THIRD METHOD OF TREATMENT—PRISM LENSES
WHEN AND HOW EMPLOYED
As stated in the preceding chapter, on ascertaining the failure of the
second muscular treatment or method, prisms are employed for constant
wear. When prism lenses are used, whether the case is exophoria or
esophoria, or right or left hyperphoria, it is always safe to prescribe
one-quarter degree prism for each degree of prism imbalance for each
eye. For example, in a case of 6 degrees of esophoria, a prism of 1½
degree base out should be prescribed for each eye; or in 6 degrees
of exophoria, employ the same amount of prism, but base in. In right
hyperphoria, place the prism base down before the right eye and up
before the left, and vice versa for left hyperphoria.
It is not always advisable, however, to allow the patient to wear the
same degree of prism for any length of time. Many authorities suggest
a constant change with the idea that a prism is nothing more than a
crutch. Should the same degree be constantly worn, even though it
afforded temporary relief, the eye would become accustomed to it and
the purpose of the prism entirely lost. Prisms should be prescribed
with extreme care, their use being identical with that of dumb-bells,
where weight is first increased to maximum and subsequently reduced,
viz.:
PRISM REDUCTION METHOD
Where prisms are prescribed, it is considered good practice to make a
binocular muscle test and the duction test (Fig. 24) at the end of each
three months’ period, employing the phorometer, Maddox rod, and rotary
prisms, as already explained.
If the condition shows any decrease, the prism degree should be
proportionately decreased. For example, in the case originally showing
6 degrees of exophoria, one-quarter degree prism for each degree of
imbalance was prescribed, or 1½ degree for each eye. If the same case
subsequently indicated 4 degrees, only one degree for each eye should
be prescribed—and so on, a gradual reduction of prism value being
constantly sought.
Except in rare cases, prisms should not be prescribed with the base
or apex at oblique angles, as the eye is rarely at rest with such a
correction. An imbalance may be caused by a false condition in one
rectus and a true imbalance in the other, giving one the impression
that cyclophoria exists, as explained in a following chapter.
Having now employed the three methods, the refractionist can readily
understand that a marked percentage of muscular imbalance cases may be
directly benefited through the aid of the Ski-optometer. If these three
methods of procedure fail, there is nothing left but the fourth and
last method—that of operative procedure.