I congratulate Lu et al. on their interesting article that discusses a staging system based on spectral domain optical coherence tomography data.1 The article is a good opportunity for a more fundamental discussion of classification systems in ophthalmology. When using a certain classification for a certain disease, one should distinguish at least between two types of classification systems: grading and staging. Unfortunately, these terms and definitions are not used in a very consistent manner throughout the ophthalmic literature.

A classification such as the present one as well as the ABCD system of Belin should be referred to as a “grading system.”2 It describes morphological characteristics of the disease without a direct clinical impact on prognosis or treatment. It is however very helpful when it comes to the question whether the disease progressed between two successive follow-ups.

On the other hand, a classification referred to as “staging system” should classify the disease regarding clinical information, such as prognosis and/or treatment recommendations. In other words, a stage 1 should result in a different prognosis for progression and/or in a different treatment recommendation compared with stage 2. Staging does not make sense at all if it is not linked to a specific treatment recommendation.

Because there exists a large variety of treatment options for keratoconus, including further follow-ups, different types of optical aids, different types of crosslinking, different types of implants, and different types of grafting, respectively, it is difficult to link a certain treatment regimen to a certain stage. Most treatment centers have different experiences as well as different understanding, knowledge, and skills. Common schemata to describe the outcome of refractive procedures effectively are not useful to evaluate and compare the outcome of different keratoconus treatment options.3 The reason is that common refractive procedures intend to remove the refractive error only to transfer the corrected visual acuity into the uncorrected one while leaving the best-corrected visual acuity more or less unchanged. This is the big difference compared with keratoconus treatment where the aim is to significantly improve the best-corrected visual acuity. To establish an objective system to compare the outcomes of keratoconus treatments of the same treatment at different centers or different treatment options in general, we have established the “performance plot” system.4 I used the performance plot system also very successfully for the measurement of the training progress of corneal surgeons.

1. Lu NJ, Hafezi F, Koppen C, Alió Del Barrio JL, Aslanides IM, Awwad ST, Ní Dhubhghaill S, Pineda R II, Torres-Netto EA, Wang L, Chen SH, Cui LL, Rozema JJ. New keratoconus staging system based on OCT. J Cataract Rafract Surg 2023;49:1098–1105 2. Belin MW, Duncan JK. Keratoconus: the ABCD grading system. Klin Monbl Augenheilkd 2016;233:701–707 3. He G, Bala C. Ray-tracing-guided myopic LASIK: real-world clinical outcomes. J Cataract Refract Surg 2023;49:1140–1146 4. Daxer A, Ettl A, Hörantner R. Long-term results of MyoRing treatment of keratoconus. J Optom 2017;10:123–129