As discussed in Chapter 3, the pupillary axis is the imaginary line that is perpendicular to the corneal surface and passes through the midpoint of the entrance pupil (see Fig 3-3). The visual axis is the imaginary line that connects the point of fixation to the fovea. The angle kappa (κ) is defined as the angle between the pupillary axis and the visual axis (which intersects the corneal surface very near the corneal apex in a regular cornea) and is measured by observing the difference between the apparent center of the pupil and the penlight reflex during monocular fixation of the light source. A large angle kappa results from a significant difference between these two. If the angle kappa is large, centering an excimer ablation over the geometric center of the cornea will effectively result in a decentered ablation. This can be particularly problematic in a hyperopic correction, in which a large angle kappa can result in a refractively significant “second corneal apex,” causing monocular diplopia and decreased quality of vision. A large angle kappa must be identified before surgery to reduce the likelihood of a poor visual outcome.
Reinstein DZ, Gobbe M, Archer TJ. Coaxially sighted corneal light reflex versus entrance pupil center centration of moderate to high hyperopic corneal ablations in eyes with small and large angle kappa. J Refract Surg. 2013;29(8):518–525.
Wachler BS, Korn TS, Chandra NS, Michel FK. Decentration of the optical zone: centering on the pupil versus the coaxially sighted corneal light reflex in LASIK for hyperopia. J Refract Surg. 2003;19(4):464–465.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.