SYMPTOMS OF HETEROPHORIA
These depend on the kind of error present as well as the degree and
widely vary.
In general, they may be said to fall into three classes—(1) defective
vision, (2) pain of greater or less degree—(3) reflex symptoms.
_Defective Vision._ The first class may be present, even though each
eye has a normal visual acuity; since, even when compensation is very
good, the brain gets the impression of two objects, nearly, though
not quite fused; and vision may be considerably worse with both eyes
together than with either eye singly.
When compensation is considerably impaired, the diplopia becomes more
and more persistent, till the brain finally makes choice of one image
as more satisfactory, entirely suppressing the other. Visual acuity
may not suffer in either eye; but vision being no longer binocular,
everything is seen in the flat, the judgments of depth and distance
being regularly more or less defective. While this is a tremendous
disadvantage in many occupations, people gradually and not infrequently
become accustomed to these visual defects and are not conscious of the
handicap.
_Pain._ It is quite different with the second set of symptoms, which
are always accompanied with pain. In fact, the character of the
subjective symptoms in refractive errors and muscular imbalance is so
similar that it is practically impossible to differentiate in many
cases.
In muscular asthenopia, however, in addition to becoming easily tired,
the patient often complains that letters seem to jump or run together
or he may contend that he sees double for an instant; or again that he
can “feel his eyes turn” involuntarily in their sockets. These pains
or conditions are sometimes present only during actual _use_ of the
eyes. At other times they persist for hours. In some cases, after days
or weeks of overstimulation, an explosion in migraine form occurs at
irregular intervals. This condition often lasts a day or two.
_Reflex Symptoms._ In the third and last case, there are other reflex
symptoms—such as dizziness, nausea, fainting, indigestion, insomnia
and pains in other portions of the body—sometimes stimulating organic
diseases.
The possibility of heterophoria as a factor in chorea, migraine,
neurasthenia and other diseases which may be primarily due to unstable
nerves, equilibrium is not to be forgotten. It is a notable fact that
when the fusion compensation fails so completely that one image is
entirely suppressed, or the diplopia is so great as to be overlooked,
the symptoms often cease entirely.
TREATMENT
The treatment of heterophoria depends on a careful study of each
individual case, but it cannot be too strongly emphasized that in the
great majority of cases the subjective symptoms disappear after a full
correction of the refraction is made.
In many cases, if the visual acuity in each eye be made normal, the
fusion impulse alone will be sufficient to restore compensation.
Many cases of esophoria result from overstimulation of the centers
for convergence and accommodation, made necessary by hyperopia and
astigmatism, entirely disappearing when glasses abolish the need of
accommodation. Cases of exophoria are sometimes due to the abnormal
relaxation of accommodation and convergence which secures the best
distant vision in myopia. Likewise the correction of myopia, by
increasing the far point, may diminish the amount of convergence
necessary for near vision.
Prisms for constant use are often prescribed, so placed as to help the
weak muscles and counteract the strong. For instance, in esophoria we
find the prism which, base in, will produce orthophoria for distance
and prescribe a quarter of it, base in, before each eye. While this
is very successful in some cases, the tendency in others is for the
externus to increase slightly from constant exercise in overcoming
the prism, while the internus decreases in proportion to the amount
of work of which it is relieved. Prisms for permanent use are very
beneficial in vertical deviations, since when the images are brought
on the same level they require much less effort to secure fusion; and
when prescribed base up or down, the effect secured is commonly an
unchanging one.
We sometimes take advantage of this tendency when we prescribe for
constant use weak prisms with the apex over the weak muscle, which
gradually becomes strong from the exercise of overcoming it. This plan
is effective only in patients who have a strong fusion impulse, and
the prism selected must be weak enough to be easily overcome. We can
accomplish the same effect by decentering the patient’s refraction
lenses.
For instance, a convex lens so placed that the visual line passes
the reverse will be the case if the lens is concave. The amount of
prismatic action depends on the strength of the lens and the amount
of decentering, the rule being that every centimeter of displacement
causes as many prism diopters as there are diopters in that meridian
of the lens. Thus +1 sphere, or cylinder axis 90, decentered one
centimeter outward, is equivalent to adding a one degree prism diopter
lens, base out.
DESTROPHORIA AND LAEVOPHORIA
These are terms denoting a condition in which both eyes are capable
of abnormal rotating toward the right or left, as the case may be.
The movement in the opposite direction is most common. The patient
can often rotate his eyes 60 degrees toward the right, and to perhaps
only 40 degrees to the left. His position of rest is parallel with his
visual lines, but to the _right_, in looking at objects directly in
front, he is much more comfortable with his head turned slightly to the
left.
It is difficult to account for, except on the theory that definite
movement of the eyes is rather to the right than to the left in most
occupations. The position of the paper in writing at a desk tends
toward dextrophoria; in reading, we move our eyes steadily from left
to right and then begin a new line by a single brief movement to
the left; the things that a man uses most—whether he be laborer or
student—are kept within reach of the right hand, and in referring to
them the eyes are constantly turned toward the right.
However, when these conditions result from other imbalances, they must
be treated more carefully. For instance, a patient whose right internus
is paralysed or congenitally defective on looking to the left, has
a cross diplopia which vanishes to the right; as a result, he soon
assumes a habit of carrying his head in this position. Ordinarily, this
will cause no discomfort; but if the left internus is so weak that it
cannot follow the right externus to its position of greatest ease, the
visual lines are evidently different and the case must be treated as an
exophoria.
If, on the other hand, the _left_ internus over-balances the right
externus, the condition is an esophoria and must be treated as such.
Similar reasoning applies to the conditions known as Anaphoria and
Kataphoria, in which the visual lines are parallel to each other but
directed up or down with regard to the horizontal plane of the body.
In the first, owing to congenital abnormalities, the eyes usually tend
upward and the individual must go about with his chin on his chest, so
that his eyes may look in front and yet remain in the position of rest.
In the second, the chin is held in the air and the body arched backward.
But, unless extreme, neither of these conditions causes more than
cosmetic difficulty and both should be undisturbed owing to the extreme
difficulty of securing the same operative effect on both eyes. Suitable
prisms are much more likely to be beneficial.
[Illustration: Supports for Holding The Ski-optometer
_Floor Stand_]
[Illustration: _Wall Bracket_]
[Illustration: _Chair Clamp_]
[Illustration: _Chair Attachment with Upright_
Choice may be made from any of the above. The Wall Bracket is
recommended, unless refractionist is provided with a specialist’s
chair, to which the Chair Attachment with Upright may be attached.]