There may not be an association between keratoconus and higher levels of axial myopia, despite prior research suggesting otherwise, reports a study published online in the journal Cornea.1 The connection was first suggested in the 1970s as a way to explain the excessive degrees of myopia seen following penetrating keratoplasty; it was later confirmed by studies that identified significant axial elongation in patients with keratoconus. Of note, however: patients with emmetropic eyes were typically used as the control groups during these studies, which may have led to bias.2-4 

Researchers in Belgium set out to repeat prior statistical analyses of the corneal and noncorneal biometry in keratoconic eyes and compare these values to those of healthy eyes with a range of refractions and to a subgroup of emmetropes. Two hundred patients were divided into three groups: those with keratoconus, those with healthy corneas and a subset of the latter with emmetropic eyes characterized by a spherical equivalent refraction between ±0.75D. Uncyclopleged refraction was determined using an autorefractometer, while keratometry values were measured using Scheimpflug tomographer. The former was used to calculate the SE and Jackson cylinders J0 and J45, while the latter was used to calculate the anterior kera­tometry Km and anterior corneal Jackson cylinders J0,c and J45,c. Axial length, as well as central corneal thickness (CCT) and aqueous depth, were also measured.

“As one would expect, the differences between the group with keratoconus and the two other groups lie mostly in the refractive and corneal parameters, with significant differences seen for SE, J45, Km, J45,c and CCT,” the researchers report. “Keratoconic eyes also had significantly deeper anterior chambers than the other two groups, but no significant differences were seen for either lens thickness or lens power.” 

These results confirm that keratoconus has a minimal influence on axial myopia and that its pathology is purely corneal in nature, the researchers add. Additionally, the increase in anterior depth might be the result of combined thinning and outward expansion of the cornea. “Clinically, these findings may be important to better understand the refractive development in patients with keratoconus, as well as for lens power calculations in patients with keratoconus, where axial length is the most important factor for lens power,” they conclude.  

1. Rozema JJ, Zakaria N, Hidalgo IR, Jongenelen S, Tassignon M, Koppen C.  How abnormal is the noncorneal biometry of keratoconic eyes? Cornea. 2016. Epub ahead of print.