Part 10

Rapid fatigue of the retina occurs in comparatively good visual acuteness. For example:

CASE 16.--Mr. W--, æt. 35, came under treatment for conjunctivitis. In testing the vision, emmetropia (or doubtful hypermetropia) was found on the left, V. = 5/6. Refraction of right eye similar to that of left, V. = 5/18 to 5/12, but with rapidly occurring fatigue of the retina. Patient had observed this fifteen years before, when shooting during his period of army service. Position and movements of the eyes are normal.

This peculiarity occurs more often in higher degrees of defective vision. For example:

CASE 17.--Mrs. von G--, æt. 60, has always seen badly with the left eye. On the right H. 1·25 D., V. 5/12. On the left with + 2 D., V. 1/12 with + 5 D. words of No. 1·75 were recognised; but the visual acuteness above stated is only present at the first moment; after a few seconds everything disappears in a fog. The left eye has a slightly conical nebulous cornea, detected only on focal illumination, which does not, however, cause the slightest irregular astigmatism, and cannot, therefore, serve as explanation of the defective sight.

This rapid fatigue, which only permits the visual acuity present to be estimated for a short period at a time, may easily result in the visual acuity being supposed to be worse than it is.

The other phenomenon above mentioned, which occurs in defective vision without being actually a necessary symptom, is the depreciation of the central visual acuity, which we designate as central scotoma in acquired amblyopia. It should be remembered that the visual acuteness which we determine under these conditions is something different from what we are usually accustomed to designate by this idea. When we simply talk of visual acuity we always imply the central visual acuity; however, in cases where the centre of the retina is so injured in its function, that the peripheral parts lying near are too often called into requisition, we do not determine the central visual acuity at all, but that of the nearest and at the same time best, excentric part. We cannot prevent patients from using that part of the retina which seems best to them for recognising the test objects. In such cases (just as in acquired central scotoma) continuous print is read badly, and with more trouble than one would expect from the visual acuteness which is specified in the recognition of single letters. Of course spelling and reading are two different things; the excentric visual acuity may perfectly suffice for the recognition of single letters, central and also excentric visual acuity is necessary for reading. There are patients who, despite full visual acuteness, are unable to read continuously, as soon as a defect in the right half of the visual field extends close to the fixation point. To read fluently, the excentric vision must work on in advance for the width of several letters, but if the first letter is seen excentrically, the excentric visual acuteness rapidly sinking in a physiological way, does not suffice for the following ones.

When testing the vision these circumstances should be carefully regarded. The apparent contradiction between the visual acuteness specified with test-letters, and the uncertainty in reading continuous print, may be taken for simulation (I have seen some sad examples of this in acquired central scotoma), and, on the other hand, if in the form of defective vision now under discussion we content ourselves by merely employing reading tests, we take the visual acuteness to be worse than it is, or than we find it later when single test-letters are used, for even though excentric, it is yet always visual acuteness. The excentricity of that part of the retina put into fixation is usually so slight, that the oblique direction of the visual axis cannot be seen with the naked eye; if considerable and extensive defect of the centre of the retina is present, either varying fixation occurs, sometimes parts lying to the nasal and sometimes to the temporal side are put into fixation; or excentric fixation exists; an inner retinal area but sometimes also a temporal then usually has comparatively the best visual acuteness.

A third peculiarity which sometimes occurs in extreme degrees of congenital amblyopia, is monocular nystagmus of the weak eye. This trembling may be so slight that it is only observable during investigation with the ophthalmoscope; in other cases it is most marked as soon as the weak eye is put into fixation by exclusion of the sound one.

Cases of congenital amblyopia in both eyes, where no explanatory cause can be traced, and no nystagmus is present, are rare, but all the more interesting. For instance:

CASE 18.--Mr. F--, æt. 56, has seen badly from childhood; the visual acuteness of each eye singly examined amounts to 1/18 to 1/12, binocular 1/12. No. 0·75 is read with difficulty at 8 cm. Ophthalmoscopic condition is normal. In mydriasis by atropine hypermetropia of 3 to 4 dioptres results. With convex 3· 5 D. on the right V. 1/18 to 1/12, on the left V. 1/12, binocular V. 1/12 to 1/9, with convex 6 D. still only 0·75 can be read, but more fluently than with the naked eyes.

Normal binocular fusion may continue to exist even in extreme degrees of monocular weak sight; I have observed it up to a visual acuteness of 1/24. The stereoscope is well adapted to prove binocular fusion in these cases; only we must then take care that sufficiently large letters are present in the visual field of the defective eye, so that they may easily be recognised with the existing visual acuteness. Binocular fusion is naturally rendered still more difficult if the weak-sighted eye is at the same time hypermetropic to a high degree, as it then receives simultaneously indistinct retinal images on account of the difference of refraction; and yet in the above table there are 117 cases with hypermetropia of at least 2 D. in the better eye, and faulty visual acuteness in the other, 7 with visual acuteness of less than 1/7 to V. 1/12, and 9 with less than 1/12 to V. 1/36.

In the highest degrees of congenital defective vision, binocular fusion cannot as a rule be proved; partly because the methods of investigation by which we are able to prove binocular fusion presuppose the existence of a sufficient visual acuteness. On the other hand, it cannot be expected that normal binocular vision can be learnt with such a large amount of monocular defective vision. If the relative strength of the muscles is normal, so also are the position and movements of the eyes, if elastic preponderance on the part of the muscles is present, which in monocular defective vision of considerable degree is no longer governed by binocular fusion, and this is frequently the case, squint is developed.

