Grade I
Grade II
– Eccentric corneal steepening
– Absence of scarring
– Myopia and/or astigmatism <5 D
– Myopia and/or astigmatism 5–8 D
– Mean central K readings ≤48.00 D
– Mean central K readings >48.00 to ≤53 D
– Minimum corneal thickness >400 μm
Grade III
Grade IV
– Absence of scarring
– Central corneal scarring
– Myopia and/or astigmatism 8–10 D
– Not reliable refraction
– Mean central K readings >53.00 to ≤55 D
– Mean central K readings >55 D
– Minimum corneal thickness 300 to 400 μm
– Minimum corneal thickness 200 μm
15.3 Optical-Based Keratoconus Grading
Alio-Shabayek classification [ 9], this grading system was created taking into account the analysis of the anterior corneal higher order aberrations in patients with keratoconus. By means of evaluation of the corneal wavefront, the authors categorize the severity of the disease assessing the asymmetric aberrations, specifically the coma and its radial orders, which are the more affected in keratoconic patients. Table 15.2 summarizes the main characteristics of this classification.
Table 15.2
Alio-Shabayek Classification
Grade I
Grade II
– Absence of scarring
– Absence of scarring
– RMS of coma-like aberration from 1.50 to 2.50 μm
– RMS of coma-like aberration from 2.50 to 3.50 μm
– Mean central K readings ≤48.00 D
– Mean central K readings >48.00 to ≤53 D
– Minimum corneal thickness >400 μm
Grade III
Grade IV
– Absence of scarring
– Central corneal scarring
– RMS of coma-like aberration from 3.50 to 4.50 μm
– RMS of coma-like aberration >4.50 μm
– Mean central K readings >53.00 to ≤55 D
– Mean central K readings >55 D
– Minimum corneal thickness 300 to 400 μm
– Minimum corneal thickness 200 μm
As we can see, there are several grading systems found in the literature aiming to classify the severity of keratoconus. Nevertheless, many of them are obsolete or are based just in isolated or morphological parameters without taking into considerations other variables closely associated with the optical quality, such as the visual acuity, which additional is related with the quality of life of the patients.
15.4 Visual Function-Based Grading Keratoconus
Recently, our research group developed a grading system based on the analysis of almost 800 cases of keratoconus in which was evaluated the visual, refractive, topographic, aberrometric, and biomechanical variables in order to classify the severity of the disease [10]. The principal features of this grading system are presented in Fig. 15.1.
Fig. 15.1
The principal features of the visual function based grading system
As shown in Fig. 15.1, this new classification system includes morphological parameters that are directly correlated with functional variables as the visual acuity of the patient. In addition, includes new parameters as the internal astigmatism and corneal asphericity which is also significantly affected in keratoconic patients. Moreover, it also analyses the biomechanical alterations on every case. Even when we do not observe a statistically significant difference on the biomechanical variables in pair of groups analyzed in the work where the classification was proposed [10], there is a clear difference between the mild and the severe cases in terms of biomechanical behavior (Figs. 15.2, 15.3, 15.4, and 15.5). As we can see from the figures, the biomechanical parameters that are found in the Ocular Response Analyzer (ORA), corneal hysteresis (CH), and corneal resistant factor (CRF) present higher levels when compared to the most severe cases. Additionally, in the severe cases the shape of the waveform is more flat and almost null when compared to the mild cases .
Fig. 15.2
Grade I keratoconus showing in C the biomechanical parameters corneal hysteresis (CH) and corneal resistant factor (CRF). We can also observe the high peaks on the waveform
Fig. 15.3
Grade II keratoconus showing in C the biomechanical parameters corneal hysteresis (CH) and corneal resistant factor (CRF)
Fig. 15.4
Grade III keratoconus showing in C the biomechanical parameters corneal hysteresis (CH) and corneal-resistant factor (CRF)
Fig. 15.5
Grade IV keratoconus showing in C the biomechanical parameters corneal hysteresis (CH) and corneal resistant factor (CRF). We can also observe the flat peaks on the waveform
15.5 Intracorneal Ring Segment Implantation Outcomes Based on Our New Keratoconus Classification
One of the main objectives of grading the severity of a disease is to be able to assess the efficacy of the different therapeutic alternatives that exists in order to treat such pathology. As commented in the previous section, one of the main limitations of the grading systems that are used to classify keratoconus is that they are based on morphological variables without taking into account the visual function of the patients. Therefore, in a recent work developed by the authors, it was assessed the outcomes of intracorneal ring segments (ICRS) implantation to treat keratoconic patients based on a new grading system.
If we revised the published literature, we will find that most of the series reported an improvement in the visual and refractive variables when treating keratoconus with ICRS [11–16]. Nevertheless and as previously commented, the majority of these works are based on grading systems whose pitfalls have been already mentioned. Moreover, most of the authors in those studies do not evaluate the results of the surgical technique independently of the severity of the disease but analyzing the outcomes of all the population under study as a whole.
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