THE MONOCULAR DUCTION MUSCLE TEST
MADE WITH BOTH ROTARY PRISMS
While the previously described binocular muscle test made with the
phorometer and Maddox rod, only determines the existence and amount
of esophoria, exophoria, and hyperphoria, neither the faulty nor
the deviating muscle is located, hence a _monocular muscle test_ is
essential in order to determine whether the muscles of the right or
left eye are faulty. Furthermore, an imbalance may possibly be due to
either a faulty muscular poise, or lack of nerve force in one or both
eyes. A “duction test” should accordingly be made of each muscle of
each eye separately, followed by a comparison of the muscular pull of
both eyes collectively.
These tests are commonly termed adduction, abduction, superduction and
subduction, and are defined in the order named. They include tests of
the vertical and horizontal muscles of each eye, made individually by
means of the rotary prisms, each being placed before the eye undergoing
the test.
LOCATING THE FAULTY MUSCLE
The phorometer and the Maddox rod should be removed from operative
position, discontinuing the use of the muscle-testing spot-light,
employed in the previously described binocular test. The optical
correction, if one is required, should be left in place, while the
patient’s attention should be directed, with both eyes open, to the
largest letter on the distant test chart; or if preferable, the Greek
cross in the Woolf ophthalmic, chimney may be used. Either one,
however, should be located on a plane with the patient’s head. As a
guide for the operator, it might be well to remember that when the
handle of the rotary prism is in a horizontal position, the lateral or
horizontal muscles are being tested. On the other hand, when the handle
is in a vertical position, the vertical muscles are undergoing the test.
ADDUCTION
Adduction, or relative convergence, is the power of the internal
muscles to turn the eyes inward; prism power base out and apex in, is
employed.
[Illustration: Fig. 22—To test adduction, base out is required. Rotary
prism’s line or indicator should be rotated from zero outwardly. To
test abduction, base in is required. Indicator should be rotated
inwardly from zero.]
To test adduction of the patient’s right eye, the rotary prism should
be placed in position before the right eye, the red line or prism
indicator being registered at zero upon the prism upper scale. The two
cyphers (0) should be placed in a vertical position with the handle
pointed horizontally (Fig. 21). The rotary prism should then be rotated
so that its red line or indicator is rotated outward from zero until
the large letter—preferably the largest letter, which is usually “E”—on
the distance test-type or the Greek cross previously referred to, first
appears to double in the horizontal plane. The reading on the scale of
measurements should accordingly be noted. This test should be repeated
several times, constantly striving for the highest prism power that the
patient will accept without producing diplopia. The prism equivalent
thus obtained will indicate the right adduction and should be so
recorded, as designated in Fig. 24. The amount of adduction ranges from
6 to 28, prism diopters, the normal average being 24.
ABDUCTION
Abduction is the relative power of the external muscles to turn
the eyes outward. Prism power base in and apex out is employed. To
determine abduction, or the amount of divergence of the external rectus
muscle of the right eye, prism power with base in or toward the nasal
side should be employed. The rotary prism will therefore remain in the
same relative position as in making the adduction test (Fig. 22), with
the two cyphers (0) or zero graduations vertical, but the indicator or
red line should be rotated _inward_ from zero, or towards the patient’s
nose.
With the patient’s attention again directed to the large letter “E,”
or the Greek cross, this inward rotation should be continued until
diplopia or double vision occurs. Like the former, this test should
be repeated several times, the refractionist continuing to strive for
the highest prism power which the eye will accept. This will indicate
abduction of the right eye and should be so recorded as designated in
Fig. 24. The amount of abduction ranges from 3 to 10 prism diopters.
The normal average is 8.
The ratio of adduction to abduction is normally rated at about three
to one. In other words, it is conceded that the power of the eye to
converge is normally three times as great as its power to diverge,
the usual measurements being eight to twenty-four respectively. While
applicable in most instances, this may vary in different cases.
SUPERDUCTION
Superduction, sometimes termed sursumduction, is the relative power of
the superior recti to turn the eyes upward. Prism power base down and
apex up is employed. To test superduction, the rotary prism should be
placed in position with the two cyphers lying horizontally, with the
handle pointed vertically (Fig. 23). The patient’s attention should
again be directed to the large letter “E”, and the indicator or red
line should be rotated downward from zero. The highest prism power that
the patient will accept before the object appears to double in the
vertical plane will indicate the degree of right superduction. This
should be recorded accordingly. Conditions of this kind do not usually
exceed two or three degrees. The test, however, should be repeated
several times before the final result is recorded, as indicated in Fig.
24. The amount of superduction ranges from 1 to 4 prism diopters. The
normal average is 2.
[Illustration: Fig. 23—To test superduction, base down is required.
Rotary prism’s line or indicator should be rotated downward from zero.
To test subduction, base up is required. Indicator should be rotated
upward from zero.]
SUBDUCTION
Subduction, sometimes termed infraduction or deorsumduction, is the
relative power of the inferior recti to turn the eyes downward.
