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Effect on keratoconus cone location of the topographic, pachymetric and zonular corneal wavefront parameters and their predictive cutoffs

Poster Details

First Author: G.Prakash UNITED ARAB EMIRATES

Co Author(s):    D. Srivastava   S. Choudhuri   S. Thirumalai   R. Bacero           

Abstract Details


The location of keratoconus apex can determine the distribution of ectatic pathology over the cornea. We wished to evaluate the effect of keratoconus apex location on the topography (central simk@3mm, maximum keratometry), pachymetry (corneal thickness: Central (CCT) and minimum (MCT)) and zonular higher-order-aberrations (HOA). Some ophthalmolgists use simpler cutoffs in the absence of a topography/tomography device to suspect keratoconus. We also wanted evaluated if the existing these cutoffs (Steep central K >47.2D or Central corneal thickness <491.6µ) would be useful in keratoconics with central cones and with non-central cones in differentiating them from normal eyes.


Department of Cornea and Refractive Surgery Services, NMC Eye Care, NMC Specialty Hospital, Abu Dhabi, United Arab Emirates.


In this prospective, comparative, cross-sectional study, 50 consecutive cases each of keratoconus (KC) with apex within 2mm (KC_Central), outside 2mm(KC_Non-central), normal cases (refractive surgery candidates fit for lasik) with apex within 2mm (normal_central) and outside 2mm (normal_Non_central) were evaluated ( total 200 eyes or 200 cases). All cases underwent detailed clinical evaluation and corneal topography (CSO, Sirius, Italy). Topographic [maximum keratometry (MaxK), simulated Keratometry@3mm( SimK steep, flat and astigmatism)] , pachymetric [CCT and MCT], corneal first surface higher-order-aberrations [Total HOA root-mean-square (HOARMS), and all polar terms for third and fourth order at 3,4,6 and 8 mm corneal diameters] were evaluated.


Inspite of having comparable MaxK , KC_central had higher SimK values, thinner CCT and MCT compared to KC_Non_central(p<0.001). The total HOARMS were worse for KC_central at 3,4mm and comparable for larger zones. Receiver-operating-curve analysis showed that the existing cutoff of (either SimK steep>47.2D or CCT <491.6µ) had a good sensitivity of 0.98 for KC_central, but poor sensitivity of 0.80 for KC_Non_central. Changing this cut-off to “either SimK steep K≥45.8D or CCT ≤503µ’ gave a combined sensitivity and specificity of 0.95, 0.87 to the KC_Non_central and 0.99, 0.87 respectively to KC_central.


Non-Central keratoconus (>2mm apex-center distance) have lesser effect on simulated keratometry, pachymetry and smaller-aperture HOARMS, and therefore may be missed in unsatisfactory numbers if screened with existing non-topographic/tomographic cutoffs. Along with a vigilant clinical suspicion, performing topography on cases with either SimK steep K≥45.8D or CCT ≤503µ can be useful.

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