Heterophorias and insufficiencies : a clinical study

Chapter I. General Considerations And Methods Of Testing

TH E class of cases to be considered here has been recognized as having had an existence since \^» the dawn of modern ophthalmology. Von Graefe attempted to solve the problems they presented, and : although he furnished some valuable ideas for his : successors to work upon, his knowledge upon the tf subject would be a poor equipment for an ophthal- .* mologist of to-day. Donders saw the fallacies of the work being done in this field, solved in general Jfc terms the problem of accommodative asthenopia,

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2 Hctcrophorias and Insufficiencies.

and found that " as soon as insufficiency of the internal or external recti muscles in binocular vision threatens to give rise to muscular asthenopia, it is of importance that the mutual distance of the glasses should not aggravate this but rather counteract it." If spherical glasses are insufficient, we are to combine with prisms or operate according to Von Graefe. There is agreement with Helmholtz's statement that displacement of glasses in a vertical direction causes more asthenopia than the same amount of lateral displacement. Except for the operative method indicated, the statements here made are entirely sound, but cannot be said to suffer from over-elaboration ; they seem, however, sufficient to more than cover what is usually done for muscular asthenopia to-day in practice upon the European continent. In England signs of interest began to be manifested regarding these cases over fifteen years ago, and thanks to the work of Maddox and others many principles of value were evolved. Previous to this, Stevens in America had succeeded in attracting attention by means of his work, his highly enthusiastic followers, and his bitter opponents. A portion of his nomenclature came into general use, and is here followed. Orthophoria, muscular balance ; heterophoria, tendency to deviation ; esophoria, convergence tendency ; exophoria, divergence tendency ; hyperphoria, upward tendency. These terms were taken to be equivalent to and supersede the old ones of insufficiency of the interni and externi, even Duane, whose masterly classification of these conditions has earned deserved recognition, taking this view ; yet heterophoria means merely a tendency of the fixation lines away from the object of fixation, while insufficiency means lack of power. The old terms for exophoria and esophoria were dynamical divergence and convergence. It is the intention of the writer to show that certain forms of heterophoria may be due to errors of refraction or other optical defects, others to habit, others to the nervous system, and others still to muscular spasm, excess, or insufficiency. Much has been written upon this subject in addition to that so briefly mentioned, some of it of value, and the writer proposes to treat all of the authors in the most impartial manner, as he appropriates their ideas whenever he can make use of them, without regard to their source. He has no instruments to exploit, no special method of treatment to push, no new general disease which he has cured through the eye muscles. If, in spite of this, an original idea or so should creep in among the others, anybody is welcome to use it as his own.

In discussing the question of correcting the refraction except as incidental to the treatment of muscular anomalies, there is of course no intention to belittle this very important consideration in the treatment of eye strain. Important as this branch of the subject undoubtedly is, correction of the refraction may be carried to the point of absurdity. When a patient with undoubted exophoria and insufficiency of the interni complains of severe headache, dizziness and nausea, excited or increased when convergence is attempted, it is perfectly silly to correct .12 or .25 D. of hypermetropia in such eyes, and yet many have done such things.

In studying the effect of correction of the refraction upon the ocular muscles, it is absolutely necessary to separate this secondary effect of glasses from the prismatic effect of a decentred lens. In order that the optical centre of a lens may be found, the lens should be held in front of a horizontal line in such a position that the line is continuous through the glass without deviation at either edge. The position of this line is then marked upon the glass and the process repeated at right angles to the first position ; the lines cross at the optical centre and show in a sphero-cylindrical lens the axis of the cylinder and the maximum curve at right angles to it. In a simple cylinder the axis only can be found, as there is no prismatic displacement of a line at right angles to the axis ; a cylinder with horizontal axis cannot be decentred horizontally, and similarly with the axis in other meridians. Two lines crossing at right angles may be used to find the optical centre instead of the above-described method, or the lens may be held in front of an object, as a candle flame, in such a position that the reflected images from the surfaces of the glass are superimposed, when the position of the images shows the optical centre. The last method may be used for rapid verification under certain circumstances, but I prefer the first for several reasons. The base and apex of a prism are marked by a line continuous through the glass, just as the axis of a cylinder is. When the cylinder is revolved upon its centre, we notice a peculiarity of prismatic action due to the varying curves, which results in torsion of lines oblique to the axis, being most marked at an inclination of 45°. While there are causes for the distortion of objects caused by glasses other than simple prismatic effect, yet this prismatic action of the transparent cylinder has not only an important bearing upon the asthenopia produced by cylindrical lenses, but, taken in connection with other prismatic effects of curved surfaces and applied to the human eye with its frequently decentred astigmatic refractive media, it throws an entirely new light upon many observations in physiological optics and opens up a new field in the explanation of muscular anomalies. A very plausible connection could be shown between apparent divergence and convergence, the decentred biconvex lens system between the anterior corneal surface and the posterior surface of the crystalline lens, and convergent and divergent squint ; its bearing upon the subject in hand will be shown later.

