Subjective Refraction: Fogging and Use of the Astigmatic Dials
JAY H. KAUFMAN
Table Of Contents
OPTICAL PRINCIPLES OF THE METHOD|
|Today's ophthalmologist is the beneficiary of a long and sometimes
controversial process that has resulted in the development of modern
refractive techniques. Historically, two methods of refraction evolved, objective
and subjective, each with its special advantages and disadvantages. In
the objective method, the examiner made measurements to determine
the proper spectacle prescription without enlisting the patient's
responses. Objective techniques ultimately evolved into the
use of retinoscopy and automated refractors. The other, subjective, method
depended most on the responses that the patient gave to the examiner's
questions. It can be a primary method of refraction, but it
also can be used as a means of verifying the refractive error as estimated
Subjective refraction offered certain advantages: refraction could be done without cumbersome equipment and in the absence of a good pupillary reflex. Because the patient decided on the most desirable prescription, uncomfortable spectacles were less likely to be prescribed. However, subjective refraction often produced variable results because of a natural tendency for the patient to accommodate during the examination. For this reason, when cycloplegia was introduced into refraction, ophthalmologists hailed it as a major asset to subjective methods, although it brought with it another set of disadvantages and complications.
Gradually a subjective refractive technique using fogging and the astigmatic dials was developed that has a high degree of reliability and has retained its usefulness. This technique minimized the problem of accommodation while providing a sensitive measure of the patient's astigmatism. It has not replaced the standard objective refractive methods that most examiners continue to use, but it does offer an alternative or adjunct that may be preferable for use in some patients. In addition, it is a useful means of verifying a prescription obtained objectively.
Lancaster1 credited Donders with using a star-shaped figure made up of radiating lines as a prac-tical test for astigmatism as early as 1860. Green -perfected the astigmatic dial while working with Don-ders and Snellen from 1866 to 1869. In 1899, Verhoeff2 suggested a chart with two principal meridians to measure the amount of astigmatism. Lancaster laid much of the important groundwork for popularizing the technique and alone and later with Regan taught the refined method.3,4
Numerous variations of the astigmatic dials became available (Fig. 1). The number 1 charts include sunburst charts, clock dials with lines at intervals of 30 degrees, and other dials with lines at more frequent intervals, most popularly at 10 degrees. The number 2 dial, or Lancaster Cross, consists of two perpendicular lines that can be rotated to any position needed.
|OPTICAL PRINCIPLES OF THE METHOD|
|There are two components to this subjective technique: fogging to eliminate
accommodation, and use of the astigmatic dials to determine the magnitude
and axis of astigmatism. Optically, fogging means artificial
blurring of vision, identical to the naturally occurring state of myopia. It
is accomplished by placing convex lenses in front of the eye. Fogging
induces relaxation of accommodation because conditions are so arranged
that accommodation would blur visual acuity further (Figs. 2 and 3).|
A myope, of course, is naturally fogged. If the fogging is too extreme because of the amount of myopia present, concave lenses may be required to reduce the blurring to a level that permits the use of this method.
With the patient's eye rendered appropriately myopic, minus cylinders may then be used to correct the inherent astigmatism. In this procedure, the astig-matic dials are used to collapse the conoid of Sturm and thus eliminate the astigmatic interval entirely. The corrective minus cylinders bring the most out-of-focus meridian closer to the retina. This concept, which may be difficult to picture, is illustrated in Figures 3 and 4. The technique is designed to push the most blurry focal line back toward the least blurry focal line. When the astigmatism has been corrected properly, the focal lines are superimposed in the vitreous. The last step in the procedure is to reverse the fogging process until a state of emmetropia is achieved (Fig. 5).
|Let us see how these basic principles became a practical technique. Regan
and Regan3 and later Regan and Boeder4 outlined the steps in this method of refraction:|
STEPS 1 TO 4
After visual acuity for distance has been measured, the fogging process is begun. Remember that the patient with normal vision unaided by lenses probably has little astigmatic error but, if young, may be significantly hyperopic. In a myope, as previously mentioned, the fog may be adequate to begin with and may even have to be reduced with minus lenses to bring the patient's vision to a level suitable for testing with the dials. In less myopic patients or in hyperopes, sufficient myopia is simulated and accommodation is relaxed by adding plus spheres in front of the eye.
The end point of the fog is difficult to determine. An old pair of the patient's spectacles is useful in determining the approximate working range. Visual acuity is often not a reliable index of the adequacy of the fog, because acuity varies considerably depending on the degree of astigmatism. One way of approaching fogging is to continue to add plus lenses, having the patient close the eyes between changes, until the denominator of the Snellen notation of visual acuity is doubled (e.g., 20/20 to 20/40 [6/6 to 6/12]).*
Metric equivalent given in parentheses after Snellen notation.
Retinoscopy also provides a useful estimate of the proper fog. Neutralize the most hyperopic or least myopic meridian and place this spherical correction in front of the eye, being careful not to deduct for the working distance. If retinoscopy is done at 2/3 meter, a fog of 1.5 diopters (D) will result.
