HETEROPHORIA may be defined as that condition in which binocular vision, being temporarily suspended, the visual lines of the two eyes do not intersect at the point of fixation. It is characterized by a change in the innervation of the ocular muscles when the binocular fusion of images is prevented. Under normal conditions, binocular vision for a given point is, maintained by the co-ordinate action of the entire group of these muscles, and in the ideal eye, at least within certain limits, the innervation of these muscles is not a necessary function of the binocular act. That is to say, the binocular fusion of images being suspended, the innervation remains unaltered. It becomes a function of this act only in states of heterophoria.
Heterophoria is due essentially to a condition of faulty innervation, which depends upon one or more of the following factors:.
1. The form and position of the eyeballs (orbits).
2. The place of insertion of the ocular muscles.
3. The essential and relative power of the ocular muscles (amplitude of convergence).
4. The ratio of the positive and negative portions of the relative accommodation, together with the ratio of the convergence and accommodation for the point in question.
Under the first head it is clear that, assuming certain ratios of tension among the muscles of the eye as normal when fixing some point at a given distance from the eye, such as 1 m. for example, these ratios must vary with the length of the basal line of the eyes. For at this distance, with a basal line of 50 mm., the angle of convergence is 1.43 degree, while with a basal line of 75 mm. it is 2.15 degree. In high degrees of myopia the alterations in the form of the eyeballs limit their mobility, and, consequently, modify the convergence tension of the muscles.
There is some variation in the place of insertion of the ocular muscles. Stilling has observed a wide variation in that of the superior oblique. We may assume as normal the following measurements, representing the distances of the insertion of the recti muscles from the cornea (Fuchs):.
mm.
Rectus internus, 5.5.
Rectus externus, 6.9.
Rectus inferior, 6.5.
Rectus superior, 7.7.
In an eye where the muscular balance is ordinarily good, one or more muscles may become weakened by fatigue or disease, necessitating an increase in the amount of nervous stimulus to these muscles in order to preserve binocular vision. Under these circumstances, if binocular vision becomes abrogated, heterophoria is an easy and necessary consequence. Here we have true “muscular insufficiency.”.
From the essential connection of accommodation and convergence, it is evident that the ratio of the positive and negative portions of the relative accommodation for any given point has an important bearing upon the muscular balance for that point. Indeed, if no other factor were operative to affect the muscular equilibrium, it seems reasonable to assume that it could be calculated from a knowledge of the relative accommodation. However, as a matter of fact other causes uniformly do exert an influence upon the position of the eyes, and moreover may be of such moment that their effects entirely negative that of the relative accommodation.
We see, then, that heterophoria may originate in a number of ways. From the variety of causes we may infer that there must a corresponding variation in the treatment of this disorder. We shall revert to this further on.
Methods of Examination.-In ascertaining the amount and character of the heterophoria present in a given case, the essential determination to be made is the position of the non- fixing or deviating eye. The common and most exact methods of making this determination are subjective. of objective methods, the only one that is practical is the old test of alternately covering and uncovering one eye with a screen. This is too crude to be of much value. Subjective methods depend upon the uniformity and congruity of retinal projection. The first instrument of precision for making the necessary measurements was Stevens’s phorometer.
With this instrument, supposing the patient to be of ordinary intelligence, it is possible to measure deviations of the eyes in any plane with much accuracy. The substitution of a “stopped” convex lens of short focus for the vertical and horizontal prisms employed as to the character of the deviation could be made. The “red test” of Maddox marked another gain in the rapidity of the examination, and made it possible to measure the deviation of the non-fixing eye by means of scales drawn upon the wall of the examining-room. Burnett’s use of a strong convex cylinder was based upon the same principle. Another gain in convenience and precision was the introduction of the rotating prisms of Stevens, by which the separate displacing prisms were done away with. I have added another instrument to this number, a rough model of which I have the pleasure of exhibiting to this section.
It consists essentially of a frame, holding upon its right side a cell containing two 6 degree prisms, with their bases in contact; or a Maddox rod, suitably mounted; and on the left a “prism mobile” of two 5 degree prisms, which, by suitable mechanism, may be rotated in the same or in opposite directions, and the amount of rotation measured upon a graduated circle so placed as to be easily seen by the observer. Behind the openings of this frame or slide are clips for holding various accessories, such as abducting or adducting prisms, a red glass, etc. With the double prism in proper position, and the “prism mobile” at 0 degree, set to give horizontal displacements, the patient looks at the point of light through both openings and sees three images of it.
The middle image is seen by the left eye, and by turning the milled head of the “prism mobile” it may be displaced horizontally either to the right or left, from 0 degree to 10 degree. If, therefore, this image is not in a straight line with the other two, it may be quickly brought into this position, and the exophoria or esophoria read of at once upon the graduated circle at the patient’s left.
To measure deviations in any other plane, the double prism is rotated into that plane, the “prism mobile” brought to zero, the small milled head in the face of the instrument pushed well up, and, the pinion of the recording disk being pulled out, the two prisms are rotated by means of this milled head in the same direction to the desired angle, when these adjustments, being reversed, the measurements are made in the same manner as at first. A little familiarity with the instrument will enable the observer to make these adjustments very rapidly.
The Maddox rod may be substituted for the double prism if desired. To measure heterophoria greater than 10 degree, a supplementary prism may be inserted in the proper position in one of the rear clips, and its value added to the readings of the instrument.
To measure abduction and adduction, or in fact, muscular power in any direction, the double prism or rod is removed from the right-hand cell, and the “prism mobile” having been set in the desired position, the muscular powers can be easily found by rotation of the milled head at the left. In these determinations also, supplementary prisms may be inserted into the clips if necessary.
It will be seen that a considerable variety of measurements may be made by this instrument with rapidity and accuracy. Almost any object of fixation may be used, and at my distance from the eyes. The value of the double prism is, that it is easier to determine whether three points are in the same straight line than whether (as in Stevens’s phorometer) two points are in an exactly horizontal of vertical line, as the case may be. I believe that the credit of suggesting this use of the double prism is due to Savage, although the first model of this instrument was made long before his suggestion came under my observation.
So far as accuracy is concerned, there is a substantial agreement in the results obtained by the use of the Stevens phorometer the rod test, and the little instrument above described. Bissell has made a serious of comparisons of the rod and prism tests. In fifty-two cases of heterophoria, the findings of the rod test were greater than those of prism test in twenty- six, the difference ranging from 0.25 to prism dioptries, from which he concludes that the rod test is the more accurate. I do not believe that this accords with the experience of observers generally.
For the determination of “insufficiencies of the oblique muscles,” Savage employs the double prism before one eye, and a horizontal stripe at a distance of eleven inches, as a test object. If there is an insufficiency present, the middle line will run obliquely between the other two, the direction of the obliquely being dependent upon the particular muscle at fault.
Heterophoria may be measured in degrees of refracting angle, or of minimum deviation; in prism dioptries, in metre-angles, or in centigrades. The most common method is to use the refracting angle of the necessary prism. There is at present, no agreement among oculists as to the most desirable of the various reforms that have been suggested.
