Читать книгу Clinical Investigations on Squint - C. Schweigger - Страница 10

Convergent strabismus.	Permanent	V. to 1/7.	V. < 1/7 to V 1/12.	V. < 1/12 to V. ⅓6.	V. < ⅓6.	Excluded.	Periodic.	V. to 1/7.	V. < 1/7 to V. 1/12.	V. < 1/12. to V. ⅓6.	V. < ⅓6.	Excluded.
Myopia	44	26	2	4	7	5	10	10	—	—	—	—
Emmetropia	85	48	6	20	7	4	13	9	2	1	—	1
H ? to H. 1 D.	30	17	2	5	2	4	8	7	1	—	—	—
H. 1 D. to H. 1·5 D.	23	13	3	3	3	1	14	12	—	1	—	1
H. 1·5 D. to H. 2 D.	41	26	3	3	2	7	20	16	2	1	1	—
H. 2 D. to H. 3 D.	58	26	5	17	4	6	30	24	3	1	1	1
H. 3 D. to H. 4·5 D.	35	18	1	9	—	7	19	14	1	3	1	—
H. 5 D. and more	9	3	3	2	1	—	7	4	3	—	—	—
	325	177	25	63	26	34	121	96	12	7	3	3

According to this the percentage of the hypermetropia (including doubtful cases) amounts to 66 per cent. Dr. Isler in his dissertation, 'The Dependence of Strabismus on Refraction,' gives the percentage of hypermetropia in convergent squint as 88 per cent.—a great difference, which can, however, be partly accounted for. Isler found in hypermetropia of 2 to 10 dioptres squinting in 75 per cent.; in my statistics H. 1·5 D. to the highest degrees of hypermetropia are likewise represented by 75 per cent. As the difference between H. 2 D. and H. 1·5 D. amounts to only half a dioptre, the results of the statistics agree perfectly within these limits; the difference lies only in the slighter degrees of hypermetropia, for the diagnosis of which refer to pp. 12 to 14.

The influence of hypermetropia is very apparent in the percentage of periodic squint. While in myopia, emmetropia, and slight hypermetropia, the sum total of permanent as compared to periodic squint is as 100: 19·5, this number mounts in hypermetropia of 1 D. to H. = 3 D. to 52·5 and in the higher degrees to 59 per cent. Despite the small number of cases it is probably no mere accident that in the highest degrees (of H. = 5 D. and more) this percentage is calculated at 77·7.

But just this undoubted favouring of periodic squint by hypermetropia, helps to show that this condition is one of the causes of squint, but not the only one, for in periodic squint just those conditions are wanting which induce a permanent deviation.

It is further proved by the table that in convergent strabismus, myopia appears just about as frequently as the higher degrees of hypermetropia (of 3 dioptres and more). The fact that these are not so strongly represented in convergent strabismus, as one would have expected according to his theory, had also struck Donders. "This cannot be wondered at," he continues, "the power of accommodation, even with increased convergence, does not here suffice to produce clear images. One gains much better ideas by practice from imperfect retinal images than by correcting, as far as possible, the retinal images by a maximum of accommodation." I can concede neither to the facts on which the theory is based nor to the theoretical structure itself.

An additional statistic which I drew up of the cases of hypermetropia which occurred during one year in my private practice, showed that the higher degrees are rare in the same proportion as cases of convergent strabismus are, with the corresponding degrees of hypermetropia. Further, however, I maintain that as a rule, at the age when squint usually begins, the accommodation really suffices to overcome even high degrees of hypermetropia. In all cases where we find full acuity of vision without correction of extreme hypermetropia—and this is frequently the case in young persons who do not squint—we may assume that the accommodation perfectly suffices to produce clear retinal images, without excessive convergence. In full acuity of vision even high degrees of hypermetropia are no trouble to children. Asthenopia, which occurs in children in connection with hypermetropia, is nearly always accompanied by defective vision. Were the increased demand on the accommodation really the cause of convergent strabismus, asthenopia would be far more common than it is among hypermetropic children who do not squint.

One can assert, with far greater right, that a sufficient ground for squint is not given by slight degrees of hypermetropia, for the latter are accommodatively overcome and binocular fixation retained by youthful persons without any difficulty, even when the additional motives enumerated by Donders are present. I have endeavoured to obtain a foundation for the depreciating influence of these circumstances favorable to squint, for I counted in my private practice, at the same time with the cases of squint, those cases also in which, despite those conditions which lessen the value of binocular vision, squinting was not present. Taking notice then of those cases in which the hypermetropia of the better or less hypermetropic eye amounted to at least 1·5 D., in order to allow the influence of the hypermetropia to be more conspicuous. The patients from which the above-cited 219 cases of convergent strabismus with a hypermetropia of at least 1·5 D. are drawn, comprised also 117 cases in which, with the same degree of hypermetropia and simultaneous difference of refraction or monocular amblyopia, no convergent squint was present; of these cases 101 had acuity of vision to 1/7; less than 1/7 to V. = 1/12 7, and V. less than 1/12 to V. ⅓6 9 cases. The percentage 219: 117 = 100: 53, which is yielded for the middle and higher degrees of hypermetropia, is not exactly convincing for the accommodative theory of squint; it would be placed still less favorably if we were to include the lowest degrees of hypermetropia in the statistics.

