SURVEY OF OPHTHALMOLOGY
VOLUME 20
PERSPECTIVES
l
NUMBER 5
l
MARCH-APRIL
1976
IN REFRACTION
MELVIN L. RUBIN, EDITOR
Lady
Luck
BENJAMIN
MILDER,
M.D.
Department of Ophthalmology, St. Louis, Missouri
Washington
University
School
of
Medicine,
Abstract. When a patient with no distance complaints is unable to read with comfort, a shift in the cylinder axes should be among the first things considered. A simple technique for measuring the axes is described, interpretation of the measurements is discussed, and various therapeutic approaches are suggested. (Surv Ophthalmol 20: 347-349, 1976) Key Words:
T
astigmatism
l
cylinder axes
he carnival! Who can forget the itinerant carnival which sprang up magically every summer on the empty lot in your
neighborhood - your shoes dragging in the grass-turned-to-mud, the cacophony of the barkers, the carousel, the cotton-candy, the games of chance - especially the “wheel of fortune.” “Step right up,” urged the pitchman, as the siren song of the wheel’s staccato clicking beckoned to the crowd “round and round she goes, and where she stops . . . ” “a winner every time!” Of course, there really never was a winner every time any more than there is a winner every time you “spin” your astigmatic dial. But I often think of the unpredictability of that carnival wheel whenever I come up a loser with a pair of astigmatic glasses for some unhappy patient. One of the more unpredictable ways of losing (both your aplomb and your patient) is the failure to recognize a change in cylinder axis as fixation is shifted from distance to near. A patient who has a significant amount 347
of astigmatism may complain to you that, although no symptoms for distance are present, reading with comfort is impossible. For some strange reason, we usually tend to think last of the possibility of a cylinder axis shift. Perhaps we should be thinking of it first! And it is not even necessary that the “significant amount of astigmatism” be all that significant. Take the case of Mrs. R., a bright, welladjusted seventy-four-year-old widow who seemingly posed only one major problem she was the mother-in-law of a physician colleague. Mrs. R. complained that when she attempted to read she was defeated by blurring of the print and pain in the eyes. Her eye examination revealed no abnormalities other than moderate nuclear sclerosis of the crystalline lenses. Her present glasses were two years old: OD - 6.25 + 1.75 X 65 = 20/30 OS - 6.25 + 1.75 X 100 = 20/40 add + 2.50 OU 8 point type at 33 cm.
348
Manifest OD OS add
Surv Ophthalmol 20 (5) March-April
1976
refraction: - 6.50 + 1.75 X 50 = 20/25 - 6.25 + 1.75 X 135 = 20/25 + 2.50 OU
It was apparent that there could not have been a dramatic change in the cataracts over the span of two years, since the refractive error was essentially unchanged in sphere and in cylinder power. It was interesting that Mrs. R. had no symptoms for distance, so we belatedly undertook a more detailed near point refraction. At a distance of 33 cm., using a binocular technique, the cylinder axes were found to be: ODx 75 OS x 120 We quickly placed the reading correction in a trial frame, using the near point axes, and the patient was able to read clearly, with no discomfort during the limited reading trial in the office. The decision was made to prescribe new bifocals, orienting the cylinders close to the near point axes: OD-6.50+ 1.75X 70 OS - 6.25 + 1.75 X 125 add + 2.75 OU There is little question that the change in cylinder axes at near contributed to the patient’s reading problem. There is less unanimity, however, about how to deal with this problem. The minimal requirements for insuring a happy result include the ability to: (1) identify the problem; (2) examine the patient properly; (3) understand the multiple etiologic factors involved; and (4) appreciate options in prescribing. To deal with the cylinder axis shift at near, the first task is to think of it! Our index of suspicion is heightened (1) if the complaints are limited to near vision; (2) if the cylinder power is significant, and (3) if the distance refraction does not differ markedly from that achieved by prior spectacles. All of these clues were present in the case of Mrs. R. There are numerous techniques for measuring the axes at near, and the one that we employ is very simple, requiring a minimum of time, specialized skills, and instrumentation. Both eyes are directed to a series of small letters (4 or 5 point type) held at the usual reading distance of 35-40 cm. The correct distance lens prescription is placed in the trial frame and, in the case of the presbyope, the appropriate add is placed before one eye
MILDER
only, leaving the other eye blurred for the test letters. The correct cylinder axis is then determined using a Jackson cross-cylinder in the usual fashion. The add is then moved to the other eye and the process repeated. It is just that simple! If this test is performed routinely, one can usually find a modest shift in axis, from distance to near fixation. Whether it is sufficient to be responsible for near-vision symptoms will depend on the amount of astigmatism, the amount of shift in axis and on that indefinable attribute known as “patient tolerance.” Earlier, we referred to the change in axis as “unpredictable,” since it is the product of several variables - anatomic, fusional and optical. Anatomically, the vergence movement in binocular fixation at near can produce alterations in the shape of the cornea. In addition, the convergence of the eyes tends to produce excyciotorsion, while infraversion of the eyes (as in the reading position) tends to produce intorsion. Whether these two opposing tendencies cancel each other, or which predominates in a specific patient, is one of the unpredictables. In addition, the glasses themselves may be responsible for a further compensatory torsional effort in order to maintain binocular comfort at near. This effect can be best understood if the eye is considered as a “lens” which is being neutralized by the correcting spectacle. You are aware, or should be, that two cylinders of like power and axis, but of opposite sign, will cancel each other out. Thus, when -2.00 X 90 is added to +2.00 X 90, the result is plano. But if these same two cylinders are separated by a small angle, the resultant axis is not midway between the two, but at an entirely new axis.* For example, if an eye requires a lens correction of +2.00 X 90, that eye may be thought of as a lens system having a built-in cylinder power of -2.00 X 90. Now, if before that “eye-lens” system of -2.00 X 90, we place the correcting +2.00 cylinder at axis 80, a new plus cylinder is created at axis 40, and a torsional effort will be required in an attempt to manage the new induced cylinder axis. So, the term “unpredictable” is something of an understatement. *For a more detailed discussion of this problem, read: Rubin M. Optics for Clinicians, Gainesville, Triad, 1974, pp 180-181.
