Only about 20% of all exotropes are constant and somewhat more than half of them have anomalous correspondence. Constant strabismics with normal correspondence typically show suppression; those with anomalous correspondence fairly often exhibit amblyopia. For constant exotropes with normal correspondence, the treatment sequence is directed at eradicating the suppression. For constant exotropes with anomalous correspondence the treatment consists of either 1) fusional vergence training --- with a fairly good prognosis or 2) establishing normal (fovea to fovea) correspondence with the eyes in the deviated position when fusional vergence training is unsuccessful --- poor prognosis.
Before beginning either of these treatments, the acuity usually needs to be in the neighborhood of 20/40 and second-degree fusion needs to be demonstrated for targets of 5 degrees subtense.
The refractive error should be corrected with the optimum minus correction to give the clearest retinal image and maximized potential for fusion. In attempting to get functional correction, it is necessary to neutralize any vertical deviation with prisms. Such prisms are usually effective because the anomalous correspondence typically does not exhibit itself vertically. Manifestation of anomalous correspondence in the horizontal dimension, however, leads to the problem of horizontal prisms inducing a horizontal fusional vergence response. Thus, prescribing base-in prisms for an exotrope with anomalous correspondence characteristically produces an increase in fusional vergence, the angle of squint, and the angle of anomaly (co- variation).
Occlusion is used for these patients in much the same way as described for constant squinters with normal correspondence. Occlusion tends to be most effective when constant and total. When the amblyopia or suppression is deep or when rapid progress is sought, orthoptics is beneficial. Amblyopia may be more common among constant squinters with anomalous than with normal correspondence; however its management is the same. The purpose of occlusion and orthoptic treatment of amblyopia and suppression in constant exotropes with ARC is to obtain sufficiently good acuity and sufficiently little suppression to permit regular and systematic changes in fusional vergence in response to the more powerful vergence stimuli that can be generated in a major amblyoscope. Thus, the initial treatment of the amblyopia and suppression need only continue until the desired fusional vergence responses can be obtained --- regardless of the degree of amblyopia or suppression. Indeed, if fusional vergence can be appropriately driven at the outset, in spite of the presence of amblyopia or suppression, then treatment of these conditions can be deferred.
As stated at the start of this section, treatment for functional correction of constant exotropes with ARC centers around either orthoptic training of fusional vergence to overcome the deviation and angle of anomaly or occasionally (for large-angle exotropes who cannot learn fusional vergence movements) orthoptic training to break down the anomalous correspondence directly. The essential elements of the fusional vergence training to overcome the angle of deviation and angle of anomaly are to use the usually simple and rapid technique of training them to initiate convergence outside an instrument, for example as when looking from distance to very proximally placed, detailed, accommodative targets. With this method, the angle of anomaly goes to zero when the vergence overcomes the oculomotor deviation.
The main idea underlying training methods that attempt to eliminate the ARC directly is to stimulate the two foveas simultaneously with sufficient vigor (especially the deviating eye's fovea) to "re-establish" a presumed dormant normal fovea-to-fovea correspondence. When this occurs in the presence of the anomalous correspondence, binocular triplopia, and sometimes monocular diplopia, results --- indicating the simultaneous presence of normal and anomalous correspondence.
Constant exotropes who have been successfully treated functionally by optics and orthoptics may need surgery if the deviation cannot be completely controlled, if there is discomfort maintaining this control, or if the (vertical) prism required to maintain fusion is excessively large and unsightly. If there is failure with optics and orthoptics, surgery can be considered primarily for cosmetic purposes, although in some cases there is a postsurgical change in correspondence sufficient to effect a cure or to justify followup orthoptics aimed at a functional cure. According to von Noorden (1985), patients having "deep-seated" anomalous correspondence without functional potential require less surgery --- since postsurgically a cosmetically acceptable residual angle of deviation is desired so that the eyes will not later drift to a large angle of the opposite type (e.g., esotropia drifting into exotropia).