Subjective Refraction Technique: Duochrome Test

Excerpt

At birth, the ocular structures are fully formed. The eys continuously develop as the neonate grows to reach full refractive status. The developmental process in which the ocular system of the child obtains the expected refractive status is known as emmetropization. Several factors, such as corneal curvature, anterior chamber depth, axial length, and lenticular thickness, are known to be the main determinants of the final refractive outcome.

In early infancy, neonates are known to possess a large corneal curvature dioptric power, which is expected to regress to progressively reach emmetropia. These developmental changes of cornea curvature are usually rapid in the first two to four weeks of life and, subsequently, become slower after eight weeks. As the neonate grows older, the lenticular dioptric power also reduces in a progressive fashion until the optimum lenticular dioptric power is attained. Regardless of the regressive dioptric changes of the cornea curvature and lenticular dioptric power during emmetropization in the early stage of life, changes in the degree of axial length relative to the initial refractive status remain the most critical factor in reaching the required refractive status.

Depending on genetic predisposition and environmental influence, the eye can be considered mostly hyperopic in the early stages of life. Changes in the refractive apparatus of the eye, as well as its axial length at this stage of life, significantly alter the initial refractive status. The relationships between the initial refractive status, subsequent axial length changes, and the consequent change in the resulting refractive error are more consistent with the visual basis of emmetropization, regardless of the significant dioptric changes in the cornea curvature and lenticular dioptric power.

Due to the prolate shape of the eye, the parallel light rays passing through the entrance pupil, when focused correctly on the macular, can exceed the retinal limit of the peripheral retinal, known as hyperopic defocus. The process of emmetropisation is theorized to be mostly controlled by the retina.

Theoretically, the retina tends to elongate when parallel light rays from an object at infinity are focused behind the retina, which is done to achieve a clear focus. Defocus, therefore, serves as the principal stimulus and the medium in which the retinal exerts control over emmetropization. There is an increased risk of anisometropia when an inequality of increase in axial length occurs in both eyes during childhood development.

The prevalence of anisometropia increases from childhood to teenage years (ages between 5 and 15). Homeostatic failure, described as a dysfunctional emmetropisation, in addition to other conditions (ie, high degree of refractive error at birth), are major risk factors in developing refractive errors.

When determining the degree of optical correction needed to compensate for the refractive deficiency of an ocular system, it is of utmost importance to refine the final spherical dioptric power obtained from prior refractive procedures. The duochrome red-green test can be employed as a standard method of verifying the final refraction. When utilizing this test in astigmatic errors, it is essential for the circle of least confusion to be on the retina to acquire maximum effectiveness.