Topographic Asymmetry Indices: Correlation between Inferior Minus Superior Value and Index of Height Decentration

PURPOSE: To investigate the correlation between decentration index (index of height decentration, IHD) automatically calculated by the Pentacam HR software and the manually calculated inferior minus superior (I‐S) value. SETTING: Al Watany Eye Hospital, Cairo, Egypt. METHODS: In a retrospective study, history taking, clinical examination, and rotating Scheimpflug camera scanning (by Oculyzer II equivalent to Pentacam HR) were done to 128 eyes: 82 normal, 24 forme fruste keratoconus FFKC (apparently normal cornea with evident keratoconus in the fellow eye), and 22 keratoconus (KC). All cases of corneal scars or previous corneal surgeries were excluded. The (I‐S) value was calculated manually from 10 points astride the horizontal meridian. The IHD is calculated automatically by the device software 1.17r119. RESULTS: The mean (±SD) of (I‐S) value in normal, FFKC, and KC eyes were 0.30 ± 0.93, 0.11 ± 2.03, and 4.62 ± 3.89, respectively, and those of IHD were 0.008 ± 0.004, 0.011 ± 0.005, 0.066 ± 0.067, respectively. The two indices were highly correlated (p < 0.0001) with a correlation coefficient (r(2) = 0.874). Deduced regression formulae linking the two indices were calculated. CONCLUSIONS: The two topographic decentration indices are highly correlated. Deduced formulae were proposed linking them.