This paper presents model asymmetries in the topography of the human cornea arising from linear gravitational distributions of the intraocular pressure. Equations for the corneal radius of curvature were used to provide an indication of the gravitational sag of the cornea when the head is in an upright position. In such case, corneal astigmatism is found along the optic axis of the eye. Ray paths inclined at a particular angle to the optic axis produce images (spot diagrams) which show a significant reduction in corneal astigmatism.
Kasprzak, H., & Pierscionek, BK. (2002). Modelling the gravitational sag of the cornea and the subsequent quality of the refracted image. Journal of Modern Optics, 49(13), 2153-2166. https://doi.org/10.1080/09500340210129248