MUSCULAR IMBALANCE
The purpose of the present chapter is to acquaint the refractionist
with the operation of the Ski-optometer as “a scientific instrument
for muscle testing”—the subject being treated as briefly and
comprehensively as is practicable.
As the reader progresses in the subject of muscular anomalies, he
may carry his work to as high a plane as desired, increasing his
professional usefulness to an enviable degree.
Through the use of the Ski-optometer, muscle testing may be accurately
accomplished in less time than a description of the operation requires.
Furthermore, tedious examinations may be wholly overcome through the
discontinuance of the consecutive transference of the various degrees
of prisms from the trial-case. In fact, the latter method has long been
quite obsolete, owing to the possibility of inaccuracy. The muscle
action of the eye is usually quicker than the result sought through the
use of trial-case prisms; hence muscle testing with the Ski-optometer
is accomplished with far greater rapidity and accuracy, thus making the
instrument an invaluable appliance in every examination.
THE ACTION OF PRISMS
Students in refraction—and one may still be a student after years of
refracting—are sometimes puzzled as to just what a prism does when
placed before an eye. They refer to every available volume and are
often confused between ductions and phorias, finally dropping the
subject as an unsolvable problem. In view of this fact, it is suggested
that the refractionist should read the present volume with the actual
instrument before him.
Before proceeding, one should first understand the effect of a prism
and what it accomplishes. To determine this, close one eye, looking at
some small, fixed object; at the same time, hold a ten degree prism
base in before the open eye, noting displacement of the object. This
will clearly show that the eye behind the prism turns toward the prism
apex.
To carry the experiment further, the following test may be employed on
a patient. Covering one eye, direct his attention to a fixed object,
placing the ten degree prism before the eye, but far enough away to
see the patient’s eye behind it. As the prism is brought in to the
line of vision, it will be seen that the eye turns towards the apex of
the prism. When the prism is removed, the eye returns to its normal
position.
Similar experiments enable the refractionist to make the most practical
use of treating phorias and ductions, as well as to comprehend all
other technical work.
[Illustration: Fig. 13—An important part of the equipment for muscular
work.]
THE PHOROMETER
As previously stated, it is practically impossible to accurately
diagnose a case of muscular imbalance with trial-case prisms. For this
reason the phorometer forms an important part of the equipment for
muscle testing in the Ski-optometer, having proven both rapid and
accurate. It consists of two five-degree prisms with bases opposite,
each reflecting an object toward the apex or thin edge. The patient
whose attention is directed to the usual muscle-testing spot of light,
will see _two_ spots.
Aside from the instrument itself, and in further explanation of the
phorometer’s principle and construction, when two five-degree prisms
are placed together so that their bases are directly opposite, they
naturally neutralize; when their bases are together, their strength is
doubled. Thus while the prisms of the phorometer are rotating, they
give prism values from plano to ten degrees, the same being indicated
by the pointer on the phorometer’s scale of measurements.
As a guide in dark-room testing, it should be noted that the handle
of the phorometer in a vertical position is an indication that the
vertical muscles are being tested; if horizontal, the horizontal
muscles are undergoing the test.
THE MADDOX ROD
The Maddox rod (Fig. 14) consists of a number of red or white rods,
which cause a corresponding colored streak to be seen by the patient.
This rod is placed most conveniently on the instrument, being provided
with independent stops for accurately setting the rods at 90 or 180
degree positions. The Maddox rod has proven of valuable assistance in
detecting muscular defects, particularly when used in conjunction with
the phorometer. Thus employed, it enables the patient to determine
when the streak seen with one eye crosses through the muscle-testing
spot-light observable by the other eye, as hereafter described.
[Illustration: Fig. 14—The Maddox Rod, a valuable aid in making
muscular tests.]
PROCEDURE FOR MAKING THE MUSCLE TEST
The Ski-optometer should be equipped with _two_ Maddox rods, one red
and one white. Their combined use is of the utmost importance since
they assist in accurately determining cyclophoria and its degree of
tortion as designated on the degree scale, and fully described in a
later chapter.
When the Maddox rods are placed in a vertical position, it is an
indication that the vertical muscles are being tested; when placed
horizontally, the horizontal muscles are being tested. It should be
particularly noted that the streaks of light observable through the
Maddox rods always appear at right angles to the position in which they
lie.
The Ski-optometer should be placed in a comfortable position before
the patient’s face with the brow-rest and pupillary distance adjusted
to their respective requirements. The instrument should be levelled so
that the bubble of the spirit level lies evenly between its two lines,
thus insuring horizontal balance. The muscle test light should be
employed at an approximate distance of twenty feet on a plane with the
patient’s head. Best results in muscle testing are secured through the
use of the Woolf ophthalmic bracket, with iris diaphragm chimney and a
specially adapted concentrated filament electric lamp (Fig. 9). This
gives a brilliant illumination which is particularly essential. The
test for error of refraction should be made in the usual manner, using
the spherical and cylindrical lenses contained in the Ski-optometer,
thus obviating the transference of trial-case lenses and the use of
a cumbersome trial-frame. The time-saving thus effected enables the
refractionist to include a muscle test in every examination and without
tiring the patient—a consideration of the utmost importance.
BINOCULAR AND MONOCULAR TEST
The test for muscular imbalance may be divided in two parts. First,
binocular test, or combined muscle test of the two eyes; second,
monocular test, or muscle test of each eye separately. The latter does
not signify the shutting out of vision or closing off of either eye,
since muscular imbalance can only be determined when both eyes are
open. These two tests are fully explained in the following chapter.