Axial Length in Keratoconus

Keratoconus Association with Axial Myopia: A Prospective Biometric Study

“…3,4 In contrast, two other studies conclude that there was no difference in axial lengths between keratoconics and emmetropes. 13,14 These two studies found keratoconic axial lengths to have a mean of 24.39 and 24.01 mm, respectively, which, interestingly, are comparable to those in this study. However, to reach their conclusion, both of these studies compared their keratoconic measurements to the same historic emmetropic value of 24.2 mm, which was derived by the method of conjugate foci calculation.…”

“…However, the question in keratoconic eyes is whether the myopia is a function of the ectatic and steep cornea, an elongated axial length, or both. Although the primary ophthalmic feature in keratoconus is the ectatic, steep cornea, our hypothesis, and those of other investigators, [2][3][4]13 is that myopia in a keratoconic eye is also because of the axial length and not just because of a steep cornea. Therefore, to prove our hypothesis, we, like the aforementioned investigators, chose to compare keratoconic eyes to emmetropic eyes as opposed to myopic eyes, and even more specifically, to remove the contribution of the cornea, we compared the posterior segment lengths between the two groups with the understanding that an ''emmetropic'' eye is the result of the interdependent contributions of the corneal curvature, lens, and axial length.…”

“…3 Prior studies measuring axial length in keratoconus were performed using contact ultrasonography, which has been shown to indent the cornea in normal eyes and even more so in ectatic keratoconic eyes. 13 Because immersion ultrasonography does not indent the cornea, we hypothesized that the improved accuracy may reveal an association between mean keratometry and axial length. However, we found no strict association.…”

“…Studies have varied in their conclusions as to whether keratoconus is associated with an increased axial length versus emmetropic eyes. [1][2][3][4]13,14 Additionally, some of these studies exhibit methodologic issues such as evaluating postkeratoplasty eyes, using historic controls, comparing to nonemmetropic eyes, and employing contact ultrasonography. 2,3,13 Therefore, we sought to circumvent some of these past methodologic issues by prospectively determining if there was a significant difference in the axial lengths and its various components between unoperated keratoconic and internal emmetropic control eyes using immersion ultrasonography; additionally, we also sought to determine if corneal ectasia itself was associated with an increased axial length.…”

How Abnormal Is the Noncorneal Biometry of Keratoconic Eyes?

“…[4][5][6][7][8] Meanwhile, 2 other studies reported finding no such difference. 9,10 It is interesting to note, however, that all studies reporting a significant axial elongation in keratoconus used patients with emmetropic eyes as the control group, [4][5][6][7][8] while the only study that did not impose refractive criteria on their control group reported no significant difference. 10 Although the choice for an emmetropic control group is understandable, it may lead to a bias in the distribution of the axial length as these group tends to have shorter eyes than the general population, which sees an ever-increasing prevalence of myopia.…”