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EW FEATURE 66 Refractive corrections • March 2019 AT A GLANCE • Subclinical keratoconus patients retain a normal anterior surface and good visual acuity. • Epithelial mapping by anterior segment OCT can differentiate epithelial distribution in keratoconus. • Integrated corneal tomography and biomechanics with artificial intelligence enhance the accuracy to detect mild ectasia and possibly to characterize ectasia susceptibility. • Post-diagnosis approaches can be tailored to the patient's age, corneal thickness, ectasia staging, and level of functioning. by Rich Daly EyeWorld Contributing Writer However, some of these VAE cases are truly unilateral. Interestingly, while keratoconus typically presents as an asymmetric disease, this is by definition bilateral. With advanced corneal imaging including the integrated tomography and biome- chanical assessment, we can detect very mild disease in the majority of these cases." Prospective studies are needed to clarify the issue, Dr. Ambrósio said. A. John Kanellopoulos, MD, medical director, LaserVision Clinical and Research Eye Insti- tute, Athens, Greece, and clinical professor of ophthalmology, New York University Medical School, New York, has published numerous studies on the issue and concluded that in its earliest stages the corneal epithelium will thicken overall. That could mean that a normally 50- to 52-micron epithelial thickness may increase up to an average of more than 58 microns prior to creating irregularities. "Overall epithelial thickening may be an early sign of corneal biomechanical instability. If the ectasia progresses, and as it is almost always associated with vigorous eye rubbing, the epithelium remodels to eventually thin over the area of thinning and 'bulging' stroma and thickening around it as a means to keep the anterior cornea surface smooth and thus the optics of the eye at their best," Dr. Kanellopoulos said. In such early stages, topography and tomography cannot yet pick up any irregularity, especially if the ectasia is central. Instead, epitheli- um mapping provided by anterior segment OCT and/or high frequen- cy ultrasound shows what is normal, what is epithelial distribution in dry eyes, and what is epithelial distribu- tion in keratoconus. "You would be surprised how many patients pass with flying col- ors the Placido disc topography and the Scheimpflug-based tomography to be flagged red with epithelium mapping, an easy capture by anteri- or segment OCT devices such as the Avanti [Optovue, Fremont, Califor- nia]," Dr. Kanellopoulos said. Available tools Identifying keratoconus at an early stage is critical, since the condition can progress if a patient undergoes Surgeons identify keys to earlier diagnosis of keratoconus patients A growing number of tools and a better under- standing of keratoconus are helping physicians diagnose the progressive disease at earlier stages. "Currently, the earliest stage that can be diagnosed with a rea- sonable degree of certainty would be what is called 'subclinical kerato- conus,'" said Michael Belin, MD, professor of ophthalmology and vision science, University of Arizo- na, Tucson, Arizona. "This is true disease and should not be confused with 'suspect' or 'form fruste.'" Subclinical keratoconus has abnormalities on the posterior cor- neal surface or changes in corneal thickness compatible with keratoco- nus, such as abnormal pachymetric progression. It remains subclinical because the anterior surface remains normal and the patient retains good visual acuity, Dr. Belin said. Renato Ambrósio Jr., MD, adjunct professor of ophthalmolo- gy, Federal University of the State of Rio de Janeiro (UNIRIO), Brazil, said the terms "subclinical," "fruste" and "mild" are confusing since the clinical literature applies them to different conditions. "There is no easy agreement on these definitions," Dr. Ambrósio said. "We have learned a lot from the very asymmetric ectasia (VAE) cases in which one eye presents with normal front surface topography. corneal refractive surgery, said William Trattler, MD, Center for Excellence in Eye Care, Miami. "In many cases it can be diffi- cult to identify early keratoconus in patients who are interested in LASIK. Advances in imaging tech- nologies help us evaluate the cornea and define keratoconus as early as possible," Dr. Trattler said. "Early keratoconus may not be obvious on a standard Placido disc topography, but there are some technologies we can use that will help us identify it at an early stage." Like many clinicians, Dr. Trat- tler uses tomography to examine the cornea for keratoconus in its very early stages. One pattern, called a truncated bow tie, can present with a normal topography without inferior steepening. The condition can be identified on the Percentage Thickness Increase (PTI) map on the Pentacam (Oculus, Wetzlar, Germa- ny), which compares the relative thickness between the central cor- nea and the peripheral cornea. An- other early keratoconus sign can be identified with epithelial thickness maps, which can highlight an area of epithelial thinning over the apex of an early cone with both OCT and high frequency ultrasound. Another diagnostic modality evaluates the dynamic strength (or weakness) of the cornea with a jet of air. "Corneas with keratoconus are more flexible, and both the Corvis ST [Oculus] and Ocular Response Analyzer [Reichert Technologies, Depew, New York] use software to interpret the movements of the cornea after exposure to a jet of air to help identify early keratoconus," Dr. Trattler said. However, Dr. Kanellopoulos said he has found the air puff technolo- gy can provide a lot of false posi- tives and false negatives for kerato- conus. Additionally, Dr. Kanellopoulos said Brillouin-based technology, which is light scattering microscopy, may provide another clinical tool to assess cornea biomechanics to ob- tain an earlier diagnosis "but in the stage we are now, it is quite early." Dr. Belin's primary means for screening and diagnosing kerato- conus is anterior segment tomogra- phy, especially Scheimpflug devices, which provide the best corneal coverage, are easy to use, and are in most corneal or refractive practic- es. Additionally, Dr. Belin uses the Belin/Ambrosio Enhanced Ectasia display on the Pentacam to detect earlier keratoconic changes. Dr. Ambrósio said the integrat- ed tomography and biomechanical assessment with the Ambrósio, Roberts & Vinciguerra display using artificial intelligence has enhanced the accuracy to detect abnormalities among cases with normal topog- raphy (front surface curvature) and tomography. This approach is available with the Pentacam and the Corvis ST. Obtaining earlier keratoconus diagnoses continued on page 68 " In many cases it can be difficult to identify early keratoconus in patients who are interested in LASIK. Advances in imaging technologies help us evaluate the cornea and define keratoconus as early as possible. " —William Trattler, MD

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