Early paradoxical increase in astigmatism after corneal allogeneic intrastromal ring segment implantation: Incidence, predictors, and clinical implications
This retrospective study of 109 keratoconus eyes demonstrates that while corneal allogeneic intrastromal ring segment (CAIRS) implantation significantly improves long-term visual and corneal outcomes, a paradoxical early increase in astigmatism occurs in approximately one-third of patients, particularly those with lower baseline astigmatism and prior collagen cross-linking, without necessarily indicating poor long-term prognosis.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
The human eye relies on a clear, dome-shaped window at the front called the cornea to focus light onto the retina. When this window is perfectly smooth and round, vision is sharp. However, in a condition known as keratoconus, the cornea weakens and begins to bulge outward into a cone shape. This distortion creates a jagged, irregular surface that scatters light, leading to blurred vision and significant astigmatism, where objects appear stretched or doubled. While glasses or contact lenses can sometimes correct this, many patients eventually find them ineffective or uncomfortable. To stop the disease from getting worse, doctors often use a treatment called corneal cross-linking, which strengthens the cornea's fibers, but this procedure does not fix the shape or the vision problems it causes. For those who need more help, surgeons have traditionally used synthetic plastic rings inserted inside the cornea to flatten it, or they have replaced the entire cornea with a donor one. A newer, more delicate approach involves using a ring made from actual human donor tissue, which the body may accept more naturally than plastic.
A team of researchers set out to understand how this newer method, using donor tissue rings, affects vision in the weeks immediately following surgery. They wanted to know if the changes in vision were predictable and whether a temporary worsening of astigmatism meant the surgery had failed. The study followed 109 eyes from patients who received these donor tissue rings at a single hospital over a four-year period. The surgeons carefully measured the patients' vision and the shape of their corneas before the operation and tracked them closely for up to three years afterward. They looked specifically at the amount of astigmatism, which is the measure of how much the eye is out of round, to see how it changed over time.
The results showed that, overall, the surgery was highly successful. The average amount of astigmatism dropped significantly, and patients saw much better with or without glasses. However, the researchers noticed a surprising pattern in the first month after surgery. While the group as a whole improved, about one-third of the individual eyes actually showed an increase in their astigmatism numbers during that first month. This seemed counterintuitive, as one would expect the numbers to go down immediately if the surgery was working. The team investigated what factors might predict this temporary rise. They found that patients who had undergone the strengthening cross-linking treatment before the surgery, and those who started with a lower amount of astigmatism, were more likely to experience this early increase. Conversely, patients with very high levels of astigmatism before the operation were less likely to see a temporary rise.
The authors explain that this early increase does not mean the surgery failed or that the eye is getting worse. Instead, they suggest it represents a complex reshaping of the eye's surface. Before the surgery, the cornea is so irregular that standard eye exams cannot measure all the distortions accurately. The surgery helps smooth out these jagged irregularities, turning them into a more regular, predictable curve. As the eye becomes more regular, the standard measurement tools can finally detect the astigmatism that was previously hidden in the chaos. This creates a situation where the measured number goes up, even though the actual quality of the eye's optics is improving and the patient's vision is getting clearer. The study confirms that this early fluctuation is a normal part of the healing process, particularly for those with prior strengthening treatments, and that it does not signal a poor long-term outcome. The key takeaway is that doctors and patients should not be alarmed if the astigmatism numbers rise slightly in the first month, as the vision itself often continues to improve as the cornea settles into its new, smoother shape.
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