The primary position of a pair of glasses in front of the eyes when it is desired to study their effects upon the ocular muscles is with the optical centres in the lines of fixation. This position can be obtained with considerable accuracy by sighting ; that is, by making each examined eye fix the examiner's, when the optical centre of the glass is placed in the line which seems to correspond to the line of fixation of the former and the line of vision of the latter. If there is a question as to the correctness of this, discs with vertical stenopaeic slits may be placed in an adjustable frame, and the distance between their centres measured when both eyes can see through the slits at the same time. This distance between the optical centres (o. c.) should not be confounded with the distance between the geometrical centres of glasses to be worn (p. d.), which should come opposite the pupils for the sake of appearance and because there will be less annoyance to the patient from the refraction and reflection at the edges of the lenses when they are symmetrical with the pupil.

The prismatic effect of a decentred spherical lens is obtained by multiplying the distance between the optical and geometrical centres, taken in centimetres and tenths, by the dioptric strength of the lens.

Thus, a 2 D. lens decentred 3 mm. equals a .6 D. prism, 2. X -3 = .6. In a pair of glasses the difference between the primary and secondary position is taken. Thus, if the glasses are 60 mm. o. c., and the lines of fixation cut them 57 mm. apart, the prismatic action of a pair of 2 D. lenses is .6 D., one half in each eye. If the number of centimetres of decentring to produce a certain prism is desired, the formula is v-~ = cms., in tenths. The base of the prism is, of course, toward the periphery of a concave lens, toward the centre of a convex one. If the curve of the lens and the position of the line of fixation are kept in mind, or put on paper, there can be no confusion. In a cylindrical lens the number of dioptrics can be found in any direction by means of the lens measure and prismatic action then figured as for a sphere.

This easy method of figuring the prismatic action of a decentred lens is one of the merits of the prism dioptry system proposed by Mr. Charles Prentice, a layman, and adopted by many manufacturing opticians throughout the country on account of its simplicity and accuracy when applied to the manufacture of prisms. I began to use it soon after it was first suggested. The unit is the prismatic power which causes I cm. of light deviation at I metre. A lens of i D. refractive power decentred I cm. = I P. D. (A D. or D.), i.X i.= i. The best scale I know for measuring prisms in this system is that of Dr. Ziegler. I propose to speak of heterophorias in terms of prism dioptrics, just as we speak of hypermetropia and myopia in terms of the correcting glass. It is not necessary in either case to add dioptrics or D. to the figures in decimals which designate the strength of the glasses, and it will generally be omitted in the following pages.

There are a few things regarding the action of prisms necessary to remember in clinical work. Prisms beyond a certain strength can not be used in correcting anomalies of the ocular muscles for several reasons, some relating to the effect upon the muscular action, others to mental effect upon the patient, and yet others to distortion due to prismatic astigmatism from the varying ratio between the angles of incidence and refraction. There is a position of minimum deviation in which a ray of light passes symmetrically through a prism and other rays suffer greater deflection. In testing for an object in the direction of the line of fixation, prisms should be placed with the surface toward the eye perpendicular to that line, in order to secure as nearly as possible the position of minimum deviation. The position of the prism may assume some importance in the case of a vertical prism, as, if this is placed in the position of minimum deviation for distance, its action is stronger for near points, the increase amounting to about one-tenth of its strength for the nearest point at which the eyes are likely to be used.

As will be shown later, patients with inefficient ocular muscles do not bear prisms as well as those with strong muscular action ; stronger prisms may be worn base in for near work than base out for distance, and with axis horizontal than vertical, and a 3 D. prism over each eye is about the limit of strength under favorable conditions.

Secondary images due to reflection from the surfaces of the glass cause more annoyance in glasses containing prisms than in those without them, but the colors due to dispersion, diminution in size and change of shape of an object, occur to a considerable extent only when a strong prism is placed before one eye, as in testing muscular power, or at least are remarked upon by patients mostly under this condition.

