Purpose: To report clinical keratoconus in only one eye of two identical female twins, along with subclinical disease in the fellow eyes, elaborating on the two-hit hypothesis of ectasia development, which relates to the combination of genetics and the impact of environment.Methods: Case report and literature review.Results: Two identical 48-year-old female twins were presented for clinical evaluation. Mild keratoconus was detected in the right eye of twin 1, characterized by classic slit-lamp findings (exacerbated corneal nerves and incomplete Fleisher's ring) and front surface curvature abnormalities, including asymmetry of the inferior-superior curvature at 6 mm (I-S value) of 2.78 D and a grade 1 Topographic Keratoconus Classification pattern. Topographic Keratoconus Classification was possible in the fellow eye of twin 1 (I-S value = 1.22 D) and negative in both eyes of twin 2 (I-S value = -0.46 OD and -0.13 OS). Ambrósio's Relational Thickness for the maximal progression meridian was 209 and 354 in twin 1 and 360 and 392 in twin 2 in the right and left eyes respectively. The final deviation value of the Belin-Ambrósio Enhanced Ectasia Display was 4.54 and 1.47 in twin 1 and 1.7 and 1.35 in twin 2. Conclusion:Corneal tomography data provide a better representation of corneal genotype in detecting mild, subclinical, or forme fruste keratoconus in the fellow eyes with normal topography of these twins. These cases present high risk or susceptibility for ectasia progression if environmental factors are associated (second hit).

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