A New Era in Keratoconus Diagnosis: The Comprehensive Belin ABCD Approach

Aishwarya Jha, B.Optom, F.Optom, M.Optom

Optometrist, Cornea and Refractive Services, Dr. Shroff’s Charity Eye Hospital

 

Keratoconus (KC) is a progressive eye disease characterized by the thinning and bulging of the cornea, which leads to visual distortion. ⁽¹⁾ First described in 1854, it has traditionally been diagnosed using older methods that focused on the front (anterior) surface of the cornea and its central thickness. However, advances in imaging technology and the need for early diagnosis have prompted a shift towards more comprehensive classification systems, like the new Belin ABCD staging system.

The Limitations of Traditional Staging Systems

For decades, the Amsler-Krumeich (AK) classification was the go-to method for diagnosing keratoconus. Developed in the 1940s, this system relied heavily on keratometry readings (curvature measurements) and pachymetry (thickness measurements) from the central cornea. However, this approach falls short in detecting early or subclinical cases, especially as modern imaging has revealed how keratoconus affects deeper layers of the cornea. ⁽²⁾

The Belin ABCD System: A Modern Solution

Recognizing these limitations, Dr. Michael Belin introduced the Belin ABCD system to address the diagnostic gaps. Unlike older methods that focus only on the anterior curvature, this system assesses four key parameters to provide a more accurate picture of corneal health: ⁽²⁾

1. A (Anterior Radius of Curvature): Measures the curvature of the front cornea in a 3 mm zone around its thinnest point.
2. B (Posterior Radius of Curvature): Focuses on the curvature of the back corneal surface in the same 3 mm zone.
3. C (Thinnest Pachymetry): Tracks the thinnest area of the cornea.
4. D (Distance Best Corrected Visual Acuity): Assesses how well a patient can see with the best possible correction (Figure 1).

Figure 1: Belin ABCD Display
(Image courtesy: Taken from pentacam machine from a test subject by the author)

These parameters are graded from 0 to 4, reflecting the severity of the disease, with specific cutoffs for curvature, thickness, and visual acuity (Table 1). By including measurements of the posterior corneal surface and focusing on the thinnest point (not just the apex), this system is much more sensitive to early signs of keratoconus. ⁽²⁾

Table 2: The New ABCD System for Classification of Keratoconus

ABCD criteria	A (ARC 3mm zone)	B (PRC 3mm ZONE)	C (Thinnest Pach µm)	D (BDVA)	Scarring
Stage 0	>7.25 mm (<46.5 D)	>5.90 mm (<57.25 D)	>490 µm	=20/20 (=1.0)	–
Stage I	>7.05 mm (<48.0 D)	>5.70 mm (<59.25 D)	>450 µm	<20/20 (<1.0)	-, +, ++
Stage II	>6.35 mm (<53.0 D)	>5.15 mm (<65.5 D)	>400 µm	<20/40 (<0.5)	-, +, ++
Stage III	>6.15 mm (<55.0 D)	>4.95 mm (<68.5 D)	>300 µm	<20/100 (<0.2)	-, +, ++
Stage IV	<6.15 mm (>55.0 D)	<4.95 mm (>68.5 D)	=300 µm	<20/400 (<0.05)	-, +, ++

 

Tracking Disease Progression with ABCD

A significant advantage of the ABCD system is its Progression Display, which allows clinicians to monitor changes over time. Up to eight exams can be plotted to track disease stability or progression. Importantly, it highlights changes in the posterior corneal surface even when the anterior surface appears stable, enabling earlier intervention. This is crucial for deciding when to perform corneal collagen cross-linking (CXL)—a treatment that can halt keratoconus progression if caught early. ⁽³⁾

Why the ABCD System is a Game-Changer

The Belin ABCD system brings several benefits:⁽⁴⁾

• Early Detection: By including both anterior and posterior corneal measurements, it identifies changes that older systems might miss, especially in early stages.
• Personalized Treatment: With a clearer picture of disease progression, clinicians can offer tailored treatments, improving outcomes for interventions like CXL or refractive surgery.
• Objective Monitoring: The Progression Display uses data from normal and keratoconic populations to provide confidence intervals, making it easier to pinpoint when a patient’s condition is worsening.

 

Conclusion

The Belin ABCD staging system marks a significant advancement in the diagnosis and management of keratoconus, offering a more comprehensive, precise, and early detection method compared to older systems. ⁽⁵⁾ By incorporating posterior corneal measurements and focusing on the thinnest point of the cornea, it allows for timely interventions that can prevent vision loss. As imaging technology continues to evolve, the ABCD system is set to become an indispensable tool in the fight against corneal ectatic diseases.

 

References

1. Nottingham, J. (1854). Practical observations on conical cornea: And on the short sight, and other defects of vision connected with it. London: J. Churchill. Retrieved from Google Scholar.
2. Belin, M. W., & Duncan, J. K. (2016). Keratoconus: The ABCD grading system. Klinische Monatsblätter für Augenheilkunde, 233(7), 701–707.
3. Grisevic, S., Gilevska, F., Biscevic, A., Ahmedbegovic-Pjano, M., Bohac, M., & Pidro, A. (2020). Keratoconus progression classification one year after performed crosslinking method based on ABCD keratoconus grading system. Acta Informatica Medica, 28(1), 18–23.
4. Kamiya, K., Ishii, R., Shimizu, K., & Igarashi, A. (2014). Evaluation of corneal elevation, pachymetry, and keratometry in keratoconic eyes with respect to the stage of Amsler-Krumeich classification. British Journal of Ophthalmology, 98(4), 459–463.
5. Choi, J. A., & Kim, M. S. (2012). Progression of keratoconus by longitudinal assessment with corneal topography. Investigative Ophthalmology & Visual Science, 53(2), 927–935.