The eye moves toward the apex of a prism in order to overcome the deviation of rays of light toward the base, and when it is unable to do this diplopia results, with the image from the eye before which the prism is placed in the direction of the apex. The action of the interni in overcoming prisms base out is called adduction, the externi with prisms base in, abduction. These terms are easily confounded, and the mind becomes so confused and fatigued by them that I propose to use instead the io Heterophorias and Insufficiencies.

terms convergence and divergence. This can cause no confusion as the amount is always given in terms of measurement. P. D's of convergence and divergence cannot possibly be confounded with metre-angles or degrees, and those who use the term degrees, except as applied to perimetric measurements, cannot be cast into any more confusion by the innovation than that under which they now are laboring.

There is one point upon which there is agreement among all investigators in this direction : it is that abduction or divergence for distance with well balanced and strong eye muscles should be 7. or 8- [These figures are the same whether the old degree system, dioptry, or centrad is used. It is beside the purpose of this treatise to argue regarding these systems and the new system in degrees of Landolt. Whatever its demerits, the dioptric system has been (unofficially) adopted.] The student who finds that his investigations seem to contradict this standard for divergence should doubt the accuracy of his observations ; with an abducting prism the double images may easily pass unnoticed when they first appear and then suddenly be observed a considerable distance apart at the time a stronger prism than represents divergence is being used. In such a case prisms of gradually decreasing strength should be used until single vision results, then gradually increased again. After some experience the examiner will begin with the prism which is to his mind most likely to represent the divergence to be expected from the examinations previously made of the refraction and muscular balance. Until this experience is acquired it is well to start with a prism of 6., 7., 8., or 9. It has already been said that divergence which varies from 7. or 8. may be considered abnormal. A certain number of cases show but 6., in which the only criticism is that divergence and convergence are neither of them at their best. In other words, amplitude of convergence is slightly below the standard, and there is no other defect. An occasional case may be found in which the amplitude of convergence is so great that divergence of 9. or even 10. is equalized by correspondingly strong convergence and muscular balance is good. Such cases are liable to develop exophoria later, but for the time being are sometimes to be complimented upon their strong ocular muscles. Cases like this are perhaps not so infrequent as would appear from our histories. Persons with ocular muscles of that sort would not be apt to be weak or neurasthenic, and would hardly be bothered by trivial errors of refraction.

When it is said that divergence for distant vision should be 7. or 8., what is really meant is that this is what should be shown by the candle test at a distance of about twenty feet, which is the usual test. It is in reality about I. more than the actual power of divergence for infinity, as I D. of convergence is necessary for binocular fixation at twenty feet. It is fortunate that there should be such a definite standard of divergence for distance, since the power of divergence for distant vision is an important matter ; divergence power for any near point theoretically would equal the amount required to bring the lines of fixation parallel, added to that which is still possible with parallel fixation lines when the eyes are adjusted for infinity. This total is obscured in making a test at a near point by the accommodative convergence and certain other factors, but while neither so definite nor important as divergence obtained for distance, it is a help in comparing the variations of the muscular action for near and far. At about thirteen inches (taken as the average reading distance) the prism test shows generally some 6 to 8 D's more of divergence than is shown by the test at twenty feet, the difference being less than this in cases of esophoria without insufficiency of convergence, greater in some cases of exophoria. Where divergence at thirteen inches is 18. or more, insufficiency of convergence power at that distance is almost sure to be present.

Convergence power for near work is of far more importance than it is for distance, which is also fortunate, as the prism test for convergence at a distance is very variable. This variability is due mostly to the much greater difficulty some persons have of disassociating convergence and accommodation than others. If the only precaution taken in making the test is that of dividing the prisms between the examined eyes (a prism over one eye not only causes dissimilar images but often convergence in the armed eye alone, the other fixing), the proportion between convergence and divergence in eyes with orthophoria is apt to be as low as three to one. Convergence 1 8., divergence 6. is about the limit for useful ocular muscles on the one hand, while 70. versus 10. or even more can in rare instances be found among the strong muscles.

Convergence does not represent the strength of the interni alone, but the nervous energy put into them as well. In testing this power, being of too impatient a nature to wait for prism exercises or other slow methods, I have my patients fix a finger, or a lead pencil, which is gradually approached while the eyes are armed with converging prisms, and often with the exclamation, " Oh, that is what you want me to do," they proceed to converge at distance at least half as much again as they had succeeded in doing but a moment before. The rapidity and simplicity of this will absolutely prevent its acceptance as a cure, so I shall not elevate it to the dignity of a method by giving it a name, as the principle itself is already in use under many high-sounding titles.

