An interesting question arose in our contact lens clinic last week: are myopic corneas steeper than those of ametropes or hyperopes? Anecdotally, it was felt that perhaps myopes had flatter corneas. Most optometrists should be aware that most refracts errors are axial: myopes have big eyes and it’s the reverse for those who are longsighted. However, myopes having flatter corneas would be counter-intuitive.
Ametropia can be due to a normal length eye hosting a cornea that’s the wrong shape (too steep or too flat). However, I wasn’t sure if this was a general trend – do myopes tend to have steep corneas?
So, to the literature. This paper describes a study in Taiwan of 500 subjects aged 40 and over. The authors measured a range of ocular parameters and whilst a strong (ish) correlation was found between refractive error and axial length (r-value -0.65), no such relationship was demonstrated between refractive error and corneal curvature (r = -0.02). Overall, then, randomly selected hyperopes will have similar corneal curvature to a group of randomly selected myopes.
A slightly different story arises from this very large study from Canada. This work, on over 3,000 eyes finds a general increasing of corneal power with more myopia. It’s not a very strong relationship (r = -0.25) but it’s the sort of finding you can get with large data sets.
So, our anecdotal feeling-in-our-water was incorrect. Overall, myopes do have steeper corneas, but it’s a rather weak relationship and they are much more likely to have longer eyes. Predicting K readings from refractive error for an individual is unlikely to be accurate.