In subjective refraction techniques, the examiner relies on the patient’s responses to determine the refractive correction. If all refractive errors were spherical, subjective refraction would be easy. However, determining the astigmatic portion of the correction is more complex, and various subjective refraction techniques may be used. The Jackson cross cylinder is the most common instrument used in determining the astigmatic correction. However, we begin by discussing the astigmatic dial technique because it is easier to understand.
Astigmatic Dial Technique
An astigmatic dial is a test chart with radially arranged lines that may be used to determine the axes of astigmatism. A pencil of light from a point source is refracted by an astigmatic eye as a conoid of Sturm. The spokes of the astigmatic dial that are parallel to the principal meridians of the eye’s astigmatism are imaged as sharp lines, which correspond to the focal lines of the conoid of Sturm.
Figure 4-15A shows an eye with compound hyperopic astigmatism and how it sees an astigmatic dial. In this example, the vertical line of the astigmatic dial is the blackest and sharpest because the vertical focal line of each conoid of Sturm is closer to the retina than the horizontal focal line is. By accommodating, however, the patient might pull both focal lines forward. To avoid accommodation, fogging is used. Sufficient plus sphere is placed before the eye to pull both focal lines into the vitreous, creating compound myopic astigmatism (Fig 4-15B).
The focal line closest to the retina can then be identified with certainty because it is now the blackest and sharpest line of the astigmatic dial. (In the example in Fig 4-15B, this is the horizontal line on the dial.) Note that the terms blackest and sharpest are more easily understood by patients and should be used in place of the word clearest.
After the examiner locates the principal meridians of the astigmatism, the conoid of Sturm can be collapsed by moving the anterior focal line back toward the posterior focal line. This task can be accomplished by adding a minus cylinder with an axis parallel to the anterior focal line (perpendicular to the blackest and sharpest line). It is helpful to remember the “rule of 30”: place the correcting minus cylinder axis along the meridian equal to 30 times the “clock-hour” of the sharpest, blackest line on the dial (reading between 0 and 6). In Figure 4-15C, the vertical focal line has been moved back to the position of the horizontal focal line and collapsed to a point by the addition of a minus cylinder with an axis at 90°. Note that 90 = 30 × 3, the clock-hour of the (horizontal) darkest line. Notice that the minus cylinder is placed with its axis perpendicular to the blackest meridian on the astigmatic dial. Also note that as the conoid of Sturm is collapsed, the focal lines disappear into a point focus.
Figure 4-15 Astigmatic dial technique. A, Conoid of Sturm and retinal image of an astigmatic dial as viewed by an eye with compound hyperopic astigmatism. B, Fogging to produce compound myopic astigmatism. C, The conoid of Sturm is collapsed to a single point. D, Minus sphere is added (or plus sphere subtracted) to produce a sharp image, and a visual acuity chart is used for viewing.
All of the lines of the astigmatic dial now appear equally black but still are not in perfect focus, because the eye remains slightly fogged to control accommodation. At this point, a visual acuity chart is used; plus sphere is removed until the best visual acuity is obtained (Fig 4-15D).
In summary, the following steps are used in astigmatic dial refraction:
Obtain the best visual acuity using spheres only.
Fog the eye to approximately 20/50 by adding plus sphere.
Ask the patient to identify the blackest and sharpest line of the astigmatic dial.
Add minus cylinder with the axis perpendicular to the blackest and sharpest line until all lines appear equal.
Reduce plus sphere (or add minus) until the best visual acuity is obtained with the visual acuity chart.
Astigmatic dial refraction can also be performed with plus cylinder equipment, but this technique must be used in a way that simulates minus cylinder effect. All of the above steps remain the same except for step 4, which becomes “Add plus cylinder with the axis parallel to the blackest and sharpest line.” As each 0.25 D of plus cylinder power is added, change the sphere simultaneously by 0.25 D in the minus direction. Doing so simulates minus cylinder effect exactly by moving the anterior focal line posteriorly without changing the position of the posterior focal line.
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.