The Peter Abel Prize Lecture: “Higher-Order Aberrations in Keratoconus”

The reduction in visual performance typically found in keratoconic patients is believed to be associated with large magnitudes of uncorrected irregular astigmatism and optical distortions, also known as higher-order aberrations (HOAs). Previous studies have suggested that correcting HOAs in patients with keratoconus may result in an improvement in visual performance. This study explores the correction of HOAs using standard and customised aberration-controlling soft contact lenses in 22 patients with varying degrees of keratoconus. The findings of this work may be useful from a clinical perspective, as some keratoconic patients cannot tolerate rigid gas-permeable (RGP) contact lenses and have few alternatives (excluding surgical intervention) for managing vision correction.

This study first describes a series of preliminary studies conducted to improve our current understanding of the HOAs manifested in keratoconus. The results of these investigations suggest that changes in aberrations, due to accommodation (a change in the eye’s focus) or variations in the tear film immediately after a blink, were unlikely to hinder the correction of HOAs for keratoconic patients. Equally, for patients with keratoconus, it was ascertained that ‘manual’ eye examination prescription data provided significantly better vision compared to ‘automated’, objective prescriptions derived using a clinically-available aberrometer (a specialised instrument used to measure the 'optical image quality' of the eye).
Our findings also showed that automated prescription data and HOA measurements, made using the aberrometer, displayed a larger degree of variability in keratoconic patients compared to those previously reported in visually-normal subjects. Furthermore, significant increases in 3rd-order coma aberrations were found after temporarily suspending RGP contact lens wear (for 1 week) for 16 keratoconic patients.

The results of two further clinical studies suggested that standard soft contact lenses can, to some extent, mask HOAs for keratoconic patients; however, the visual performance achieved was found to be poorer compared to RGP lenses; this is likely to be due to the poor correction of astigmatism with soft lenses. Equally, the results showed that RGP lenses provided superior visual performances compared to customised aberration-controlling lenses, in spite of the customised lenses providing comparable reductions in uncorrected HOAs.

The inducement of superfluous HOAs through customised soft contact lens movements and rotations on-eye was modelled using MatLAB (version 7.6.0.324; The Mathworks, Natick, MA, US). These results confirmed that minimising the movement of customised aberration-controlling contact lenses, to less than 5 degrees of rotation and to less than 0.50 mm of movement, will help to achieve an optimal correction of HOAs. However, this level of on-eye lens stability is not possible using current contact lens technologies or materials.

In conclusion, the correction of HOAs for patients with keratoconus is possible using customised soft contact lenses; however, several factors will govern their success, including the repeatability and accuracy of the HOA measurements and the stability of the customised lenses on-eye.