In this literature review, the authors describe the inconsistencies in definition, application and usage of ocular reference axes and angles, and propose a precise, reproducible, clinically defined reference marker and axis for centration of refractive treatments and devices.
They write that the inconsistent definitions and usage of the current ocular axes as derived from eye models – the optical axis, visual axis, line of sight, pupillary axis and corneal topographic axis – limit their clinical utility. As a solution to this, they propose a new axis that is independent of pupillary dilation and phakic status of the eye: the subject-fixated coaxially-sighted corneal light reflex axis, which is formed by connecting the subject-fixated coaxially-sighted corneal light reflex and the fixation point.
They also propose the term “foveal-fixation axis” to describe the relationship between the subject-fixated coaxially-sighted corneal light reflex axis and a refined definition of the visual axis without reference to nodal points. The definitions of the two axes refer to a very similar locus of points but approach that description from two different perspectives; the subject-fixated coaxially-sighted corneal light reflex axis uses a clinical approach by identifying visible reference markers, while the fovea-fixation axis uses a theoretical approach by referencing a simplified model of the eye.
They write that the need for a new clinical reference marker and axis is paralleled by the need for a new description of its relationship to other axes. Since the quantity being described is a displacement and not an angle, they propose the term “chord mu (μ)” to denote the two-dimensional displacement of the entrance pupil center from the subject-fixated coaxially-sighted corneal light reflex. They write that the use of the term “chord” instead of “angle” emphasizes the entity described, as well as its uniqueness in the literature, and the letter “mu” replaces previously used terms that had historically conflicting or misused definitions. Since the pupil center can shift with miosis and mydriasis, the description of chord mu should optionally (and preferably) include the state of the pupil.
They write that as a clinically defined marker, the subject-fixated coaxially-sighted corneal light reflex can be a useful reference for centering refractive treatments and devices. However, use of the subject-fixated coaxially-sighted corneal light reflex does have some limitations. Corneal refractive surgery can alter its appearance or location. Treatments that require applanation or a liquid interface on the cornea preclude its visualization. Also, the precise relationship of the subject-fixated coaxially-sighted corneal light reflex axis and the fovea-fixation axis, particularly in eyes with large chord mu or highly aberrated corneas, is unclear.
They conclude that while the debate on where best to center refractive treatments and devices continues, the addition of the terms “subject-fixated coaxially-sighted corneal light reflex”, “fovea-fixation axis” and “chord mu” can help to clarify the relevant concepts and to unify these discussions.