A modified risk assessment scoring system for post laser in situ keratomileusis ectasia in topographically normal patients

Variability of CLMI-X parameters, zonal Kmax, and single-point Kmax in keratoconus progression

OBJECTIVES

To compare the repeatability of Cone Location and Magnitude Index expanded (CLMI.X) parameters of 2 mm diameter zone of greatest corneal curvature (Cspot-Axi) and 1 mm diameter zone of thinnest pachmymetry (Spot-Pach) with the maximum single point keratometry (Kmax), 3 mm Zonal Kmax (Z-Kmax3), and thinnest single point pachymetry (TP) in keratoconus (KC).

METHODS

In this Comparative repeatability study, data from 36 eyes of 36 normal individuals and 72 eyes of 72 KC patients (28 eyes with ≤50.0D and 44 eyes with >50.0D Z-Kmax3) were analyzed. For each enrolled eye, imaging was done 6 times (3 consecutive acquisitions with a half hour break). For each parameter, the within-subject standard deviation (Sw) was calculated from the data of the six exams.

RESULTS

In the normal group, Cspot-Axi-Sw was different from Kmax-Sw (p = 0.0004). Also, Z-Kmax3-Sw was different from Kmax-Sw (p = 0.0297). The difference between Cspot-Axi-Sw and Z-Kmax3-Sw was statistically significant (p = 0.0482). In the KC group, Sw were significantly different between Cspot-Axi and Kmax (p < 0.0001), and between Z-Kmax3 and Kmax (p < 0.0001). In the Z-Kmax3 ≤ 50.0D subgroup, Sw were different between Cspot-Axi and Kmax (p = 0.0002). In the Z-Kmax3 > 50.0D subgroup, Sw were different between Cspot-Axi and Kmax (p < 0.0001), and between Z-Kmax3 and Kmax (p < 0.0001). Sw differences between Spot-Pach and TP were not significant in any of the study groups (all P > 0.05).

CONCLUSIONS

In the diagnosis and follow up of KC, the zonal averages of Cspot-Axi and Z-Kmax3 are more reliable than the single point Kmax. The repeatability of Spot-Pach is similar to TP and both variables should suit the purpose equally.

Ectasia After Corneal Refractive Surgery: A Systematic Review

INTRODUCTION

The incidence of ectasia following refractive surgery is unclear. This review sought to determine the worldwide rates of ectasia after photorefractive keratectomy (PRK), laser-assisted in situ keratomileusis (LASIK), and small incision lenticule extraction (SMILE) based on reports in the literature.

METHODS

A systematic review was conducted according to modified Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Publications were identified by a search of eight electronic databases for relevant terms between 1984 and 2021. Patient characteristics and preoperative values including manifest refractive spherical refractive equivalent (MRSE), central corneal thickness (CCT), anterior keratometry, postoperative residual stromal bed (RSB), and percent tissue altered (PTA) were summarized. In addition, annual rates of each refractive surgery were determined, and incidence of post-refractive ectasia for each type was calculated using the number of ectatic eyes identified in the literature.

RESULTS

In total, 57 eyes (70 eyes including those with preoperative risk factors for ectasia) were identified to have post-PRK ectasia, while 1453 eyes (1681 eyes including risk factors) had post-LASIK ectasia, and 11 eyes (19 eyes including risk factors) had post-SMILE ectasia. Cases of refractive surgery performed annually were estimated as 283,920 for PRK, 1,608,880 for LASIK, and 96,750 for SMILE. Reported post-refractive ectasia in eyes without preoperative identifiable risk factors occurred with the following incidences: 20 per 100,000 eyes in PRK, 90 per 100,000 eyes in LASIK, and 11 per 100,000 eyes in SMILE. The rate of ectasia in LASIK was found to be 4.5 times higher than that of PRK.

CONCLUSION

Post-refractive ectasia occurs at lower rates in eyes undergoing PRK than LASIK. Although SMILE appears to have the lowest rate of ectasia, the number of cases already reported since its recent approval suggests that post-SMILE ectasia may become a concern. Considering that keratoconus is a spectrum of disease, pre-existing keratoconus may play a larger role in postoperative ectasia than previously accounted for in the literature.

Keratometric indices for detecting the type of keratoconus: a combined discriminant analysis

BACKGROUND

This study sought to determine the diagnostic ability of anterior, posterior and total keratometric indices in discriminating keratoconus (KCN) stages 1 to 4 from normal corneas.

METHODS

Anterior and posterior simulated keratometry (sim-Ksteep and sim-Kflat), maximum keratometry (Kmax), Kmax y co-ordinate, anterior and posterior radii of curvature centred on the thinnest point (ARC-3 mm and PRC-3 mm), and anterior, posterior, and total asphericity (Q-value) measured by Pentacam were extracted from electronic medical records of 200 KCN and 200 normal cases. Stepwise leave-one-out cross-validation and areas under receiver operating characteristic curves (AUROC) were used to detect the best set of indices for differentiating normal from low-grade (stages 1-2, Kmax ≤ 53.00 D) and high-grade (stages 3-4, Kmax > 53.00 D) KCN.

RESULTS

Mean age in KCN and normal groups was 33.10 ± 7.48 and 32.24 ± 9.00 years (p = 0.332), respectively. In the low-grade KCN group, PRC-3 mm (AUROC = 0.986), Kmax (AUROC = 0.979), and Kmax y co-ordinate (AUROC = 0.824) performed best, and the difference in AUROC between PRC-3 mm and Kmax was not significant (p = 0.153). In the high-grade KCN group, Kmax (AUROC = 1.000), PRC-3 mm (AUROC = 0.998), posterior Ksteep (AUROC = 0.970), posterior Q-value (AUROC = 0.940), and posterior Kflat (AUROC = 0.894) performed best, and there was no significant difference in AUROC values between Kmax and PRC-3 mm (p = 0.307) or between posterior Ksteep and posterior Q-value (p = 0.113).

CONCLUSIONS

Among the studied keratometric indices, PRC-3 mm and Kmax appear to have the best ability for detecting mild to severe KCN. The next best factors which showed good discrimination ability were Kmax y co-ordinate in low-grade cases and posterior Ksteep and Q-value in high-grade cases.