PERSPECTIVES IN REFRACTION
Finally, there are a variety of ways to manage the problem and since the goal of all refraction is “patient comfort,” (a very subjective yardstick for measuring therapeutic triumphs) the same corrective measures will not necessarily benefit each patient. First of all, if the finding is incidental to the examination and the patient is symptom-free, nothing should be done to change the status quo. If the symptoms are limited to near-vision activities, the axes should conform closely to those found in near point testing, as was done for Mrs. R. If the axis disparity between distance and near is formidable, separate reading glasses may be necessary. Occasionally, a presbyope will be well served by using only one bifocal add. Most often, however, attempts are made to
349
resolve the problem by reducing the power of the cylinder. In such an endeavor, the spherical equivalent must remain unchanged, and the patient may be willing to accept some reduction in visual acuity as the price of a reduction in symptoms when reading. Just how much decrease in cylinder power will be required, in any given case, to achieve comfort without sacrificing too much sharpness of vision requires experience - and a smile from Lady Luck.
Reprint requests should be addressed to Benjamin Milder, M.D., Department of Ophthalmology, Washington University School of Medicine, 660 South Euclid Avenue, St. Louis, MO. 63110.
VOLUME 20
PERSPECTIVES
l
NUMBER 5
l
MARCH-APRIL
1976
IN REFRACTION
MELVIN L. RUBIN, EDITOR
Lady
Luck
BENJAMIN
MILDER,
M.D.
Department of Ophthalmology, St. Louis, Missouri
Washington
University
School
of
Medicine,
Abstract. When a patient with no distance complaints is unable to read with comfort, a shift in the cylinder axes should be among the first things considered. A simple technique for measuring the axes is described, interpretation of the measurements is discussed, and various therapeutic approaches are suggested. (Surv Ophthalmol 20: 347-349, 1976) Key Words:
T
astigmatism
l
cylinder axes
he carnival! Who can forget the itinerant carnival which sprang up magically every summer on the empty lot in your
neighborhood - your shoes dragging in the grass-turned-to-mud, the cacophony of the barkers, the carousel, the cotton-candy, the games of chance - especially the “wheel of fortune.” “Step right up,” urged the pitchman, as the siren song of the wheel’s staccato clicking beckoned to the crowd “round and round she goes, and where she stops . . . ” “a winner every time!” Of course, there really never was a winner every time any more than there is a winner every time you “spin” your astigmatic dial. But I often think of the unpredictability of that carnival wheel whenever I come up a loser with a pair of astigmatic glasses for some unhappy patient. One of the more unpredictable ways of losing (both your aplomb and your patient) is the failure to recognize a change in cylinder axis as fixation is shifted from distance to near. A patient who has a significant amount 347
of astigmatism may complain to you that, although no symptoms for distance are present, reading with comfort is impossible. For some strange reason, we usually tend to think last of the possibility of a cylinder axis shift. Perhaps we should be thinking of it first! And it is not even necessary that the “significant amount of astigmatism” be all that significant. Take the case of Mrs. R., a bright, welladjusted seventy-four-year-old widow who seemingly posed only one major problem she was the mother-in-law of a physician colleague. Mrs. R. complained that when she attempted to read she was defeated by blurring of the print and pain in the eyes. Her eye examination revealed no abnormalities other than moderate nuclear sclerosis of the crystalline lenses. Her present glasses were two years old: OD - 6.25 + 1.75 X 65 = 20/30 OS - 6.25 + 1.75 X 100 = 20/40 add + 2.50 OU 8 point type at 33 cm.
