In this study, we established a prediction formula for HCL-BCVA based on clinical parameters measured by AS-OCT, with the formula showing a reliability sufficient for prediction of HCL-BCVA for keratoconus patients. We also identified clinical parameters that are linearly related to HCL-BCVA and determined threshold values for these parameters associated with maintenance of a favorable HCL-BCVA. Our results thus suggest a new approach to determination of treatment strategy for progressive keratoconus.
The correlation coefficient between predicted HCL-BCVA and actual HCL-BCVA was 0.641 for the validation group. Furthermore, the prediction error ratio for HCL-BCVA within logMAR values of ± 0.1 or ± 0.2 was 71.9% and 96.9%, respectively, which is sufficient for application to the clinical setting. With the use of our prediction formula, it is thus possible to predict HCL-BCVA without HCL wear. Similar studies have previously been performed to predict BCVA for keratoconus patients wearing glasses.23−26 For keratoconus patients, however, because of their corneal irregular astigmatism, HCL-BCVA is usually better than BCVA with glasses.3 Prediction of HCL-BCVA is thus more relevant to the clinical setting for individuals with keratoconus.
We also performed simple regression analysis to identify parameters that are linearly related to HCL-BCVA of keratoconus patients. Our analysis revealed that the relation between actual HCL-BCVA and anterior corneal refractive power, posterior corneal refractive power, or HOAs was linear. On the basis of the regression lines for these relations, we calculated cutoff (threshold) values for the three parameters corresponding to a logMAR value of 0.15 for HCL-BCVA, which is the value required for renewal of a driving license in Japan and is a good indicator for quality of life. The cutoff values for anterior corneal refractive power, posterior corneal refractive power, and HOAs were 57.18D, − 8.16D, and 1.71 µm, respectively.
CXL is currently applied to prevent the progression of keratoconus,21 with many studies having supported its clinical efficacy.21, 27–35 Several criteria for performance of CXL have been proposed,22 but they are all based on the degree of progression of keratoconus. The development of keratoconus tends to occur at a relatively young age, with affected individuals being able to maintain an active lifestyle if their vision remains good. Young patients are more tolerant of HCLs, and it is important that they be treated to prevent the progression of keratoconus while they are still able to achieve good vision while wearing such lenses. While we agree that confirmation of disease progression is a requirement for administration of CXL treatment, we have now shown that several clinical parameters manifest threshold values with regard to loss of HCL-BCVA. In particular, if keratoconus patients cross thresholds of 57.18D for anterior corneal refractive power, − 8.16D for posterior corneal refractive power, or 1.71 µm for HOAs, then they may no longer be able to maintain an adequate HCD-BCVA and may therefore become candidates for corneal surgery such as keratoplasty. Although clinical results for keratoplasty in individuals with keratoconus are good,36–39 such surgery is associated with several complications and should be avoided if possible.40, 41 We thus propose that the threshold values for clinical parameters identified in the present study on the basis of clinical data of many keratoconus patients should be considered in selecting treatment strategy for this condition. These thresholds should thus be taken into account in determining the surgical indication for CXL, so that the procedure can be performed before patients cross the line with regard to the potential for achieving a good HCL-BCVA.
Our study has at least a couple of limitations. First, AS-OCT was performed with the CASIA system, which is not able to provide separate values for anterior and posterior HOAs and for anterior and posterior SA, but instead provides only corresponding total values. In addition, the parameters considered for development of our prediction formula did not include coma aberration, a specific parameter of keratoconic corneas, as a result of limitations of the AS-OCT settings. Second, given the study setting, we evaluated HCL-BCVA only with conventional spherical HCLs, not with keratoconus-specific HCLs. The correction efficacy of keratoconus-specific HCLs usually differs from that of conventional spherical HCLs, with further studies with keratoconus-specific HCLs thus being warranted.
In conclusion, we have developed a formula to predict HCL-BCVA in keratoconus patients. We also identified three clinical parameters—anterior corneal refractive power, posterior corneal refractive power, and HOAs—that are linearly related to HCL-BCVA in such patients, and we determined cutoff values for these three parameters that are associated with the ability to achieve a favorable HCL-BCVA. Our approach to prediction of HCL-BCVA in keratoconus patients on the basis of AS-OCT measurements should prove helpful for management of this condition, and the cutoff values of the three identified clinical parameters should be taken into account when considering the indications for CXL and other treatments.