Barry Milder, MD
Cyclopegia
To ensure that accommodation does not degrade the results of refraction, it is sometimes necessary to eliminate the patient’s ability to accommodate, albeit temporarily. Cycloplegic eye drops provide an anticholinergic effect, relax the ciliary muscle, and reduce lens curvature (Table). Drops with only sympathomimetic effect may dilate the pupil by stimulating the pupillary dilator fibers, but these drops do not reduce accommodative tone. Cycloplegic refraction is most often indicated in adolescents younger than teenagers, but may be necessary in any patients in whom voluntary relaxation of accommodation for the refractive target is not easily achieved. The choice of cycloplegic agent is tailored to the age of the patient and such other factors as previous comparison of manifest and cycloplegic refractions, presence of esotropia, and eye color. Eyes with dark irides do not respond to cycloplegic drops as well as light irides.
| Table. Cycloplegic Eye Drops | ||||||||||||||||||||||||
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Cycloplegic refraction is not always a perfect indicator of the patient’s refractive error, even if the cycloplegic drops have been effective. Especially with atropine or scopolamine, the refractive power of the lens may have less plus than ever occurs in the noncyclopleged state, and the eye, therefore, may refract as more hyperopic than is truly present without cycloplegia. In some patients with significant astigmatism, the astigmatic correction of the cycloplegic refraction may not accurately identify the astigmatism the patient has when not dilated.
Even if a manifest refraction is performed prior to cycloplegic refraction, finding significantly greater hyperopia when a patient is cyclopleged warrants requesting the patient’s return for a postcycloplegic refraction, unless full cycloplegic correction is to be prescribed, as may be required in an esotropic patient. Frequently, a postcycloplegic manifest refraction performed days to weeks after cycloplegic refraction results in the patient accepting more plus power than during the initial manifest refraction. This may allow a 30- to 40-year-old hyperope to avoid bifocals for a few more years.
Checking Accuracy
Patient responses during subjective refraction can be, and often are, tainted by the patient’s preconceptions of the refractionist’s goal, the pattern of the refractionist’s questions (always making the second choice the more minus of the two choices), patient fatigue, patient confusion over the difference between increased acuity versus increased contrast, and other factors. Therefore, it is important to verify the accuracy of subjective refractions, preferably by two different methods, to ascertain the most effective prescriptions. The accuracy can be confirmed by the monocular duochrome test and a binocular balance of the results of the two eyes.
Duochrome Test
The duochrome test is based on the presence of chromatic aberration in the human visual system. This chromatic aberration breaks white light into a spectrum of colors (Slide 1). Short wavelength light (the blue end of the spectrum) is refracted more than the longer wavelengths (red end of the spectrum), so within the eye, the blue light will always be closer to the front of the eye than the red light (Slide 2). Unfortunately, blue is psychophysiologically a poorly perceived color and tends to appear dark except in very pale hues. Therefore, the clinical duochrome test uses green and red filtered light, rather than blue. The dioptric interval between the two colors is approximately 0.50 D in the average patient.
The duochrome test is performed monocularly, following the refinement of the corrective sphere and cylinder (see Subjective Refraction, Part I Tutorial). The patient should continue to view the line of letters used for cylinder axis and power refinement, and the chart is divided in half, one side green and one side red. An eye will perceive black letters as being at the position of the surrounding colors. The patient should report seeing the letters sharper, bolder, or clearer on the side of the color closer to the retina.
If corrective lenses result in the patient’s vision being slightly overplussed and the focus is in the vitreous, when the patient views the screen, the letters on the red side of the chart will be reported as clearer (Slide 3). Addition of -0.25 D lens power would bring the focus closer to the retina. If the red side is still clearer, another -0.25 D lens power may be added. If the patient still prefers the red side letters, the test must be disregarded.
If the corrective lenses have overminused the patient’s vision and the focus is behind the retina, the letters on the green side of the screen will be perceived sharper (Slide 4). In that case, +0.25 D lens power is added, until the patient prefers the letters on the red side. Again, if the preference does not change with the addition of two +0.25 D steps, the test should be disregarded.
In a patient with correct refraction, the chromatic interval straddles the retina, and the patient may report the letters as equally clear on the two sides of the chart (Slide 5). However, because the green side is slightly closer to the center of the spectrum than the red side, a correct refraction may also result in a report that the letters are seen better on the green side. A single addition of +0.25 D lens power should convert the patient’s preference to the red side.
Many patients have difficulty responding to the duochrome test, because the differences between the letters on both sides of the chart may be minimal, the screen or the projector slide may be dirty or smudged irregularly across its surface, the patient’s lens may be discolored by nuclear sclerosis, or the patient may have a strong preference for one color. The duochrome test must be ignored when its results do not correlate well with the refractionist’s impression of the accuracy of the previous steps in the refraction. Also, it is useless to attempt a duochrome test in a patient whose visual acuity is less than 20/40 at the end of cylinder refinement.
Despite the limits of the duochrome test, it is often a useful method of confirming the accuracy of the previous steps in subjective refraction, especially in patients younger than presbyopic age. Because it is based on the presence of chromatic aberration and not on color perception, the duochrome test is also effective in congenitally colorblind patients. In colorblind patients, the optical effect is present and the retina observes the same differences between the two sides of the red and green screen. The refractionist refers to the right and left halves of the chart, rather than the red or green sides.
