Important Axes of the Eye
Careful analysis of the orientation of the eye is frustrated by the imprecision of the alignment of the main optical elements. Ideally, one might imagine that the fixation target, the optical center of the cornea, the optical center of the lens, and the fovea should all fall on a single line, which one might envision as the “axis” of the eye. In fact, no 3 of these points typically lie on the same line. As a line is determined by 2 points, there are many choices, each of which determines a different “axis.” The following are definitions of selected terms used to describe the various axes of the eye:
The principal line of vision is the line passing through the fixation target, perpendicular to the corneal surface. (When the patient fixates a luminous target, the reflection in the cornea of the light source will lie on this line.)
The pupillary axis is the line perpendicular to the corneal surface and passing through the midpoint of the entrance pupil. (The entrance pupil is the image of the pupil as seen through the cornea from the outside of the eye.) If an observer is aligned with a small light source, and centers the corneal reflection in the entrance pupil, he will be sighting along the pupillary axis. If the pupil is eccentric, this will not coincide with the principal line of vision. The visual axis is the line connecting the fixation target and the fovea.
The optical axis is the line passing through the optical centers of the cornea and lens. This line typically does not pass through the center of the fovea.
The angle alpha (α) is the angle between the visual axis and the optical axis. This angle is considered positive when the visual axis in object space lies on the nasal side of the optical axis.
The angle kappa (κ) is the angle between the pupillary axis and the visual axis (Fig 3-3). The angle kappa is considered positive when the fovea lies temporal to pupillary axis, as is typically the case.
Figure 3-3 Angle kappa (κ). The pupillary axis (red line) is represented schematically as the line perpendicular to the corneal surface and passing through the midpoint of the entrance pupil (E). The visual axis (green line) is defined as the line connecting the fixation target (O) and the fovea (F). If all the optical elements of the human eye were in perfect alignment, these 2 lines would overlap. However, the fovea is normally displaced from its expected position. The angle between the pupillary axis and the visual axis (leader line) is called angle kappa (κ) and is considered positive when the fovea is located temporally, as is the usual case. Conditions that cause temporal dragging of the retina, such as retinopathy of prematurity, can lead to a large positive angle kappa. Clinically, this will present as pseudoexotropia. A large positive angle kappa may also mask a small-angle esotropia, which can be detected by the cover-uncover test. Angle alpha (α) is the angle between the optical axis and the visual axis of the eye and is considered positive when the visual axis in object space lies on the nasal side of the optical axis, as is normally the case.
(Courtesy of Neal H. Atebara, MD. Revised illustration based on a drawing by C. H. Wooley.)
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.