February 2018

COVER FEATURE

Laser vision correction
Advances in preoperative testing for refractive surgery


by Liz Hillman EyeWorld Staff Writer


Diseased eyelid showing atrophic, shortened meibomian glands
Source: Jennifer Loh, MD

Basic LASIK preoperative evaluation
Ronald Krueger, MD, discusses the salient points of the basic preoperative
evaluation for refractive surgery.

Determining a patient’s image quality, ruling out keratoconus and ectasia risk, and assessing dry eye

Preoperative 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 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 vision but quantifying it is important for objectively accurate refractive surgery.
To do this, Karolinne Rocha, MD, assistant professor of ophthalmology, director of cornea and refractive surgery, Medical University of South Carolina, Charleston, uses Scheimpflug imaging (Pentacam, Oculus, Wetzlar, Germany), which provides 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 possible 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 aberrometry 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 sensitivity 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 “degradation of their optics” after refractive surgery compared to those with smaller myopic correction due to increased values of higher order aberrations (HOA) with more flattening 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 accommodation and presbyopia, the study authors wrote that manipulating higher order aberrations could expand depth of focus (DOF) in the hope of improving near and intermediate vision.
“In this context, patients who have undergone LASER refractive surgery achieve the same optical 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 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, using the Pentacam. Though these technologies are not new, Dr. Rocha explained that she has more recently combined her Pentacam measurements with that of the Corvis ST (Oculus), a non-contact tonometer that has a dynamic ultra-high speed Scheimpflug camera to show real time deformation of the cornea. The corneal tomography combined with the corneal deformation make up the tomographic biomechanical index (TBI).3
“By combining the biomechanical properties and the tomographic findings, you have this TBI index that is more sensitive in detecting patients who are at higher risk for developing ectasia,” Dr. Rocha said, noting that an optimized TBI cut-off value of 0.29 provided 90.4% sensitivity with 96% specificity in eyes with normal topography and very asymmetric ectasia in the fellow eye. In a recent paper on which she was an author, TBI was applied to clinical cases,4 and Dr. Rocha said she now uses TBI parameters to evaluate all of her refractive surgery candidates.
“Corneal topography and tomography provide different indices that look at the thickness profile and elevation maps … but the Corvis will give us information on corneal biomechanics [cornea deformation parameters], and if that cornea is strong enough for surgery, PRK, LASIK, or SMILE,” Dr. Rocha explained. “Sometimes you can have a thin cornea but it’s a normal strong cornea … and some patients have a thick cornea but that cornea is weak.”
Daniel Reinstein, MD, MA (Cantab), FRCSC, London Vision Clinic, London, U.K., acknowledged the Corvis ST and Pentacam as a biomechanical diagnostic option, but said that while it might increase sensitivity of identifying keratoconus, its specificity is still too low to be an attractive tool in a refractive surgery clinic, in his opinion.
“Too many false positive diagnoses of keratoconus would likely be picked up,” he said. “On the other hand, I see the most important advances happening in epithelial mapping for keratoconus detection, particularly given that we now have a device that combines this with tomography.
“Currently, the most advanced OCT device for screening for keratoconus in my view is the MS-39 [CSO, Firenze, Italy],” Dr. Reinstein said. “The most accurate epithelial mapping device available is still the ArcScan Insight 100 System [ArcScan, Golden, Colorado], with a measurement precision of less than 1 µm. It is also the only system with an integrated keratoconus screening automatic epithelial profile classifier, which has a 94.6% sensitivity and 99.2% specificity for detecting keratoconus.5
“The MS-39 combines mapping of the epithelium, a Placido front surface, and OCT tomographic back surface information, which are all captured simultaneously and spatially registered,” he continued. “While epithelial maps by OCT are not as accurate as those by very high-frequency ultrasound, the MS-39 provides an excellent integration of all modalities.”6,7
Because of the epithelium’s ability to remodel, masking early keratoconus that might not be identified by other devices, Dr. Reinstein said epithelial thickness mapping may be used to confirm suspected keratoconus or show thickening over a suspicious area to help rule out keratoconus and enable corneal refractive surgery to be performed.
In addition to epithelial thickness mapping with the MS-39 or RTVue (Optovue, Fremont, California), Dr. Reinstein said his clinic uses a 20-point keratoconus screening protocol on every refractive surgery consult. This includes Placido topography, tomography, corneal OCT, and corneal hysteresis.
“Any patient in whom there is a question based on the above testing protocol undergoes ArcScan Insight 100 scanning, which is then used to make a final decision. Some patients are ‘saved’ from corneal surgery by the ArcScan Insight, but a significant number of patients are cleared for cornea surgery by the confirmation of a ‘normally’ classified epithelial profile,” Dr. Reinstein said.
“As you may surmise from our scanning protocol, we think every patient should have epithelial thickness mapping prior to surgery,” Dr. Reinstein said. “Given the significant change in diagnostic category afforded by epithelial thickness mapping, I think that no refractive surgery clinic can afford not to use it. The ArcScan Insight 100 increases our annual surgical volume by 7% by providing confirmation of normality when things are equivocal.
“Finally, by having the ability to map epithelium we are also equipping ourselves with the ability to perform layered pachymetric mapping of the cornea for flap, residual bed and other interface biometry (such as scars, etc.) when evaluating postoperative corneas.”

