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 nebulae 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 _role_ 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.
PERIODIC CONVERGENT SQUINT.
The opinion is prevalent that convergent strabismus usually begins in
the form of periodic squint, and that a permanent deviation is developed
in this way only. In many cases it may be so; on the other hand I have
sometimes seen convergent strabismus arise suddenly, without a
preliminary stage of periodic squint. This question, however, is of no
special interest. It is more important to note that periodic squint
frequently continues to exist unchanged, without ever becoming
permanent.
Like the whole doctrine of strabismus, opinions on periodic squint have
been governed by Donders' theory, regardless of facts, but as the
accommodation frequently exercises a perceptible influence, it is
judicious to consider first of all the cases in which this does not
happen.
Convergent squint in myopia begins as a rule with periodic squint, and
may continue to exist in this form: some patients who would not be
operated upon have been under my observation for years; sometimes a
correct position was retained for a long time, and sometimes strong
convergent squint was present, proving that accommodation had nothing
whatever to do with it. In myopia of higher degree the accommodation is
scarcely used--unless concave glasses are worn; still periodic squint
occurs under these circumstances. For example:
CASE 1. Miss B--, aet. 22, possesses in both eyes myopia of 6.5 D. with
full visual acuteness and without posterior staphyloma. A concave
eyeglass of 4.5 D. is used off and on for distance, and the eyes have
never been over-exerted in looking at near objects. For a long time
tendency to convergent squint, which is combined with diplopia, has
existed on the left side. The eyes generally have a perfectly normal
position, but occasionally convergent squint occurs, remains in
existence a few hours, perhaps for a whole day even, and disappears
again. The deviation here amounts to 4 or 5 mm. As the patient did not
wish for an operation, I have been able to observe the condition for
years without any change in it or without the squint becoming permanent.
The cause of periodic squint is certainly not to be sought for here, in
the accommodation.
Many cases of convergent strabismus with myopia constantly offer such a
peculiar phase of the defect, that one has accepted the statements which
ascribe to short-sightedness a determining influence on this form of
squint, without asking for further proof. It may, therefore, be useful
for our purpose to cite a few cases of periodic convergent strabismus
with emmetropia. For instance:
CASE 2. Louise S--, aet. 6-1/2, came under treatment for follicular
conjunctivitis, convergent strabismus appearing simultaneously on the
right side; the investigation showed the acuity of vision of left eye =
5/12, right V. = 5/36, the ophthalmoscope, and also mydriasis by
atropine, proved the presence of emmetropia. The squint had first been
observed when the child was about two years old, then it disappeared
spontaneously and returned again three or four months ago.
In the course of treatment, which extended over about six months, the
child came repeatedly into my consulting room, sometimes with squint,
sometimes without, in the periods during which correct fixation existed,
no squint occurred even when working. Examination with the stereoscope
showed no normal binocular fusion even during normal position of the
eyes.
CASE 3. Vera von K--, aet. 6; tendency to convergent strabismus, mostly
on right side, has existed one and a half years. Normal position as a
rule, on covering the eye immediate convergence, with a deviation of 5
mm.; with additional aid of a red glass and weak prisms deviating in a
vertical direction, homonymous diplopia is very easily provoked. Visual
acuteness on both sides 5/12, the left slightly better than the right;
emmetropia in mydriasis by atropine. A year later a repeated examination
gave the same result.
The cause of periodic squint in these cases can only be sought in the
bearing of the ocular muscles; an elastic preponderance of the interni
existed, which ceased, as a rule, on using the externi. A special
influence of the accommodation was not traceable, which does not of
course prevent this from acting differently in other cases. But in
periodic squint it may frequently be observed that the deviation
commences under influences which have nothing to do with the
accommodation, but, on the contrary, under those which weaken the
muscular energy generally, for example, fatigue, anxiety, &c.
Like convergent squint generally, the periodic form is also more
frequent in hypermetropia than in emmetropia or myopia, and we admit
that in hypermetropia the strain on the accommodation has more influence
in producing the deviation. But as the appearance of periodic squint in
emmetropia or myopia is proved without participation of the
accommodation, solely on the ground of the muscular forces--so the
presence of the same forces in hypermetropia ought not to be ignored.
