When the AC/A ratio in prism diopters of vergence per diopter of accommodation numerically equals the patient's interpupillary distance in centimeters, the angle of oculomotor deviation (phoria or squint) will be the same at all distances up to the nearpoint of convergence; meaning that this patient has a very small heterophoria at distance and near. Thus, the symptomatic patient having a normal distance phoria and a normal AC/A generally has symptoms, not from the small heterophoria, but rather from coexisting vertical heterophorias or accommodative dysfunctions.
The average adult interpupillary distance is in the neighborhood of 6 centimeters and the average calculated AC/A ratio is only slightly less at about 5.5 p.d./D. Thus, for patients with such P.D.'s and AC/A ratios, the distance and near heterophoria will be about the same. The importance of this situation is that these patients tend to have the same kinds of symptoms or visual problems at distance and near, and treatment tends to be equally effective for both distances.
After correcting the refractive error to improve sensory and motor fusion one can consider vertical prisms (that neutralize any vertical deviation present with the eyes in the fused position) in addition to added lens power for further improvement of sensory and motor fusion.
Prisms prescribed to neutralize even small amounts of vertical phoria can overcome symptoms and enhance horizontal fusion. Measures of the associated phoria (prism power that reduces fixation disparity to zero) usually indicate that nearly the full amount of any (disassociated) vertical phoria or esophoria needs to be corrected with prism.
Whether to consider prescribing lateral prisms at this point depends on the distance phoria. In general, horizontal prism is of little benefit for the patient with a normal distance phoria and a normal AC/A and its use should be deferred until other modes of therapy have been completed.
We have accepted the average AC/A range to be from 3 to 7 p.d./D., which is symmetric around a value of 5 p.d./D. For patients with such AC/A ratios added lenses produce significant changes in vergence. Thus, added plus lenses can be effectively used for these patients, especially if there is a concurrent accommodative dysfunction. Of course, the change in vergence per diopter of added lens power depends on the AC/A ratio. In our experience, the calculated AC/A gives a better indication of the ultimate vergence change than does the gradient AC/A ratio.
The different sequence of treatment for phoria patients having low, normal, and high AC/A ratios consists mainly of when to consider the use of added lenses and prisms. For the patient having a normal distance phoria and a normal AC/A (3 to 7 p.d./D) under consideration here, added lenses are a useful treatment to be considered after deciding on the refractive correction. In the model we have indicated considering prisms before added lenses mainly because of the important effect that vertical prisms can have in the treatment. Although added lenses are generally more effective than lateral prisms for these phoria patients, the advantages and disadvantages of both should be considered for an individual patient.
Once the optical considerations have been dealt with in the format described here for normal AC/A patients, the other treatment considerations are similar for all phoria patients.
Any amblyopia should be treated with passive occlusion and/or active training, primarily under binocular conditions. Suppression, which is usually slight and confined to the central retina, must also be treated whether or not amblyopia is present. The ultimate goal here is to eliminate any amblyopia and suppression so that sensory and motor fusion training can be maximally effective; however, sensory and motor fusion training can be introduced as the amblyopia and suppression are being eliminated.
For patients with normal distance phorias and normal AC/A ratios, vision training of fusional vergence is typically equally effective for distance and near. Indeed, fusional training can also be additionally effective in breaking down suppression and amblyopia. For the patient with a normal distance phoria and a normal AC/A be especially aware of the need to train accommodative facility in addition to both of the vergence functions (base-in and base-out). Accommodative facility is readily achieved and overall clinical success can often be achieved through training alone or in combination with a near plus addition.
Surgery is not the problem for esophoric patients with a normal AC/A as it is for the low and high AC/A patients. Even so, surgery is only considered in the occasional instance of a very large esophoria when optical and training management are unsuccessful.