Ординатура / Офтальмология / Английские материалы / Relearning To See_Quackenbush_2000

WHAT D O T H E XX/YY T O P A N D B O T T O M

N U M B E R S R E F E R T O ?

One way to think about the xx/yy numbers on an eyeglass prescription or an eye chart is to consider xx to be the number of feet a per­ son with xx/yy vision is standing from the eye chart, while yy is the number of feet a person with 20/20 vision is standing from the same chart. Both people are looking at the same xx/yy line.

For example, a person with 20/200 sight would need to stand at twenty feet to see the same з У г " letters a person with 20/20 sight could see at 200 feet. A person with 20/40 sight would need to stand at twenty feet to see the same и/гб" letters a person with 20/20 sight could see at 40 feet.

Letters smaller than гАп on the eye chart correspond to better than 20/20 sight. For example, a person with 20/15 sight can read at twenty feet the same Ул" letters a person with 20/20 sight would need to stand at 15 feet to see. Reading the Мб" letters at twenty feet is 20/10 vision; %г" letters is 20/5 vision, which is four times better than "normal" 20/20 sight. A person with 20/5 vision can see at twenty feet what a person with 20/20 sight would need to stand at 5 feet to see!

Another way to think of the xx/yy num­ bers is to consider the ratio: 20/20 is 1/1, or "normal" sight. 20/200 sight is Moth of 20/20 sight. 20/10 is two times better than normal sight.

How can a person see better than 20/20 if the cones in the fovea pick up the Vs" letters? The cones in the fovea are not distributed evenly. As the center of the fovea is approached, the density (cones/mm2) increases to it maximum density. People with superior vision have their attention in a very tiny central area—a key natural vision prin­

ciple called centralization. This will be explained further in Chapter 17, "The Retina."

People are typically given corrective lenses that bring the eyes back to "normal" 20/20 sight, or better. The more the refractive error, the more diopters of correction are needed to let that person see 20/20 again. For exam­ ple, a person with 20/40 uncorrected vision might need only - 1 D to read the 20/20 line, but a person with 20/200 might need -4.00 D to read the same 20/20 line.

Figure 3-6: Table for Distance Eye Chart

Bates offers the following suggestion to those with nearsightedness in Perfect Sight Without

Glasses:

It will be well... to have two... cards, one to be used at the near point, where it can be seen best, and the other at ten or twenty feet. The student will find it a great help to shift from the near card to the distant one, as the unconscious memory of the letters seen at the near point helps to bring out those seen at the distance.

The opposite approach can be used by those with farsightedness. Both approaches can be used by those with astigmatism.

P A R T O N E : F U N D A M E N T A L S

There are identical Distance and Near Eye Charts in Appendix E

DRIVING REQUIREMENTS

20/40 sight is the driving requirement for eye­ sight in most states. Although 20/40 is "half" of normal sight, this is still quite functional sight. Your eye doctor can tell you the vision requirement for your state.

BATES AND THE SNELLEN EYE CHART

Here Bates discusses the limitations of test­ ing vision with the subjective Snellen eye chart, and the advantages of objective simul­ taneous retinoscopy.

Quoting from Perfect Sight Without

Glasses:

Much of my information about the eyes has been obtained by means of simultaneous retinoscopy. The retinoscope is an instrument used to measure the refraction of the eye....

This exceedingly useful instrument has possibilities which have not been generally realized by the medical profession. Most ophthalmologists depend upon the Snel­ len3 card, supplemented by trial lenses, to determine whether the vision is normal or not, and to determine the degree of any abnormality that may exist. This is a slow, awkward, and unreliable method of test­ ing the vision, and absolutely unavailable for the study of the refraction of the lower animals, of infants, and of adult human beings under the conditions of life.

» Herman Snellen (1835-1908). Celebrated Dutch ophthalmologist, professor of ophthalmology in the University of Utrecht, and director of the Netherlandic Eye HospitalЛпе present standards of visual acuity were proposed by him, and his test types became the model for those now in use.

