Together with congenital defective vision we must consider the depreciation in visual acuteness usually present in nystagmus, although it might be asserted that it can neither be the cause nor the result of the nystagmus, for we find very considerable degrees of congenital defective sight in both eyes without nystagmus, as well as nystagmus with remarkably good visual acuteness. Not to complicate the question, however, I have excluded all cases of nystagmus from the following investigation. All cases of myopia of higher degree (i. e. of more than 6 D.) have also been excluded, as in such cases for various well-known reasons the full visual acuteness is never present. In the case of individual patients who remained for years under my observation I have been able to convince myself that visual acuteness decreased in accordance with the increase of myopia; on the other hand, however, it appeared to me very probable that just those cases of myopia, which from the beginning do not possess full visual acuteness, have a special tendency to increase quickly.
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 glasses. 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 glass, 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 corneæ 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 corneæ 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 anæsthesia retinæ, 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 hæmorrhage 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—, 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.