In face of these facts I do not consider it a happy question, that of seeking after "reasons for the prevention of squint." We do not want to quarrel with Donders over the question why all hypermetropes do not squint. Here, of course, I quite agree with Ulrich that squint does not occur if the necessary muscular conditions are absent. The identity of the fields of vision, on the other hand, seems to me to be of no importance for the age at which squint usually commences. This identity presupposes the habit of binocular fusion; but convergent squint arises, as a rule, before this habit is acquired. But even if binocular fusion were already learnt, it is given up with astonishing rapidity by children as soon as squint develops itself (see Case 16). The fixed habit of binocular fusion and the identity of the fields of vision dependent on it, is contracted only when squint does not occur, notwithstanding the presence of conditions favorable to it.

However, the number of cases is so considerable in which, despite the presence of the causative motives suggested by Donders, no convergent strabismus is present, that the co-operation of other causes is necessary for the production of squint, and the first thing we do is to think of those causes which lead to squint even without hypermetropia.

The attempt has really been made to attribute the commencement of convergent strabismus to the accommodation even in emmetropia, and offers fresh proof how easily facts are overwhelmed by theories. Donders originally gave it as his opinion, that loss of power or paresis of the accommodation produces strabismus just as little as the decrease in the amount of accommodation which comes with increase of years; a year later, because he could not agree with Donders' theory, Javal declared the principal cause to be due to weakening of the accommodation and not the refraction, but without producing any other ground for the assertion than that of his own good pleasure. Afterwards, Donders sought to explain the occurrence of convergent strabismus in emmetropia by paresis of accommodation, which must indeed, according to his theory, produce the same result as hypermetropia.

I content myself by reminding my readers, that at the age when convergent strabismus usually arises, between the second and third year of life, a determination of the near point is utterly impossible; a foundation in fact is therefore wanting to the theory. But, further, if paresis of accommodation really had the significance assigned to it, atropine, which is so frequently used in the ophthalmic treatment of children, would be followed by convergent strabismus. This is still more the case with diphtheritic paralysis of accommodation, which is present more frequently than we are aware of, for it is only a trouble to children in the schoolroom, in younger children it passes through its natural uninterrupted course of recovery unobserved, in hypermetropia as well as in emmetropia. If the accommodation were really of great importance in the occurrence of squint, convergent strabismus would frequently be an after symptom of diphtheria, which, as is known, is not the case. The few cases of squint which I have seen after diphtheria, had their origin in paresis of the external rectus, which was proved by the objective defect in movement, as well as by the disappearance of the squint, with the recovery of the paralysis of the abducens.

That the accommodation can play a part, is shown by the rarity of periodic accommodative squint, but for the great majority we must seek the chief cause of squint in emmetropia and myopia, in elastic preponderance of the internal recti and insufficiency of the externi, and it is apparent that the same causes will also be influential in hypermetropia.

In hypermetropia, if one causes fixation at about 30 cm. and then covers the eye with the hand, it frequently deviates inwards. Donders infers from this, that most hypermetropes prefer to sacrifice comfortable and clear vision in order to retain binocular vision. Now, it is easy to convince oneself that youthful hypermetropes see distinctly even without correction of their hypermetropia, and we may assume that they see comfortably if they do not complain of asthenopia; but that is by no means always the case, for the appearance of asthenopia is conditional on the relation of the degree of the hypermetropia to the amount of the accommodation, which, apart from a few other causes, depends chiefly on the age of the patient.

Just as we refer the deviation outwards of the covered eye to insufficiency of the interni or preponderance of the externi, we may conclude an inward deviation of the covered eye to be due to insufficiency of the externi or preponderance of the interni, and this all the more, as in hypermetropia the covered eye very frequently remains in fixation, and falls away exceptionally into relative divergence.

Just as in myopia even in the lesser degrees, insufficiency of the interni or preponderance of the externi is not rare, so in hypermetropia insufficiency of the externi or preponderance of the interni appears to be frequent; and if this disturbance of the muscular balance be followed even in myopia or emmetropia by convergent strabismus, this will of course happen still more easily if at the same time hypermetropia, or even without hypermetropia, the remaining favouring conditions mentioned by Donders are present. Of course I do not deny the effect of the hypermetropia and of those other favouring conditions, but only wish to draw attention to the fact with reference to them, that as a rule they do not of themselves suffice to produce convergent strabismus.

Nebulæ have always been regarded as one of the causes of squint; here I quite agree with Donders that they may operate, firstly, as general causes of weak sight; secondly, through this, that the irritated condition, combined with the keratitis, may produce a spasmodic, afterwards a trophic shortening of the muscles; but this seldom happens.

Whether nebulæ are found rarely or often in squint, depends in great measure on the statistic materials which are worked out. In my statistics they do not occur in any quantity worth mentioning, because in private practice purulent ophthalmia keratitis, and in short, the whole army of external inflammations of the eye is much rarer, than in that portion of the populace which fills public clinics. Further, it is to be observed that the mere occurrence of nebulæ in squint proves nothing—even squinting eyes may develop keratitis. We must at least require to be assured that the squint began after the keratitis.

Among the causes which promote the occurrence of squint, Donders mentions also conditions which diminish convergence. We have ascribed a very important rôle to the muscles, and have only to occupy ourselves here with the relation between the visual line and the axis of the cornea, which we have already mentioned on page 2. Donders has measured the angle a in ten cases of hypermetropia with convergent strabismus, and from the comparison with hypermetropic non-squinting eyes draws the conclusion, that in similar degrees of hypermetropia a higher amount of a specially disposes to strabismus. I will not repeat here the witty deduction by which Donders seeks to point out that a higher value of a must be followed by insufficiency of the externi and preponderance of the interni; the concession is enough that these circumstances exist and are the cause of the squint.