The impact of exposure on the magnitude of astigmatism formed within the precorneal tear film over the central optical zone of the cornea in ocular surface disease

To estimate the astigmatic power and axis of the tear film over the central optical zone of the cornea by vector analysis of topographic data, in ocular surface disease (OSD) and controls, during blink suppression. Video-keratoscopic images were captured on opening the eyes after a single blink 5, 10 & 15 s later during blink suppression in OSD patients (mixed aetiology, group 1 age 20 ­ 50 years, n = 12, group 2 > 50 years, n = 38) and controls (group 3, n = 19). The SimK and axis values were used to calculate the astigmatism (power and axis) that formed in the precorneal tear film during each period. Data were aggregated into 3 periods; T0-T5 (between 0 & 5 s after the blink), T5-T10 (5 & 10 s later, T10-T15 (between 10 & 15 s later). Mean (± SD, 95%CI) astigmatic power (DC) formed in the tear film over each period was respectively : Group 1, −0.81 DC (0.99, −1.44 to −0.17), −2.65 DC(1.36, −3.52 to −1.79), −1.37 DC (2.15, −2.73 to −0.01). Group 2, −0.33 DC (0.38, −0.45 to −0.20), −0.57 DC (0.97, −0.91 to −0.24) −0.96 DC (2.10, −1.68 to −0.24), Group 3, −0.57 DC (0.55 −0.76 to −0.38), −0.56 DC (0.57, −0.76 to -0.37), −0.31 DC (0.44, −0.46 to −0.16). Changes were significant in groups 1 (p = 0.013) & 3 (p = 0.033) but not in 2 (p = 0.078). Intergroup differences were significant at all periods (p < 0.05). Significant correlations were detected following vector analysis, e.g Group2 between the astigmatism formed during T5-T10 (y) and ocular surface astigmatism at 5 s was y = 0.281x - 0.834 (r = 0.328, n = 38, p < 0.05). In all three groups apparent changes in the axes of astigmatism were not significant (p > 0.05). Changes in the precorneal tear film after blinking are predominately astigmatic indicating that changes in the central region of the tear film following the natural blink are quasi-orthogonal.