Читать книгу Clinical Investigations on Squint - C. Schweigger - Страница 9

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To Donders belongs the merit of having pointed out the presence of hypermetropia in about two thirds of all cases of convergent strabismus. The fact is undeniable, the theories built upon it are doubtful. Donders declares no other conclusion to be possible, than this, that the hypermetropia is the cause of the squint. "To see clearly, the hypermetrope must accommodate vigorously for each distance. In looking even at distant objects he must overcome his hypermetropia by exerting his accommodation, and in proportion as the object approaches him, he must add to it as much accommodation as the normal emmetropic eye would use. The inspection of near objects requires then a special amount of exertion. There exists, however, a certain connection between accommodation and convergence of the visual lines. The stronger one converges the more one has to put into action the accommodation. A certain tendency to convergence cannot then be absent during any effort of the faculty of accommodation."

Right as these conclusions may appear, and as they really are, as far as emmetropia is concerned, they leave out of sight the fact, that the connection between accommodation and convergence is an individual and acquired one. The weak side of the theory lies in the fact, that that relation between accommodation and convergence which is developed in emmetropia in consequence of daily practice, is given as being in itself normal and the one for all conditions of refraction. The relation between accommodation and convergence depends on the state of refraction, and alters with any of its changes in the course of life. In proportion as myopia is gradually developed in originally existing emmetropia, myopes learn to converge to the neighbourhood of their far point without allowing their accommodation to come into action. With hypermetropia it is just the contrary. By far the greater number of hypermetropes learn to use their accommodation without difficulty, even with parallel lines of vision, for they see distant objects clearly, while they neutralise their hypermetropia by accommodation, without sacrificing the parallelism of the visual lines.

It is important to notice that Donders' theory makes convergent squint appear as almost a necessary consequence of hypermetropia. According to Donders, hypermetropes have to choose between the advantages of binocular vision with an effort of accommodation corresponding to the hypermetropia, and relief to the accommodation by too strong convergence with the sacrifice of binocular fixation; and the decision will tend to the latter condition, if circumstances exist which deprecate the value of binocular vision.

The demand for binocular fusion of the retinal images will be greater if both eyes are of equal value; on the contrary it will be less, if the retinal image or the visual acuteness of one eye is less perfect than that of the other. Varieties of weakness; when one eye always receives a clear retinal image, the other an indistinct one; lowering of the visual acuteness of one eye by nebulæ, astigmatism or any other cause. According to Donders all these furnish a reason why, in existing hypermetropia, binocular fixation should be abandoned and convergent strabismus developed.

It cannot be denied that the relation existing between convergent strabismus and hypermetropia may be as Donders represents it; the only question is, whether it really is so. A theory may appear very acceptable, and may rest on a firm physiological basis; it will, however, be more perfect if it answers to facts. Physiological possibility is not always pathological reality, for other unusual causes besides physiological ones acquire value, and so things become pathological. If Donders' theory is right, convergent strabismus must really begin, as soon as double hypermetropia meets with causes which depreciate the value of binocular vision. The theory may be tested then by statistics, which confront the cases of hypermetropia and convergent strabismus with those cases in which hypermetropia meets with Donders' conditions and normal binocular vision still remains.

The statistics, which I have collected, relate to all the cases which have appeared in my private practice during the last ten years. The number would be much more considerable if I had included the patients of the University Clinic; however, the reliability of the single elements of which the statistics are composed was to me more important than the number. In my private practice I have myself examined every case with reference to these statistics for at least five years.

In a large clinic, where more than 5000 new patients annually come under treatment, one must frequently content oneself by satisfying the demands of the moment; thus the sources of inaccuracy in the statistics would be augmented.

Included in the statistics were not merely the cases which came under treatment for squint, but all in which squinting was present or those in which it could be objectively proved (for example, by scars left by previous operations for squint), that squint had formerly existed.