It happens, indeed, that in considerable degrees of hypermetropia a
slight convergent deviation occurs only from time to time, the cause of
which, on closer investigation, can only be sought in the ocular
muscles. For example:
CASE 4. Paul F--, was first introduced to me in 1872 as a child of three
years and two months, with a tendency to convergent strabismus on the
right side of two months' standing, which was sometimes greater,
sometimes less, and sometimes was not present at all. In 1877 I saw him
again suffering from conjunctivitis, without perceiving any squint; no
examination respecting it was made. In 1880 his elder brother came under
treatment for apparent myopia, which with the ophthalmoscope proved to
be hypermetropia, and my attention, being again drawn to the eyes of the
family, I requested the younger brother to come for examination. At
first sight the position of the eyes appeared to be quite normal, on
more careful inspection slight convergent squint of the right eye showed
itself occasionally. On both sides apparent emmetropia or very slight
hypermetropia, acuity of vision on left side 5/9, on the right 5/18,
ophthalmoscopic diagnosis of refraction was impossible on account of
restless fixation.
With the addition of a red glass diplopia cannot be produced, the left
field of vision is observed in the stereoscope, then the right one on
covering the left eye; never both together. In mydriasis by atropine
hypermetropia of high degree (about 4 dioptres) is ophthalmoscopically
detected on both sides, with convex 4.5 D., V. = 5/9 with slight
convergent deviation of the right eye.
What has here prevented the transition to permanent squint with a
deviation corresponding to the great strain on the accommodation? That
the accommodation was really in action is proved simply by the apparent
emmetropia and the school-work, that no retention of binocular single
vision took place is shown by the proved incapacity for binocular fusion
of the retinal images. Nothing then remains but to accept the fact that
in the ocular muscles inducement was only given for a slight periodic
squint, not for a permanent one answering to the amount of accommodation
used.
As further proof that periodic squint may occur even in hypermetropia
quite independently of the accommodation, I should like to cite a case
of intermittent convergent strabismus which a number of other oculists
have seen besides myself.
CASE 5. Sophie S--, aet. 7-3/4, has suffered for two years from a strong
convergent squint on the left side, occurring every other day. The
deviation amounts to 7 mm. (the same deviation is transferred to the
left eye, when the right is put into fixation). On the intervening days
the position of the eyes is quite normal, on covering one only a slight
deviation takes place. The visual acuteness amounts to 5/12 on the left,
5/24 on the right, ophthalmoscopically with atropine hypermetropia of
two dioptres. Quinine has been given without avail, a convex glass of 2
D. also, which has been worn for the last half year, has not affected
the deviation.
Diplopia was not present--on the intervening days free from squint, with
the aid of a red glass, homonymous diplopia could be detected without
perceptible deviation, still it was impossible to bring about a union of
the double images by prisms. In the stereoscope the left field of vision
was first inspected, then both, still fusion of the fields of vision was
not traceable. The statements, moreover, as indeed could not be expected
otherwise in a child of such tender age, were not free from
contradictions, but the existence of normal binocular vision was very
doubtful. I therefore performed tenotomy of the left internal rectus,
after which normal position continued to exist on the following squint
days. After three quarters of a year I saw the child again; the squint
was perfectly cured, even on looking down, convergence was no longer
present. Whether a permanent cure was thus obtained, seems to me
doubtful, owing to the rare peculiarities of this case.
Mannhardt also describes a similar case of intermittent squint; that of
a girl aged eight years, in whom periodic convergent strabismus had
begun four years previously, and for two years had occurred regularly
every other day. On undecided vision the eyes were normally placed, but
as soon as a near or distant object was fixed, a considerable deviation
inwards of the left eye occurred. Under the covering hand both eyes
deviated inwards equally. On the non-squinting days strabismus could in
no way be produced even by fixation of the nearest objects, only under
the covering hand a deviation inwards ensued. The squint could not be
removed by quinine, but only by correction of the hypermetropia of 3 D.
In any case, then, hypermetropia was one of the causes of the squint,
but not the only one, as it cannot operate on alternate days only.
Javal, who tries to make this case coincide with his theory, accepting
an intermitting paresis of accommodation as the cause of squint, is
manifestly in error, as Mannhardt particularly mentions that acuity of
vision, refraction and accommodation remained perfectly equal on both
days.
