About 75% of all esotropes 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 squinters with normal correspondence, the treatment sequence is directed at eradicating the suppression. For constant esotropia with anomalous correspondence the treatment consists of either 1) fusional vergence training for small-angle (less than 15-20 p.d.) esotropes -- - for whom the prognosis is fairly good or 2) establishing normal (fovea to fovea) correspondence with the eyes in the deviated position for large-angle (greater than 20 p.d.) esotropes --- for whom the prognosis is poor.
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 poor prognosis for functional correction of constant ARC esotropes with squint angles greater than 20 p.d. means that cosmetic management is a frequent consideration. The preponderance of esotropes among constant squinters with ARC dictates that hyperopia will be the most common refractive error, necessitating attention to latent hyperopia. Such attention is required for either functional or cosmetic correction since maximum plus correction will relax accommodation and reduce the esodeviation which is desirable for functional treatment or for cosmetic reasons.
After correcting the refractive error, treating for functional correction is very different from treating for cosmetic correction. 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-out prisms for an esotrope with anomalous correspondence characteristically produces an increase in fusional vergence, the angle of squint, and the angle of anomaly (co-variation). (Cosmetically, the result is doubly bad because the angle of squint has actually increased and additionally the eyes look displaced nasalward through the base-out prisms.) Putting base-in prisms before the eyes of an esotrope with anomalous correspondence will produce a fusional divergence and an associated reduction in the angle of anomaly.
Unfortunately, with prisms the divergence response is limited to a few prism diopters. The strong vergence stimuli available on the major amblyoscope permit larger fusional divergence responses (than with prisms) so that eso- deviations up to 20 p.d. can be overcome with a simultaneous decrease in the angle of anomaly through covariation.
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 esotropes 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 esotropes 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 esotropes with ARC centers around either orthoptic training of fusional vergence to overcome the deviation and angle of anomaly or (for large-angle esotropes) 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 second- or preferably third-degree targets that are large, detailed, and high contrast and to change the vergence disparity slowly enough (perhaps 1 p.d. per minute) to drive the slow vergence (and maybe the tonic vergence system).
For esotropes whose deviation is less than 20 p.d., this training is conveniently accomplished with a major amblyoscope using third-degree "swing" targets (set at the subjective angle of directionalization) that are rapidly flashed while the instrument arms are slowly diverged and the patient continues to see the target singly, without suppression and ideally with stereopsis, until the eyes are physically straight and can be maintained when the patient sits back from the instrument first in a dark room and then as the illumination is slowly raised to normal (Wick, 1974). 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. Quite a few constant large-angle esotropes with ARC can be treated to this point. The difficulty in this orthoptic method occurs at this point when one attempts to eliminate the anomalous correspondence between the fixating eye's fovea and the associated peripheral area in the deviating eye.
If either orthoptic treatment of the ARC succeeds, it should be followed by training that improves the qualitative aspects of sensory and motor fusion, and perhaps by surgery for functional correction. If treatment of the ARC fails, cosmetic improvement needs to be considered using optical methods (described above) or surgery.
Constant squinters 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).