Heterophoria

Heterophoria (from Greek heteros—different and phero—I strive) is a condition in which the eyes, solely under the influence of mechanical forces arising from differences in anatomical relationships, are positioned such that their visual lines cease to be parallel, as they are in the ideal resting position (orthophoria) given absolute identity of anatomical structure on both sides. If the disproportion of structure on one side or the other does not exceed known limits, then the impulse for fusion, which accompanies the unpleasant sensation of double images inherent to most people, corrects the position of the eyes. The resulting involuntary change in muscle tone counteracts the eyes' tendency toward a different alignment and keeps this tendency in a latent state. For this reason, heterophoria is also called latent strabismus. The tendency toward a different alignment in heterophoria can be directed in any direction: a tendency to turn such that the visual line of one of the eyes would be directed toward the midline of the body is called esophoria (latent convergence); a tendency to turn in the opposite direction is exophoria (latent divergence); a tendency to turn upward or downward is hyper- and hypophoria (latent vertical divergence). When fixing on a close distance, a latent tendency toward asymmetry may manifest where it was not present for a far distance, and take on the opposite character where it was; thus, for example, orthophoria or esophoria may be accompanied by latent divergence for near vision. The fear of double images forces the eyes in heterophoria to remain in a position of symmetry. Such a forced position is accompanied by a certain expenditure of effort. The more significant the obstacle that the muscles have to overcome in this process, and the less the reserve of strength in the organism in general, the sooner phenomena of overexertion (muscular asthenopia) occur. These phenomena can reach significant degrees and are expressed in pain, dizziness, and nausea. It is characteristic of them that they disappear immediately as soon as one eye is excluded from the act of binocular vision. In this case, with the cessation of the danger of binocular double vision, the excluded eye immediately occupies its natural position, and the need for muscle overexertion thereby disappears. As soon as the eye has occupied its characteristic position, the latent tendency toward asymmetry turns into a manifest deviation. In order to determine the character and degree of the deviation, one allows the inevitably associated diplopia (see) to manifest. For this purpose, the eye is excluded from binocular vision not by simply closing it, but by placing a prism or a densely colored glass or so-called Maddox rods in front of it. When examining muscle balance for distance, the patient is asked to fixate on an object located 5-6 m from the eyes; when examining for near, the fixation point is moved to a distance of 30 cm. The character of the deviation is judged by the arrangement of the double images; by eliminating the double vision with the help of prisms, the magnitude of the angle of deviation is judged by the strength of the prism found necessary for this. To combat the distressing sensations that heterophoria can cause, one resorts to general strengthening treatment and prescribes prisms that correct the deviation. However, in doing so, one must consider the impossibility of prescribing prisms stronger than 3-4° for each side, and in those cases where there is a need for stronger prisms, one moves on to surgical measures.