Further, in the following statistics, only those cases were included, where an exact determination of the amount of error was possible; in most cases this was also verified objectively with the ophthalmoscope. In many cases, especially in children, the objective determination of refraction alone is possible, and is practicable only with the greatest difficulty and by the use of atropine.

Those cases deserve particular mention, in which it remained doubtful whether hypermetropia of slight degree or emmetropia was present. Even in full visual acuteness it is not unusual that with weak convex glasses (of less than a dioptre) binocular vision is just as clear as with the naked eyes, while in monocular investigation convex glasses cause a slight indistinctness of vision. Are we to recognise hypermetropia here or not? Opposed to the objection that in covering one eye the hypermetropia is more easily neutralised by accommodation, stands the observation that binocular is, as a rule, clearer than monocular vision, wherefore, in the usual method for testing the sight, unless special precautions are taken, full binocular visual acuteness does not prove the presence of absolutely distinct retinal images. These doubts arise much oftener in lowered visual acuteness. All conclusions which we derive from visual acuteness become very inexact as soon as it is lowered. In such cases, in determining anomalies of refraction we are accustomed to consider the strongest convex—relatively, the weakest concave glass, with which the visual acuteness individually present is reached, as the most correct expression of the hypermetropia or myopia, and with good reason if it is a case of ordering spectacles, as all sources of error in the method of examination are then avoided as far as possible; but it is quite another question if in such cases an exact measurement of the amount of error is required solely for diagnostic purposes; investigation with the ophthalmoscope is then alone decisive and furnishes proof at the same time of how unreliable the determination of the error by testing the vision is, in cases of short sight. One can realise this most readily in cases of myopia with congenital amblyopia; one gets frequently with the most exact correction possible of the objectively determined myopia no better visual acuteness than with a very imperfect one. In one case, for instance, which I have repeatedly examined in the course of years, the degree of myopia determinable by means of the ophthalmoscope amounted to at least 6·5 D., while the weakest concave glass with which the full visual acuteness of 5/24 was attainable was 2·5 D. Under these circumstances, if one relies merely on the trial of vision, the degree of myopia appears too small, that of the hypermetropia, on the contrary, just as much too great.

But even the ophthalmoscopic diagnosis of refraction has its limits of error. It is a question of determining the conditions under which the image of the fundus of the eye still appears distinct. We will except those circumstances which prevent our obtaining a clear erect image of the fundus of the eye, as, for example, high degrees of astigmatism, nebulæ, &c.—even under normal circumstances the fundus of the eye does not always present such sharply-defined lines, that one could form a perfectly safe opinion from the clearness of the image.

When we call the ophthalmoscopic diagnosis of refraction objective, we only mean to say that we count the subjective opinion of the patient to be of less value, than that of the physician who examines him. The determination of the glass even, with which we believe we are able distinctly to see the fundus of the eye, is also an objective one. Whoever, for instance, is firmly convinced that convergent strabismus depends on hypermetropia, will, in doubtful cases, very easily carry his subjective conviction into the objective examination, and will still see clearly the fundus of even an emmetropic eye with a weak convex glass—the objective signs for the clearness of the image have no absolutely defined limits. But apart from this, other sources of error are possible. A person using the ophthalmoscope, for instance, who, without knowing it—and such a thing may happen—possesses a slight degree of latent hypermetropia, will find his own hypermetropia everywhere, just also as a myope, who deceives himself slightly about the degree of his myopia in the calculation of the ophthalmoscopic diagnosis of refraction, lays rather too high a value on his own myopia.

Finally it must be added, that if the ophthalmoscopic estimation of refraction is to be exact, mydriasis by atropine is required, when, as is known, even emmetropic eyes may show a slight degree of hypermetropia. Enough, we must not over-rate the value of the objective determination of the error of refraction, and I would estimate the limit of error at half a dioptre at least. If the examination is rendered more difficult, as is frequently the case with children, by a restless and impatient demeanour of the patient, even the objective diagnosis may afford very doubtful results; such cases were, of course, excluded from the statistics. Moreover, ophthalmoscopic determination of the error in convergent strabismus is specially difficult, for one cannot advise the patient as to a suitable direction for the eye not under investigation. It is generally best to keep the eye not under investigation closed.

