for fifteen years. Statement in August, 1877: Convergence to a pencil
held before patient on the median line is only retained to about 50 cm.,
nearer, crossed diplopia occurs. In reading, binocular fixation is
possible with an effort at a nearer point. The facultative divergence
does not amount to more than 3 deg.; even by convergence to a distance of 4
mtr. prisms of 3 deg. only are overcome. Emmetropia and full visual acuity
on both sides. In Sept., 1880, three years after, the statement remained
unaltered. Patient has only used the prismatic spectacles then
prescribed off and on, as the symptoms are sometimes more troublesome,
sometimes less so, and he exerts his eyes but little on the whole.
A restriction of movement of the internal recti did not exist in these
cases; the absence of the convergent movement is not then to be set down
to the interni not possessing the proper power for acting, but only to
the fact that the impulse for their simultaneous innervation was
wanting. We frequently find this absence of innervation in divergent
squint, and then generally consider it to be a consequence of the
squint, which, however, as the above cases show, need not necessarily be
the case. If preponderance of the externi is at the same time present,
absolute divergence is the result, but not always permanent squint,
frequently only the periodic form. The anomaly of innervation may also
usually be proved in such cases, in that after the removal by operation
of the absolute divergence it continues to exist in the relative form;
it can indeed happen that for a few days after the operation convergent
squint is present for distance, together with relative divergence for
near objects.
The highest phases of this anomaly, as represented in Cases 13 and 14,
are seldom seen. Slighter degrees, which, like so many other things, are
usually designated as "insufficiency of the interni," are more
frequently met with and are combined with asthenopia. On the one hand,
in looking at near objects a tendency exists to the formation of double
images, which are removed by the action of the interni; on the other
hand, however, the habit of binocular single vision is relinquished on
account of the frequent diplopia. In all forms of squint we see that
binocular fusion is forgotten; still it seems more natural to assume
this to be the result, and not the cause of the squint, as Krenchel
does.
Another form of relative divergence is that which is brought about in
consequence of extreme myopia. The change in form of the myopic eye
diminishes its mobility, associated movements of the eyes may be
replaced by turning the head, but this is not possible for the movement
of convergence. Further, in extreme myopia the far point is generally
used for reading, &c., and sometimes even a somewhat greater distance,
because on account of the close proximity of the objects the retinal
images are so large that they are sufficiently clearly recognised even
if they are not quite distinct. At all events accommodation certainly
does not take place, hence one motive favouring convergence is removed.
Finally, however, such considerable convergence as clear vision demands
in high degrees of myopia, would be difficult even for a normally
movable eye. Reasons enough therefore exist for giving up binocular
fixation and using only the more convenient eye for reading, without
effort to the accommodation and convergence. In myopia of high degree
patients almost always read with relative divergence, and these myopes
do just what we must advise them to do, they avoid strain of the
accommodation and convergence of the visual axes and thus keep well.
Notwithstanding that this condition necessarily results from the nature
of extreme myopia, it is frequently held to be pathological, which it
certainly is not in itself. At most, the short-sightedness and change in
form of the eye are pathological; the relative divergence on the other
hand is simply a harmless result of the above conditions.
No doubts whatever exist about this relative divergence. The theory that
the demands on the working eye must be very much increased is quite
unfounded. If any harmful influence were to be feared for the fixing
eye, one would observe the same in convergent squint, when, as a rule,
one eye only is used for fixation even after operation.
In convergent strabismus, however, no one, at least no ophthalmologist,
thinks of entertaining such fears for the eye used in fixation, and
where is the physiological basis of this whole idea to be found? Is the
visual purple more active in monocular than in binocular vision, or what
physiological activity is thereby taxed in increased degree?
I have found no confirmation of Alfred Graefe's theory that in myopia
the eye chiefly used in fixation is frequently affected with choroiditis
of the macula lutea, &c., but have only observed that patients to whom
this happens seek the advice of a physician more eagerly than when the
same intra-ocular troubles befall the other usually neglected eye in
connection with myopia.
Muscular asthenopia undoubtedly occurs; it is only a question whether it
is as frequent as it is diagnosed. It has its foundation in that the
convergence necessary for reading, writing, &c., can only be sustained
by an effort of the internal recti, which exceeds their strength, and
finally results in painful fatigue of the muscles, just as accommodative
asthenopia depends on painful fatigue of the muscles of accommodation.