Prism power base up and apex down is employed. To test subduction,
the rotary prism should be operated with zero graduations placed
horizontally, as in the superduction test (Fig. 23), but the indicator
should be slowly rotated in the reverse direction, or upward from zero.
With the patient’s attention again directed to the large letter “E,”
or the Greek cross, the strongest degree prism thus secured without
diplopia will indicate the right subduction. The amount of subduction
ranges from 1 to 4 prism diopters. The normal average is 2.
Any difference between superduction and subduction, usually denoting
the existence of hyperphoria, should be given careful consideration.
PROCEDURE FOR MONOCULAR MUSCLE TESTING
As previously explained, after a duction test of each of the four
muscles of the right eye, the rotary prism before that eye should be
placed out of position and the procedure for adduction, abduction,
superduction and subduction repeated by means of the rotary prism
before the left eye. In case of an existing imbalance, after testing
the muscle of both right and left eyes, the refractionist can quickly
determine which muscle or muscles may be lacking in strength (Fig. 24).
In practically every instance muscle exercises or correcting prisms
may then be prescribed with definite knowledge of requirements, as
further described in the following paragraphs.
A binocular muscle test made with the phorometer, Maddox rod and
distant muscle-testing point of light might quickly indicate six
degrees of exophoria, both before and after the optical correction is
made. While this would doubtless be the correct amount of the manifest
imbalance, it would be a difficult matter to ascertain which muscles
caused the disturbance. To determine this important question, the
monocular or duction test should be invariably employed.
DIAGNOSING A SPECIFIC MUSCLE CASE
Assuming, for example, a specific case where six degrees of exophoria
was determined in the binocular test that the muscle findings in
the duction test show right adduction of twenty-four degrees, with
an accompanying abduction of eight degrees; likewise a superduction
and subduction of two degrees for each eye. With the aid of a chart
or diagram—which should be made in every case—a comparison of these
figures would indicate an exophoria of approximately six degrees, with
a corresponding weak left internus (Fig. 24). This not only shows the
muscle pull of each eye individually, but a comparison of the two eyes
as indicated by the dotted lines. Thus the relationship of the two
eyes, and their corresponding muscles is quickly indicated.
[Illustration: Fig. 24—Duction chart should be made in every case.
Above readily shows existence of muscular imbalance and proves
subduction and superduction for both eyes are equal; otherwise
hyperphoria would be disclosed. Also note abduction for both right and
left eye are equal, otherwise esophoria would be disclosed. Also note
adduction for right eye is 24° while left is but 18°, proving a case of
6° of exophoria with a left weak internus.]
A glance at the above diagram discloses the following three important
facts, all of which should be known to the refractionist before a
single thought can be devoted to the correcting of the case:
1. 6° exophoria is the amount of the insufficiency.
2. 18° adduction (which should be 24°).
3. Left weak internus.
As previously stated, the power to converge is normally rated 3 to 1,
or 8 to 24, as shown above, while the power of the eye to look upward,
is equal to the power to look downward. The diagram accordingly proves
that the muscles of the right eye are in perfect balance, having equal
muscular energy.
A comparison of the left eye shows adduction of 18 degrees with an
abduction of 8 degrees, proving a lateral insufficiency because the
ratio is less than 3 to 1; and the muscles of the left eye are at
fault. The power of 2 degrees superduction and 2 degrees subduction,
proves that no weakness exists in the vertical muscles.
After making the duction test for each eye individually, a comparison
of both eyes in relationship to each other may be more readily
determined by following the dotted lines (Fig. 24).
As previously stated, it is the inability of the two eyes to work
together that causes the imbalance, so that if both eyes were normal,
the adduction, abduction, superduction and subduction of the two eyes
would agree.
The duction chart (Fig. 24.) also shows that the corresponding muscles
of each eye agree—with the exception of the adduction of the right eye
and the left eye. This proves that the left internus is weak, measuring
only 18 degrees instead of 24 degrees; it further proves the 6 degrees
of exophoria in the monocular test, as was quickly and more readily
determined in the binocular test.
Likewise, in cases of esophoria, hyperphoria, or cataphoria, the making
of definite muscle measurements independently through the prescribed
method would show through the merest glance at a similar diagram which
muscle or muscles were relatively out of balance. Heterphoria of almost
any type, or tendencies other than normal, may be fully investigated by
making a thorough and separate test of each muscle.
Where an imbalance exists, a rapid test may be employed to distinguish
a pseudo or false condition from a true condition. This is accomplished
by first placing the two Maddox rods (both the red and white) so that
the rods lie in a vertical position. If the two lines fuse, we have
determined the existence of a false condition caused by a possible
error of refraction or nerve strain. If the lines separate, we have
determined a true muscular condition, and then only should the second
method of muscular treatment follow.
[Illustration: _Same as Model 215 but Automatic Cylinder Arrangement
omitted._
SKI-OPTOMETER MODEL 205
Embodying Spherical Lenses Combined with Appliances for Testing and
Correcting Muscular Imbalance.]