Addressing the ocular surface

All patients seeing Jennifer Loh, MD, Loh Ophthalmology Associates, Miami, will receive ocular surface/dry eye testing.
“We place it at a very high importance; it’s one of the first things I evaluate when examining patients, especially if they’re coming in for a surgery consult,” she said.
“I think the tear film and ocular surface are critical, and every refractive surgeon should be paying attention to it as a poor tear film and ocular surface will lead to refractive misses,” said Preeya K. Gupta, MD, associate professor of ophthalmology, Duke University School of Medicine, Durham, North Carolina. “To that end, it is important to screen pre-surgical patients, especially those who have a refractive goal, to make sure they don’t have dry eye disease or meibomian gland disease (MGD).” 
To assess the ocular surface quality of a patient, Dr. Loh said she will perform fluorescein staining, tear breakup time testing, topography, and meibography. The latter, she said, is one of the newer dry eye diagnostic tools that she thinks “can give us a lot of clues to the health of the ocular surface, and it gives the patient an image of their disease, if they do show such signs.”
Dr. Gupta said the most common tests to screen for dry eye in her practice are osmolarity testing, MMP-9 (an inflammatory marker) testing, and meibomian gland imaging.
“These tests give me a great understanding of how the ocular surface is functioning at the time of my evaluation,” Dr. Gupta said. “Hyperosmolarity is a core mechanism in dry eye disease and with prolonged hyperosmolarity inflammation is triggered; this is where the MMP-9 test is helpful as it identifies critical levels of inflammation on the ocular surface. Last but not least, MGD is a big part of DED. Often clinical exam is insufficient to allow us to see how advanced MGD is. On many occasions, I have examined a patient clinically and found mild to moderate MGD, only to find severe gland atrophy on MG imaging. Understanding the degree of gland atrophy helps to understand disease severity and allows the physician to set realistic expectations with the patient.”
Dr. Gupta will perform tear osmolarity testing and meibography on all patients. If either of those tests are positive for disease she will advance to MMP-9, Sjögren’s syndrome, and/or allergy testing.
Most patients, after their dry eye is managed, can go on to have successful refractive surgery, Dr. Loh said. While there has been increased awareness efforts on the importance of treating the ocular surface prior to refractive surgery, including for refractive cataract surgery, Dr. Loh said taking the time to really treat these patients can be a hurdle.

References

1. Zhao J, et al. Effect of higher-order aberrations and intraocular scatter on contrast sensitivity measured with a single instrument. Biomed Opt Express. 2017;8:2138–2147.
2. Sarkar S, et al. Image quality analysis of eyes undergoing LASER refractive surgery. PLoS One. 2016;11:e0148085.
3. Ambrósio R Jr, et al. Integration of Scheimpflug-based corneal tomography and biomechanical assessments for enhancing ectasia detection. J Refract Surg. 2017;33:434–443.
4. Haddad J, et al. First clinical impressions on the integrated corneal tomography and corneal deformation with Scheimpflug imaging. Int J Kerat Ect Cor Dis. 2017;6:101–109.
5. Silverman RH, et al. Epithelial remodeling as basis for machine-based identification of keratoconus. Invest Ophthalmol Vis Sci. 2014;55:1580–7.
6. Reinstein DZ, et al. Comparison of corneal epithelial thickness measurement between Fourier-domain OCT and very high-frequency digital ultrasound. J Refract Surg. 2015;31:438–45.
7. Urs R, et al. Comparison of very-high-frequency ultrasound and spectral-domain optical coherence tomography corneal and epithelial thickness maps. J Cataract Refract Surg. 2016;42:95–101.

Editors’ note: Dr. Gupta has financial interests with Johnson & Johnson Vision (Santa Ana, California), TearLab (San Diego), and TearScience (Morrisville, North Carolina). Dr. Loh has financial interests with Allergan (Dublin, Ireland) and Shire (Lexington, Massachusetts). Dr. Reinstein and Dr. Rocha have no financial interests related to their comments.

Contact information

Gupta
: preeyakgupta@gmail.com
Loh: jenniferlohmd@gmail.com
Reinstein: dzr@londonvisionclinic.com
Rocha: karolinnemaia@gmail.com

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