J^jl^U^L J j J 1882.] Theobald, Insufficiency of the Internal Recti. 7r Abl ^ tb Article XXI. ^°^pll. l°^s What Constitutes Insufficiency of the Internal Recti Muscles ? "7ienh. By Samuel Theobald, M.D., of Baltimore.' During several years a conviction has been gradually developing in my mind that I did not know what constitutes insufficiency of the internal recti muscles, or, at least, that I did not know where the line between sufficiency and insufficiency should be drawn. When, in seeking to ac- count for the existence of asthenopia, I discovered by the usual means a considerable degree of hypermetropia or of astigmatism, I felt that I had found something which was definite and real, and I had little doubt that the correction of this defect would be followed in due time by the disap- pearance of the asthenopic symptoms; but, on the other hand, when, fail- ing other causes, I was led to examine the state of the muscles of conver- gence, and by means of the tests usually relied upon, discovered, accord- ing to the accepted standards, insufficiency of these muscles, I felt that I Avas dealing with something which Avas far less definite, and I could not escape the suspicion that the therapeutic measures which this discovery prompted me to take were, perhaps not unfrequently, directed against an evil Avhich had only an imaginary existence. So far as I can account for this state of mind, it Avas due, first, to the fact that I seldom sought for insufficiency of the internal recti muscles without finding it, unless hypermetropia existed; secondly, to the fre- quency with which I observed the disappearance of asthenopic symptoms without any diminution in the apparent insufficiency of the muscles ; and, thirdly, to the contradictory character of the results which I obtained in many cases from the different tests upon which we are taught to rely to determine the existence and to measure the amount of this anomaly. The important influence Avhich errors of refraction exert upon the behaviour of the muscles of convergence; the necessity of correcting such errors or of allowing for their influence, in applying the tests for in- sufficiency ; and the propriety of distinguishing between real insufficiency or weakness of the internal recti-muscles and the apparent insufficiency which is so commonly associated with myopia, and which is due to dis- turbance of the normal relation between accommodation and convergence, were forcibly impressed upon my mind some years ago, through a study of the behaviour of my own myopically formed eyes, and in a paper en- titled " An endeavour to show that insufficiency of the internal recti muscles and myopia have been erroneously associated; and that the mus- cular asthenopia of myopia is not the result of such insufficiency, but of the anomaly of refraction," published in the Am. Journal of the Med. 1 Read before the American Ophthalmological Society at its annual Newport, R. I., July, 1881. fl&Al&V' 458 Theobald, Insufficiency of the Internal Recti. [April Sciences, in January, 1874, I dwelt at some length upon these points. At that time I argued that the Graefe tests for insufficiency of the internal recti muscles, though trustworthy when applied to emmetropic eyes, are calculated to give deceptive results in ametropia, because of the disturb- ance of the normal parallelism between accommodation and convergence, unless as preliminary to their application the precaution is taken to cor- rect the error of refraction. That, owing to the disposition which ame- tropic eyes always exhibit to restore this normal parallelism, an effort to do so is made so soon as the production of vertical diplopia annuls the still stronger desire for single vision, and that as a result of this the ver- tical prism tests show " in hypermetropia, an excess of poAver in the in- ternal recti muscles which is not real," and, " in myopia, an insufficiency Avhich is only apparent," being merely the expression of " the ever pre- sent, but for the moment unrestrained desire, to exact as little work of the internal recti as is at the same time required of the ciliary muscles.'' In support of this view I mentioned that an apparent insufficiency of 12°, which my own eyes, with M^, exhibited at 8", disappeared entirely with total correction of the iriyopia, diminished with partial correction, and Avas increased by convex glasses ; and, further, that when the experi- ment Avas tried of placing before one eye a convex glass, and before the other a concave glass, which more than neutralized the myopia, so that objects could not be seen distinctly at the same time with both eyes, the images of the dot in the vertical diplopia test changed their positions in such a manner, as to indicate excess of power in the internal recti when the dot was seen distinctly with the over-corrected eye, with strained accommodation, and marked insufficiency Avhen looked at, Avith relaxed accommodation, with the eye before which the convex glass was placed. Two other possible sources of error in Von Graefe's dot and prism test, which Avere detected during my experiments, Avere also pointed out, and a means of getting rid of them suggested. The relation betAveen accommo- dation and convergence being so intimate, it becomes important that dur- ing the examination the eyes should be accurately accommodated for the distance at which the test object is held. Now the round dot 6f the Graefe test, since it remains a round dot, and is seen almost as distinctly whether brought exactly to a focus upon the retina or not, does not offer a sufficient incentive to accurate accommodation, and is, therefore, ill-adapted to the purpose for Avhich it is employed. For this reason the substitution of a star for the dot was proposed—a cross, which otherAvise might have answered still better, being discarded because of the confusion to Avhich it Avould be likely to give rise should astigmatism happen to be present. The other source of error lies in the undue length of the vertical, bisect- ing line. The two images of this line overlap ; and this Avas found in a measure to annul the influence of the vertical diplopia, an involuntary inclination to blend the overlapping portions of the images being felt. 1882.] Theobald, Insufficiency of the Internal Recti. 