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CLINICAL INVESTIGATIONS ON SQUINT
A MONOGRAPH
BY
DR. C. SCHWEIGGER,
PROFESSOR OF OPHTHALMOLOGY AT THE UNIVERSITY OF BERLIN
TRANSLATED FROM THE GERMAN
BY
EMILY J. ROBINSON
EDITED BY
GUSTAVUS HARTRIDGE, F.R.C.S.
LONDON
J. & A. CHURCHILL
11, NEW BURLINGTON STREET
1887
TRANSLATOR'S PREFACE
The subject of Squint is so interesting that we venture to think an
English rendering of this exhaustive monograph will be acceptable to
many ophthalmic surgeons and students.
While adhering as far as possible to the spirit and style of the
original we have not hesitated here and there to give a somewhat free
translation. This has been partly necessitated by the difficulty of
finding an exact equivalent in English for all the terms used in the
original text.
In the German Edition the old system of inches is used. We have (with
the consent of the author) altered these to the dioptric system.
E. J. R.
G. H.
PREFACE
_Amicus Plato, amicus Socrates, magis amica veritas._ May my friends and
colleagues, whose views differ from mine, read the following
observations without prejudice. A fact, which does not agree with the
system, is generally worth more than theory, still it is very difficult
for even the most important fact to find recognition if it contradicts
received opinion. For theories and dogmas are narcotics, which are
necessary to men; some flatter themselves by composing them, while
others content themselves by satisfying their own craving for a creed.
Reasonably applied, they may be useful, but the boundary line is only
too easily over-stepped. It is the task of science to observe also
whether theories correspond with the progress of facts. The present
reigning theory on strabismus will have to submit to various
limitations; on the other hand, we are ready to leave to the scholastic
science of medicine and its followers certain dogmas which remain
unproved and which have nothing but the fact of their existence to
recommend them.
The small compass of the following treatise proves that it was not
intended to exhaust the rich literature on the subject; I have only
referred to the same where it appeared to me necessary for the interest
of the work in hand.
Above all, it has been my endeavour to treat the subject of this
treatise (which occurs so frequently in practice) in a way intelligible
to every physician, at the same time, however, to bring sufficiently
into notice those facts and views which are of value to my special
colleagues.
C. SCHWEIGGER.
BERLIN.
INDEX TO CONTENTS.
INTRODUCTION. PAGES
Ordinary use of the word squint and its meaning. Apparent
squint. Paralytic and typical squint. Law of association.
Squint angle and linear measure of the deviation.
Permanent, periodic, latent, monolateral, and alternating
squint 1-8
CONVERGENT SQUINT.
Donders' theory and the test of it by statistics. Limits
of error in the subjective and objective determination of
hypermetropia. Statistics of convergent squint. Hypermetropia
and favouring circumstances. Participation
of the accommodation. Preponderance of the interni
and insufficiency of the externi. Nebulae of the cornea. 9-26
PERIODIC CONVERGENT SQUINT.
In myopia, emmetropia, and hypermetropia. Intermittent
squint. Accommodative squint 27-35
CONVERGENT SQUINT IN MYOPIA 36-38
SQUINT FROM PARALYSIS OF THE ABDUCENS 39-40
HYSTERICAL SQUINT 41-43
DIVERGENT SQUINT.
Absolute and relative divergence. Statistics of divergent
squint. Causes 44-49
DYNAMIC SQUINT, INSUFFICIENCY OF THE INTERNI AND
MUSCULAR ASTHENOPIA.
Diplopia and power of overcoming prisms. Facultative
divergence. Dynamic absolute divergence. Parallel
strabismus. Relative divergence in myopia. Muscular
asthenopia. Dynamic relative divergence. Treatment
of muscular asthenopia 50-63
BINOCULAR VISION IN SQUINT.
Single vision in squint. Theory of exclusion. Forms of
binocular vision in squint 64-74
VISUAL ACUTENESS OF THE SQUINTING EYE.
The trial of vision and its results. Appearance, diagnosis.
Peculiarities and statistics of congenital defective vision.
Relation of the same to defective vision in squint 75-104
CURE OF SQUINT.
Spontaneous cure. Voluntary loss of the habit. Cure of
convergent squint by means of convex glasses. Strabotomy.