Sometimes other congenital anomalies are present at the same time with congenital defective vision (for example, congenital dermoid growths on the edge of the cornea), and undoubtedly hereditary influences play a considerable rôle therein.

In order to determine the relation of congenital defective vision without squint, to defective vision with squint, I have taken those cases where congenital defective vision without squint was observed, together with the cases of squint, from the diaries of my private practice for the last ten years. I have personally investigated every case, and the observations on each were carefully examined before being included in the statistics; all cases with myopia of six or more dioptres, all cases of double nystagmus, and, finally, all those cases where the previous existence of squint might be suspected, were excluded, as above stated. I must also remark that before the last ten years I had not begun to collect these cases. In order to find monocular congenital defective vision one must seek for it, as patients usually come under treatment for quite different disorders, and in the consulting-room there is not always time carefully to investigate what possesses interest for us but none for the patient. In cases of squint the opportunity for investigating the power of vision does not escape us so easily, and yet the same list, which contains among 629 patients 177 cases of squint with a visual acuteness of 1/8 to less than 1/36, furnished at the same time 98 cases with undoubted congenital defective vision of the same high degree without squint, which I place together in the following review.

Cases of congenital amblyopia with visual acuteness of 1/7 are so frequent, that I have not drawn up special statistics of them. I was not anxious to collect a large number of cases but only material for evidence. I have therefore divided the 98 cases I observed into 3 groups. (1) Cases with visual acuteness of less than 1/7 to V. 1/12; (2) V. less than 1/12 to V. 1/36; (3) visual acuteness less than 1/36. The limits between these groups are of course not very sharply defined, for what is designated as "measurement" of visual acuteness contains, even if we accept the statements of patients as trustworthy, not an inconsiderable number of sources of error; and we often find a remarkable absence of conformity in the analysed causes of congenital amblyopia, according as we seek to determine the visual acuteness by means of single test-letters or by reading printed matter. In a case of visual acuteness of 1/12 No. 0·75 with convex 6 was the smallest type that could be read, and that with difficulty, larger type was usually required; and in one case where at first only single words of No. 2·25 were read with difficulty--this test was on that account repeated in myosis by eserine--No. 1·75 was finally the smallest print which could with the same difficulty be deciphered. In the division of the groups here arranged the best visual acuteness ascertained in the various examinations was always used as the basis.

A. Vision less than 1/7 to V. 1/12 38 cases. The examination of the better eye showed:

(a) Emmetropia in 18 cases. A determination of refraction, mostly ophthalmoscopic, of the weaker eye is submitted in 11 cases, which divide themselves into, 4 with emmetropia, 3 with hypermetropia (of H. 2 D. and 2·25 D.), 3 with hypermetropic astigmatism, I with myopic astigmatism.

(b) Myopia in 5 cases (3 of M. 1 D. to 1·5 D., 2 of M. 4·5 D. and 4 D.), the condition of the defective eye was determined in 3 cases, and was twice hypermetropic, once astigmatic.

(c) Hypermetropia in 8 cases, hypermetropic astigmatism in 3. In 4 cases an exact determination of refraction even of the better eye was for some reason impracticable.

There are 4 cases in this group where the visual acuteness in both eyes did not exceed the above-stated small amount, and one which was interesting from another point of view.

CASE 19.--Max L--, æt. 8-1/2, recognises No. 24, and a few letters of 18 at 5 metres with the better eye with convex 6 D.; at 1 metre V. 1/4 to 1/3, the left eye recognises only No. 60 at 5 m. with + 6 D. at 1 m. No. 0·75 is read with difficulty. If we exclude one eye it lapses into now less, now greater convergence, and still no squint is present, but diplopia as well as binocular fusion can be proved by the aid of prisms. The theory of Donders that squint is less frequent in hypermetropia of high degree because too strong convergence would not suffice to furnish clear retinal images, is scarcely tenable in the face of such cases. If indistinct retinal images are added to a visual acuteness of only 1/3 to 1/4 still, even with faulty accommodation, it is difficult to believe how a child could learn to read if it did not hold the book close to its eyes, which was not the case here, and indeed seldom happens. Therefore, in spite of defective vision the accommodation must have sufficed, without sacrificing binocular fusion, whilst in all probability accommodative convergence followed on exclusion of one eye.

B. 48 cases had visual acuteness from 1/12 to 1/36. The better eye was--

(a) Emmetropic in 16 cases; in 6 of them the refraction of the defective eye was determined, which showed in one case emmetropia, 3 hypermetropia, 2 astigmatism.

(b) Myopia of the better eye was present in 7 cases (in 3 myopia of 1 D., in 4 M. 3 D. to 6 D.).

(c) Hypermetropia in 18, astigmatism in 4 cases. In 3 cases the condition of the better eye was, for some reason or other, indeterminable.

In this group I should like to point out the following cases as worthy of attention:

CASE 20.--Margarethe T--, æt. 16, has hypermetropia 2 D. in the right eye, V. 5/6, in the left the ophthalmoscope shows with an otherwise normal condition a higher degree of hypermetropia, with + 6·5 D., V. 1/18, with + 10 D. No. 3·0 is read. No spectacles have been used until now; for the past few years school tasks have been performed with a certain effort, only during the last year the asthenopia has increased. Squint is not present, and with prisms as well as with the stereoscope (by the use of objects, whose size corresponds to the defective sight on the left side) binocular fusion can be proved.

The case is the same as regards divergent strabismus.