Understanding higher-order aberrations in keratoconus

Keratoconus is characterized by progressive thinning and increasing irregularity of the cornea, leading to substantial degradation in optical quality through the induction of both lower- and higher-order aberrations (HOAs). In addition to inducing myopic and astigmatic refractive error, asymmetric corneal steepening generates complex optical distortions of the ocular wavefront, resulting in persistent reductions in image quality with conventional optical corrections including spectacles and soft contact lenses (CLs). In keratoconus, HOAs that comprise the ocular wavefront are significantly elevated, with marked variability in composition both across the population and between fellow eyes. Largely this is dominated by vertical coma, reflecting the asymmetric deformation of the cornea, with smaller contributions from trefoil and spherical aberrations. Rigid gas permeable CLs, including scleral lenses, can mask anterior corneal irregularity and partially reduce these aberrations, to improve vision quality. However, image quality in eyes with keratoconus remains reduced relative to nondiseased eyes due to residual HOAs, arising predominantly from the irregular posterior corneal surface and internal optics. This narrative review aims to translate the complex optics relating to the ocular wavefront in keratoconus into clinically relevant insights, examine current evidence on the origins of HOAs, describe related visual consequences, and outline implications for optical correction. An introduction to Zernike polynomials is presented, aiming to define the mathematical representation and facilitate a deeper understanding of wavefront data. These concepts provide a foundation for optimizing clinical care and guiding future approaches to customized optical correction in keratoconus.