Tenotomy. Advancement. Result of the
operation and choice of methods. After-treatment by
means of influence on the ocular muscles and on the
accommodation. Aim of more extended results of the
operation. Artificial strabismus. Operation for periodic
convergent squint. Strabotomy in homonymous diplopia.
Operation for squint after paralysis of the abducens.
Operation for divergent squint and for periodic divergence.
Degree of the result of the operation. Determination
on the age best suited for operation 105-141
SQUINT
INTRODUCTION
By squinting, in the German vocabulary, is understood every oblique
direction of the visual axes. We prefer that the eyes which turn towards
us should do so in a straight line, and feel it to be something ugly and
out of harmony, if anyone squints at us. AEsthetic feeling is, however,
too individual and uncertain a guide to be laid down as a foundation for
the decision of questions of medicine. Parents have repeatedly brought
to me children said to squint, when frequent and careful examination of
them showed normal position of the eyes and perfect binocular vision;
the over-anxious parents had taken mere physiological convergence or
side glances for squinting.
On the other hand, cases appear in which such a strong semblance of
squinting is present, that at the first glance one cannot say whether
absolute fixation takes place or not. A very simple examination suffices
to determine these doubts:--Cause the patient to gaze at a certain point
on the horizon and cover first one eye and then the other. If the
covered eye remains stationary, no squint exists, but if it is observed
that when giving one eye its freedom and covering the other, the first
must make a movement in order to fix the object to be looked at, it is
only a question of discovering whether the squint does not simply ensue
from the covering up of the eye. We will return to these cases at
greater length, in order to occupy ourselves now with the fact, that the
examination above referred to proves the non-existence of strabismus,
while appearance still allows us to suspect its existence.
This apparent contradiction finds its explanation in the fact that the
scientific notion of squinting is determined by the direction of the
visual axes. Strabismus is present when one eye only is directed to the
fixed point, while the visual line of the other eye deviates from it.
But we cannot see the direction of the visual line, we can only judge of
it from the position of the cornea. It is exactly that line which joins
the point fixed with the centre of the fovea centralis. We can determine
the position of the cornea by a perpendicular line passing through the
centre of the cornea; this does not coincide with the visual line but
deviates from it about 5 deg. outwards. In the case of parallel lines of
vision the corneae are directed slightly outwards, a position which we
are accustomed to consider as the normal one. If the angle formed by the
above-mentioned perpendicular and the visual line is larger than usual,
_i. e._ if the corneae move further outwards than usual, the unusual
appearance strikes us, and gives us the impression of a divergent
squint. The enlargement of this angle, which is usually indicated as
Angle a, is a peculiarity of the hypermetropic eye; and where we have an
apparent divergent squint we may expect to find also hypermetropia,
while an apparent convergent squint occurs occasionally in myopia of
high degree.
If we turn now to those cases in which a real deviation of the visual
line occurs, we must first consider the cause, and afterwards
distinguish it from paralysis of the ocular muscles. The faulty position
may be constantly present or it may only occur when the paralysed muscle
is called into action. It is almost invariably combined with double
vision; sometimes the latter is the prevailing symptom, whilst the
faulty position of the eye is in no way obtrusive, and can only be
proved by careful investigation.
In contrast to paralysis of the ocular muscles stands the typical
concomitant squint, in which the squinting eye normally accompanies the
movements of the other. Transitional forms may thus be brought about, in
some of which the paralysis recovers, with complete or almost complete
restoration of movement, but with continuance of the squint. On the
other hand, in concomitant strabismus, restriction of movement towards
the opposite side not unfrequently develops itself.
This impairment of movement has its origin generally in a want of use.
Those who squint have less need for movement, since one of their eyes is
already directed obliquely. In divergent strabismus this is apparent,
but in convergent strabismus the squinting eye governs the field of
vision on the side to which it turns. When the fixing eye is turned
towards the side of the squinting eye in convergent strabismus, the
latter, it is true, makes a concomitant movement, which does not,
however, bring it by a long way to the limit of the movement of which it
is capable. The defect of motion is therefore generally present in both
eyes, and is usually most marked in the squinting eye. Often, indeed,
there is present at the same time a congenital or acquired insufficiency
of the antagonistic muscle, but that want of use has also much to do
with it, is shown by the improvement of mobility that often follows even
short practice.
