In the detection of heterophoria I regard the rod as the most convenient and trustworthy instrument used.
The distance at which the test is made and the dissimilarity of the images seen usually eliminate all actual effort to hold the eyes in any particular position other than that in which they stand the most easily. Accordingly the deviation is quickly noted and readily measured.
So satisfactory has this modest little instrument been in my examinations that I now rarely resort to other methods. The amount of deviation sometimes shown between this and other instruments is so slight as to make little or no difference in the measures employed for correction.
It is to be noted that cases not unfrequently occur in which a hyper-sensitive, or, on the contrary, an enervated condition exists, which is not fully indicated by any instrument. An educated judgment will here have to supply conclusions not to be drawn by any hard-and-fast rules.
After the rod and the phorometer came into use and an opportunity was presented to compare the results obtained by each, I made a careful test of eighty pronounced cases of errors of refraction accompanied by heterophoria. Of this number only nine showed a persistent difference of deviation and in none of them a difference greater than 1½°. But this was not always on the one side or the other, as six out of the nine showed a higher degree of deviation by the rod than by the phorometer. Eighty cases may not be enough upon which to base an orthodox conclusion; but my experience with the rod has been so satisfactory that I now seldom use the phorometer at all. It appears quite possible practically to estimate the degree of heterophoria as accurately with the one instrument as with the other; and while it is true that a correction of the error of refraction will commonly correct the deviation, still all cases of optical defect should be tested with the rod or phorometer before the lenses are prescribed.
Wm. A. Phillips.
My Dear Dr. Deady: Dr. Hubbell limits the discussion “to the comparative value of the diplopia test, by Stevens’ phorometer” and the Maddox rod test.
It would be interesting to follow out the idea with other phorometers,—and with the Wilson phorometer my records do not show quite such a marked difference in results,—but I have not taken pains to get comparative results in any considerable number of cases.
Dr. Hubbell says: “In the diplopia test, the dissociation is effected by changing the visual axis of one eye by means of a prism. The displacement of one image cannot be done without associating with it, more or less, an impulse to some form of ocular effort.... In the obscuration test (Maddox rod) no such effort is invited, no change of innervation takes place.” But in the rod test the light seems nearer to the patient than in the prism test. This may account for much of the difference in results and amount to “an extraneous impulse to muscular contraction.”
Dr. Hubbell is entirely justified in his conclusion as made upon experiments with the Maddox rod and the Stevens phorometer. I shall watch cases along similar lines with the Wilson phorometer and report later.