The Snellen card and trial lenses can be used only under certain favorable condi­ tions, but the retinoscope can be used any­ where. It is a little easier to use it in a dim light than in a bright one, but it may be used in any light It may also be used under many other unfavorable conditions.

It takes a considerable time, varying from minutes to hours, to measure the refraction with the Snellen card and trial lenses. With the retinoscope, however, it can be deter­ mined in a fraction of a second. By the for­ mer method it would be impossible, for instance, to get any information about the refraction of a baseball player at the moment he swings for the ball, at the moment he strikes it, and at the moment after he strikes it. But with the retinoscope it is quite easy to determine whether his vision is normal, or whether he is myopic, hypermetropic, or astigmatic, when he does these things; and if any errors of refraction are noted, one can guess their degree pretty accurately by the rapidity of the movement of the shadow.

With the Snellen card and trial lenses con­ clusions must be drawn from the persons statements as to what he sees; but the person often becomes so worried and confused dur­ ing the examination that he does not know what he sees, or whether different glasses make his sight better or worse; and, more­ over, visual acuity is not reliable evidence of the state of the refraction. One person with 2 D of myopia may see twice as much as another with the same error of refraction. The evidence of the Snellen card is, in fact, entirely subjective; that of the retinoscope is entirely objective, depending in no way upon the statements of the person.

In short, while the testing of the refrac­ tion by means of the Snellen card and trial lenses requires considerable time, and can be done only under certain artificial con­ ditions, with results that are not always reli-

able, the retinoscope can be used under all sorts of normal and abnormal conditions on the eyes both of human beings and the lower animals; and the results, when it is used properly, can always be depended upon. This means that it must not be brought nearer to the eye than six feet; otherwise the subject will be made nervous, the refraction, for reasons which will be explained later, will be changed, and no reliable observations will be possible. In the case of animals it is often necessary to use it at a much greater distance.

N E A R "20/20" V I S I O N

A different chart is used by eye doctors to test the near vision of farsights. The Near Eye Chart has different sizes of type and is held 14 inches away.

When a person can read the line consisting of 3-point type letters, she has normal 20/20 near vision. Reading 7-point type is 20/40 near vision, half of normal near vision. If the small letters cannot be read, the eye doctor may prescribe converging lenses, "magriifiers," to focus near objects onto the retina.

There is a Near Eye Chart in Appendix F, "Eye Charts." There are also samples of small print in Chapter 22, "Reading—For All Ages."

O V E R - C O R R E C T I O N : A S T R A I N ;

U N D E R - C O R R E C T I O N : A R E L I E F

Prescription glasses are sometimes "over-cor- rected" to give sharper acuity than 20/20— for example, 20/10 correction. Unfortunately, 20/10 lenses create a greater strain on the eyes than 20/20 lenses.

Glasses can be "under-corrected" to give functional but not 20/20 sight—for example, 20/30 or 20/40 correction. Some eye doctors intuitively under-correct prescriptions to prevent 20/20 glasses from straining a person's eyes.

Students improving vision often choose to get glasses that are weaker than 20/20. Overand under-corrections are discussed further in the next two chapters.

UNDERSTANDING PRESCRIPTIONS

NOTE: Prescriptions are always obtained from an eye doctor. Bates teachers do not prescribe, diagnose, treat, or determine function.

Many people who wear glasses have seen numbers associated with their glasses. The numerical description of the lenses made for glasses or contact lenses by an eye doctor is called a prescription.

Figure 3-8 is a theoretical sample single lense (non-bifocal) prescription for nearsightedness and astigmatism in both eyes, and strabismus (crossed eye) in the right eye.

In nearsightedness the eye is too long; in farsightedness the eye is too short. In both cases, the eye is not spherical when viewed from the side. Diverging and converging

lenses compensate for the nearsighted and farsighted eye's "non-spherical" shape, respec­ tively. This correction is indicated in the "Spherical" column of a prescription. Spher­ ical corrections for nearsightedness and far­ sightedness are measured in diopters.