The similarity reaches still further. We occasionally find that despite
considerable degrees of hypermetropia no asthenopia occurs even in
persons who strain their eyes; while, on the other hand, asthenopic
troubles appear in hypermetropia which are not removed by correction of
the refraction and must consequently have some other motive. Yet still
more is this the case with those disorders, of which muscular asthenopia
may be supposed to be the cause. Notwithstanding the existence of a
considerable preponderance of the externi, muscular asthenopia may be
entirely absent. If we find, for example, that as soon as we do away
with binocular single vision absolute divergence occurs even on looking
at a distant fixed point, and that prisms of 12 deg. to 30 deg. are overcome by
divergence, we may safely assume that the elastic preponderance of the
externi must be overcome in reading, &c., in the interest of binocular,
single vision by a stronger muscular effort of the interni, which is,
however, very frequently accomplished without fatigue. Asthenopic
disorders are also frequently present together with preponderance of the
externi, which continue to exist despite the removal of the same by
operation, and must consequently have some other cause. The diagnosis of
accommodative asthenopia is as a rule confirmed _ex juvantibus_; this
cannot be asserted for the muscular form.
For example, Case 15.--Mathilde F--, aet. 21, has suffered from
asthenopic disorders for three years. The investigation at the beginning
of January, 1880, shows: On the left, myopia 4 D., V. = 5/18, No. 0.3 is
read at 10 cm.; on the right, myopia 6 D., V. 5/24, 0.3 is read with
difficulty, cylindrical glasses cause no improvement. Patient converges
to about 8 cm., on exclusion absolute divergence of 3 to 4 mm. follows,
with slight upward deviation of the right eye.
On correction of the myopia the facultative divergence amounts to = 26 deg.
Here one might easily have concluded the asthenopia to be a result of
fatigue of the interni, but this opinion was refuted by the effect of
the treatment. The double tenotomy of the externi performed on January
2nd was first followed by convergent squint with homonymous double
images, which were united by a prism of 12 deg. with the base inwards. In
the course of a few days single vision was again restored. A fortnight
after the operation, on correction of the myopia, patient could see
singly to 3 mtr.: towards both sides homonymous double images were still
present, and in fixation to 30 cm. relative divergence on exclusion of
one eye. Six months after the operation, on correction of the myopia and
application of red glass to one eye, crossed double images occur close
together, which become homonymous by means of a prism of 3 deg. with the
base inwards. Patient sees double images always, without being much
disturbed by them, yet they cannot be united by means of prisms. The
habit of binocular single vision has also gradually been lost. In
reading (without correction) a movement of convergence takes place (it
cannot be determined whether this answers exactly to the distance of the
object). If, on the other hand, one asks the patient to fix binocularly
larger objects, such as a pencil close to her, she is unable to do it,
relative divergence occurs then, as well as on exclusion of one eye. The
asthenopic disorders remain unchanged and are not removed even by
prismatic spectacles. Despite all reasons then for the supposition the
asthenopia was certainly not of a muscular nature.
The uncertainty as to diagnosis is still greater in those cases which,
according to v. Graefe, were to be designated as dynamic relative
divergence; cases in which with parallel visual axes a disturbed
balance is not present but occurs on convergence in such a way that the
interni only perform their destined work with difficulty, and are
nevertheless urged on in the interest of binocular single vision, till
they give way in painful fatigue.
According to v. Graefe the diagnosis of this condition must be carried
out in the following way. First of all the convergence must be fixed on
a near object in the median line; if one eye remains behind in the
movement it may be accounted for in various ways, for example, the
impediment of movement caused by the change in form of the eye in myopia
or the faulty innervation of the interni mentioned on p. 54. In both
cases for the most part no dynamic, but manifest relative divergence is
present in viewing near objects. It may also happen that the patient
does not converge sufficiently, merely because accommodation is absent.
This experiment does not then prove the presence of dynamic relative
convergence, and v. Graefe came to the conclusion, therefore, that a
normal position of the eyes obtained only by the habit of binocular
single vision must be relinquished so soon as we cause binocular single
vision to cease. Just as under these circumstances dynamic absolute
divergence is manifested in the observance of distant objects, so must
this be the case in dynamic relative divergence in the observance of
near objects. One eye is first excluded while looking at an object about
25 cm. distant, to determine whether it still remains in a proper
position for fixation. We have reason to believe that the position which
occurs in the excluded eye answers to the given conditions of tension of
the muscles. Still it is not necessary to cause binocular vision quite
to cease, it is sufficient and even more advantageous, simply to make
binocular single vision impossible, which we are able to do by means of
prisms. If, for example, a point be fixed lying at the usual distance
for work of 25 to 30 cm., or, according to v. Graefe, a large spot
intersected by a vertical line, and one then applies a vertically
deviating prism to one eye, the influence of binocular single vision on
the ocular muscles is removed, as the fusion of the double images
standing above one another is impossible; and nothing prevents the
assumption of a relative position of divergence instead of a proper
convergent one; as a result of this the double images show a crossed
lateral position as well as the difference in height produced by the
prism. The extent of this lateral deviation may be measured by means of
prisms, which being applied to the eyes with the bases inwards place the
double images again perpendicularly above one another. Von Graefe holds
it to be of importance to determine the strongest prisms which can be
overcome for the given distance by means of convergence and by the
outward movement of the eyes.
