TREATING CONSTANT ESOTROPIA with NORMAL CORRESPONDENCE

The main point in treating constant esotropes with normal correspondence is to recognize that they do not fuse at any time. The challenge in treating these patients is initially to reduce suppression and any amblyopia to the point that fusion can be obtained at some distance (usually at near) at least some of the time. To this end, occlusion therapy and vision training for the amblyopia and suppression usually precede the use of optics (added lenses or prisms) or vision training to obtain fusion.

Refractive Error Correction and Occlusion

Occlusion to reduce the amblyopia and suppression is facilitated by maximum plus correction of the refractive error to provide the sharpest possible retinal image and to maximize the potential for fusion. Occlusion for constant esotropia is generally most effective when it is constant and total. It needs to be monitored frequently. When the amblyopia is deep or rapid progress is sought, orthoptics to break down the amblyopia and suppression is beneficial.

Amblyopia Therapy

Orthoptic training consists initially of monocular fixation training, with feedback, if the eccentric fixation is greater than about 4 p.d.; the amblyopia is treated by direct stimulation of the fovea with resolution targets presented under monocular conditions followed by dichoptic stimulation which breaks down the suppression. Moving the anti-suppression targets from periphery to the central field is effective. Target size and element separation are gradually reduced. Other stimulus parameters that can be manipulated in the treatment are: brightness, contrast, color, on-period, and movement. The orthoptic training goal is foveal fixation, normal acuity, and pathological diplopia when the eyes deviate and physiological diplopia when the eyes are fused. When the acuity and suppression improve to the point where sensory fusion can be demonstrated, occlusion can be reduced over time as sensory fusion is correspondingly being increased. The level of acuity that just permits sensory fusion is quite variable across patients; few can fuse with monocular amblyopia of 20/200, and almost all patients can fuse (at near with the aid of optics) with amblyopia of 20/40. For most patients the threshold for sensory fusion is in the 20/60 to 20/100 range.

Added Lenses and Prisms

After acuity reaches the threshold level where sensory fusion is possible, added lenses and prisms become a major factor in treatment. For constant esotropes, the angle of deviation tends to be large; therefore fairly large amounts of added lens power are often needed as training lenses (up to 4 or 5 diopters of plus). Prisms that correct a vertical deviation that remains WHEN THE EYES ARE FUSED should be considered in conjunction with added lenses. While a plus add can be very effective in reducing an esotropia at near, it does nothing for the distant angle. When the esotropia is about the same magnitude at distance and near (namely, the calculated AC/A ratio equals the PD in centimeters), base-out prisms approximating the angle of deviation should be considered instead of an add to provide fusion at the angle of deviation. For AC/A ratios greater than the PD (esotropia larger at near than at distance), added plus power can be prescribed to neutralize the near eso that remains after prescribing base-out prism that nearly equals the distance angle. The large amounts of base-out prism initially needed to provide fusion for constant esotropes can be reduced as orthoptics successfully improves sensory and motor fusion.

Sensory/Motor Function Therapy

Sensory fusion training needs to precede fusional vergence training. Traditionally, one thinks about gaining amplitude of fusional vergence, but fusional vergence and accommodative facility, as well as transfer of training responses to free space, are probably more important than fusional amplitude in obtaining functional cure of a strabismus.

Surgery

If the optics necessary to provide constant fusion are large or unwieldy, or if fusion is not well maintained, surgery should be considered. It needs to be controlled, possibly with an adjustable suture, to avoid onset of a vertical deviation or noncomitancy. Post-surgical follow-up with optics and orthoptics can be important to deal with resulting vertical deviations, noncomitancy, or diplopia.