start the dry eye treatment before surgery." The point spread function of the HD Analyzer, Dr. Rocha said, can describe how patients see a source of light, which could indicate if they experience coma, halo, or glare in low light situations. Wavefront aber- rometry is also useful in that it gives an objective refraction and higher order aberration measurements. "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 diplopia," Dr. Rocha said. "Plus, someone with higher amounts of coma preoperatively may be a red flag. … He or she may have irregular astigmatism, 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 ab- errations (HOA) with more flatten- ing 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 performance (high- and low-contrast logMAR 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 optical ef- fect of a multifocal lens in that their expanded DOF might also support useful intermediate and near vision without exerting much accommoda- tive effort—a scenario that is useful with the onset of presbyopia," Sark- ar 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 that can cause changes in quality of vision." Ruling out keratoconus Dr. Rocha performs Placido-based corneal topography (Atlas, Carl Zeiss Meditec, Jena, Germany) and Scheimpflug corneal tomography, Diseased eyelid showing atrophic, shortened meibomian glands Source: Jennifer Loh, MD

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