Changes in the orientation of the axis of astigmatism associated with age.

A new regression equation for the change in the axis of astigmatism with age, as deduced from cross-sectional studies, is presented. Possible artefacts arising from the methods used to obtain mean prescriptions are discussed; it is shown that these may result in over-estimation of the occurrence of oblique cylinders in middle-age and that a previously demonstrated gradual change of orientation of the axis of astigmatism may have no basis in reality. The need for further longitudinal studies is emphasized.