Unlike the nearsighted and farsighted eye, the astigmatic eye is oval (not spherical) from the front point of view. The amount of cor­ rection for astigmatism is indicated in the "Cylindrical" column, in diopters. The angle of the astigmatism correction is indicated in the "Axis" column, in degrees. The axis is sometimes preceded by an "X"; a degree sym­ bol is usually not written.

In strabismus, an eye turns away from the point of interest. The amount of correction for strabismus is indicated in the "Prism" col­ umn. The direction of the correction is indi­ cated in the "Base" column.

"O. D." stands for oculus dexter, which means "right eye." "O. S." stands for oculus sinister, which means "left eye."

The top two rows indicate corrections for "Distance" vision. Distance vision correction is commonly given to nearsights. Distance vision correction may also be given to farsights if their distance vision becomes suffi­ ciently blurred.

The bottom two rows are for "Near" vision correction, commonly given to farsights.

In bifocals, the bottom two rows indicate a correction that has been "Added" to the Distance/Spherical diopters (in the top two rows) to create the prescription in the bot­ tom part of bifocals. For nearsights, the cor­ rection on the bottom part of the bifocal is less than the distance correction. For farsights, the correction on the bottom part is greater than the distance correction.

Sometimes the correction given by the pre­ scription, e.g. 20/20 or 20/40, is written on the prescription. The uncorrected visual acuity may also be indicated, e.g. "WOG 20/400"; "WOG" means without glasses

PRESCRIPTIONS FOR NEARSIGHTEDNESS

Refer to Figure 3-8: Theoretical Sample Sin­ gle Lense Prescription for Nearsightedness, Astigmatism, and Crossed Eye.

A prescription for nearsightedness has a negative (-) number in the Spherical column. Usually this is the number of diopters needed to correct the nearsighted eye back to 20/20 normal sight. If less diopters are given, the sight can be corrected to less than 20/20, e.g. 20/40.

In our example, the Distance vision has a

correction of - 7 . 2 5 D for the right eye, and -6.75 D for the left eye. This is not a bifocal prescription because there is no Near/Add diopter measurement in the bottom two rows.

Sometimes the prescription is written with "DS" after the number of diopters, e.g. - 7 . 2 5 DS, or -7.25 D. S. DS means diopters of spherical correction. The plus or minus sign in front of the number of diopters indicates whether the correction is for nearsightedness or farsightedness.

Nearsights can also be given bifocals. In a bifocal prescription, there might be two +1.25 D Near/Add numbers in the bottom two rows. The bottom part of this bifocal would then be -6.00 D (-7.25 D "Add" +1.25 D) for the right eye, and - 5 . 5 0 D ( - 6 . 75 D "Add" +1.25 D) for the left eye. Notice that the plus and minus signs of the diopters are taken into account when adding diopters in the Spherical column. The amount of nearsighted correction is less on the bottom part of the bifocal because the nearsighted eye does not need the full distance correction to see clearly up close. (In fact, there is usually a point up close where no correction is needed to see clearly up close.) Though the bottom part of a nearsighted bifocal prescription is reduced in power, a person could still have 20/20 or sharper near vision with this lower correction.

As your vision changes, a prescription adjusted originally for 20/20 correction would no longer correct a person to 20/20. When the vision improves, the prescription would be sharper than 20/20, i.e., too strong. When the vision lowers, the prescription would be less sharp than 20/20.

D I O P T E R S A N D 20/xx D I S T A N C E

N U M B E R S : A L O O S E C O R R E L A T I O N

Many students ask about the relationship between the number of diopters in their prescriptions and their acuity, e.g. 20/20,20/40,

20/200, etc.

There is no absolute correlation. The number of diopters in a prescription relates to the refractive error of the eyeball. As Bates stated above, the Snellen eye chart test is a subjective test and can vary from individual to individual. Squinting can change acuity quickly and dramatically. One person with -3.00 D of nearsightedness may read the 20/100 line, while another person with -3.00 D may only be able to read the 20/200 line.

That being said, there is a loose correlation between diopters and 20/xx numbers. At approximately -1.00 D of nearsightedness, a person might have difficulty passing a "20/40" driver's test. -2.00 D might be 20/80;-3.00 D

may be between 20/100 to 20/200. -5.00 D

may be around 20/500 sight.

Beyond -3.00 D distant objects usually appear very blurry. When improving vision, the most pronounced experiences of sharper vision occur from -3.00 D to zero D.

P R E S C R I P T I O N S F O R A S T I G M A T I S M

Figure 3-8: Theoretical Sample Single Lense

Prescription for Nearsightedness, Astigma- tism, and Crossed Eye shows a prescription with -1.25 D of astigmatism correction for the right eye. The angle of the astigmatism (axis) is 170 degrees. The left eye has a correction of - 1 . 75 D at an axis of 145 degrees.

Trying to correct for astigmatism can be a problem because not only can the magnitude (diopters) change (as with nearsighted-

P A R T O N E : F U N D A M E N T A L S

ness and farsightedness), but the angle (axis) can change as well. If either one changes, the original correction will be incorrect and can create a strain.

PRESCRIPTIONS FOR STRABISMUS

(CROSSED EYE, WALL EYE, ETC.)

Figure 3-8 also shows a correction for a crossed (inward turning) right eye.

1Д, called "one prism," in the Prism column is the amount of the prism correction. "OUT" in the Base column indicates this prism is cor­ recting for a right eye that turns inward. (The direction of the correction is the opposite of the direction the eye is turning.)

In terms of eyestrain, one might equate r4 with 1D, 2Д with 2 D, and so on.

PRESCRIPTIONS FOR FARSIGHTEDNESS

In Figure 3-9, we see that a theoretical sam­ ple prescription for farsightedness has a pos­ itive number in the Spherical column. This is the number of diopters needed to correct the farsighted eye, usually back to 20/20 near vision in a single lense prescription.

Since this is a bifocal prescription, there are two parts: the "Distance" and the "Near/Add."

The top part of this bifocal has a +1.50 D correction for the right eye and a +1.75 D cor­ rection for the left eye. Both of these correc­ tions are for distance vision.

The bottom part of a bifocal is for near vision. In this prescription there is an "Add" of +1.25 D for each eye. The "Add" diopters are "added" to the diopters in the top (Dis­ tance) part of the bifocal. In this example, the right eye near prescription is +2.75 D (+1.50

D "Add" +1.25 D). The bottom part of a farsighted bifocal is stronger than the top part, because a farsight's near vision is more blurred than the far vision. A farsight with this pre­ scription might have difficulty passing the dri­ ver's vision test without corrective lenses.

There is also -1.25 D of astigmatism cor­ rection in each eye. Sometimes astigmatism correction is indicated by DC, or D. C, eg., -1.25 DC. DC means diopters of cylindrical correction. The axis in the right eye is 4O°;the axis in the left eye is 95 °. Remember, since the axis is not a measure of the magnitude of

the astigmatism, 950 is not more blurred than

400.

At about +1.00 D of farsightedness, a per­ son often begins to need glasses for reading small print, especially in dim light. However, such a person might be able to pass the dri­ ver's vision test without glasses

Many people obtain "magnifiers" at the "5&10" store when their near vision first becomes blurry. Often the number of diopters of farsighted correction is written on the inside of one of the frame's temples. 5&10 glasses do not have astigmatism correction. Also, a prescription from an eye doctor is not needed to buy 5&10 "readers."

