MOVEMENTS OF THE EYEBALLS AND THEIR ANOMALIES
After a careful study of the foregoing chapters, the refractionist may
desire _further_ knowledge concerning muscular imbalance—a matter in
which the Ski-optometer plays an exceptionally important part.
It should be remembered that it is only the general utility of the
instrument, _plus_ one’s knowledge of refraction and individual
diagnosis that enables the refractionist to attain maximum efficiency
in every examination, a fact which largely accounts for the following
chapter.
MONOCULAR FIXATION
When we view an object _directly_, so that it appears to be more
distinct than surrounding objects, we are said to “fix” or “fixate” it.
As the fovea is normally the most sensitive part of the retina,
affording by far the most distinct vision, “fixation,” in the great
majority of cases, is so performed that the image of the object that
is “fixated” falls upon the fovea of the eye that is “fixing.” This is
known as central or muscular “fixation.”
When central vision is absent, however, the patient is compelled to
see with a portion of the retina _outside_ of the fovea. The eye must
then be so directed as to cause the image of the object to fall on this
outlying portion of the retina. This is termed “eccentric fixation,”
and usually denotes that vision is exceptionally poor.
The ability to “fix” is apparently acquired in early infancy by
constant practice in looking at objects. Any marked interference with
vision, particularly with central vision—present at birth or soon
thereafter—will tend to prevent the acquisition of this ability, and in
_extreme_ cases the eye does not learn to “fix” at all, but aimlessly
wanders in all directions.
BINOCULAR FIXATION
We habitually use the eyes together, fixating with both at once; that
is, we direct the eyes in such a way that the image of the object to
which the attention is directed falls on the fovea of each eye.
Where both eyes are accurately directed to an object at which one or
both are looking, the condition is known as “binocular fixation,” which
is commonly understood to mean that _both eyes are straight_.
The ability to produce and maintain binocular fixation—to keep both
eyes directly straight—is acquired early in life. The impulse to
maintain it grows with exercise, and soon becomes so strong that after
the age of infancy binocular fixation is present in the great majority
of persons, and in most of them is present all the time.
Binocular fixation must be distinguished through three
conditions—_orthophoria_, _heterophoria_ and _squint_.
ORTHOPHORIA
This is the condition in which both eyes look straight at the same
object, whether both see it or not. There is not the slightest tendency
of deviation.
HETEROPHORIA
This is the condition in which both eyes keep looking straight at
the same object so long as both _see_ it; but as soon as one eye
is excluded from vision (as by a screen) that eye deviates. This
is then a tendency of deviation which is strong enough to become
manifest when either eye is covered, but which is abolished or
overcome by the compelling impulse of binocular fixation as soon as
both eyes are used for seeing. A heterophoria thus produces a maximum
deviation. The deviation is also said to be _latent_, since it is
absent under ordinary conditions and is brought to light only under
special conditions. A common though improper term for heterophoria is
“insufficiency.”
SQUINT
Squint is the condition in which there is so great a tendency to
deviation that even when both eyes are uncovered, one deviates and
only one “fixes.” It differs, therefore, from heterophoria in that the
deviation it produces is obvious under ordinary conditions.
Squint is also called strabismus, or heterotropia. In other words, in
orthophoria there is binocular fixation _all the time_ and under all
conditions; in heterophoria it is present only when the two eyes are
uncovered, so that both see the object looked at; while in squint it is
not present at all.
Or, in still plainer terms, in orthophoria both eyes are straight
all the time; in heterophoria both are straight, but only so long as
both are uncovered; and in squint only one eye is straight, no matter
whether both eyes are uncovered or not.
In squint, while binocular fixation is altogether absent, the ability
to perform monocular fixation is almost always preserved; i.e., the
squinting eye will “fix” at once if the other eye is covered. It is
only when there is marked amblyopia, particularly as the result of a
central scotoma (or spot on the cornea in the line of vision) that the
squinting eye loses its power to fix at all, and wanders uncertainly
about, receiving impressions now on one, now on another portion of the
retina.
The term imbalance is often used to denote the two conditions opposed
to orthophoria; i.e., to denote collectively heterophoria and squint.
