Peripheral nasal-temporal corneal asymmetry in relation to corneal thickness: a Scheimpflug imaging study.

PURPOSE: To investigate the asymmetry of the peripheral cornea up to 5 mm nasally and temporally from the centre and to assess correlations with regional peripheral corneal thickness. METHODS: Central and peripheral corneal thickness was measured by Scheimpflug imaging (Pentacam) in 113 eyes of 113 healthy, pre-presbyopic Caucasian subjects. Absolute and relative corneal thickness were analysed in 1 mm steps up to 5 mm to the nasal and temporal sides with the corneal apex as the central reference point. Nasal-temporal asymmetry was calculated as the thickness ratio between corresponding off-centre thickness measurements. RESULTS: The mean (±SD) central corneal thickness was 552 ± 36 µm. CT increased by 22% at 4 mm temporally to 672 ± 44 µm, and 32% at 4 mm nasally to 731 ± 45 µm. The nasal-temporal asymmetry became greater with increasing distance from the corneal centre, with a mean difference of 59 ± 22 µm at 4 mm from the apex. The nasal-temporal thickness ratio, based on this difference, was significantly related to the relative temporal (r = -0.41, p < 0.001, simple linear regression) and nasal corneal thickness (r = 0.61, p < 0.001). CONCLUSIONS: A substantial and progressively increasing nasal-temporal asymmetry in corneal thickness has been confirmed by Scheimpflug imaging, which is related to the magnitude of corneal thickness at peripheral locations. Pachymetry output data and models, including volume calculations, that assume symmetry to the corneal thickness profile may not provide optimum metrics for planning and predicting the outcome of corneal refractive surgery procedures.