In practice it is immaterial whether emmetropia or a minimum degree of hypermetropia is present; for statistics essentially devoted to theoretical questions it seemed more suitable to unite these cases in a separate group.

Accurately taken, the statistics should give the condition of refraction at the age at which the squint begins. But, if there is a thankless task, it is that of examining the erect image in children from two to three years of age. To furnish accurate results this method requires a certain tractability on the patient's side, which is never present at this age, and not always in adults. A number of the cases surveyed in the following table also came under observation long after the squint commenced, and in some short-sighted persons in particular, the degree of myopia at the time when squinting began, may have been less than it was at the time of the examination.

Further, it seemed to me desirable to keep periodic, separate from permanent squint; this, however, could not be accomplished with exactness. It may easily happen that children with periodic squint always squint just when one sees them, and in those cases which had already been operated on when they came to be examined, it was quite impossible to determine whether periodic or permanent squint had formerly been present. Therefore I have represented separately in each particular group the number of those previously operated on.

In the following table the refraction of the fixing eye and the visual acuteness of the squinting eye are given. In alternating squint the refraction of the emmetropic eye was taken, as determining it for insertion in the lower division of the statistics.

A. Convergent squint with myopia:

1. Slight myopia to M. = 1·75 D.

(a) Permanent squint 11 cases (3 previously operated on). Anisometropia in 2 cases (one with M. 1·25 D. of the fixing, M. 4 D. of the squinting eye; the other with M. 1·25 D. of the fixing, H. 4 D. and V. = 1 of the squinting eye). The examination of the visual acuteness of the squinting eye showed:

V. more than 1/7	4 cases.
V. 1/12 - 1/18	1 case.
V. ½4 - ⅓6	1 case.
V. Less than ⅓6	4 cases (among them one with H. 2 D. in the squinting eye.)
V. indeterminable	1 case.

(b) Periodic squint 2 cases with very slight anisometropia and good vision.

2. M. 2 D. to M. 3 D. 11 cases, all permanent (6 cases previously operated on), anisometropia with good vision in both eyes in 2 cases (in both, the less myopic eye squints). V. of the squinting eye more than 1/7 in 6 cases.

	V. 1/12 - 1/18	1 case.
	V. ½4 - ⅓6	2 cases.
	V. less than ⅓6	2 cases (one with H = 5 D).

3. M. 3·5 D. to 6 D.

(a) Permanent 11 cases (one previously operated on). Anisometropia in 2 cases, of which one consisted of alternating squint, while the other possessed in the fixing eye M. 4 D., in the squinting one M. 7·5 D. with good vision on both sides.

V. more than 1/7	7 cases.
V. ½4	1 case.
V. ⅓6	1 case (in fixation with this eye; the visual axis shows a linear deviation of 2 mm. The presence of emmetropia is detected with the ophthalmoscope).

Two cases were excluded from the statistics of vision, one on account of congenital capsular cataract, covering almost the whole pupil area, the other on account of choroiditis of the macula lutea.

(b) Periodic squint 4 cases with good vision, anisometropia in 2 cases.

4. M. 6·5 D. and more.

(a) Permanent 11 cases, among them 9 with V. more than 1/7, 2 excluded from the statistics, one on account of complication with corneal nebulæ, cataract, &c., the other possessed in the fixing eye M. 6·5 D. V. = 10/70 and slight nystagmus, in the squinting eye a smaller amount of sight not accurately noted, and strong nystagmus in fixing with this eye.

(b) Periodic squint in 4 cases with good vision.

5. Myopia with nystagmus and congenital amblyopia on both sides, 2 cases (not included in the statistics of vision). Altogether 56 cases, among them 10 with periodic squint.