If it is thus proved, that also in periodic inward squint the deviation
may occur quite independently of the accommodation, on the other hand it
is apparent, that if once a tendency to squint exists, a
disproportionately strong convergence may very easily unite itself with
the accommodation. Particularly of course in hypermetropes, who are able
to fix nothing without using their accommodation, a remarkable
fluctuation of the squint angle very frequently takes place. Sometimes
the deviation is exceedingly strong, sometimes so slight that it seems
to be absent. It is usually impossible to determine if it is really
absent, for as soon as we single out a point for fixation to make the
investigation feasible, strong deviation sets in. If in such cases we
perfectly atropise both eyes, restore the attainable acuity of vision by
neutralisation of the hypermetropia with convex glasses, and yet,
nevertheless, as is generally the case, the customary strong convergence
takes place on fixation of a distant object, there can be no talk of a
strain on the accommodation; at most we can say, that the impulse for
accommodation, habitually united with the intention to see distinctly,
and the too strong convergence combined with it, also takes place,
though by paralysis of the accommodation the participation of the same
has become impossible. As accommodative squint those cases are chiefly
indicated in which the deviation only takes place when there is a claim
on the accommodation. In most cases of this kind hypermetropia is
present. I have occasionally seen periodic accommodative squint with
emmetropia of the fixing eye.
CASE 6 may serve as an example: H. B--, aet. 15, shows a considerable and
very varying periodic inward squint. Sometimes correct position is
present, sometimes strong deviation, indeed the latter only occurs on
looking at distant objects, while for near ones correct position of the
eyes generally takes place. The examination showed for the right eye
hypermetropia 1.5, for the left myopia 3.5 D.; full acuity of vision on
both sides. The squint occurring in the left eye on looking at distant
objects was therefore accommodative; the effort of the accommodation
necessary for correcting the hypermetropia united itself to an
excessively strong innervation of the interni, as the interests of
binocular vision came but slightly into consideration on account of the
myopia in the left eye. For near objects the myopic eye is used without
accommodation and therefore also without convergent strabismus of the
right. But if one caused a point about 25 cm. distant to be fixed first
with the right (hypermetropic) eye while the left was covered and then
caused fixation to be transferred to the left, the accommodative
convergent strabismus induced was alternately transferred to the left
eye and continued, although the left eye fixed without any effort of the
accommodation on account of its myopia. Double tenotomy of the interni
and correction of the hypermetropia effected the cure of the squint.
The clearest cases of accommodative strabismus are those in which
usually a correct position and sometimes even binocular fusion is
present, while squint occurs only during the strain on the accommodation
necessary for distinct vision.
CASE 7. Miss Bertha v. Pr--, aet. 27, shows strong accommodative squint
of the right eye, said to have been observed by her parents when she was
fifteen months old. Correct position of the eyes is generally present
with indistinct vision; the endeavours to see clearly immediately causes
striking convergence of the right eye. On the left hypermetropia 3.5 D.,
vision normal; on the right the same degree of hypermetropia, vision not
more than 1/12 of the normal, no ophthalmoscopic report. On correction
of the hypermetropia and with aid of a red glass crossed diplopia
immediately appears, which is corrected by a prism of 5 deg. base inwards;
prisms of 12 deg. with the bases inwards are overcome on fixation of an
object about 12 ft. distant by divergence. The elastic tension of the
ocular muscles necessitates then a preponderance of the externi, and an
effort of the accommodation necessary to overcome the hypermetropia,
which on account of the congenital amblyopia of the right eye unites
itself with excessive convergence. Had the elastic tension of the ocular
muscles made a preponderance of the interni a condition, permanent
convergent squint would have been the result, and one would have called
the weak sight of the right eye amblyopia from want of use.
Typical accommodative squint occurs quite independently of the will on
each effort of the accommodation, and is not combined with diplopia. It
is otherwise in those cases of hypermetropia of high degree in which
patients voluntarily call forth convergent squint, and retain it for a
short time for the purpose of distinct vision. They are then perfectly
conscious of the squint, and perceive also as a rule the double images
which occur at the same time; I have seen such cases in adults who could
only produce the accommodation necessary for distinct vision by the aid
of a too strong convergence; they, however, only now and then made use
of this help. Although differing much from the typical form, these cases
of voluntary accommodative squint were also included in the statistics.
In involuntary periodic (even if not accommodative) squint, the patient
as a rule is not conscious of the occurrence of the false position; that
exceptions to this occur Case 1 has given us an instance.
CONVERGENT SQUINT IN MYOPIA.
For the aetiology of convergent strabismus it is of interest to ascertain
the age at which it is developed, and one of the first results we obtain
is the exceptional position which the union of myopia with convergent
strabismus takes in this category. Of the 56 cases contained in the
above statistics I possess reliable information of the time of
commencement in 11 cases; the squint was twice observed before the
fourth year of life, once between four and ten years of age, eight times
between the tenth and thirty-third years of life.
I must first state prominently with regard to the connection of myopia
with convergent squint that I see no reason for holding short sight to
be the cause of the squint, as v. Graefe does.