Binocular Balance
Though both refractionist and patient may perform their tasks to the best of their abilities, it is not rare for the two eyes to have been refracted with slightly different accommodative tone. The effect leaves the patient with one eye slightly overminused or overplussed when both are open and the patient attempts to see a target. Because the visual system is not arranged such that a different accommodative message can be sent to each eye, any attempt to compensate the imbalance by further accommodative changes is doomed to fail (Slide 6). An unbalanced prescription can not provide a patient with equal vision in both eyes. To avoid this error, the refractionist must search for evidence of unbalanced refractive results and bring the eyes into balance.
Balancing is achieved with binocular testing conditions. All of the steps previously discussed were performed under monocular conditions. Once the best monocular results have been obtained for each eye individually, binocular balance is performed. All binocular balance methods have similarities and involve the following steps:
- Fogging both eyes equally
- Determining which eye sees better. This eye has its focus closer to the retina.
- Adding minus sphere to the worse eye, until it becomes the better eye.
- Removing the last -0.25 D of change.
- Removing fog from both eyes simultaneously in -0.25 D increments until best visual acuity is reached.
“Fogging” Technique:
Power of +1.75 D is added to the refractive end result for each eye, bringing the focus of each eye into the vitreous (unless one eye was more than 1.75 D overminused) (Slide 7). If the refraction was balanced, the two eyes will be equally blurred (center pair on Slide 8). Otherwise, one eye has its focus closer to the retina and will see the chart better than the other (outer pairs on Slide 8). With fogging of +1.75 D, the patient should be able to identify 20/200 letters and possibly a line or two better. It is courteous to let the patient know that the letters are intentionally significantly blurred. If acuity of either eye is better than 20/70, the refractionist should surmise that the eye is slightly overminused, and repeat the refraction for that eye.
The refractionist alternates covering the right and left eyes, asking if one eye sees the letters slightly clearer than the other. To the worse eye, -0.25 D is added, and the cover test and questions are repeated (Slide 9). Additional -0.25 D changes are made to the originally worse eye until it becomes the better eye. Then the last -0.25 D, which made it the better eye, is removed. The two eyes are then balanced, though still fogged. The fog is removed in -0.25 D steps on both eyes until the patient achieves best visual acuity. If the patient reported equal vision with the two eyes with the +1.75 D fog, the original refraction is presumed to have been balanced. To confirm this assumption, -0.25 D is added to each eye sequentially (Slide 10). If the original refraction was truly balanced, addition of -0.25 D should make each eye the better eye.
The fogging method of binocular balance works well in most patients, even patients with poor or absent binocular vision resulting from suppression associated with childhood strabismus. The fogging method can be effective in patients whose best visual acuity is not identical in both eyes, but there is little benefit in trying to balance eyes with greater than two lines difference in acuity, or in patients whose best vision is less than 20/40 in either eye.
“Split Image” Technique:
To the refractive end result for each eye, +1.00 D is added (see Slide 7). The chart is reduced to a single 20/40 line, and a 4 D prism is placed base down before one eye. The patient should see two sets of 20/40 letters, one higher than the other, and both somewhat blurred. If the patient does not see double, the prism may be increased to as much as 6 D. If diplopia is still not present, this method cannot be used.
The refractionist asks the patient whether the higher or lower line of letters is sharper. The higher image will be seen by the eye with the base down prism, so the refractionist can immediately know which eye has its focus closer to the retina. The worse eye is adjusted in -0.25 D increments until it becomes the better eye, and then the last -0.25 D is removed (see Slide 9). The dissociating prism is then eliminated, and the fog is removed in -0.25 D increments from both eyes simultaneously until best visual acuity is reached.
Because the patient can see both images simultaneously, comparison of the quality of the letters is easy and accurate. This method can balance to 0.12 D tolerance in cooperative patients with excellent visual acuity. Because the letters are not blurred as much as in the fogging technique, most patients find this the easier method. However, it is so sensitive that any difference of greater than one-half line in the two eyes’ best visual acuity may render them too asymmetric to balance by this technique. Furthermore, patients who lack simultaneous binocular perception cannot see the two lines and cannot be balanced this way.
In this discussion of Subjective Refraction (both Parts I and II), no discrimination has been made between refracting patients with phoropter or trial frame. The refraction steps are identical with either. However, phoropter and trial frame are not equally valuable. Refraction with phoropter is fast and accurate, unless the refractive error is more than 6 D. Above 6 D, the vertex distance at which the corrective lenses are positioned becomes a significant factor in the refractive result, and vertex distance cannot be measured accurately with phoropter refractions. Anyone with high ametropia should be refracted in trial frame if spectacles may be prescribed from the refraction. Vertex distance should be measured with a distometer and recorded with the refraction results. Otherwise, it is impossible to determine the degree of changes in future refractions. Refractions performed for the sole purpose of determining best visual acuity may be done with a phoropter, as long as the patient is steady behind the phoropter aperture.
Bibliography
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