4o9 The omission of the line, therefore, since it is not an essential part of the test, was recommended.1 1 An interesting and instructive article by Dr. E. G. Loring upon " Tests for the In- sufficiency of the Recti Interni Muscles," Avhich appears in the Transactions of this Society for 1868, has been brought to my notice, since the preparation of this paper was begun, by my friend Dr. Russell Murdoch. In this article Dr. Loring discusses the trustworthiness of the several tests proposed by Von (Jraefe, and especially con- siders whether the vertical diplopia tests do away with all voluntary control over the muscles of convergence, and whether the prism which reduces crossed images to the same vertical line expresses the whole amount of the insufficiency of the interni. His conclusions upon these two points are, that the production of vertical diplopia does not always prevent such control, and that the prism which reduces the images to the same vertical line, in some instances, indicates only a part of the total insufficiency, in support of the former conclusion, he mentions a case in which a patient during the application of the test was able to produce at will either crossed or homonymous images in both near and distant vision, and cites his own ability to do this with the test card at twelve inches. This faculty of changing the position of the images is, I think, due rather to voluntary control of the accommodation than to direct command over the internal recti muscles, the alteration in the direction of the visual Hues being brought about by increasing at one moment and diminishing at another the tension of accommoda- tion ; at all events the ability to control the position of the images, without a corres- ponding change of accommodation, must be of extremely rare occurrence. AVe have it in our power, therefore, to prevent this perplexing accident, by insisting that the test object be accurately accommodated for; and this the change in the form of the object which I have proposed renders more easy of accomplishment. The second conclusion is sustained by reference to cases which he had frequently met with, in which " after Ave have brought the images from being crossed into the same vertical line, we can go on adding prisms, sometimes those of considerable de- gree, and yet the images remain exactly over each other, instead of becoming homony- mous." The explanation offered of this occurrence, that it is due to the existence of latent insufficiency, comparable to latent hypermetropia, does not seem to me very satisfactory. In the first place, latent insufficiency to be comparable to latent hyper- metropia should be insufficiency which cannot be immediately rendered manifest by glasses, whereas the insufficiency which Dr. Loring describes as latent, expressed by the difference betweeu the weakest prism which causes the images to stand in a verti- cal line, and the strongest which does not induce homonymous separation, is brought out at once by this means. Furthermore, how does this explanation help us to com- prehend the singular circumstance, that with each increase in the strength of the prisms the latent insufficiency is brought out, or the internal recti muscles yield, to exactly such an amount as to keep the images directly one above the other'. In hyper- metropia, exactly a forty-eighth, next a forty-second, and then a thirty-sixth is rend- ered manifest by the application of glasses, because the ciliary muscle is prompted each time to yield to just that extent to accomplish a definite purpose—the mainte- nance of distinct vision ; but why should the internal recti muscles, with apparently no definite purpose to be served by keeping the images in the same vertical line, yield in the regular manner described ? Dr. Loring's suggestion of latent insufficiency cer- tainly affords no explanation of this. The true explanation, it seems to me, is to be found in quite a different direction—in the overlapping of the images of the unduly- long bisecting line of the test, to which I have referred as a possible source of error, and the obviation of which I have proposed because of the likelihood of its giving rise to just such confusing results. Suppose in a case of insufficiency of con- siderable degree this inclination to fuse the overlapping portions of the liue6, exists, the images being widely separated this disposition Avould probably remain in abeyance, but so soon as they were brought somewhat nearer together, by a 460 Theobald, Insufficiency of the Internal Recti. [April Though these several sources of error in the determination of insuffi- ciency of the internal recti muscles attracted my attention early, and though subsequently they were kept constantly in mind, and in practice Avere guarded against by the adoption of the precautionary measures to which I have referred,1 a feeling of uncertainty as to the trustworthiness of the results Avhich I obtained in dealing with this condition has, as I have said, with the increasing experience of the