Keratoconus is a progressive eye disorder that leads to the thinning and bulging of the cornea, resulting in distorted vision. Over the years, numerous modalities have emerged to improve the vision of affected patients, but the primary method for stabilizing the cornea remains corneal cross-linking (CXL). With the advent of cross-linking, it has become increasingly vital to identify progression early to ensure timely intervention and preservation of vision. This article, based on a presentation given by Dr. Steven Greenstein, delves into the latest research, case studies, and best practices in diagnosing and treating progressive keratoconus.
How Quickly Does Keratoconus Progress? Lessons from Clinical Trials
Historically, the definition of keratoconus progression has been based on criteria from the FDA’s multi-center pivotal trial. Progression was characterized by:
- A diopter or more increase in K-steep.
- A diopter or more increase in manifest cylinder.
- A worsening of the spherical equivalent by about half a diopter over 24 months.
However, these metrics present challenges, particularly in severe cases, where refractive changes fluctuate significantly, even hour by hour. A review of over a thousand keratoconic eyes revealed that 40% exhibited hyperopia (farsightedness) or emmetropia (no glasses prescription at all), while another 40% had mixed or no astigmatism—factors that complicate the reliance on refraction alone for diagnosing keratoconus and keratoconus progression.
Diagnosing Keratoconus: The Limitations of Traditional Metrics
In diagnosing keratoconus, curvature measurements have been a primary focus. On average, K-steep is eight diopters flatter than K-max. In mild to moderate cases (50-55 diopters), K-steep is approximately 47 diopters—easily mistaken for a normal cornea. A strong correlation between K-steep and K-max only emerges in advanced keratoconus (65 diopters or steeper), emphasizing that basic central curvature measurements alone are insufficient for diagnosis and monitoring progression.
Tomography as the Gold Standard
Tomography has become the most reliable method for diagnosing and tracking keratoconus progression. K-max remains a widely reported metric in the literature, but its limitations necessitate additional approaches.
The Role of the Thinnest Point and the Bellin ABCD Progression Display
Recent advancements have led to a shift in focus toward the thinnest point of the cornea and the three-millimeter zone surrounding it. The Belin ABCD Progression Display integrates multiple metrics through regression analysis to track changes in:
- A: Anterior corneal curvature.
- B: Posterior corneal curvature.
- C: Corneal thickness at the thinnest point.
- D: Visual function (entered manually).
The Importance of Cross-Linking and Monitoring Progression
In post-cross-linking evaluations, the Belin ABCD display provides a third analytical layer, comparing patients to a cross-linked population. The system sets a baseline one year post-CXL and assesses whether further progression has occurred.Case Study: An 18-year-old male showed no clear progression based on traditional axial curvature and K-max metrics. However, the Belin ABCD analysis revealed subtle but statistically significant posterior curvature steepening, suggesting early progression. As a result, the patient was monitored closely and cross-linked at the appropriate time.
The Role of the Thinnest Point and the Bellin ABCD Progression Display
Recent advancements have led to a shift in focus toward the thinnest point of the cornea and the three-millimeter zone surrounding it. The Belin ABCD Progression Display integrates multiple metrics through regression analysis to track changes in:
- A: Anterior corneal curvature.
- B: Posterior corneal curvature.
- C: Corneal thickness at the thinnest point.
- D: Visual function (entered manually).
The Importance of Cross-Linking and Monitoring Progression
In post-cross-linking evaluations, the Belin ABCD display provides a third analytical layer, comparing patients to a cross-linked population. The system sets a baseline one year post-CXL and assesses whether further progression has occurred.
Case Study: An 18-year-old male showed no clear progression based on traditional axial curvature and K-max metrics. However, the Belin ABCD analysis revealed subtle but statistically significant posterior curvature steepening, suggesting early progression. As a result, the patient was monitored closely and cross-linked at the appropriate time.
Challenges in Diagnosing Severe Cases
For patients with advanced keratoconus, determining progression is even more complex. Consider a patient with a K-max of 77 diopters in 2016, increasing to 78.7 in 2019. This seemingly clear progression required a deeper dive:
- Anterior curvature maps showed inferior steepening.
- Posterior elevation maps remained unchanged.
- Significant corneal thinning was noted.
However, a crucial observation was that the K-max location had shifted from central to more peripheral over time. Additionally, densitometry measurements revealed increased corneal scarring, indicating that the observed changes were due to scarring rather than true disease progression. In such cases, cross-linking may not be necessary.
The Impact of Long-Term Follow-Up
Lost-to-follow-up cases highlight the importance of consistent monitoring. A 35-year-old patient first examined in 2009 presented with mild keratoconus but did not return until 2019, by which time severe disease was evident. However, using comparative analysis over multiple visits, it was determined that progression had stabilized after 2017, eliminating the immediate need for cross-linking. The patient was advised to return for frequent monitoring instead.
Conclusion
No single metric is sufficient to determine keratoconus progression. Over time, the field has evolved from relying on K-steep and refractions to utilizing comprehensive analyses such as multi-metric tomography. The Belin ABCD Progression Display has become an invaluable tool, synthesizing data to allow clinicians to:
- Detect early progression in mild cases.
- Assess severe cases with greater accuracy.
- Provide clearer communication with patients about their disease status.
By improving early diagnosis and more accurately monitoring progression, clinicians can intervene at the right time to prevent vision loss and improve patient outcomes. With the continued refinement of diagnostic tools and treatment approaches, keratoconus management is now more precise and effective than ever before.
Watch Dr. Greenstein’s full presentation here:
