CHAPTER XIX HeEapACcHE, NEURALGIA, AND EYESTRAIN (Part 2)

upon it in daily life. As a rule we may accept the standard of 20/20 as the minimum of perfect vision, ascertain the best possible in each individual case, and if it cannot be brought up to this minimum it is incumbent upon us to learn the reason why. We call to our aid the ophthalmometer to determine whether the curvature of the cornea is normal, the ophthalmoscope and the retinoscope to measure the refraction objectively in the different meridians of the eye, yet the case of test lenses must remain the court of last resort until some method is devised by which we can compute objectively the normal activity of the ciliary muscle in the individual. . Ophthalmic literature is full of instructions how to use these instruments for the measurement of refraction, each writer AAA DIAGNOSIS FROM OCULAR SYMPTOMS advocating a method that seems to him to give better results than any other. Some employ cycloplegics as a routine measure, others use them only in selected cases, and it is probable that each method is the one best suited to the person who advocates it; that is, by its means he is enabled to make fewer mistakes than he would if he used any other. Yet none of these methods are perfect. very eye surgeon knows that he has been successful in giving comfort to a large majority of his patients, and that he has failed to do so in a minority, but he has no data from which he is able to form even a reasonable conjecture how many of his failures were due to a wrong measurement of the refractive error, and how many to a wrong diagnosis in ascribing the symptoms to the refractive error, because most of those patients went elsewhere for relief. He only knows from his successes where others have failed that no one always measures refractive errors rightly. Without entering into the controversy as to the merits and demerits of the various ways of determining the refraction of the eye, it will suffice to say that the fundamental prin- ciples are to be obtained from any one of a great number of text- books, some devoted wholly to this subject, and that it is best to adopt some one method with a view to becoming as nearly perfect as possible in its application, for in that way we shall make the fewest mistakes. We shall examine very few eyes in which we are not able to find an error of refraction, but the positive demonstration of such an error is not sufficient. One of the most difficult things for us to appreciate is the widely varying effects produced by the same error of refraction upon different individuals. In the great majority it produces no effect of which the person is conscious, unless it is so great as to impair the vision. Even when it does cause trouble the eyes are so very tolerant that they are made perfectly comfort- able by an approximate correction, and remain so when the mount- ings are bent, when a spherical lens is tilted so as to produce an astigmatic effect, or when the axis of a cylindrical is altered five, ten, or even fifteen degrees. Persons with such tolerant eyes are easily satisfied, as might naturally be expected. A comparatively small number of eyes are intolerant to even a very slight error of refraction, and to a slight variation of the correcting lens from its proper position. These demand accuracy not only in the measure- ment of the refractive error, but also in the adjustment of the cor- recting lenses, which should be so placed in a plane perpendicular HEADACHE, NEURALGIA, AND EYESTRAIN MAS to the line of vision that each eye looks through the optical center of its own lens, while, if the correction is cylindrical, the axis is exactly right. Faults in which the optical center does not coincide with the midpoint, in which the visual lines pass to one side of, above, or below the optical center, and in which the axes vary some degrees from their correct position, are very common in glasses as they come from the opticians, who have learned by experience that slight faults of this nature are not noticed by the great majority of people, and see no profit to themselves from the expenditure of the time and money necessary to secure accuracy. <A certain propor- tion of our failures to give relief is to be ascribed to such faults as these, so we need to investigate each pair of glasses furnished as an essential for success not only in treatment, but also in diagnosis. After we have measured the refractive error we can obtain positive evidence that we have discovered the source of trouble in many pa- tients, though not in all, by having them wear the properly adjusted correction for a short time. If the symptoms of eyestrain are com- pletely, or even partially relieved, we have reason to feel confident that the corrected error is at least one of the sources of the trouble, while if no relief is felt we are guided to search elsewhere. When the patient is made comfortable by the test lenses, but the same comfort is not secured by the glasses provided by the optician, the probability is that the latter are at fault either in the situations of the optical centers, in the positions of the axes, or in the adjustment of the lenses before the eyes. When no relief is given by the test lenses the probability is that the source of trouble is elsewhere, but sometimes we eventually find the refractive error to be if not the primary, at least a contributory cause. At this point mention may be made of two widely prevalent erroneous ideas that we should do our best to controvert. We meet with many people who delay and avoid the wearing of glasses at the expense of much trouble, annoyance, and sometimes injury to the eyes, because they