From the law of equal innervation, which governs the movements of the
eyes, it follows that the fixing eye lapses into the associated
deviation as soon as the squinting eye is directed straight forwards.
If, for example, a convergent squinting eye is put into fixation, an
innervation of the external rectus, with which just as strong an
associated contraction of the internal rectus of the other eye, is
called forth; the direction of the squint then, as well as the degree of
deviation, is transferred from one eye to the other. It is naturally the
same with divergent squint.
Squinting upwards or downwards seldom occurs as a symptom by itself;
more frequently it is associated with convergent or divergent squint.
According to the law of associated movements, when an eye squinting
upwards is put into fixation, the other eye should make a movement
downwards, as normally both eyes move together up and down, yet this is
not always the case. For example, when an upward deviation is present in
convergent squint, it not uncommonly follows that the secondary
deviation of the eye which usually fixes is also inwards and upwards;
only exceptionally in cases of deviation in height of the squinting eye
does the sympathetic movement take place without change of height.
Sometimes with deviation of height, I found combined a distinct rotation
of the eye, generally thus, that together with the movement upwards was
combined a rotation of the vertical meridian outwards and _vice versa_;
in fixing the eye a rolling inwards was combined with the movement
downwards. The other eye then usually showed a similar rotation (thus
the meridian of both eyes rotated simultaneously to the right or left),
but the deviation in height was not always the same.
The law of equal innervation requires in alternate fixation, first with
one eye, then with the other, that the same degree of deviation be
transferred to the non-fixing eye. When exceptions appear, and the
deviation in the two eyes is unequal, it is (provided the inequality has
not been caused by attempted operation, or is the result of paralysis),
usually to be explained by the fact, that an accommodative movement
takes place when we are expecting an associated one. For example, if
there is convergent squint and hypermetropia in both eyes, but more
hypermetropia in one than the other, in alternate fixation it will be
found that the least hypermetropic eye always undergoes the greatest
deviation, because in fixation with the more hypermetropic eye a
stronger effort of accommodation unites itself with a corresponding
innervation of the internal rectus, which is transferred equally to the
other and non-fixing eye. Thus it happens frequently in divergent
strabismus, when one eye is myopic, the other emmetropic. If the latter
fixes an object stationed near the "far point" of the myopic eye, the
internal recti and the accommodation act simultaneously; on the other
hand if the myopic eye fixes, it wants no accommodation and the
emmetropic eye sinks into divergence.
With regard to the immutability of the squint; it must not be understood
that the squint angle always remains the same with the same individual;
in most cases the amount of deviation varies, the squint is now less,
now greater; it is desirable however, to know the bounds within which it
fluctuates.
To determine the degree of the squint one can either ascertain the angle
of the squint, or use v. Graefe's so-called linear measure of deviation.
The squint angle is that angle, which the visual line of the squinting
eye encloses with the direction it ought normally to take--it may be
measured with the aid of a perimeter. The patient's head is so placed by
means of a chin rest, that the axis of the squinting eye is in the
centre of the arc of the perimeter; a distant point in the centre of the
field of vision is fixed. Behind the patient is a candle, the reflection
of which is thrown into the squinting eye by means of a plane mirror;
now slide the mirror along the arc of the perimeter, till the reflection
on the cornea stands in the centre of the pupil of the eye which is
under observation. The point which the mirror occupies on the arc of the
perimeter, indicates the squint angle. In deviation in height of the
squinting eye, bring the arc of the perimeter into the corresponding
direction and so measure at the same time the degree of deviation in
height. Were the method more exact than it is, one would be able to
measure the angle formed by the visual line and the axis of the cornea.
To find the linear measure of the deviation, cover the fixing eye and
allow the squinting eye to fix. Hold a millimetre measure close to the
under lid, so that a chosen portion of it stands under the centre of the
pupil; uncover the other eye and when the squinting eye returns to its
deviation, it can be seen over which point the centre of the pupil
stands, and the linear measure of the deviation is thus obtained. The
secondary deviation of the other eye is measured of course in the same
way. If, in consequence of amblyopia, the squinting eye possesses no
certain fixation, the measure may be so held that the _nil_ point of the
division coincides with the lower punctum, and then in unchanged
fixation the portion lying under the centre of the pupil is determined,
first in the sound and then in the squinting eye.