A specially severe strain of the eyes, as v. Graefe assumes, was not
traceable in the cases observed by me. Excessive convergence and strain
on the accommodation is often enough present in weak sight, for example,
in astigmatism without the existence of squint; were short sight in
general an inducement to convergent squint these cases would appear much
oftener than they actually do, owing to the frequency of myopia. In my
opinion the cause of their rarity lies in the fact that myopia is
frequently combined with insufficiency of the interni and preponderance
of the externi, but only rarely with the reverse condition of the
muscles. If, however, a preponderance of the interni develops itself
together with the myopia, convergent strabismus is easily produced, for
without correction of the myopia by spectacles, the desire for retaining
binocular single vision for everything beyond the far point is lessened
by the indistinctness of the retinal images. Within the range of their
field of distinct vision these squinting myopes frequently retain
binocular vision, while the capacity for accepting parallel rays or
retaining them for long, is lost.
Strictly speaking, the periodic squint present in these cases is of a
peculiar kind, for the binocular single vision present within range of
the convergence excludes the notion of squint; the latter only occurs
when an object lying outside the point of convergence is fixed.
Moreover, according to the common use of language, I have only used the
expression periodic convergent squint for the change between a parallel
direction of the visual axes and pathological convergence.
As squint in myopia usually commences at an age when binocular fusion
has already become a fixed habit, diplopia regularly takes place with
it, but patients become more easily accustomed to this than in paralysis
of the ocular muscles, because the retinal images are indistinct and the
double images in the field of vision always keep at about the same
distance, while in paralysis of the ocular muscles the distance is
constantly changing.
The myopia, in these cases, is not the cause of the squint, but only a
favouring circumstance. If the same preponderance of the interni is
developed at the same age in emmetropia, squint is not so easily caused,
as the distinct retinal images present in the whole field of vision
render it easy to retain binocular single vision. Therefore we see the
same form of squint arise less often in emmetropia (see Case 45) when
childhood is past, than in myopia. As a rule preponderance of the
interni in hypermetropia leads eventually to convergent squint even in
childhood.
In emmetropia and hypermetropia convergent strabismus seldom arises
after the tenth year (paresis of the abducens of course excepted),
therefore in my investigations as to the time of commencement of typical
squint I have only considered those patients who came under my treatment
before their tenth year. We must rely for the most part on the vague
statements of the parents, which lose in exactness in proportion as the
origin of the squint is of distant date; moreover, I have myself seen a
great many of the children before they were four years old. In this way
I have collected reliable information respecting the origin of the
squint in 193 cases, and of these (_a_) 88 cases occurred in children
one to three years old, (_b_) 53 in children three to four years old,
(_c_) 35 cases in children of over four years of age. It is thus at once
seen that in the great majority of cases, convergent strabismus
commences in children under four years of age, who have not yet begun to
read and write, and have no inducement to use their accommodation
severely, and still less continuously.
SQUINT FROM PARALYSIS OF THE ABDUCENS.
Convergent squint as a result of paralysis of the abducens is not very
often seen. It is first to be observed that a convergent squint,
including the whole field of vision, occurs by no means in all cases; in
about half the cases binocular fusion is retained towards the healthy
side, diplopia then only occurs when the weak abducens is exerted beyond
its strength. In those cases in which convergent squint is present in
the whole field of vision paralysis of the abducens cannot be the sole
cause, but some other cause than the most apparent one must co-operate.
An insufficiency of the externi of previous existence, or an elastic
preponderence of the interni may be considered. I have not been able to
persuade myself of the fact that hypermetropia can play any part
therein.
In by far the greater number of cases paralytic convergent squint
recovers together with the paralysis of the abducens, the field of
single vision transfers itself gradually from the healthy side to the
side of the weak abducens, and at length governs the whole field of
vision. In proportion as the muscle again fulfils its normal functions,
the habit of binocular fixation regains its power, and it seldom happens
that the elastic tension of the muscles has so changed during paralysis
that the desire for binocular single vision does not suffice to overcome
it. Case 48 furnishes an example of the fact that although the squint
occurred as a consequence of paralysis of the abducens, it certainly
remained in existence after healing of the paralysis on account of
previously existing insufficiency of the externi.
Congenital paralysis of the abducens seems more frequently to have
convergent squint as a result. If, for example, convergent squint is
observed in the first year of life, and we find a complete defect of
motion on the part of one abducens when the children become old enough
to be examined, we may certainly assume that the case is one of
congenital paralysis of this muscle, or at least that the paralysis
originated soon after birth. Doubtless, however, cases appear, of
congenital paralysis of the abducens without squint, and as these cases
are so rare I will describe two which I observed in adults.