Chapter XITHIRD METHOD OF TREATMENT—PRISM LENSES
Asstated 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 andthe 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.:
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 thatcyclophoria 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.