Keratoconus and Astigmatism: Why Your Vision Keeps Changing

Each time the cone steepens, the axis and strength of your astigmatism shift. What worked three months ago may already be off. In early keratoconus, these changes can resemble ordinary prescription updates, but over time the updates become more frequent and each new pair of glasses provides less improvement than the last.

With regular astigmatism, the right prescription typically restores vision to 20/20. With keratoconus, even the most accurately measured glasses prescription may only bring your vision to 20/30 or worse, and this threshold tends to drop as the disease progresses. When your eye doctor notes that your best corrected visual acuity is declining, it often signals that the irregular component of your astigmatism has outgrown what glasses can handle and that specialty contact lenses may be needed.

Keratoconus typically affects both eyes, but rarely at the same pace. One eye may need a prescription update every few months while the other stays relatively stable. This unequal progression is a hallmark of the disease and creates its own challenges, because the brain must try to merge two images of very different quality. When the mismatch is significant, you may notice visual fatigue, difficulty with depth perception, or a tendency to favor one eye. Correcting each eye independently with the appropriate lens type helps reduce this imbalance.

Standard soft toric lenses are designed for regular astigmatism along a single axis and are generally not sufficient once keratoconus has produced meaningful irregularity. Specialty soft lenses designed for keratoconus can manage mild to moderate irregularity better than standard torics, but they have limits. If your eye doctor has recommended rigid or scleral lenses, it is because the degree of irregular astigmatism in your case exceeds what any soft lens can fully address.

Corneal topography, a detailed map of your corneal curvature, is the most reliable way to distinguish keratoconus astigmatism from ordinary astigmatism. Keratoconus produces an asymmetric pattern with localized steepening, typically in the lower part of the cornea, that looks distinctly different from the symmetric pattern of regular astigmatism. If you have a rapidly changing prescription or vision that does not correct well with glasses, ask your eye doctor about getting a topography scan.

The primary goal of cross-linking is stabilization, not improvement. Some patients do experience modest flattening of the cone in the months following the procedure, which may slightly reduce astigmatism, but this outcome is not the norm and cannot be predicted in advance. If your astigmatism does improve, it is a welcome benefit. More importantly, cross-linking prevents the further worsening that would otherwise continue without treatment.

Laser refractive surgery is not appropriate for keratoconus and should not be pursued as a correction option. The procedure works by removing corneal tissue to reshape the eye's surface, but in keratoconus the cornea is already structurally compromised and too thin. Removing additional tissue would worsen the underlying weakness and could lead to a dangerous condition called post-surgical ectasia, where the cornea continues to bulge and thin even more rapidly. If you have keratoconus, always inform any provider you see about your diagnosis before any vision correction procedure is considered.

When keratoconus is more advanced in one eye, your brain is receiving two images of very different quality. Closing the more affected eye removes the distorted input and allows the clearer eye to work on its own. This is a common experience and a practical indicator that your two eyes have significant vision mismatch. Fitting both eyes with the appropriate specialty lenses can equalize image quality and reduce the strain that comes from the brain trying to merge those two different images.

Most patients with active keratoconus benefit from more frequent monitoring than the standard annual exam. Your eye doctor will recommend an individualized schedule based on how quickly your disease is progressing. During periods of active change, visits every three to six months allow for timely prescription updates and early identification of when intervention such as cross-linking may be warranted. Once the disease is stable, monitoring intervals can be extended.