The execution of one or other of these forms of measurement is in every
case to be recommended, and if their exactness is not as perfect as can
be desired, still, on the other hand it should be remembered, that for
surgical treatment, an exact measurement of the deviation does not
possess the importance sometimes assigned to it, as in most cases the
squint angle shows considerable variations.
In a large number of cases these variations are so great, that a correct
position of the eyes alternates with a more or less considerable squint,
which as the case may be, appears seldom or often, sometimes only under
certain conditions, and sometimes quite unexpectedly (periodic squint).
In some cases stationary or permanent squint begins with the periodic
form, however, one must not conclude that periodic squint is invariably
the precursor of the permanent form. In by far the greater number of
cases periodic squint continues unchanged without ever becoming
permanent.
The transition from squint to the normal condition is formed by those
cases, in which the proper position of the eyes is maintained by a
desire for binocular single vision, while the elastic tensions of the
muscles are such, that squinting sets in as soon as binocular single
vision is rendered impossible (latent squint).
The squint is generally one sided (monolateral), for the eyes in this
case are usually of unequal value, and the best is always preferred for
use. The eye which has the acuter vision is always made use of when
something has to be carefully observed. But when the acuteness of vision
is equal, and one eye is emmetropic and the other hypermetropic, or if
both are hypermetropic but in varying degree, the most hypermetropic eye
is always the squinting one; for with a greater power of accommodation
it does not accomplish more than the emmetropic or less hypermetropic
one with slighter expenditure of strength. Why should a man strain his
accommodation when no advantage is thereby gained?
In most cases the squinting eye has also an available power of vision
and is on that account used for fixing objects which lie in the
direction of its visual axis; it can also be made to fix objects in
front, this occurs as soon as the other eye is covered; it remains as
the fixing eye till the next blinking of the lids, or movement to
another object for fixation, or till both eyes are closed for a short
time, when it returns to its former deviation.
A true alternating strabismus, _i. e._ alternate use of first one eye
and then the other to fix objects straight ahead, only occurs when both
eyes are of equal value as regards weakness and acuteness of vision, or
when one is more conveniently used for near, and the other for distant
vision. In these circumstances one eye is always short-sighted and is
used for near objects, while the other is emmetropic (or in less degree
near-sighted or long-sighted) and is preferred for distant things. The
reason for the alternation lies in the necessity for the act of vision
itself; it begins regularly whenever distant and near objects are
alternately fixed. Alternating squint is usually divergent, with short
sight on one side, still convergent strabismus may occur under these
conditions.
CONVERGENT SQUINT
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 nebulae, 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,
nebulae, &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. 1/24 - 1/36 1 case.
V. Less than 1/36 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. 1/24 - 1/36 2 cases.
V. less than 1/36 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. 1/24 1 case.
V. 1/36 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 nebulae, 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. = 1/36 20 cases; V. less than 1/36 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. = 1/36 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. = 1/36 5. V. < 1/36 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. = 1/36 3. V. < 1/36 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. = 1/36 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. = 1/36 3; V. < 1/36 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 1/36 1; V. < 1/36 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. = 1/36 17; V.
< 1/36 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. = 1/36 1; V < 1/36 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 1/36 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. = 1/36 3; V. < 1/36
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. = 1/36 2; V. < 1/36 1 case.
(_b_) Periodic 7; V. to 1/7 4; V. < 1/7 to V. = 1/12 3
cases.
_Table of Refraction and Acuity of Vision in Convergent Strabismus._
[Transcriber's note: Key created to make table fit page]
KEY:
A: Permanent
B: V. to 1/7.
C: V. < 1/7 to V 1/12.
D: V. < 1/12 to V. 1/36.
E: V. < 1/36.
F: Excluded.
G: Periodic.
H: V. to 1/7.
I: V. < 1/7 to V. 1/12.
J: V. < 1/12. to V. 1/36.
K: V. < 1/36.
L: Excluded.
--------------------+---+----+---+----+----+----+----+----+----+----+----+----
Convergent | | | | | | | | | | | |
strabismus. | A | B | C | D | E | F | G | H | I | J |K | L
--------------------+---+----+---+----+----+----+----+----+----+----+----+----
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. 1/36 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.
Nebulae 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.