On the strength of this method of inquiry there is a prevalence of
opinion that the asthenopic disorders common in myopia are caused by
over-exertion of the ocular muscles; indeed people believe this so
strongly that they assume the presence of muscular asthenopia even in
individuals in whom the habit of working with relative divergence is
already firmly rooted. Relative divergence may perchance cause annoyance
through double images, though this really seldom happens, but it can
never cause muscular asthenopia, for the internal recti muscles protect
themselves by means of relative divergence from any stronger exertion.
Asthenopic disturbances are certainly frequent in myopia, but the above
method of inquiry does not at all prove that their cause lies in the
ocular muscles, for those appearances from which one concludes dynamic
relative divergence and muscular asthenopia, are found in almost all
myopes, even when the latter have no asthenopic troubles, for they owe
their origin to the nature of the myopia. Myopes learn to converge to
the distance of their far point, without exerting the accommodation; if
we now cause a point at this distance to be fixed and then exclude one
eye, or make binocular fusion impossible by means of vertical prisms,
what imaginable reason is there for the excluded eye to remain in proper
fixation? In emmetropia the habitual relation between accommodation and
convergence will be able to ensure that the excluded eye also remains
covering the fixed object, convergently as well as accommodatively; in
myopia, every discretionary relative divergence up to parallelism of the
lines of vision is perfectly justified, because no effort of the
accommodation takes place. How in the world can it be held to be
pathological that a movement of convergence does not occur, when one has
just artificially removed all those physiological conditions which could
possibly have brought it about? If one now likes, as v. Graefe proposes,
to determine the prisms, which can be overcome by means of the outward
movement, there is no doubt about the fact, that with the aid of prisms
the lines of vision may be made parallel or even divergent, the retinal
images indeed, always retaining the same distinctness, in so far as they
are not injured by the prismatic diffusion of colours. There is just as
little reason why the convergence usually attainable should not also be
restored by the aid of prisms with the bases outwards, the retinal
images are not only impaired by the prisms, but the accommodation united
with the convergence, no longer corresponds to the real distance of the
fixed point.
Enough, all these incidents, which are to prove the presence of muscular
asthenopia in myopia, occur when the investigation is carried out as
usual in the region of the far point, entirely on a physiological basis,
and must not therefore be held to be pathological without further proof.
The proof of muscular asthenopia in slight degrees of myopia,
emmetropia, or hypermetropia, is somewhat more certain; a deviation from
physiological laws is certainly present, if we find that the
corresponding convergence does not unite itself with the accommodation
for a near object, we must be quite sure that an exact accommodation for
the fixed point is also really present. It by no means follows because
one causes a large black spot to be observed at a distance of 25 to 30
cm., that an exact accommodation takes place; one can see these things
even with circles of diffusion, the retinal images are already dimmed by
means of the prisms, and one can easily convince one's self that, on the
renunciation of clear retinal images, normal eyes can reach every
attainable convergence or relative divergence by means of prisms.
Insufficient accommodation and defective convergence are, however,
easily caused by all painful sensations situated near the eye, which
make the accommodation uncomfortable and fatiguing. This applies to
every common head- or tooth-ache, and in the same manner to disturbances
arising in the conjunctiva, or which depend on the stretching of the
collective tunics of the eye in myopia, or which allow any other
so-called "nervous" origin to be suspected.
We must place the same claims to the diagnosis of muscular asthenopia as
to that of the accommodative form. Just as the latter is only detected
if convex glasses really give the expected relief, so the proof of
muscular asthenopia is only furnished when relief to the interni is
brought about by means of the appropriate remedies. For myopes, who do
not fall back on the aid of relative divergence, notwithstanding that
they possess a clear field of vision only attainable with difficulty
through convergence, it is the simplest plan to remove the far point to
about 25 to 30 cm. by specially adapted concave glasses. If only slight
myopia or none at all is present, but the relation between accommodation
and convergence is disturbed, the latter can be corrected by means of
prisms with the bases inwards--to be sure, only in a slight degree, as
prisms of more than 4 deg. are scarcely suited for spectacles, partly on
account of their weight and partly on account of the diffusion of
colours. Prisms may be ground with concave or convex surfaces, according
to the requirements of refraction or accommodation.