A moment's reflection will show anyone that the prism test at distance does not necessarily represent the real convergence power as shown by the near point for convergence. For the latter test I use a lead pencil or a penholder, as being an object which is always at hand during an examination, necessitates accommodation on the part of the patient in order that it shall be clearly seen, and allows the examiner to watch the action of the eyes without distracting his attention. The ease with which convergence for the near point is accomplished, and the steadiness with which the eyes hold an object, are of more importance than the exact measure of its distance from the eyes, and the sensations produced by this muscular action, with the attitude of the patient regarding them, are valuable diagnostic indications. I would, moreover, as soon trust my observation as to when convergence is no longer possible as a patient's statement regarding the production of diplopia. Tests for the near point usually will show more power of convergence than the prism test at distance, because in the latter instance the convergence without accommodation is an unwonted, as it is usually an unnecessary, accomplishment. Teaching a patient how to converge with prisms is simply training for a test, and has something of the same relation to a cure of weak convergence as training a sensitive throat for the tongue depressor has to the treatment of hypertrophied tonsils.

The near point of convergence varies with the refraction and becomes more and more distant as age advances and accommodation and the interni become weaker. Undoubtedly valuable as the test for the near point of convergence may be, I confine my use of it mostly to certain purposes already mentioned, and as a check upon other tests. Thus if convergence with prisms for a distance seems below normal, I should reject a diagnosis of insufficiency of the interni, or of excess of the externi, if the same eyes showed easy and comfortable convergence up to two inches or less from the root of the nose, with or without the correction of the refraction. The prism test for convergence at the reading distance is unsatisfactory, although some information may be gained thereby, as in cases in which hyperphoria occurs only with convergence.

Sursumduction, or upward movement of an eye, is tested on the same principle as convergence and divergence : right sursumduction with the prism base down right eye, or base up left eye ; left sursumduction base down left eye, base up right eye. It is hardly necessary to call attention to the necessity of verifying the base apex line of the prism and having this axis exactly horizontal or vertical in the above mentioned tests.

Although it has been convenient to consider here the muscular strength as shown in divergence and convergence before the muscular balance or tendency to deviation, the latter should be tested before the former and preferably after examination of the refraction. The test for balance which is most time-honored is that by which vertical prisms produce diplopia, when if the images have a homonymous or crossed position as regards each other the correcting prism which brings them into equilibrium is considered the measure of the esophoria or exophoria. This test should be made at a distant and near point, first without, then with, the refraction corrected. I give the conclusions for a near point based upon tests at 13 inches, as this is the distance I have chosen for years, although now satisfied that it is somewhat too near for most cases. The near test would be best taken at reading distance, which varies in different individuals.

The test for equilibrium with prisms has been perfected, especially by Stevens, and has resulted in the phorometer with its two prisms of 5 D's base in, which can be rotated, after the test for vertical equilibrium has been obtained, for the test of the lateral tendency, and shows the amount and nature of the deviation upon its anterior surface in prism dioptrics when the images show equilibrium. It thus gives the novice information of the deviation with which he has to deal with some certainty, as he can hardly turn the instrument upside down or otherwise misplace it in making a test. When the axis of the prism is horizontal, if single vision still obtains it is a fair conclusion that divergence is excessive, and another prism may be added in order to obtain the lateral images, the prisms in the phorometer only being rotated in obtaining the measurements. An arm upon the instrument is very handy for getting the muscular balance at a near point, although the test object is more horizontal to the eyes than the position of the other objects most often seen at this distance in ordinary work. As a rule, deviations remain latent with this test to an extent that they do not with the rod test, although occasionally the reverse is true.

The Maddox glass rod, or series of rods, will produce a line of light at right angles to the axis when placed before an eye. This test has the advantage of indicating the fixing eye (the unarmed one) and allows it to be changed at will by changing the rod from one eye to the other. When the correcting prism is obtained its amount and direction are patent, thus saving the beginner in ophthalmology some mental effort in ordering correcting prisms. When a deviation has been corrected with a prism in one direction the rod test will show the effect upon the deviation at right angles to it, a very important consideration in certain instances, as when with a correcting prism for hyperphoria we wish to test exophoria or esophoria as compared to the same deviation before the correction. The rod test also allows of an examination toward the periphery of the field more accurate than that of the phorometer, when we wish to see if the deviations are comitant, but is of no definite use in testing for reading distance. I can hardly see how either of these tests can be dispensed with.

The old screen test, in which one eye is covered, then uncovered, and its deviation noted while the other fixes, was in my hands so contradictory and unreliable long ago, that when Duane modified its use by adding a subjective element, giving the parallax test, I lacked sufficient confidence in the latter to use it, in which I may be wrong. Maddox rejected the double prism with bases meeting at the centre for the rod test, and various attempts have been made to revive its use for certain purposes, with but little success.

