Binocular mechanisms in small-angle strabismus.

THE purpose of treatment of all patients with strabismus is to reduce the angle of deviation to the smallest possible value and to develop the co-operation between the eyes to the highest possible degree. Normal binocular use of the eyes may be achieved in patients with fully accommodative squints of late onset and relatively short duration, but this is not always the case even in patients who show strong fusion and stereopsis on the major amblyoscope. A certain number of such patients have a definite slight deviation on cover test at all distances; others have a manifest deviation for one distance only, arousing the suspicion that binocular vision may not be perfect even at those distances at which the cover test reveals no deviation; the subsequent history may demonstrate a tendency for the strabismus to recur; and in others the depth perception may be surprisingly poor (Naylor, Shannon, and Stanworth, 1956). One possible obstacle to perfect binocular vision in such patients is abnormal retinal correspondence with a very small angle of anomaly; this may be very difficult to detect and to interpret, so that there is little unanimity of opinion regarding its importance. There is no doubt that a number of patients with small-angle strabismus show, on a variety of tests, a difference between the objective and subjective angles of squint (Jampolsky, 1951; Levinge, 1953), but this in itself does not prove that abnormal retinal correspondence is present during everyday use of the eyes, and some surgeons (e.g. Cashell, 1954) consider that a more likely explanation of the condition is that an abnormally large degree of fixation disparity is present. Fixation disparity, which is a normal condition in most subjects with heterophoria, is a minute strabismus occurring in binocular single vision, the slightly disparate images from the two eyes being nevertheless fused to give binocular single vision. An obvious deduction is that patients who have apparently had their squint cured may in fact be left with fixation disparity and a squint which, though small, is nevertheless of clinical significance. Careful performance of the "cover-uncover" test has indeed shown that some such patients have a small manifest deviation, sometimes described as a "flick". Others show the even more characteristic partial recovery of the squinting eye towards the straight position; the small degree of convergence still remaining will be seen if the other eye is then covered, for the squinting eye will then make a further movement to take up fixation. Since these movements may be small, it is conceivable that some patients may have fixation disparity of