For instance, if the examination of a hypermetropic eye, whose defect can be exactly determined by means of the ophthalmoscope, shows very faulty visual acuteness which is but slightly or not at all improved on correction of the hypermetropia, it is clear that the cause of defective sight is not to be sought in the hypermetropia. It is just the same with astigmatism. In defective vision with astigmatism proved by means of the ophthalmoscope, how frequently it is the case that not even the slightest improvement can be obtained with cylindrical gla.s.ses. This is usually attributed to the presence of an irregular astigmatism situated near the asymmetric meridian. If we illuminate the eye by means of a plane mirror, and observe one spot on the pupillary area which looks sometimes bright, sometimes dark, during slight rotations of the gla.s.s, this appearance can only be caused by the above-mentioned irregularity of the refraction of light, and it will be easy to determine whether the same takes place in the cornea or in the lens. But if this appearance is not present then irregular astigmatism cannot be proved. It is purely intentional, or a play upon words, if we refer an existing defective sight to an optic cause which cannot be proved. For instance, if haziness of the cornea exists, it is not difficult to learn to estimate by practical experience whether the amount of visual disturbance corresponds to the optic irregularities caused by the opacities and irregular refraction of the cornea. Slightly nebulous corneae with disproportionately bad vision were therefore included in the following statistics; however, they do not influence the result as there are only ten cases in all. On the other hand, considerable opacity of the corneae or cases which were complicated with anterior synechia, &c., were excluded from the statistics.

If then we find defective vision, the development of which has not been noticed by the patient, together with normal ophthalmoscopic condition and full visual field, and if it is further seen that the condition remains unchanged for years, we have every reason for considering the defective sight to be congenital. The statements of patients must of course be received with caution. If congenital amblyopia of moderate degree exists in both eyes, patients do not usually know that it is possible for anyone to see better; if the congenital defect is one sided, it is generally only casually noticed on closing the better eye.

We can scarcely doubt that it is a case of congenital amblyopia if it happens in children. Acquired defective sight without ophthalmoscopic cause seldom occurs among children. I have seen a few cases as a result of severe cerebral disease (hydrocephalus, for example); so-called anaesthesia retinae, or amblyopia marked by contraction of the visual field is not quite so rare. It is easy to avoid confounding both these cases with congenital amblyopia.

One must be more careful about drawing conclusions with regard to adults, for on the one hand it happens that gradually developed monocular visual disturbances are only accidentally observed by patients after they have reached a high degree, and it is very difficult then to persuade these attentive observers that it is not a case of sudden blindness of one eye. (Only a few people seem to be really aware that they have two eyes, and still fewer appear to suspect the existence of a visual field.)

In all these cases opportunity is hardly given for mistakes with reference to the diagnosis of congenital amblyopia, as slowly developed monocular defect scarcely occurs without ophthalmoscopic cause. On the other hand, ophthalmoscopic symptoms (such as haemorrhage of the retinal artery in the macula lutea) may disappear without leaving a trace, while defective vision remains. The law of habit, however, usually helps us here. In congenital monocular defect patients are generally accustomed to this condition, and only notice it when special claim is made on the visual faculty of this eye,--he, on the other hand, who is accustomed to see with two equally good eyes, may not observe a gradually occurring blindness of one eye, if his talent of observation be faulty, but I have never had reason to suppose that a rapid depreciation of the central visual acuteness of one eye is also overlooked. Rapidly occurring monocular visual disturbances are noticed, whether detected with or without the ophthalmoscope.

Two peculiarities appear in isolated cases of congenital amblyopia, which may render the testing of vision difficult: rapid fatigue of the retina, and depreciation of the central visual acuteness in such a way, that an adjoining part of the retina possesses a better visual faculty than the centre.

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

CASE 16.--Mr. W--, aet. 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--, aet. 60, has always seen badly with the left eye.