Having attempted to indicate the methods of examination which were followed in obtaining the clinical data here used, I shall treat the subject of heterophorias under the following heads: Hyperphoria; Esophoria, first, the accommodative, second, esophoria from habit, lastly, esophoria from insufficiency of the externi or excess of the interni; Exophoria, accommodative and muscular; and lastly Insufficiency of Convergence, Inefficiency of the Ocular Muscles, or Neurasthenic Muscular Asthenopia. It may be observed that this class of cases is not intended to comprise, except as occasional reference may occur, those which have spontaneous diplopia, since I regard these as either cases of periodic squint, paresis of an ocular muscle, or of an associated movement ; neither are other cases of evident strabismus without diplopia to be considered in detail. Although in practice it is difficult to draw the dividing line, it is primarily my desire to further definite knowledge regarding a class of cases in which individual muscles may of course be affected, yet the deviation can only be demonstrated by the heterophoria shown with appropriate tests.

It is lack of definite knowledge concerning deviations of both eyes above or below the horizontal plane, or inclinations of the physiological vertical meridian, which prevents consideration of these subjects here; at present, although the germs of ideas may be working in the brains of investigators upon these subjects, we can hardly accept information as definite where the diagnosis depends upon unproven points in physiology and optics and treatment consists of muscular exercises with oblique prisms, or

2O Heterophorias and Insufficiencies.

tenotomy of the upper half of one lateral muscle and lower half of its opponent. Torsion will receive some incidental consideration in dealing with muscular deviations connected with oblique astigmatism. Before proceeding to the symptoms of the special deviations to be considered, it seems well to draw attention to certain signs of ocular asthenopia common to the different errors. They are mainly headache, dizziness and sensations of nausea, pain in the eyes, retinal asthenia or hyperesthesia, congestion of the palpebral or ocular conjunctiva. Any of these may be due to muscular anomalies; pain in the eyeballs seems often to occur from nutritive disturbances, as may conjunctival congestion, although the latter is generally connected with chronic nasal catarrh. Relief of conjunctival congestion by glasses may be due to protection of the eyes from dust and chemical rays of light, has occurred with plane glasses, or ridiculously weak spherical lenses, and is no proof of relief of eye strain. Asthenopic symptoms sometimes disappear when conjunctivitis is successfully treated. Temporal headache is usually due to astigmatism, the pain being most severe on the side of the functionally better eye, this being in men the right when refraction and vision are practically equal, because their habit of "sighting" objects with this eye renders them more " right eyed " than women. Next to eye errors, tobacco and whiskey in men, uterine trouble in women, are the most important factors in temporal headache. Frontal headache, when dull, may be due to errors of refraction, but if severe and persistent suggests insufficiency of the interni, or disease of the frontal sinus. Supra-orbital neuralgia with tenderness over the nerve is usually unilateral and is best treated with quinine unless the pain is severe, radiating and persistent, with much lacrimation, when it should be referred to nasal obstruction, ethmoidal disease, or disease of some other of the accessory sinuses. In this latter condition there may be ciliary pain and lowered intra-ocular tension. A feeling of pressure over the top of the head about the fronto-parietal suture is from anaemia, not eye strain, and localized pain over one parietal protuberance is not an eye symptom, although its presence with accompanying tenderness is not a positive diagnostic sign of tumor of the brain, as is sometimes stated. Very severe headache with vomiting, coming on suddenly in persons who have not been subject to headaches, should always be regarded with suspicion, and if persistent, it will perhaps be necessary for a diagnosis between beginning cerebral disease and some lesion of the digestive organs to wait for further developments.

In " bilious attacks " there is sometimes paresis of an ocular muscle or of an associated movement at the time of an attack which recurs with such attacks for years, and just such conditions of the muscles may occur and prove very misleading to the oculist in cases of central disease of the nervous system. Take, for instance, a case of recurrent pachymeningitis in an early stage, with headache and spasm of convergence from irritation of the convergence centre, or some other muscular trouble from central lesion which resembles some muscular excess or insufficiency in which treatment usually causes relief, and in such a case relief to the headache follows treatment of the eye muscles. It is hard to avoid mistakes in such cases, yet in them mistakes are very awkward ; it weakens a man's local influence as an authority on muscular asthenopia when some of his medical neighbors have attended autopsies upon the late subjects of some of his late cures, and have not exercised that charity regarding faulty diagnosis which seems so often to begin at home.