have the idea that by so doing they will keep these organs in a better condition than they would be in if glasses were worn continually. The bugbear of becoming so habituated to their glasses that they cannot do without them is a constant source of worry. These people need to be assured that the constant strain upon their eyes when they refuse to wear a needed correction tends to weaken rather than to strengthen these organs; that no one can become habituated to glasses that they do not need so as to make 446 DIAGNOSIS FROM OCULAR SYMPTOMS their aid a necessity, and that anyone can at any time cease to wear his needed. glasses if he will do what they are doing—accept the discomfort and trouble from which they are freed by such aid. An- other class, composed mainly of young ladies, decline to wear glasses because they think the latter do not accord with their own particular styles of beauty. They accept present discomfort and possible dan- ger in the future with a cheerfulness we envy, and welcome the sug- gestions of the former class that they are conserving their eyes by not following directions. We can do little for these beyond present- ing the facts as plainly as possible. The other erroneous idea is that the wearing of glasses when they are not needed tends to conserve the eyes. Parents sometimes bring children who are in perfect health and present no symptoms whatever, to have their eyes ex- amined, in order to forestall future trouble. Almost invariably it is possible to prescribe glasses that can be worn, but the wearing of them is useless when they neither alleviate symptoms, nor improve vision; they neither conserve the vision, nor protect the eyes from ill, except that in some cases they may help to guard against the de- velopment of school myopia. Presbyopia When a patient 45 or more years old tells us of an inability to use his eyes for reading, sewing, or other fine close work, we make a positive diagnosis of presbyopia, because we know that the near point recedes with the advance of age, and that usually in the fifth decade of life it gets so far away as to render near work impracti- cable. This is a purely physiological condition, but the claim of some writers that it never gives rise to symptoms of eyestrain is much too broad. It is true that in most cases of uncomplicated presbyopia an approximate correction is satisfactory to the person, who is quite as well pleased with the results obtained by picking out glasses for himself, or by having them chosen by an optician, as by having them fitted accurately through a scientific examination, but occasionally the eyes rebel. An example of this nature was furnished by a gentleman 46 years old, who complained that the ++ 1 D lenses which he wore for reading strained his eyes unbearably, and was given perfect comfort by a pair of + 1.25. Presbyopes who complain of eyestrain are quite apt to have other refractive errors, or some form of muscular imbalance, so the examination of the eyes should HEADACHE, NEURALGIA, AND EYESTRAIN 4AT always be thorough, but we must beware of certainty in ascribing the symptoms to these faults. If the correction of the presbyopia gives complete relief to the symptoms the diagnosis of eyestrain from presbyopia is positive, no matter what other abnormalities may be present, and it is worth noting in this connection that the simul- taneous correction. of an astigmatism, which has never given rise to symptoms, may plunge an elderly patient into trouble that can be allayed only by the removal of the cylindrical correction. The rule is that presbyopia becomes manifest about the age of forty-five, and that its presence then can be demonstrated after the correction of a hypermetropia or a myopia, just the same as in emmetropia, although it often seems to appear earlier in hyper- metropia, and sometimes not at all in myopia, when these conditions are uncorrected. The reason of the last is that the far point of the eye is at about the reading distance. The cause is supposed to be a sclerosis of the lens, which begins in early life and about this age reaches a point at which it has deprived the lens of so much of its ability to change its shape readily with the contraction of the ciliary muscle as to reduce the range of accommodation enough to make near work difficult. The possible exceptions to be met with are too few and too uncertain to invalidate this rule in the least. Cases occur in which the accommodation is sluggish in its action, so that the symptoms appear sooner than we expect them, and I have seen two cases in which only a single diopter of correction was needed at sixty-seven and sixty-eight, although both patients assured me that they had always enjoyed sharp vision for distance, had never needed glasses before, and the refraction in both was slightly hyperme- tropic. This history and the low degree of presbyopia together seemed to indicate a late onset of the latter, though the possibility of a former slight myopia could not be excluded objectively. It is by no means unusual for the presbyopia to be unequal in the two’ eyes, but whether this inequality is to be referred to differences in the lenses or in the ciliary muscles is unknown. Hypermetropia When a person has equally as good vision with a convex glass as without it, and when his vision is improved by such a glass, he has hypermetropia. If he suffers from eyestrain this may or may not be the cause, and our problem then is not simply to measure the de- 448 DIAGNOSIS FROM OCULAR SYMPTOMS gree of the hypermetropia, but to ascertain whether it is associated with an abnormal strain upon the accommodation that is respon- sible for the suffering. We