In this episode of Interventional Mindset, Preeya K. Gupta, MD, Steven Greenstein, MD, and John D. Gelles, OD, FIAO, FCLSA, FSLS, FBCLA, discuss the use of optical coherence tomography (OCT) and epithelial thickness mapping in keratoconus (KC) care.
Dr. Greenstein is the Medical Director, Director of Refractive Surgery, and Director of the Center for Keratoconus at the Cornea and Laser Eye Institute (CLEI) in Teaneck, NJ. He is also a co-inventor with Dr. Gelles and Peter Hersh, MD, of corneal tissue addition keratoplasty (CTAK).
Dr. Gelles is the Director of the Specialty Contact Lens Division of the CLEI and the CLEI Center for Keratoconus in Teaneck, NJ.
The evolution of diagnostic imaging for corneal pathologies
In recent years, advancements in surgical interventions for KC have necessitated multimetric imaging modalities to visualize the posterior and anterior surfaces of the cornea to diagnose subclinical KC as well as screen for corneal refractive surgery.1
With anterior segment OCT (AS-OCT), clinicians can now map the epithelium to identify early epithelial changes, such as subtle thinning over the cone, to intervene earlier in the disease process and stabilize the cornea before visual sequelae occur. OCTs also enable surgeons to determine if there is any retinal pathology to consider in these patients.
As he often performs CTAK in his practice, Dr. Greenstein explained that if patients have post-LASIK ectasia, AS-OCT gives him the ability to account for the depth of the LASIK flap by measuring it precisely to ensure the CTAK inlay is set to a depth below it, rather than estimating and assuming the depth of the flap.
In addition, AS-OCT is beneficial for corneal refractive surgery patients, as it allows for visualization of anterior chamber depth, the crystalline lens, implantable collamer lens (ICL) vault, and, as mentioned earlier, determining the depth of LASIK flaps.
Dr. Greenstein stated that he has used AS-OCT in cases where a Kamra Inlay was implanted after LASIK, and it was unclear whether it was situated under the LASIK flap or if a new pocket was created.
To learn about refractive surgery procedures that can improve vision in KC patients, check out Refractive Surgery Options for Vision Correction in Keratoconus!
Surgical planning with epithelial thickness mapping
Epithelial thickness mapping can be particularly useful for KC patients undergoing photorefractive keratectomy (PRK), as it allows the surgeon to understand how the epithelial thickness varies across the keratoconic cornea to fine-tune the corrections to the patients’ exact anatomy.
Dr. Greenstein highlighted that he finds it indispensable for planning out surgery, especially in more complex cases, such as in post-LASIK patients. Moreover, he expressed that AS-OCT has fundamentally changed the understanding of epithelial changes following myopic and hyperopic ablation.
While refractive regression was once perceived to be a result of LASIK or PRK correction, it has since been proven that these are in fact remarkably stable. Instead, epithelial remodelling, whether it is around the hyperopic ablation or inside of the myopic ablation, has been directly implicated in causing these refractive changes.2
Identifying refractive surgery candidates with epithelial thickness maps
Dr. Gelles mentioned a recent study by Randleman et al. that investigated the impact of epithelial thickness maps derived from OCT on screening for refractive surgery candidacy, wherein they ruled in more patients (10%) than they ruled out (6%) for LASIK.3
He added that epithelial thickness maps are especially helpful for patients to discern if a patient has KC or epithelial hypertrophy in the inferior cornea that may be related to ocular surface dryness. By taking a closer look at the epithelium, clinicians may find that these patients are good candidates for refractive surgery once their ocular surface has been optimized.
Pearl: What can look like inferior steepening on tomography may in fact simply be hypertrophy. The diagnosis can be confirmed using epithelial thickness maps by looking for an epithelial donut pattern, which indicates the presence of an underlying stromal cone.4
Which diagnostic devices should I use in a cornea practice?
Dr. Gelles remarked that, due to the limited space in clinics, having multifunctional devices that can image the cornea, lens, and macula (if possible) is critical to having a comprehensive understanding of the patient’s ocular health.
For example, last year, Visionix launched a new topography module in their Optovue line of OCT systems to provide an extensive assessment of the anterior and posterior corneal surfaces while differentiating between stroma and epithelial cells.5
Dr. Greenstein noted that he uses the OCULUS Pentacam as a benchmark device for corneal imaging and the Optovue Solix Essential SD-OCT by Visionix to get detailed images of the cornea and macula for an all-in-one image of the eye. He added that there is still a place for Placido imaging to get a clear view of the tear film, though epithelial maps can also aid in identifying ocular surface disease.
For example, if there aren’t clear patterns of KC or hypertrophy from warpage, but a patient presents with a scattered appearance of thinning and thickening on epithelial maps, they may have epithelial basement membrane dystrophy (EBMD) or simply ocular surface dryness. Ultimately, epithelial mapping can hone in on signs that are easy to overlook at the slit lamp to confirm or rule out a diagnosis.
To learn more about developments in multifunctional OCTs, watch the full interview!
Key parameters to use in advanced diagnostic imaging
For the Pentacam, Dr. Gelles explained that he most often uses the Belin-Ambrósio Enhanced Ectasia Display (BAD) to visualize front and back elevation and corneal thickness in a single view.
The BAD is a powerful screening tool for KC and refractive surgery because it highlights if there is progressive thickening of the cornea that requires further investigation. He also uses the Belin ABCD Progression Display to track disease progression in patients with KC and for some patients with post-refractive ectasia.
He added that he is excited to see the upcoming OCT-based multimetric analysis for KC on the Optovue Solix by Visionix that will add features to its existing epithelial mapping capabilities.
From a surgical standpoint, Dr. Greenstein noted that he looks at the overall tomography, BAD analysis, and epithelial maps to get a full picture of the cornea and select the optimal modality to address patients’ corneal pathology. For example, in KC patients, AS-OCT can show the depth of apical scarring, and be used to determine which appear more visually significant.
With this information, he can decide if CTAK or deep anterior lamellar keratoplasty (DALK) is sufficient to reshape the cornea, if he needs to proceed with removing the scarring, or if the scarring is so deep that penetrating keratoplasty (PK) is required.
Conclusion
Dr. Gupta noted that AS-OCT and epithelial thickness mapping can aid surgeons in being better diagnosticians because they can provide direct answers to ambiguous signs and symptoms.
Further, OCT imaging continues to change how surgeons approach surgical cases, as they can now tailor the treatment to the patient and have more confidence in the outcome.
Also, as the saying goes, “a picture is worth a thousand words;” the images produced from OCT and epithelial thickness mapping can be used as patient education tools to discuss any relevant diagnoses and walk them through the rationale behind any required surgical interventions.
