Is It Really Progressing? Why Repeat, Averaged Corneal Maps Matter

If you have keratoconus, you've probably sat in front of a corneal imager more than once in a single appointment, and then been asked to come back in a few months to do it all again. It can feel repetitive. But there is a good reason for it, and a 2026 study puts some numbers on that reason.

The big question in keratoconus care is often simple: is it changing? Cross-linking, a treatment performed by an ophthalmologist, is generally considered when keratoconus is shown to be progressing, because it aims to slow further change. So the measurements used to judge progression need to be trustworthy. Every measuring device has some natural wobble, and the key is knowing how big that wobble is so a chance variation isn't mistaken for real change.

What the researchers did

A team based mainly at the Eye & ENT Hospital of Fudan University in Shanghai, China, with co-authors in Kunming and in Rome, studied the MS-39, a corneal imager that combines optical coherence tomography with a Placido-ring map. They looked at eyes across the range of keratoconus severity: 27 with forme fruste (very early) keratoconus, 29 mild, 34 moderate and 41 severe, which is 131 eyes by our count.

Two trained operators each took three consecutive scans of every eye. The researchers then worked out how much the reading at the thinnest point of the cornea varied between repeats and between operators, comparing single scans with the average of three. This was a precision study done in one sitting. There was no treatment, no follow-up over time and no comparison with normal corneas.

What they found

Overall, the device was consistent. All intraclass correlation coefficients (ICCs) for corneal thickness were above 0.90. An ICC is a consistency score from 0 to 1, where 1 means repeated measurements agree perfectly, so above 0.90 is generally regarded as high.

From the size of the wobble, the authors suggested how much the thinnest corneal thickness would need to change before it is worth taking seriously:

• Using the average of three scans: 4.43 µm for forme fruste, 6.58 µm for mild, 8.34 µm for moderate and 16.71 µm for severe keratoconus.
• Using a single scan: 6.68 µm, 10.89 µm, 10.20 µm and 23.31 µm respectively.

(A micrometre, µm, is one-thousandth of a millimetre. The published single-scan figure for moderate keratoconus is slightly lower than for mild; we've reported it as published.)

Two patterns stand out. First, the more advanced the keratoconus, the bigger the natural wobble: in severe keratoconus a single-scan change of up to 23.31 µm could still be measurement noise. Irregular corneas are simply harder to measure precisely. Second, averaging three scans narrowed the range at every stage. In the authors' words, exceeding these values "should prompt careful re-evaluation of measurement error or progression of the disease". They concluded that averaging three measurements may improve reliability and that thresholds should be matched to the stage of keratoconus.

The abstract does not report confidence intervals or P-values for these thresholds, so we haven't quoted any.

What this looks like in practice

Imagine someone with mild keratoconus whose thinnest reading is 8 µm lower than last time. On single scans, that is within the 10.89 µm wobble, so it could easily be noise. On averaged scans, it is beyond the 6.58 µm threshold, which is a signal to look again carefully: repeat the scans, check the image quality and compare with the other measurements. Even then, it is a prompt for re-evaluation, not proof of progression.

That is why a careful optometrist takes several scans, keeps the conditions consistent, and compares maps over time before suggesting a referral. Thickness is only one piece; corneal shape and how you are seeing also matter, and decisions rest on the overall trend rather than a single number.

Where this fits in your care

Cross-linking is performed by an ophthalmologist, but the front door is a keratoconus-focused optometrist. At Rose Optometry in Hamilton, our optometrists, including Jagrut Lallu and Jacqueline Rowe, diagnose keratoconus, document progression with sequential corneal maps, refer to an ophthalmologist for cross-linking at the right time, and fit scleral lenses. Our guide to cross-linking in New Zealand covers the treatment itself, and our article on early detection in teenagers explains why monitoring matters most in younger people. You can also read about our keratoconus monitoring in Hamilton.

What the studies can't tell us

• Only the abstract was available to us. We could not read the full paper, so details such as funding, conflicts of interest, exclusions and how the thresholds were calculated could not be checked.
• Noise thresholds are not progression criteria. The study measured how much readings vary by chance. It did not follow anyone over time or test whether these thresholds correctly identify real progression.
• Device-specific. The figures apply to the MS-39 only and shouldn't be applied to other corneal imagers.
• Unknowns about the patients. The abstract doesn't report how many people (rather than eyes) took part, their ages, how severity was graded, or whether operators were masked.
• Industry ties unknown. Funding and any links with the device maker are not stated in the abstract.

If you've been told your keratoconus "might be changing" and want a careful, repeatable baseline, you can book a keratoconus assessment with our optometrists in Hamilton.

References

• Ning R, Wang Y, Yang X, et al. Pachymetric Thresholds for Keratoconus Progression Based on MS-39 Measurement Precision. Ophthalmic Physiol Opt. 2026. PMID: 42667347. https://doi.org/10.1007/s44402-026-00174-5