Predicting Manifest Astigmatism in Keratoconus Patients.

Keratoconus is one of the leading causes of visual impairment worldwide. Its main therapeutic options include corneal cross-linking, intracorneal ring segments, penetrating keratoplasty, and lamellar keratoplasty.

Keratoconus is a progressive condition with onset in childhood, resulting in increasing irregularity of corneal shape. The cornea's role is to refract incoming light to focus it onto the retina to produce a sharp image. Irregularity of the cornea degrades this image. Keratoconus develops throughout the second and third decades of life and usually stabilises in the fourth decade. It leads to progressive vision loss. When detected early, progression can be halted or stabilised with collagen cross-linking treatment. Once the condition progresses to impair the quality of vision, rigid contact lenses or surgical procedures are often required for correction. Spectacles and soft contact lenses are frequently inadequate due to the cornea's irregular shape in keratoconus.

Surgical approaches to keratoconus that do not involve corneal transplantation include implantable collamer lenses (ICL) or toric intraocular lenses (tIOL). There is little published evidence on the use of these approaches, and no consensus exists regarding the extent of astigmatism to correct or the proper axis of astigmatism to select.

In addition to lower-order aberrations (such as those correctable by spectacles, including short-sightedness, long-sightedness, and astigmatism), higher-order aberrations are present, which cannot be corrected with glasses. Due to irregular corneal astigmatism and higher-order aberrations, accurate refraction is often challenging in patients with keratoconus. Eyes with keratoconus exhibit approximately five to six times more higher-order aberrations than normal eyes, particularly vertical coma. The manifest refractive cylinder (the astigmatism component in a glasses prescription) includes both regular and irregular astigmatism. Additionally, the axis of astigmatism in the prescription often differs significantly from the topometric axis derived from three-dimensional corneal imaging. In managing astigmatism for suitable patients, such as those receiving implantable collamer lenses (ICL) or toric intraocular lenses (tIOL), it is crucial to better understand the shape factors contributing to manifest refraction. Improved understanding can help neutralise post-operative refraction more effectively and achieve greater predictability. For non-surgical patients, determining the optimal glasses prescription to achieve the best vision correction often involves a time-consuming trial-and-error process. Improved predictability could reduce the time required to identify the optimal prescription, thereby enhancing productivity and patient care.

The aim of this study is to use data already collected from more than 800 patients to compare topographic elements (three-dimensional modelling of corneal shape), such as anterior and posterior curvatures and higher-order aberrations, to subjective manifest astigmatism (the amount of astigmatism in the glasses prescription) in keratoconic eyes. The impact of keratoconus severity on disparities between these methods will also be evaluated.

Inclusion criteria

• All patients with a diagnosis of keratoconus who have had topography performed at the West of England Eye Unit with contemporaneous manifest refraction.

Exclusion criteria

• Poor quality topography scans.
• History of ocular surgery.
• Age <16 years.