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First Author: B.Lopes BRAZIL
Co Author(s): I. Ramos A. Luz B. Valbon M. Salomão R. Ambrósio Jr.
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To report topometric, tomographic and biomechanical indices on cases with clinical keratoconus, but central steepest simulated keratometric value (K2) lower than 45.7D and maximum front curvature (KMax) lower than 47 D and to compare such parameters with IOP and age matched normal corneas and keratoconus (KMax > 47D) cases.
Instiuto de Olhos Renato Ambrósio, Rio de Janeiro, Brazil
In a retrospective study, 13 eyes from 13 patients were identified among 177 patients with clinical bilateral keratoconus (KC) as having KMax lower than 47D - "Low K Keratoconus (LKKC)". One eye randomly selected from the remaining 164 patientes with bilateral standard keratoconus (KMax>47D), and from 170 patients with normal corneas that were age- and IOP-matched were enrolled for comparative analysis. All patients underwent a full ophthalmic examination including corneal and anterior segment tomography with Pentacam HR (Oculus) and biomechanical assessment with Ocular Response Analyzer (ORA, Reichert). Pentacam keratometric values (central simulated Ks and KMAx), topometric indices (derived from the front surface: ISV, IVA, IS-Value, KI, CKI, IHA, and IHD), and tomographic indices (posterior elevation at the thinnest point [PETP], pachymetry at the thinnest point (TP), average and maximal pachymetric progression [PPI-Ave, PPI-Max], average and maximal Ambrósio"s Relational Thickness [ART-Ave, ART-Max], and Belin-Ambrósio Deviation [BAD-D]), pressure-derived ORA biomechanical indices (corneal hysteresis [CH] and corneal resistance factor [CRF]) and 38 waveform-derived metrics were computed. Kruskal-Wallis non-parametric test was used to assess the difference between groups with post hoc Dunns test to assess pairwise differences between groups. The ability of distinguishing LKKC and normals was tested using receiver operating characteristic (ROC) curves.
We found that 13 eyes of 13 patient in a population of 177 patients with clinical diagnosis of bilateral keratoconus had relatively flat corneas (K2<45.7 and Kmax<47) it corresponds to a prevalence of 7.3%. In the LKKC group there were 6 males and 7 females, mean age 33ḟ9 years. Corneal indices for this subgroup were: keratometric K1=41.74ḟ1.82, K2=43.56ḟ1.32 and KMax=45.23ḟ1.13; topometric ISV= 35.77ḟ15.55, IS-Value=2.29ḟ1.78 and KI=1.08ḟ0.06; tomographic TP=480.54ḟ39.93, PETP=21.15ḟ15.24, PPI Max=1.76ḟ0.37, ART Max=282.92ḟ53.27 and BAD-D=3.25ḟ1.16; and biomechanical CH=8.41ḟ1.4, CRF=7.64ḟ1.97, KC Score=0.34ḟ0.31, p1area=2321ḟ532, p2area=1764ḟ495. All keratometric, topometic, tomographic and biomechanical indices had statistically significant differences among the 3 groups (p<0.0001). Differences (p<0.05) were found in all pairwise comparisons for topometric indices (ISV, IVA, IS-Value, KI and IHD), with the KC group presenting higher values, LKKC group intermediate values and normal group lower values. Tomographic indices (TP, PETP, PPI, ART and BAD-D) and biomechanical indices (CH, CRF, KC Score, p1area) had significant differences (p<0.05) between KC and normal groups, LKKC and normal group but not between the KC and LKKC groups. The parameters that exhibited grater areas under the ROC curves (AUC) were BAD-D (0.997), ART-Max (0.995), p1area (0.912), PETP (0.910), pachy min (0.885), KI (0.870), CH (0.860) and CRF (0.840).
Keratoconus may occur despite of relatively normal keratometric values. Topometric, tomographic and biomechanical indices can be useful in separating kerotoconic corneas with low front curvature from the normal ones.
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