last feAv years forced itself, more and more, upon me. The disappearance of asthenopic symp- toms in certain cases, without any diminution in the insufficiency of the internal recti muscles, upon which they Avere supposed to depend, and the existence of which had been demonstrated by the usual tests, especially sug- gested the inquiry, whether, even in emmetropia and with accommodation properly regulated, a considerable relative divergence of the visual lines might not occur in connection with vertical diplopia, and still no insuf- ficiency of the internal recti muscles be present, or, if this be interpreted as proof of insufficiency, whether this so-called insufficiency might not exist to a marked degree, and the eyes be none the worse for it. In order to determine whether or not this was the case, I concluded to examine with especial reference to this point a number of strong-eyed, non-asthenopic individuals; and in selecting suitable subjects for this purpose, to make the experiment more decisive, I chose only those whose occupations led to frequent use of the eyes in near vision. My examina- tions were not confined to emmetropic eyes, but Avhenever errors of prism representing perhaps only one-half of the true insufficiency, they would be fused by an unconscious effort. Now it is evident that with each increase in the strength ot the prisms, until the one was reached which truly represented the insuffi- ciency, the fusion of the lines would be rendered easier of accomplishment, and that after this point had been passed, the prisms might be progressively increased for some time before this tendency would be again annulled by a too wide homonymous sepa- ration of the images. In this persistent blending of the overlapping images of the line, it is evident, is implied exactly such exceptional behaviour of the interni with prisms of different strength as Dr. Loring describes ; and which exceptional behaviour I may add, in confirmation of this view, I have never observed with the test modified as I have proposed. While, therefore, endorsing much that is contained in Dr. Lor- ing's art.cle, especially what is said concerning the influence of errors of refraction in modifying the results of the Graefe tests, I am compelled to differ with him as to the significance of these two observations, to which he, especially directs attention. During the discussion which followed the reading of this paper Dr. R. H. Derby, formerly a pupil of Von Graefe, stated that in 1869 Von Graefe, himself, had modified his.original vertical diplopia test for insufficiency in almost the identical manner which I afterwards suggested (in my paper in the Am. Journal Med. Sciences in 1874), and for reasons almost precisely the same, and that he had described these modifications in an article in the Klinische Monatshlatter fur Augenheilkunde, in 1869. It would seem that the suggestions contained in this paper of Von Graefe, referred to by Dr. Derby, have attracted comparatively little attention, at least they are not alluded to in any text- book upon diseases of the eye with which I am familiar; even Soelberg Wells, Von .^tT: dPle' d°eS QOt meQti0n them' but descri^ only the original test. Dr. Derby's mention of them at Newport, last summer, first brought them to No. Sex Age Re- I frac- 1 F. 31 Hm i ^5 2 F. 11 Hm l 3 F. 23 II in i 3S 10 F. 12 JH. l ! ] a 6 F. ! 37 Hm. ! i 1 "J3 7 ' M. , 21 E. ■ I 8 M. 41 j E. 9 F. i 29 Hin ' i l M. j 23 E. 11 i F. ! 15 E. 12 M. 21 E. Results of Prism Tests. With error of i et'i corrected. Insuf. extern. recti. Intern. recti o\ er-come. In-uf. intern. recti. I usui. extern. recti. 14° 4° .... 34 3 46 7 70 2 34 41 141 60 34 ! 34 27 At 20', Insuf. , Insuf. intern, extern. recti. recti. 0 11 0 Less than 1 13 24 0 10 1° 12 1 16 1 10 1 16 Inlern recii over- come 10° 25 18 49 With error of refraction corrected. Insuf. Insuf. intern. extern. recti. recti. 1° 0 0 0 Less than 1 1° 1 Upon second trial, after acquiring the nack of contracting the internal recti, he was able to overcome at 12" 55°. Intern. recti over- come. Has strong eyes. Reads and sew a grout deal, and has never win glasses. Never (omplains of eyes; j goes to school; does not wear glasses. j Eyes '• very strong." Reads and sews a gi eat deal, and has never worn glasses. AA'ears glasses irregularly, for distanc" only. Eyes never trouble some, though she reads and does fine needlework. School girl. Eyes strong. Does not wear glasses. Teaches school. Has worn glasses to correct Hm. for three years. Physician. Reads a great deal. Eyes never trouble- some. Bookkeeper. Wears, in near vision, + J_ ' oo School teacher. Reads a | great deal. Has never worn glasses. j Bookkeeper. Writes con- stantly and never has any trouble with eyes. Attends sclmol, and is a constant reader. Eyes never troublesome. Med. student. Reads a great deal, and never experi- ences discomfort from it. 00 00 to H X s o K > a 33 4G2 Theobald, Insufficiency of the Internal Recti. [April refraction were found to exist the tests were applied, first without, and then Avith correcting glasses. The vertical diplopia tests were made at 12", and at 20'; at the former distance the test object employed Avas a small star with short, vertical bisecting line, held a little below the level of the eyes ; and at 20' a candle-flame, similarly placed. The prism used for producing the diplopia was one of only 7°, which was found quite strong enough, and more convenient for the purpose than those of greater poAver. As supplementary to the diplopia tests the cover test was applied at 12", and the capacity of the internal recti muscles to overcome prisms producing lateral