BLOG: Beyond the Baylor nomogram
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How much astigmatism does your patient have?
This question becomes extremely important before cataract surgery, when patients with more than about 0.5 D of astigmatism have the option to choose a toric lens. Doug Koch’s Baylor nomogram suggests that posterior corneal astigmatism negates about 0.5 D of with-the-rule astigmatism (steep near 90°) and accentuates by about 0.5 D against-the-rule astigmatism (steep near 180°). This adjustment applies to about 88% of patients, according to Doug’s well-done study, and it has improved astigmatism correction for thousands of patients.
The Baylor nomogram’s findings don’t, however, apply to every patient. We studied 450 eyes that underwent cataract surgery in which intraoperative aberrometry was used to assist in planning astigmatism correction. We sought to determine how intraoperative aberrometry compared with Baylor nomogram-adjusted preoperative measurements. In our study, published in Clinical Ophthalmology, intraoperative aberrometry measured astigmatism greater than 0.5 D in 75% of patients vs. 70% with adjusted preoperative keratometry alone — a statistically significant difference and an indication that more patients qualify for surgical astigmatism correction than adjusted preoperative measurements alone would indicate. The finding of higher intraoperative astigmatism was particularly noted among patients whose Baylor-adjusted preoperative measurements showed with-the rule and oblique astigmatism and among those whose preoperative magnitude of astigmatism was between 0.5 D and 1 D. In other words, the Baylor nomogram can misguide us in a meaningful percentage of patients, and if your patient has about 0.5 D of with-the-rule or oblique astigmatism on preoperative measurements, you might still want to offer surgical astigmatism correction, knowing that intraoperative measurements are likely to show a higher degree of cylinder.
Finally, our study asked whether we get a better final refractive result by following the guidance of aberrometry even when it disagrees with preoperative measurements. Indeed, we do, according to our findings, but only if the amount of disagreement in measurements is less than 1 D. In cases in which aberrometry disagreed with adjusted preoperative measurements by more than about 1 D, it was no more accurate than preoperative measurements alone. I suspect that unknown confounding variables are playing a role when a measurement disagreement of greater than 1 D occurs.
Astigmatism is a messy business, yet every successful refractive surgeon will tell you that managing it effectively is the key to satisfied patients. Taking the time to understand our measurements — and adjust our treatment plans accordingly — is an investment in outcomes that’s worthy of our best efforts.
- Reference:
- Hovanesian JA. Clin Ophthalmol. 2021;doi:10.2147/OPTH.S314618.
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