Finally, if an elastic preponderance of the externi can be proved by
means of considerable facultative divergence, the same may be lessened
by tenotomy of one or both externi; still after my own experience I
cannot advise the performance of this operation unless prisms of at
least 16 deg. are overcome by absolute divergence, for I have seen many
patients in other practices who have acquired convergent squint and
diplopia for distance as the sole result of the operation, while the
asthenopic troubles for near objects continue. The proof that it is not
a case of muscular asthenopia is sometimes only obtained by the
operation.
BINOCULAR VISION IN SQUINT.
The fact that those who squint do not as a rule have diplopia, while
squints depending on paralysis of the ocular muscles are combined with
diplopia, was difficult to explain as long as the view was adhered to of
identical retinal areas founded on anatomical construction. The first
explanation hit upon was that a false identity became established, an
inequality of the retinae; were this the case diplopia must of necessity
occur on correction of the squint by tenotomy.
Commencing with the assumption of a congenital identity which led under
all circumstances to the occurrence of diplopia as soon as the images of
the same object fell in both eyes upon non-identical points of the
retinae, the hypothesis was next advanced that the image of the squinting
eye was not perceived, that a constant suppression of the sensations in
the squinting eye took place. Suppression of sense-impressions does take
place; as soon as our attention is entirely engrossed upon anything, we
are in a position to disregard the impressions upon all other organs of
sense; they do not reach our consciousness. That visual sensations are
easily disregarded may be proved by experiments. Hold a small plane
mirror obliquely before one eye, with the brim pressed into the angle of
the nose so that the objects lying at the side and behind are seen in
the mirror. If the other eye is now used to read with, it is quite easy
to disregard the objects seen in the mirror provided that our attention
is not attracted to places by a particularly bright light. No doubt
those who squint also possess this physiological power, and it is
therefore certain that they make use of it under certain circumstances;
but the suppression theory necessitates that they should constantly and
always do so, since diplopia is bound to occur directly they do not do
it.
The absence of double vision is in fact the only evidence that can be
adduced in favour of the exclusion theory; this negative fact, however,
proves nothing, and is, moreover, capable of other explanations, as soon
as one abandons the theory of congenital retinal identity. The
examination of those who squint demonstrates the untenability of this
theory. People who squint seldom complain of diplopia, but double images
can be rendered apparent in a comparatively large proportion of cases,
usually with the greatest ease, by covering the best eye with a red
glass and squinting with a vertically deviating prism. Many squinters
now admit the presence of double images, but their position by no means
corresponds to the identity theory, their lateral displacement is far
too slight, or patients find themselves unable to localise the position
of the image. It sometimes happens that alternating vision with both
eyes is mistaken for diplopia, the images are then invariably specified
as homonymous; however, with attention it is easy to distinguish this
alternating vision from the simultaneous perception of two images of one
and the same object.
There can be no doubt that in most cases the position of the double
images does not correspond to the principle of identity, and just as
little doubt that one to whom double images are easily made apparent
cannot possess the confirmed habit of always suppressing the image of
the squinting eye. A certain number of cases remain in which it is
impossible to produce diplopia; that these, however, do not constantly
suppress the image of the squinting eye may be proved in the very simple
way I have indicated. An object of fixation is placed in a darkened
room, on one side of and behind the squinting eye is placed a small
flame, the reflection of which, by means of a plane mirror before the
squinting eye is thrown upon its retina. The reflection of the flame is
seen on the cornea of the squinting eye, by slight rotation of the glass
it can be brought into the area of the pupil, and at the same instant
the patient sees the light, the reflection of which can easily be made
to coincide with the image of the fixation object seen by the other eye.
The experiment has then an entirely objective basis, it always succeeds,
a fact on which I lay special stress, even in eyes whose vision is very
defective; therefore here also the habit of suppression of the retinal
images of the squinting eye is not present.
That the squinting eye really possesses its full share of the visual
field can easily be proved (especially in divergent squint) by the aid
of a perimeter. The best eye is covered with a red glass, so that the
objects projected from the fixation point, as well as the excentric
field of vision of this eye, appear red. As soon as the test object
moves towards the side of the squinting eye and enters the visual area
covered by the latter, it appears in its natural white colour, and this
in most cases before it has reached the centre of the retina of this
eye.
Another proof that the squinting eye is really used for vision appears
to me to lie in the fact that persons who squint, provided of course
that the vision of the eye concerned is not very defective, do not show
that uncertainty in the estimation of distance, which is apt to prove so
troublesome to those who have only monocular vision.
[Illustration: FIG. 1.]