B. Convergent squint in emmetropia, including simple myopic astigmatism, 98 cases.

(a) Permanent 81 cases (13 previously operated on). Visual acuteness more than 1/7 in 44 cases. V. less than 1/7 to V. = 1/12 6 cases; V. less than 1/12 to V. = ⅓6 20 cases; V. less than ⅓6 7. Excluded from statistics of vision 4 (3 on account of complications, 1 on account of lack of accurate information).

(b) Alternating convergent squint with emmetropia in one, myopia in the other eye, 4 cases. The degree of the myopia was 3·75 D., 5 D., 6 D., 12 D. Vision good on both sides.

(c) Periodic squint 13 cases (in 6 of them the refraction was objectively and subjectively determined in mydriasis by atropine). No anisometropia worth mentioning was present in any of these cases. Visual acuteness more than 1/7 9 cases. V. < 1/7 to V. = 1/12 2. V. < 1/12 to V. = ⅓6 1; one case with choroiditis excluded.

C. Convergent squint with doubtful hypermetropia to H. = 1 D., including simple hypermetropic astigmatism, 38 cases.

(a) Permanent 30 cases (5 previously operated on). Visual acuteness more than 1/7 7 cases. V < 1/7 to V. = 1/12 2. V. < 1/12 to V. = ⅓6 5. V. < ⅓6 2 cases. 4 excluded (3 complicated with cataract, one on account of impossibility of a trial of vision).

(b) Periodic squint 8 cases. V. more than 1/7 7. V. < 1/7 to V. = 1/12 1 case.

D. Hypermetropia 1 D. to 1·5 D. 37 cases.

(a) Permanent 23 (4 cases previously operated on). V. more than 1/7 13, V. < 1/7 to V. = 1/12 3. V. < 1/12 to V. = ⅓6 3. V. < ⅓6 3. One case excluded (choroiditis of the macula lutea).

(b) Periodic squint 14 cases. V. more than 1/7 12. V. < 1/12 to V. = ⅓6 1 case. One excluded on account of choroiditis.

E. Hypermetropia 1·5 D. to 2 D. 61 cases.

(a) Permanent 41 (3 previously operated on). V. more than 1/7 26 cases. V. < 1/7 to V. = 1/12 3; V. < 1/12 to V. = ⅓6 3; V. < ⅓6 2; (7 cases excluded, 2 as complicated, 5 on account of the impossibility of testing the vision).

(b) Periodic 20 cases. V. more than 1/7 16; V. < 1/7 to V. = 1/12 2; V. < 1/12 to ⅓6 1; V. < ⅓6 1 case.

F. Hypermetropia 2 D. to 3 D. 88 cases.

(a) Permanent 58 cases. V. more than 1/7 26 cases; V. < 1/7 to V. = 1/12 5 cases (among them one with V. = 1/12 in both eyes); V. < 1/12 to V. = ⅓6 17; V. < ⅓6 4 cases. Six cases excluded as indeterminable.

(b) Periodic 30 cases. V. to 1/7 24; V < 1/7 to V. = 1/12 3; V. < 1/12 to V. = ⅓6 1; V < ⅓6 1. One case excluded as indeterminable.

G. Hypermetropia 3 D. to 4·5 D. 54 cases.

(a) Permanent 35 cases (9 previously operated on). V. more than 1/7 18 cases; V. < 1/7 to V. = 1/12 1 case; V. < 1/12 to ⅓6 9; 7 cases excluded.

(b) Periodic 19 cases. V. more than 1/7 14; V. < 1/7 to V. = 1/12 1; V. < 1/12 to V. = ⅓6 3; V. < ⅓6 1 case.

H. H. 5 D. and more, 16 cases.

(a) Permanent 9; V. to 1/7 3; V. < 1/7 to V. = 1/12 3; V. < 1/12 to V. = ⅓6 2; V. < ⅓6 1 case.

(b) Periodic 7; V. to 1/7 4; V. < 1/7 to V. = 1/12 3 cases.