MIXED PRESCRIPTIONS

The total correction for one eye is the sum of the absolute values (magnitudes) of the nearsighted (or farsighted) diopters, the astigmatism diopters, and the prism base. The plus or rninus signs in front of the cylindrical diopters are ignored when adding diopters to determine the total correction in one eye. For example, if a person has -3.00 D of near-

sightedness and +2.00 D of astigmatism for distance correction in the left eye, the total distance correction in the left eye is 5.00 D (3.00 D plus 2.00 D).

Interestingly, the sum of the spherical and cylindrical diopters for the right eye is often the same as, or close to, the sum in the left eye. For example, a person may have -3.00 D spherical/+i.oo D cylindrical correction in the right eye, and -2.25 spherical/+i.75 cylindrical in the left eye. In this case, the total correction in each eye is 4.00 D.

INADEQUATE PRESCRIPTIONS

If there is too much blur, or if there are pathologies involved, like cataracts, lenses may not be able to correct a person to 20/20 or even usable vision.

I am weary of glasses—

/ have worn them so long, I wonder as time passes

Will my eyes—ever be strong? Time was when I

Could read in dim light; Now even with glasses

That light must be bright....

Too many are wearing glasses, They put them on too soon;

For looks, for sight, for pain and strain, They wear them night and noon.

I've done my best to tell the rest That of glasses I would beware;

You'll find it's true, I'm telling you Glasses will not get you there.1

—Joseph J. Kennebeck, optometrist

G L A S S E S A N D C O N T A C T L E N S E S : A R T I F I C I A L S O L U T I O N S T O B L U R

Rather than identifying the cause of their initial blur and reversing it, most people obtain glasses or contact lenses. Even stronger "crutches" are used when vision becomes worse. "Strong" glasses are those that correct one's eyes to 20/20 vision or stronger, for near or far vision.

B A T E S : " W H A T G L A S S E S D O T O U S "

Contact lenses were not generally available when Bates taught his students how to improve their vision. Even though he limited his discussion to the problems with eyeglasses, many of these problems apply to strong contact lenses.

The opinions of Bates regarding the strain caused by wearing strong glasses are now being echoed by more and more eye doctors, including ophthalmologists.

From Perfect Sight Without Glasses:

On a tomb in the Church of Santa Maria Maggiore in Florence was found an inscription which read: "Here lies Salvino degli Armati, Inventor of Spectacles. May God pardon him his sins." Nuova Enciclopedia Italiana, Sixth Edition 2

WHAT GLASSES DO TO US

The Florentines were doubtless mistaken in supposing that their fellow citizen... was the inventor of the lenses now so commonly worn to correct errors of refraction. There has been much discussion as to the origin of these devices, but they are generally believed to have been known at a

P A R T O N E : F U N D A M E N T A L S

period much earlier than that of Salvino degli Armati. The Romans at least must have known something of the art of supplementing the powers of the eye, for Pliny tells us that Nero used to watch the games in the Colosseum through a concave gem set in a ring for that purpose. If, however, his contemporaries believed that Salvino of the Armati was the first to produce these aids to vision, they might well pray for the pardon of his sins; for while it is true that eyeglasses have brought to some people improved vision and relief from pain and discomfort, they have been to others simply an added torture, they always do more or less harm, and at their best they never improve the vision to normal.

The "relief from pain and discomfort" provided by glasses is not necessarily beneficial in the long term. A drag used to relie ve stomach pain may cause serious problems if used continuously. A drug does not necessarily remove the cause of the stomach ache. A drug or mechanical crutch may even lead a person to ignore the real cause of a problem.

Continuing with Bates' words:

That glasses cannot improve the sight to normal can be very simply demonstrated by looking at any color through a strong convex or concave glass. It will be noted that the color is always less intense than when seen with the naked eye; and since the perception of form depends upon the perception of color, it follows that both color and form must be less distinctly seen with glasses than without them. Even plane glass lowers the vision both for color and form, as everyone knows who has ever looked out of a window.