On the right H. 125 D., V. 5/12. On the left with + 2 D., V. 1/12 with + 5 D. words of No. 175 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 circ.u.mstances 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--, aet. 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. 075 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 075 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 role 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 a.n.a.lysed 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. 075 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. 225 were read with difficulty--this test was on that account repeated in myosis by eserine--No. 175 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 225 D.), 3 with hypermetropic astigmatism, I with myopic astigmatism.

(_b_) Myopia in 5 cases (3 of M. 1 D. to 15 D., 2 of M. 45 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--, aet. 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.

075 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--, aet. 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 + 65 D., V.

1/18, with + 10 D. No. 30 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.

CASE 21.--Mr. H--, aet. 28, has myopia 6 D., V. 6/9 in the right eye; the left eye has always been weak sighted, emmetropia is detected with the ophthalmoscope, with normal fundus, V. 1/18. No squint, binocular fusion can be proved with prisms.

CASE 22.--Mr. B--, aet. 47, has hypermetropia 5 D., V. 5/9 in the right eye. Left eye with + 5 D., V. 1/18 (a few letters of 12 also were recognised at 1 m.). It seems, however, that the patient is not able exactly to indicate the position of the retinal images of his left eye, he does not know, as he expresses himself, "whether the letters stand here or there." Patient observed the defective sight long ago; the ophthalmoscopic condition is normal. Patient really comes on account of his son, aged 7-1/2, in whom hypermetropia of 35 dioptres is detected with the ophthalmoscope, right eye with + 35 V. 5/9. Left eye has convergent squint, V. 1/36, No. 30 is read with + 65 D.

The hereditary tendency is seen also in the following case:

CASE 23.--Mrs. S--, aet. about 46, on the left H. 4 D., V. 5/18 to 5/12, has used no spectacles until now, and reads No. 075 without gla.s.ses at about 15 cm. R. with + 4 D., V. 1/18, with + 65 D. large letters of No.

50 are recognised.

Two sons, present at the same time, are hypermetropic. One has in either eye V. 1/4, the other a slighter degree of congenital amblyopia.

CASE 24.--Johanna L--, aet. 4, came under treatment for a congenital fibroma covered with hair, about the size of a cherry-stone, situated on the outer corneal margin of the left eye, which was removed. Three years later, when the child had learnt to read, emmetropia and full visual acuteness was observed in the right eye, with the left No. 40 only is read with difficulty. The ophthalmoscope shows a slight degree of irregular astigmatism of the cornea, which in no way explains the defective vision; the image of the fundus is perfectly clear and quite normal.

CASE 25 afforded me a not altogether pleasant surprise. Martin M--, aet.

58, has matured cataract in the right eye, with perfectly satisfactory light reflex, proper projection, &c. On the left progressive cloudiness of the lens has begun. The course of operation and cure were regular in every respect, but the power of vision finally was so small that with a clear pupillary area, and otherwise normal condition, only single words of No. 30 were recognised with difficulty at 10 to 15 cm. with convex 20 D. For the first time the patient remembers that he noticed the defective sight in his right eye at the age of sixteen, and was for this exempt from army service. The operation performed later on the left eye procured satisfactory vision.

C. Visual acuteness of less than 1/36 12 cases.

Determination of refraction of the better eye is given in 6 cases, and showed twice emmetropia, twice slight myopia, twice hypermetropia. I only possess an exact ophthalmoscopic determination of the condition of the defective-sighted eye in one instance with H. 25 D.

This group is of special interest in that it represents the extreme degrees of congenital amblyopia, and, on the other hand, because it contains 5 cases of children under 10 years of age.

CASE 26.--Constanze von M--, aet. 9-1/2. Defective vision on the left side had been noticed long before by the child's parents. On May 1, 1879, emmetropia was observed in right eye, V. 5/12 to 5/9. No. 04 is read at 15 cm. On the left, only movements of the hand are seen, fingers cannot be counted even when close to the eye; the visual field is good, that is, on moving the hand in the periphery of the visual field the child sees "something" without being able to state what it is. Reaction of the pupils as rapid and equal as usual. The ophthalmoscopic condition (even with dilated pupils) is perfectly normal. All tests for simulation were of course applied.