Unless large astigmatic errors make it necessary, it is better not to produce a blur worse than 20/100 to 20/200 (6/30 to 6/90). For the average patient, a wait of 1 or 2 minutes after fogging induces relaxation of accommodation. However, in a hyperope who has never worn glasses, 10 or 15 minutes may elapse before satisfactory relaxation of accommodation occurs.
Once the desired degree of fogging has been induced, the process is reversed by gradually removing the fogging lenses. As the fog is reduced, watch carefully for an increase in visual acuity with each decrease in fogging. If acuity does not increase as expected, attention should be directed to the astigmatic dial; the astigmatism is probably decreasing the vision. In a person with 3 or 4 D astigmatism, reducing the fog by as much as 1 D may not help acuity, although the patient will still be able to select the axis of astigmatism accurately.
If fogging has been correctly performed, enough myopia will have been induced to suppress accommodation but not enough to impair the intensity of the black lines on the astigmatic dial. Because it is easier for a patient to recognize the differing sharpness of radiating lines than to appreciate differences in visual acuity using letters as test objects, this is a highly sensitive test for astigmatism. In regular astigmatism, lines that run in the same direction as the meridian of the greatest refractive error appear sharply defined. A line therefore appears sharply outlined when it is accurately refracted in the meridian at right angles to it. (Another way of stating this is that the clearest lines are in the axis of least refractive error, the axis closest to the retina.) Thus, a patient with uncorrected myopic astigmatism at the 180-degree axis sees the vertical line at the 12- and 6-o'clock positions most clearly.
Make sure the patient's head is not tilted. Superimpose a pointer over the number 1 chart and ask the patient to select the darkest and clearest lines on the dial and to indicate his or her choice. Hold the pointer parallel to and alongside the axis you are pointing out, and ask the patient how many lines on either side of the pointer appear equally dark. Adjust the pointer to the center of any such group of lines. With larger degrees of astigmatism (e.g., 2 D), one to three lines are clearer. When astigmatism is small (e.g., less than 1 D), the patient often selects a larger number of lines as darker or clearer.
Sometimes a patient may not discern any difference among the lines on the dial. If no darker or clearer lines are noted, check the accuracy of the patient's observations by placing a -0.50 D cylinder at the 90-degree axis before the fogging lens. The patient should report that the vertical lines are darker. Likewise, when the cylinder is at the 180-degree axis, the horizontal lines should come into focus. Often such a maneuver clarifies what the examiner is testing for and makes astigmatism apparent for the first time. Now when the cylinder is removed, the patient can recognize slightly darker or clearer lines on the number 1 dial.
Set the number 2 dial so that one line is parallel to the axis determined previously. This line, of course, appears sharper than the line at right angles to it. Now introduce minus cylinder in 0.25 D steps with the cylinder axis parallel to the less dark or less clear line. (A good way to choose the proper minus cylinder axis is to multiply the lowest clock hour of the sharp line by 30 degrees. For instance, with a line running from the 2-o'clock to the 8-o'clock position, 2 × 30 = 60-degree axis.) Minus cylinder is added until both lines on the number 2 dial are equally sharp and clear. At this point the patient's astigmatism has been corrected.
The axis of astigmatism may be double-checked by using a Jackson cross cylinder. The amplitude of astigmatism may be checked by adding -0.25 D of cylinder first at one axis and then at the perpendicular axis for comparison. This ensures that the patient sees equally clear lines and does not require more or less cylinder to equalize the lines.
Remember to protect the fog as minus cylinder is added by adding + 0.25 D of sphere for every 0.50 D of minus cylinder that is given. Protecting the fog means keeping the astigmatic focal lines in the vitreous by maintaining the myopic state. If this is not done, the focal lines may be moved too close to the retina so that the adequate fog is lost and accommodation is stimulated.
When astigmatism has been corrected, reduce the fog in gradual steps. The goal is to improve visual acuity to 20/15 (6/5) by the least reduction in sphere that can accomplish this. Remove 0.25 D of fog, and check the resultant visual acuity. Repeat this procedure until no further improvement occurs.
If desired, the final prescription may be checked by reusing the dials. Removal of the correcting minus cylinder (the power of which has already been checked) produces the so-called perfect fog. The posterior focal line is now on the retina, and refinement of the axis to within 2.5 degrees is now possible by repeating the use of the dials.
Examination of the second eye is done in the identical manner and is facilitated by the patient's familiarity with the test and the fact that the examiner has a good idea of what to expect, because most patients have similar refractive errors in both eyes.
Balance the refraction.
|Several circumstances may render this subjective method of refraction inaccurate. If the patient's vision is so poor that 0.50 D of cylinder is not recognizable, or if the fog is too strong to allow discrimination of the proper axis, then the method will fail. If the fog is insufficient, accommodation will interfere with measurement of astigmatism. A patient with miotic pupils creates a pinhole effect, and one who squints induces a stenopeic slit; therefore, both miosis and squinting influence the final refraction. Finally, some patients cannot or will not cooperate with the technique. Children who cannot respond accurately to the examiner's questions cannot be refracted well with this method. Senile or mentally deficient patients are also poor candidates. Some patients are simply no good at making decisions, and their responses to subjective testing are therefore unreliable. In all these patients, techniques that rely more on the examiner's objective measurements (e.g., retinoscopy) are more reliable.|