displacement Avas determined in each instance. In ex- amining the state of refraction only the manifest hypermetropia was determined, as it was not considered important to ascertain the total amount. TAvelve persons, selected as I have said Avith especial reference to their freedom from asthenopia, in spite of their eyes being much used in near work, were examined in this manner. The result obtained in each in- stance is given in the table on the preceding page. Although the number of eyes which I have examined is not'large, it is sufficient for the end in view, since the results obtained show that relative divergence of the visual lines, such as has been regarded as proof of insuf- ficiency of the internal recti muscles, does, in fact, occur very frequently, as an accompaniment of induced vertical diplopia, even in the strongest- eyed persons. A glance at the table will show that this took place, to a greater or less extent, not only in every emmetropic person examined, but that it happened in connection with Ioav grades of manifest hypermetro- pia, uncorrected by glasses, in three instances in five. Of the five emme- tropic persons, with the test object at 12", one exhibited a divergence indicating an insufficiency of the internal recti muscles of 2°, and the others of 3°, 5°, 8°, and 11° respectively; and even Avith the object at 20', three of these showed slight relative divergence. In the hyperme- tropes, Avith the test at 12", an apparent insufficiency of 3° was associated with Hm Ol0, 2° in one instance, and 6° in the other with Ilm ^'s, and of the two individuals in whom no insufficiency was discovered Ilm ^ was present in one and Hm ^ in the other. As might be expected, none of the hypermetropes showed any apparent insufficiency of the interni at 20', but, on the contrary, one of them showed apparent insufficiency of the externi. The influence of the state of refraction upon the behaviour of the internal recti muscles, and the way in which neutralizing glasses modify the results of the vertical diplopia tests, are well slioAvn in the table. Thus, Avliile the five hypermetropes show at 12" an average apparent insufficiency of ~§-°, which with correction of the manifest error of refraction is increased to 3|°, the emmetropes sIioav an average of ")|°, and the tAvo myopes of 12°. Furthermore, it will be seen that the 22° of apparent insufficiency which 1882.] Theobald, Insufficiency of the Internal Recti. 463 is associated (in No. 4) with M T»„, is reduced to 2° when the test is repeated with neutralizing glasses, and that (in No. 5) an apparent insuf- ficiency of the interni of 2° to 3° is changed to apparent insufficiency of the externi of 1° by the correction of M ^V Another point to which 1 have alluded, the contradictory character of the results which are obtained* in many cases from the different tests for determining the strength of the internal recti muscles, is also Avell shown. Thus, No. 4, though exhibiting at 12" (with M^ uncorrected) an appa- rent insufficiency of 22°, tAvice as much as any of the others, was able to overcome, at the same distance and without neutralizing glasses, prisms with bases outwards amounting to 70°, while No. 1, with 3° of insuf- ficiency, overcame but 14°, No. 12, with 2° of insufficiency, but 27°, No. fi, with no insufficiency, but 41°, and No. 9, also Avith no insufficiency, only 34°, less than half as much. The explanation of this paradoxical result is, that the ability to overcome prisms of high power thus placed does not, in fact, depend upon the strength of the internal recti muscles, but simply upon the individual under examination possessing the " knack" of causing these muscles to contract strongly—a faculty Avhich we often meet with in persons who can look cross-eyed at pleasure. Evidently, then, upon this method of ascertaining the strength of the internal recti muscles but little dependence can be placed, and since it has been shown that, in the strongest-eyed persons, relative divergence of con- siderable degree frequently accompanies vertical diplopia—even when every precaution is taken to prevent confusion in applying the tests—the question arises, Hoav are Ave to determine the existence of actual insuf- ficiency of these muscles ? Of course, we shall have little difficulty in rec- ognizing the higher grades of insufficiency, those which are not far removed from divergent strabismus, but the difficulty occurs in dealing with the slighter degrees—in deciding, indeed, where to draw the line between suf- ficiency and insuffciency. For myself, I confess I do not knoAV Avhere the line should be drawn, whether at eight or at eleven, or, perhaps, at tAvelve or fifteen degrees of relative divergence ; but of one thing I am convinced— that the tests proposed by Von Graefe, as I have understood them, and as I believe they have usually been interpreted, art; calculated to lead to erroneous conclusions regarding the strength of the internal recti muscles; and that to prevent this it is necessary, not only to take into consideration the state of refraction, to be sure that the accommodation is properly regulated, and to be careful that an incentive to binocular fixation does not arise through overlapping of portions of the vertically separated images, but, in addition, to recognize the fact that relative divergence of the visual lines, even of considerable degree, occurring in connection with vertical diplopia, and after these several precautions have been taken, does not necessarily indicate insuffciency of the internal recti muscles, or, at least, that it is not incom- patible with entire freedom from functional disorder.