... That glasses must injure the eye is evident from the facts given in the preced-

ing chapter. One cannot see through them unless one produces the degree of refractive error that they are designed to correct. But refractive errors, in the eye which is left to itself, are never constant. If one secures good vision by the aid of concave, or convex, or astigmatic lenses, therefore, it means that one is maintaining constantly a degree of refractive error which otherwise would not be maintained constantly. It is only to be expected that this should make the condition worse, and it is a matter of common experience that it does. After people once begin to wear glasses their strength, in most cases, has to be steadily increased in order to maintain the degree of visual acuity secured by the aid of the first pair. Persons with presbyopia who put on glasses because they cannot read fine print too often find that after they have worn them for a time they cannot, without their aid, read the larger print that was perfectly plain to them before. A person with myopia of 20/70 who puts on glasses giving him a vision of 20/20 may find that in a week's time his unaided vision has declined to 20/200, and we have the testimony of Dr. Sidler-Huguenin, of Zurich,8 that of the thousands of myopes [under his care] the majority grew steadily worse, in spite of all the skill he could apply to the fitting of glasses for them. When people break their glasses and go without them for a week or two, they frequently observe that their sight has improved. As a matter of fact the sight always improves, to a greater or lesser degree, when glasses are discarded, although the fact may not always be noted.

Archiv t Augenh., vol. lxxix, 1915, translated in

Arch. Ophth., vol. xlv, No. 6,1916.

A person's sight would not necessarily improve if a person practiced worse vision habits—especially straining to see—than the ones he had before discarding glasses. This is why releaming relaxed vision habits is essential for improving eyesight. Still, many people have found that their vision improves by wearing glasses less.

Continuing from Perfect Sight Without

Glasses:

That the human eye resents glasses is a fact which no one would attempt to deny. Every oculist knows that people have to "get used" to them, and that sometimes they never succeed in doing so. People with high degrees of myopia and hypermetropia have great difficulty in accustoming themselves to the full correction, and often are never able to do so. The strong concave glasses required by myopes of high degree make all objects seem much smaller than they really are, while convex glasses enlarge them These are unpleasantnesses that cannot be overcome. People with high degrees of astigmatism suffer some very disagreeable sensations when they first put on glasses, for which reason they are warned by one of the "Conservation of Vision" leaflets published by the Council on Health and Public Instruction of the American Medical Association to "get used to them at home before venturing where a misstep might cause a serious accident."8 Usually these difficulties are overcome, but often they are not, and it sometimes happens that those who get on fairly well with their glasses in the daytime never succeed in getting used to them at night.

All glasses contract the field of vision to a greater or lesser degree. Even with very

a Lancaster: Wearing Glasses, p. 15.

weak glasses people are unable to see distinctly unless they look through the center of the lenses, with the frames at right angles to the line of vision; and not only is their vision lowered if they fail to do this, but annoying nervous symptoms, such as dizziness and headache, are sometimes produced. Therefore they are unable to turn their eyes freely in different directions. It is true that glasses are now ground in such a way that it is theoretically possible to look through them at any angle, but practically they seldom accomplish the desired result.

The difficulty of keeping the glass clear is one of the minor discomforts of glasses, but nevertheless a most annoying one. On damp and rainy days the atmosphere clouds them. On hot days the perspiration from the body may have a similar effect. On cold days they are often clouded by the moisture of the breath. Every day they are so subject to contamination by dust and moisture and the touch of the fingers incident to unavoidable handling that it is seldom they afford an absolutely unobstructed view of the objects regarded.

Reflections of strong light from eyeglasses are often very annoying, and in the street may be very dangerous.

Soldiers, sailors, athletes, workmen, and children have great difficulty with glasses because of the activity of their fives, which not only leads to the breaking of the lenses, but often throws them out of focus, particularly in the case of eyeglasses worn for astigmatism.

The fact that glasses are very disfiguring may seem a matter unworthy of consideration in a medical publication; but mental discomfort does not improve either the general health or the vision, and while we have gone so far toward making a virtue of what we conceive to be necessity that some of us have actually come to consider