VARIETIES OF HETEROPHORIA AND SQUINT
1. _Classification According to Direction of Deviating Eye_:
Heterophoria and squint may be classified according to the direction
assumed by the deviating eye. Thus we have the following varieties of
heterophoria:
HETEROPHORIA
LATERAL DEVIATIONS
Either eye deviates
In, or toward the nose Esophoria
Out, or toward the temple Exophoria
VERTICAL DEVIATIONS
The right eye goes up and the left down Right Hyperphoria
The left eye goes up and the right down Left Hyperphoria
In rare cases of vertical heterophoria, each eye has either an upward
tendency (anophoria) or a downward tendency (cataphoria). These cases
must not be confused with anatropia and catatropia. In anaphoria and
cataphoria, there is binocular fixation when both eyes are uncovered,
while in anatropia and catatropia _one_ of the eyes squints. This shows
the following squint condition:
SQUINT
LATERAL SQUINT
The deviating eye turns in, or toward the nose:
Esotropia (Strabismus convergens—Convergent Squint)
The deviating eye turns out or toward the temple:
Exotropia (Strabismus divergens—Divergent Squint)
VERTICAL SQUINT
The deviating eye turns up:
Hypertropia (Strabismus sursumvergens) (Right or left)
The deviating eye turns down:
Hypotropia (Strabismus deorsumvergens) (Right or left)
In addition to these lateral and vertical deviations, conditions exist
in which the vertical meridian of one eye, instead of maintaining its
parallelism with the vertical meridian of the other, either forms (or
tends to form) an angle with it (cyclotropia), but is kept in position
through muscular effort (cyclophoria.)
Cyclotropia is usually due to paralysis of one of the ocular muscles,
causing the vertical meridian of the affected eye to be tilted out
or toward the temple (extorsion) or in toward the nose (intorsion).
A tilting of the vertical meridian toward the right is also called
dextrotorsion (or positive declination); and to the left, levotorsion
or negative declination.
2. _Constant, Intermittent and Periodic Deviations_: A deviation,
whether squint or heterophoria, may be present at all times (constant),
or occasionally present and occasionally absent (intermittent). In
this case we may have heterophoria alternating with orthophoria, or
heterophoria alternating with squint; or squint alternating with
orthophoria. We also find variations such as a squint for near and
a heterophoria or orthophoria for distance; or a heterophoria for
near and orthophoria for distance; or a constant squint for near and
an intermittent squint for distance, etc. Again, a deviation may be
periodic, in that its amount for distance may greatly exceed that for
near, or vice versa.
Opposed to a periodic deviation is one which is present, and in about
equal amount, both for distance and near. Such a deviation, whether
squint or heterophoria, is called “continuous.”
3. _Alternating and Uniocular Squint_: An alternating squint is one
in which when both eyes are uncovered, so that both have a chance to
“fix”; sometimes the right eye will deviate, sometimes the left. In
uniocular (less properly monocular) squint, under the same conditions,
one eye, either the right or the left, always “fixes” and the other
always deviates. A uniocular squint is denoted as right or left,
according to whether it is the right or left eye which deviates.
[Illustration: A TYPICAL REFRACTION ROOM—THE WOOLF SANITARY ALL-METAL
EQUIPMENT
Installation comprising: Ophthalmic Chair, complete with Ski-optometer,
Test Letter Cabinet, Asceptic Trial-Case Cabinet, Muscle Testing
and Skioscopic Lamp, Ophthalmometer, Perimeter, Adjustable Tables,
Adjustable Stool.]
4. _Comitant and Non-Comitant Deviations_: In some varieties of
heterophoria and squint, the amount of deviation is the same in all
directions of the gaze, so that the angle between the visual line of
one eye and that of the other remains the same, no matter which way the
eyes are turned. Such deviations are called comitant or non-comitant,
because one eye accompanies and keeps pace with the other in all its
movements. In other cases, the deviation changes as the eyes are moved
in different directions, so that the angle between the two visual lines
constantly varies. Such deviations are termed concomitant. Usually in a
non-comitant squint the angle of deviation increases in a regular way
as the eyes are moved in one direction and decreases as they move in
the direction opposite.
In cases of long standing, however, the squinting eye, particularly
when very amblyopic, wanders in an uncertain way and apparently quite
without reference to the movements of the other eye.