must not assume that hypermetropia is of itself abnormal, even though we commonly speak of it as an error of refraction, nor that the tonic contraction of the ciliary muscle which accompanies it is physiologically wrong. We are obliged to concede both to be phy- siological rather than pathological when we take into account the development of children’s eyes, and the fact that the great majority of the people who have never had any reason to think that there was anything the matter with their eyes have a certain degree of hyper- metropia. With few if any exceptions babies are born with eyes that are too short for convenient use in the seeing of either near or dis- tant objects; these eyes lengthen, rather rapidly at first, then more and more slowly as the resisting power of the sclera increases, until the process comes to a standstill. If this process of lengthening. ceases when the eye is just long enough to allow parallel rays of light that fall upon the cornea to come to a focal point on the fovea without any assistance on the part of the ciliary muscle, the condi- tion is one of emmetropia, which is commonly accepted as the nor- mal, or at least as the ideal refraction of the eye. Perfect emme- tropia is very far from common. If the eye has become lengthened beyond this point, so that parallel rays cross before they reach the retina, a condition of axial myopia has been produced, over which the ciliary muscle has no control whatever. If the cessation of the process occurred before the eye had attained its ideal length, the parallel rays of light that enter can be focussed on the fovea only through the active intervention of the ciliary muscle, if at all, be- cause they tend to meet behind the retina, and the condition is that of hypermetropia. The presence of good vision in a hypermetropic eye when it is looking at a distant object proves that the ciliary muscle is in a state of activity which we call its tone, or tonic con- traction. This tone differs in no way from the normal tension to be observed in all healthy tissues, is called by the same name, and should not be looked upon as abnormal. Although theoretically we should find the best vision in eyes that are emmetropic, my observation has been that the greatest acuteness of vision is to be met with in eyes that are slightly hypermetropic with a normal tone of the ciliary muscle, and the best explanation of this that has occurred to me is that possibly the constant call to action on the part of the latter HEADACHE, NEURALGIA, AND EYESTRAIN 449 enables it to make a finer adjustment of the accommodation than is made when its relaxation is complete at times. It does not seem right to speak of eyes as faulty that have perfect vision, give their owners no trouble, and are in a condition which is known to be physiologically normal in all eyes during early childhood and remains in most of them throughout life. At the same time the condition is one that gives rise to eyestrain occasionally through an abnormal variation of the activity of the ciliary muscle from its normal tone. We divide hypermetropia into the manifest, or the portion repre- sented by the strongest convex lens through which the eye retains perfectly distinct distant vision while the accommodation is intact, and the latent, which is hidden by the contraction of the ciliary mus- cle and is revealed when this muscle is paralyzed, either by disease, traumatism, or drugs. The manifest and the latent together con- stitute the total hypermetropia. The ratio of manifest to latent hypermetropia varies not only with the amount of the total; but also with the age of the patient, and with the strength or weakness of the general muscular tone throughout the body, so it cannot be stated in any definite figures which are applicable to individuals. It is generally true in a person who enjoys ordinarily good health and strength and has a moderate degree of hypermetropia, that the latter is almost wholly latent in early childhood, that the manifest steadily increases at the expense of the latent until the two are about equal at the age of twenty-five, and that the latent has nearly disappeared at forty. About the last age it happens very often that a patient com- plains of difficulty in reading, writing, or other near work, some- times that his distant vision also has become more or less indistinct, but of no other trouble, and then we almost invariably find that these symptoms are relieved perfectly by glasses through which his distant vision is perfect. The symptoms are exactly the same as those of presbyopia, but they are caused wholly by the hypermetropia. When the degree of hypermetropia is so great that the ciliary muscle is unequal to the task of focussing on the fovea the rays of light which enter the eye, the vision is subnormal, but can be brought up to or above the accepted standard by means of convex lenses. Such a de- gree of hypermetropia seldom induces symptoms of eyestrain, it simply renders distant vision indistinct, perhaps because the ciliary muscle refuses to attempt to maintain habitually a sufficient degree of contraction. As long as the normal tone of the ciliary muscie is sufficient to 450 DIAGNOSIS FROM OCULAR SYMPTOMS overcome easily the degree of hypermetropia present the person is absolutely unaware of any effort, and is inclined to boast of his ex- cellent eyes, but when the tone is modified so that the contraction of the muscle is less or greater than normal, symptoms of eyestrain are apt to appear. Possibly these symptoms are likely to be pro- duced when the normal tone of the ciliary muscle is habitually taxed to its utmost, but we are not sure of this. As age