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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

Purpose:

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.

Setting:

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

Methods:

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.

Results:

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.

Conclusions:

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.

Financial Disclosure:

NONE

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