Many exotropes who appear to be constant at initial examination are found upon subsequent evaluation to be able to fuse at some distance (usually extremely close) or under some circumstances (such as with extreme voluntary effort) --- thus making them technically occasional exotropes for whom the prognosis is considerably better and the treatment considerably easier.
The main point in treating constant exotropes 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.
Occlusion to reduce the amblyopia and suppression is facilitated first by optimum minus correction of the refractive error to provide the sharpest possible retinal image and to maximize the potential for fusion. Occlusion for constant squinters 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.
Amblyopic training consists initially of monocular fixation training, with feedback if 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 anti-suppression 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. When acuity reaches the threshold level where sensory fusion is possible, added lenses and prisms become a major factor in treatment. For constant squinters, 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 added minus). Prisms that correct a vertical deviation that remains WHEN THE EYES ARE FUSED should be considered in conjunction with added lenses.
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. For exotropes the rapid improvement in fusional vergence with orthoptics tends to eliminate the need for large amounts of base-in prisms; therefore, orthoptics should be considered for constant exotropes after or in conjunction with the added minus lenses and before, or in place of, prescribing any base-in prism.
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.