348
Manifest OD OS add
Surv Ophthalmol 20 (5) March-April
1976
refraction: - 6.50 + 1.75 X 50 = 20/25 - 6.25 + 1.75 X 135 = 20/25 + 2.50 OU
It was apparent that there could not have been a dramatic change in the cataracts over the span of two years, since the refractive error was essentially unchanged in sphere and in cylinder power. It was interesting that Mrs. R. had no symptoms for distance, so we belatedly undertook a more detailed near point refraction. At a distance of 33 cm., using a binocular technique, the cylinder axes were found to be: ODx 75 OS x 120 We quickly placed the reading correction in a trial frame, using the near point axes, and the patient was able to read clearly, with no discomfort during the limited reading trial in the office. The decision was made to prescribe new bifocals, orienting the cylinders close to the near point axes: OD-6.50+ 1.75X 70 OS - 6.25 + 1.75 X 125 add + 2.75 OU There is little question that the change in cylinder axes at near contributed to the patient’s reading problem. There is less unanimity, however, about how to deal with this problem. The minimal requirements for insuring a happy result include the ability to: (1) identify the problem; (2) examine the patient properly; (3) understand the multiple etiologic factors involved; and (4) appreciate options in prescribing. To deal with the cylinder axis shift at near, the first task is to think of it! Our index of suspicion is heightened (1) if the complaints are limited to near vision; (2) if the cylinder power is significant, and (3) if the distance refraction does not differ markedly from that achieved by prior spectacles. All of these clues were present in the case of Mrs. R. There are numerous techniques for measuring the axes at near, and the one that we employ is very simple, requiring a minimum of time, specialized skills, and instrumentation. Both eyes are directed to a series of small letters (4 or 5 point type) held at the usual reading distance of 35-40 cm. The correct distance lens prescription is placed in the trial frame and, in the case of the presbyope, the appropriate add is placed before one eye
MILDER
only, leaving the other eye blurred for the test letters. The correct cylinder axis is then determined using a Jackson cross-cylinder in the usual fashion. The add is then moved to the other eye and the process repeated. It is just that simple! If this test is performed routinely, one can usually find a modest shift in axis, from distance to near fixation. Whether it is sufficient to be responsible for near-vision symptoms will depend on the amount of astigmatism, the amount of shift in axis and on that indefinable attribute known as “patient tolerance.” Earlier, we referred to the change in axis as “unpredictable,” since it is the product of several variables - anatomic, fusional and optical. Anatomically, the vergence movement in binocular fixation at near can produce alterations in the shape of the cornea. In addition, the convergence of the eyes tends to produce excyciotorsion, while infraversion of the eyes (as in the reading position) tends to produce intorsion. Whether these two opposing tendencies cancel each other, or which predominates in a specific patient, is one of the unpredictables. In addition, the glasses themselves may be responsible for a further compensatory torsional effort in order to maintain binocular comfort at near. This effect can be best understood if the eye is considered as a “lens” which is being neutralized by the correcting spectacle. You are aware, or should be, that two cylinders of like power and axis, but of opposite sign, will cancel each other out. Thus, when -2.00 X 90 is added to +2.00 X 90, the result is plano. But if these same two cylinders are separated by a small angle, the resultant axis is not midway between the two, but at an entirely new axis.* For example, if an eye requires a lens correction of +2.00 X 90, that eye may be thought of as a lens system having a built-in cylinder power of -2.00 X 90. Now, if before that “eye-lens” system of -2.00 X 90, we place the correcting +2.00 cylinder at axis 80, a new plus cylinder is created at axis 40, and a torsional effort will be required in an attempt to manage the new induced cylinder axis. So, the term “unpredictable” is something of an understatement. *For a more detailed discussion of this problem, read: Rubin M. Optics for Clinicians, Gainesville, Triad, 1974, pp 180-181.
PERSPECTIVES IN REFRACTION
Finally, there are a variety of ways to manage the problem and since the goal of all refraction is “patient comfort,” (a very subjective yardstick for measuring therapeutic triumphs) the same corrective measures will not necessarily benefit each patient. First of all, if the finding is incidental to the examination and the patient is symptom-free, nothing should be done to change the status quo. If the symptoms are limited to near-vision activities, the axes should conform closely to those found in near point testing, as was done for Mrs. R. If the axis disparity between distance and near is formidable, separate reading glasses may be necessary. Occasionally, a presbyope will be well served by using only one bifocal add. Most often, however, attempts are made to
349
resolve the problem by reducing the power of the cylinder. In such an endeavor, the spherical equivalent must remain unchanged, and the patient may be willing to accept some reduction in visual acuity as the price of a reduction in symptoms when reading. Just how much decrease in cylinder power will be required, in any given case, to achieve comfort without sacrificing too much sharpness of vision requires experience - and a smile from Lady Luck.
Reprint requests should be addressed to Benjamin Milder, M.D., Department of Ophthalmology, Washington University School of Medicine, 660 South Euclid Avenue, St. Louis, MO. 63110.
